A11102 1=1777

NBSIR 87

Measurement of the Setting Time and

Strength of Concrete by the Impact-Echo

Method

Stephen P. Pessiki

Nicholas J. Carino

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Building Technology

Structures Division

Gaithersburg, MD 20899

July 1987

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

-QC

100

. U56

#87-3575

1987

NBSIR 87-3575

MEASUREMENT OF THE SETTING TIME AND

STRENGTH OF CONCRETE BY THE IMPACT-ECHO

METHOD

Research Information Center

Kational bureau oi Standards

Gaithersburg, Maryland 20899

auoo

Stephen P. Pessiki

Nicholas J. Carino

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Building Technology

Structures Division

Gaithersburg, MD 20899

July 1987

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

ABSTRACT

Tests were performed to evaluate the feasibility of using the impactecho method to determine setting time and monitor strength development of

concrete. In the impact-echo method, the test object is subjected to point

impact and the surface displacement adjacent to the impact point is monitored.

From the measured displacement waveform and the thickness of the object, the

P-wave velocity is determined. Changes in the P-wave velocity with time

reveal information about the development of mechanical properties

Setting time tests ware made on concrete mixtures of two water -cement

ratios and with and without set-controlling admixtures. A strong correlation

was found between the time of initial setting of mortars sieved from the

concrete, as determined by penetration resistance (ASTM C 403), and the time

when the P-wave velocity began to increase. Two approaches for using the

impact- echo method to define the setting time of concrete are presented.

Tests were performed to examine the relationship between P-wave velocity,

as determined by the impact-echo method, and the compressive strength of

concrete. At early ages (up to about 3 days at standard temperature), the

relationship was independent of curing temperature and water cement ratio.

It is concluded that the impact-echo method is a promising technique for

nondestructively monitoring the development of mechanical properties in

concrete from initial setting to ages of several days.

KEYWORDS: Concrete; compressive strength, early-age, impact-echo method,

nondestructive testing, mortar, setting time, wave propagation.

iii
TABLE OF CONTENTS

Page

Abstract iii

List of Tables vii

List of Figures .... ................... viii

CHAPTER 1: INTRODUCTION ..... ... 1

1.0 Introduction 1

1 . 1 Background 1

1.2 Objectives of Research 3

1.3 Scope 3

CHAPTER 2: LITERATURE REVIEW 5

2.0 Introduction . 5

2.1 Pulse Velocity and Setting 5

2.2 Pulse Velocity and Compressive Strength .......

2.2.1 Curing Temperature . ..... 7

2.2.2 Water -Cement Ratio ...... . 8

2.2.3 Content, Type, and Maximum Size of

Coarse Aggregate 8

2 . 3 Summary 8

CHAPTER 3: BACKGROUND 10

3.0 Introduction 10

3.1 Impact-Echo Technique 10

3.2 Vibration of a Cylinder Caused by Impact 11

3.2.1 Elastic Impact on a Solid 12

3. 2. 1.1 Stress Waves Generated by Impact ........ 12

3. 2. 1.2 Force-Time Function and Contact Time

of the Elastic Impact of a Sphere

on a Plate 14

3. 2. 1.3 Frequency Content of Impact 15

3.2.2 Mode Shapes of Vibration of a Free Cylinder 15

3.2.3 Experimental Study of the Vibration of a Free Cylinder Caused by Impact 16

3.3 Development of Elastic Modulus in Cement Paste 18

3 . 4 Summary 20

CHAPTER 4: MEASUREMENT OF SETTING TIME 29

4.0 Introduction 29

4.1 Development of Measurement Technique ....... 29

4.1.1 Experimental Set-up 29

4.1.2 Signal Interpretation 30

4.1.3 Sampling Frequency and Voltage Range 30

4.1.4 Transducer Resonance 31

iv
4.2 Prepared Mortar Tests 31

4.2.1 Objective and Scope 31

4.2.2 Materials 32

4.2.3 Specimen Preparation 32

4.2.4 Results and’ Observations 32

4.3 Series Model 34

4.3.1 Description of the Series Model 34

4.3.2 Experimental Evaluation of the Series Model 36

4.4 Concrete and Sieved Mortar Tests 38

4.4.1 Objective and Scope 38

4.4.2 Materials 39

4.4.3 Specimen Preparation 39

4.4.4 Results and Observations 39

4.4.4.

Velocity Measurements on Concrete . 40

4.4.4.

Influence of Set -Controlling Admixtures 42

4.4.4.

Influence of Changes in Water -Cement

Ratio 43

4. 4. 4.Specimen Temperatures 44

4.5 Defining Setting Time Using Impact -Echo

Velocity Measurements 45

4.5.1 Defining Setting Time at the Onset of

P-Wave Velocity Development 45

4.5.2 Defining Setting Time at a Specified

P-Wave Velocity 46

4 . 6 Summary 47

CHAPTER 5: MEASUREMENT OF COMPRESSIVE STRENGTH 77

5.0 Introduction 77

5.1 Objective and Scope 77

5.2 Experimental Set-up and Procedures 78

5.2.1 Materials 78

5.2.2 Mixture Proportions 78

5.2.3 Specimen Preparation 78

5.3 Specimen Testing 79

5.3.1 Velocity Measurement 80

5.3.2 Compressive Strength Determination ..... 81

5.4 Anticipated Compressive Strength - Velocity

Relationship 81

5.5 Experimental Results 81

5.5.1 Regression Model: Room-Cured Specimens 82

5.5.2 Curing Temperature Variation 84

5. 5. 2.1 Hot-Cured Specimens 84

5. 5. 2.Cold-Cured Specimens 84

5.5.3 Water-Cement Ratio Variation 84

5.5.4 Aggregate Content Variation 85

5.6 Discussion and Summary 85

v
CHAPTER 6: SUMMARY, CONCLUSIONS, AND FUTURE RESEARCH ... 99

6.0 Introduction 99

6.1 Summary . 99

6.1.1 Setting Time Study ......... 99

6.1.2 Compressive Strength Study ............... 99

6.2 Conclusions 100

6.3 Practical Applications 100

6.4 Future Research 101

CHAPTER 7: REFERENCES 102

APPENDIX A: Setting Time Data • 105

APPENDIX B: Strength Data 109
LIST OF TABLES

Table Page

3.1 Frequency parameters for a cylinder (aspect ratio - 2) 21

4.1 Mixture proportions (by weight) for prepared mortar tests 49

4.2 Material properties for prepared mortar tests ; 49

4.3 Mixture proportion for concrete -sieved mortar tests 50

4.4 Material properties for concrete -sieved mortar tests 50

4.5 Results of slump tests 51

4.6 Comparison of setting times 51

5.1 Mixture proportions for compressive strength tests 87

5.2 Summary of strength test program 87

vii
LIST OF FIGURES

Figure Page

1.1 Idealization of the early-age strength development of

concrete 4

2.1 Development of pulse velocity with time 9

3 . 1 Displacement transducer 22

3.2 Typical impact-echo measurement: a) surface displacement

waveform; and, b) frequency spectrum 22

3.3 Schematic of experimental set-up .......... 23

3.4 Wavefronts of P- , S- and R-waves caused by point impact 23

3.5 Characterization of P- , S-, and R-waves: a) P-wave;

b) S-wave; and, c) R-wave ............... 23

3.6 Force-time function of the elastic impact of a sphere

on a solid 24

3.7 Frequency content of the force -time function shown in

Figure 3.6 24

3.8 Approximate form of vibration of 20 of the first 22

modes of vibration of solid cylinders 25

3.9 Frequencies of the first six modes of vibration

circumferential order 0 .......... . 25

3.10 Frequency content of 60 and 30 us duration impacts 26

3.11 Response of a hardened concrete cylinder to impact by a 12.7 mm diameter ball (tc - 60 us): a) surface displacement

waveform; and, b) frequency spectrum 27

3.12 Response of a hardened concrete cylinder to impact by a

5.56 mm diameter ball (tc - 30 us): a) surface displacement

waveform; and, b) frequency spectrum 27

3.13 Structure of cement paste at various stages of

hydration: a) after initial mixing; b) post- induction

period structure; c) a later stage of hydration;

and, d) advanced stage of hydration 28

3.14 Structure of cement paste at various stages of

hydration - higher w-c ratio: a) after initial mixing;

b) post- induction period structure; c) even later stage

of hydration; and, d) advanced stage of hydration 28

viii
4.1 Schematic representation of experimental set-up used

to monitor the development of P-wave velocity in

early- age concrete 52

4.2 Impact and receiver plates 52

4.3 Typical surface displacement waveforms at ages of:

a) 7.3 hours; and, b) 3.1 hours 53

4.4 Frequency spectra of the waveforms shown in: a) Figure

4.3 (a); and, b) Figure 4.3 (b) 53

4.5 Results of prepared mortar test ML2: a) P-wave velocity

vs age; and, b) penetration resistance vs age 54

4.6 Results of prepared mortar replicates MH4 and MH5:

a) P-wave velocity vs age; and, b) penetration

resistance vs age 55

4.7 Results of prepared mortar replicates ML1 through ML4:

a) P-wave velocity vs age; and, b) penetration

resistance vs age 56

4.8 Results of prepared mortar replicates MH1 through MH3:

a) P-wave velocity vs age; and, b) penetration

resistance vs age 57

4.9 Results of prepared mortar tests ML1, ML2 , MH4, and

MH5 : a) P-wave velocity vs age; and, b) penetration

resistance vs age 58

4.10 Description of the series model: a) single ray

propagating through the concrete; b) idealization of

the path as a series of paste and aggregate regions; and,

c) equivalent two -phase model 59

4.11 Two-phase series model, consisting of paste and

aggregate phases, used to model the concrete 59

4.12 Gain in velocity in concrete with age: a) gain in

velocity in the cement paste; b) constant velocity in

the aggregate; and, c) gain in velocity in concrete

predicted by Eq. 4.2 for different aggregate contents 60

4.13 Temperatures of the cement paste, 40 % aggregate, and

60 % aggregate specimens vs age 61

4.14 Linear regressions to determine (CpP - C 0 )

K, and M0: a) Equation 4.9; and, b) Equation 4.10 62

4.15 P-wave velocity vs maturity for the cement paste, 40 %

aggregate, and 60 % aggregate specimens 63

ix
4.16 Results from test CL2 (low w-c ratio mixture):

a) penetration resistance vs age; and,

b) P-wave velocity vs age 64

4.17 Frequency spectra from test CL2 at ages of: a) 1.78

hours; b) 2.10 hours; c) 2.42 hours; d) 2.73 hours;

e) 2.88 hours; f) 3.00 hours; g) 3.13 hours; h) 3.45

hours; i) 4.07 hours; j) 4.60 hours; k) 6.00 hours;

and, 1) 7.50 hours 65

4.18 Results of replicate tests of a low w-c ratio mixture

( tests CL1, CL2, and CL3) : a) penetration resistance

vs age; and, b) P-wave velocity vs age 67

4.19 Results of replicate tests of an accelerated mixture

(tests CAl, CA2, and CA3) : a) penetration resistance

vs age; and, b) P-wave velocity vs age 68

4.20 Results of replicate tests of a retarded mixture

(tests CR1, CR2, and CR3):a) penetration resistance

vs age; and, b) P-wave velocity vs age .............. 69

4.21 Results of tests CA3 and CR1: a) penetration resistance

vs age; and, b) P-wave velocity vs age 70

4.22 Results of replicate tests of a low w-c ratio mixture

(tests CHI, CH2 , and CH3): a) penetration resistance

vs age; and, b) P-wave velocity vs age 71

4.23 Results of tests CHI and CL2 : a) penetration resistance

vs age; and, b) P-wave velocity vs age 72

4.24 Specimen temperature vs age: a) test CL3 ; b) test CA3

c) test CR1; and, d) test CHI 73

4.25 Two approaches for using impact-echo velocity

measurements for defining the setting time of concrete 75

4.26 Comparison of setting times - penetration resistance

initial setting time vs start of velocity gain 76

4.27 Comparison of setting times - penetration resistance

initial setting time vs target velocity of 670 m/s 76

5.1 Methodology of in-place strength estimation .... 88

5.2 Typical velocity measurement: a) surface displacement

waveform; and, b) frequency spectrum 88

5.3 Anticipated fourth-power relationship between

compressive strength and P-wave velocity 89

x
5.4 Room-cured specimens: strength vs velocity 89

5.5 Results of regression analysis using Eq. 5.5: a) linear

fit of Eq. 5.5; b) residuals in log space; c) residuals

in real space; and, d) best-fit of Eq. 5.5 90

5.6 Results of regression analysis using Eq. 5.7: a) linear

fit of Eq. 5.7; b) residuals in log space; c) residuals

in real space; and, d) best- fit of Eq. 5.7 ; 92

5.7 Hot-cured specimens: a) strength vs velocity; and,

b) residuals in real space about best-fit of Eq. 5.7 to

room- cured results 94

5.8 Cold- cured specimens: a) strength vs velocity; and,

b) residuals in real space about best- fit of Eq. 5.7 to

room- cured results . 95

5.9 Low w-c ratio: a) strength vs velocity; and,

b) residuals in real space about best- fit of Eq. 5.7 to

high w-c ratio results 96

5.10 High aggregate content: a) strength vs velocity; and,

b) residuals in real space about best-fit of Eq. 5.7 to

low aggregate content results 97

5.11 Room-cured specimens: velocity and strength vs age 98
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. wri

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CHAPTER 1

INTRODUCTION

1 . 0 INTRODUCTION

The purpose of this research was to evaluate a new test method for

measuring the setting time and strength of concrete. The technique uses

transient stress waves and is called the impact-echo method. This technique

has previously been used for flaw detection in hardened concrete (10,31). In

this method, stress waves are introduced into a test object by mechanical

impact on its surface. The multiple reflections (echoes) of the stress waves

from the boundaries of the object are monitored and used to determine the wave

velocity. The change in wave velocity with age is related to setting and

strength gain.

1 . 1 BACKGROUND

Upon initial mixing, fresh concrete is a fluid- like material. Through

the physical and chemical processes collectively termed hydration, the fresh

concrete gradually changes from this fluid- like material into a hardened

mass. Figure 1.1 is an idealization of the early-age strength development of

concrete. As can be seen in the figure, strength gain does not begin

immediately after mixing. Instead, there is an induction period, during which

the concrete remains fluid- like. After this induction period the concrete

gradually increases in stiffness. This increase in stiffness occurs during

what is known as the setting period. By the end of the setting period the

concrete has changed to a rigid state, but still has little appreciable

strength. Appreciable strength gain begins during the hardening period, after

setting has occurred.

Setting is a gradual stiffening process, and it is difficult to define

precisely when setting begins and when it has ended. The practical approach

that has been adopted in standard test methods is based on monitoring the

development of resistance to penetration by specified test needles. The times

required to reach different levels of penetration resistance are used to

define initial and final setting times of paste or mortar specimens. Since

arbitrary levels of penetration resistance are used in these definitions, the

setting times correspond to arbitrary levels of stiffening rather than to the

start and end of some physical or chemical process. However, setting times

based on penetration resistance are very useful in studying the effects of

pertinent factors on the setting behavior of cementitious systems. The

technique addressed by this research is an alternative to tests based on

penetration resistance.

The need for being able to measure the setting time of concrete in-place

can be understood with reference to Figure 1.1. Shown in the figure as a dashed line is a similar strength development curve, but with a delayed

setting time. Such a delay could be caused by reduced temperature or by

chemical admixtures. It is seen that the delay in setting also causes a delay

in the subsequent strength gain and may have a great effect on the value of

strength at a very early age. As an example, consider the strengths indicated

by each curve at the time, t]_, indicated with a dashed line in the figure.

Although the shapes of the two strength development curves are similar, the

1
concrete with a delayed setting time has a much lower strength at this age.

Thus , detection of an unexpected delay in setting time may influence early

construction operations such as the time of formwork removal, slipforming, or transfer of prestressing. Further, the ability to measure concrete setting

times in-place can be used as a means of quality control when set-controlling

admixtures are being used. The ability to determine when a concrete has set

can also be used to reduce formwork pressures from freshly placed concrete.

Pressures on tall wall forms can be reduced if earlier placed lifts have set

prior to the placement of later lifts. Lastly, the ability to monitor the

setting behavior of concrete may be used to determine when finishing

operations can proceed.

Currently, there is no standard test (either a field or a laboratory

test) for measuring the setting time of concrete that is performed on the

concrete itself. The existing concrete setting time test, ASTM C 403 Standard

Test Method for Time of Setting of Concrete Mixtures by Penetration Resistance

(3), is performed on mortar sieved from the concrete. The resistance offered

by the sieved mortar to penetration by various diameter needles is measured.

Initial and final setting times are defined by penetration resistances of 500

and 4000 psi (3.5 and 27.6 MPa), respectively. Clearly, any definition of

setting time based on some measure of strength (whether performed on paste,

mortar, or concrete) must be arbitrary, because the strength development is a gradual process

The penetration resistance setting time test is based on a soil testing

technique adapted by Tuthill and Cordon for studying retarding admixtures (35)

in concrete. Tuthill and Cordon concluded that the vibration limit of

concrete, or point during the setting process when it can no longer be made

plastic by revibration, is reached when the sieved mortar reaches a penetration resistance of 500 psi. Further, they concluded that at a penetration resistance of 4000 psi, the accompanying concrete had a strength

of about 100 psi. The 4000 psi penetration resistance was the limit of their

penetration device.

There are two limitations to using the penetration resistance test to

define the setting time of concrete. First, the test is not performed on the

concrete itself. Differences between the behavior of the concrete and sieved

mortar are reported in Chapter 4. Second, it is not an in-place test, but is

performed on small prepared specimens. Such specimens generally do not

experience the same temperature and moisture conditions as the concrete in the

structure , and thus are not necessarily representative of the concrete in the

structure

The ability to determine in-place concrete strength can be used to

improve construction safety and economy. Reliable measurements of in-place

strength can be used to determine when it is safe to remove formwork, transfer

prestressing, apply post- tensioning, and to terminate curing. In-place

strength information allows each of these operations to begin safely at the

earliest possible time, leading to more economical and efficient construction.

Because of the importance of these safety and economic considerations,

many methods of estimating in-place strength already exist. These methods

include; rebound hammer (ASTM C 805), probe penetration (ASTM C 803), pullout

(ASTM C 900)

ultrasonic pulse velocity, maturity (recently adopted as an ASTM

Standard Test Method), and cast- in-place cylinders (ASTM C 873) (3). A review

of each of these methods, discussing their underlying principles and

limitations can be found in Reference (1) . As will be explained, the

2
:

impact-echo method offers another alternative means for estimating concrete

strength at early ages

1.2 OBJECTIVES OF RESEARCH

The objectives of this study project were to experimentally evaluate the

usefulness of the impact-echo technique as a means to;

1. define and measure the setting time of concrete

2. estimate the strength of concrete

The work is presented as a first step towards reliable in-place setting time

measurement and strength estimation tests for concrete using the impact- echo

method.

1.3 SCOPE

Chapter 2 reviews previous work that used stress wave propagation to

monitor the setting behavior and strength gain of concrete.

Background information is presented in Chapter 3 . The experimental

set-up and technique for making impact-echo velocity measurements is

described, and the fundamentals of wave propagation in elastic solids are

reviewed. As will be discussed, the setting time and strength experiments

were performed using cylindrical concrete specimens. Thus, Chapter 3 also

discusses the vibrations of a cylinder caused by impact. The importance of

the frequency content of the impact on the resulting cylinder vibrations is

examined.

Chapter 4 presents the results of the setting time study. The adaptation

of the impact-echo technique for measurements on early- age mortar and concrete

is discussed. Results of impact-echo velocity measurements made on prepared

mortar and concrete specimens are presented. In these tests, the development

of wave velocity was monitored from the time of initial casting up to ages of

about 12 hours. A simple model is used to explain the wave velocity in the

concrete in terms of the velocities in the paste and aggregate . Two

approaches for using the impact-echo method to define and measure the setting

time of concrete are presented.

Chapter 5 presents the results of the compressive strength study. The

influence of changes in curing temperature, water to cement (w-c) ratio, and

aggregate content on the relationship between compressive strength and wave

velocity are discussed. As will be discussed, strength-velocity tests were

made as early as 6 hours after initial mixing and continued up to an age of

about 28 days

Chapter 6 summarizes the important results and conclusions of the

research, discusses applications, and outlines future research needs.

3
Strength

Figure 1*1 Idealization of the early-age strength development

of concrete

4

i
CHAPTER 2

LITERATURE REVIEW

2 . 0 INTRODUCTION

Stress wave propagation has been used in the past to study the setting

behavior and strength gain of concrete. The testing method used is the

ultrasonic pulse velocity technique, which can briefly be described as follows. A pulse, or short train of stress waves, is introduced into a test

object at one surface by a transmitting transducer (source). The arrival of

this pulse is detected at a second location with a receiving transducer

(receiver) . The time interval between the initiation and reception of the

pulse is measured, and the pulse velocity is determined by dividing this time

interval by the known distance between the source and the receiver.

The present study uses a stress wave propagation method known as the

impact-echo technique, which differs from the pulse velocity technique in the

following manner. The pulse velocity technique is a through transmission

method; the source and receiver are generally located on opposite sides of a test object. In the impact-echo technique, the source and receiver are

located on the same surface of the test object. Stress waves are introduced

at the source. The periodicity of multiple reflections of these stress waves

within the boundaries of the test object, along with the dimensions of the

object, are used to calculate wave velocity.

The two methods are similar in that both are stress wave propagation

testing techniques . Thus , this chapter reviews previous work that used the

ultrasonic pulse velocity method to monitor setting behavior and strength gain

of concrete. Section 2.2 reviews work related to setting behavior, and

Section 2.3 reviews work related to compressive strength.

2.1 PULSE VELOCITY AND SETTING

Many attempts were reported in the late 1940 's and early 1950 's on the

measurement of pulse velocity in early-age concrete (4,14,15,19,20,21,38).

These investigations examined the influence of variables such as mixture

proportions, curing temperature, etc. on the development of pulse velocity.

It was not until 1957 that the penetration resistance test (ASTM C 403) became

a standard test method for measuring the setting time of concrete. Thus most

of these early studies compared the results of pulse velocity measurements to

setting time measurements performed on cement pastes. More recent work (12)

has compared the development of pulse velocity and penetration resistance in

mortar sieved from concrete.

Jones (19,20) and Arndt (4) reported some of the earliest attempts to

monitor the development of pulse velocity in concrete . From the results

presented by Jones it appears the earliest measurements were obtained at about

10 hours after initial mixing, and the corresponding pulse velocities ranged

from 1200 to 1600 m/s. As will be shown in Chapter 4, the values of these

earliest measurements are high compared to the earliest measurements obtained

in the present study.

Arndt presented curves showing the development of pulse velocity in

concretes made with three different cements and cured at two different

temperatures. He reported that the pulse velocity- age curves reflect the

5
influence of temperature on setting time, as well as differences in setting

times occurring between the different cements. Arndt did not evaluate the

curves insofar as defining a setting time, but presented them to show the

usefulness of the pulse velocity method as a means of "describing the

processes" occurring during setting.

One of the earliest reported efforts specifically directed at evaluating

the usefulness of the pulse velocity method as a means of determining the

setting time of concrete was reported by Whitehurst (38) . Whitehurst

monitored the development of pulse velocity for several concretes, and

compared the results with setting times of cement pastes determined by

Gillmore needles (ASTM C 266) (2) . Whitehurst used 100 mm x 100 mm x 400 mm

beam specimens, the 400 mm dimension being the transmission length. The

concrete mixtures were very stiff with slumps ranging from 3 to 6 mm. The

stiff mixtures were used so that the ends of the beam molds could be removed

after casting and the transmitting and receiving transducers placed directly

against the fresh concrete. Even with these stiff mixes, successful

measurements could not be made for the first two to four hours after mixing,

depending on the cement used. Figure 2.1 shows typical results from

Whitehurst's experiments. Whitehurst measured initial velocities of 1200

m/s. He reported that during the period from 4-1/2 to 8-1/2 hours after

initial mixing there was a rapid increase in pulse velocity. After this

period of dramatic increase, there was a decrease in the rate of velocity

increase. The elapsed time from initial mixing to when the rate of velocity

increase slowed was taken by Whitehurst as the setting time of the concrete.

These setting times were in good agreement with setting times measured using

the Gillmore needles. Whitehurst also reported that in all tests the signals

received during the setting period (first 8 hours) were very weak. Attempts

to perform the tests on concretes with wetter consistencies were unsuccessful

because cracks developed in the beams when the end plates of the forms were

removed.

Cheesman (14,15) also reported results from a study of the development of

pulse velocity with time in concrete. Cheesman tested 100 mm slump concretes

placed in 100 mm x 100 mm x 510 mm molds. Measurements were made through the

100 mm dimension as early as 10 minutes after mixing. The transmitting and

receiving transducers were placed against rubber windows fitted into the

specimen molds. Cheesman noted that the use of these windows lowered the

measured pulse velocities. For the concretes tested, the pulse velocity did

not increase as rapidly, or stop increasing as abruptly as observed by

Whitehurst. Cheesman stated that because of this more gradual development of

pulse velocity, it was difficult to correlate inflection points in the pulse

velocity development curves to setting time measurements made on cement pastes

with Vicat needles (ASTM C 191) (2).

Casson and Domone (12) monitored the development of pulse velocity and

penetration resistance in mortar sieved from concrete. Pulse velocities were

determined through a 30 mm path length. While detailed comparisons were not

presented, Casson and Domone concluded that the effectiveness of admixtures on

setting times could be assessed using pulse velocity measurements. In

addition, they reported results of pulse velocity measurements through

concrete, using a 300 mm path length. They reported that it was only after

hydration created a "semi-viscous/plastic paste matrix" that successful

measurements could be made. The earliest measurements made ranged from 1000

to 2000 m/s

6
2.2 PULSE VELOCITY AND COMPRESSIVE STRENGTH

Many researchers have studied the relationship between pulse velocity and

compressive strength. In these studies, the effect of the following variables

on the pulse velocity-compressive strength relationship have been

investigated:

- curing temperature

- content, type, and maximum size of coarse aggregate

- water to cement (w-c) ratio

- moisture content

- air content

A report by Sturrup, Vecchio, and Caratin ( 33 > provides a comprehensive

review of these investigations. The following sections focus on work

performed to study the effects of curing temperature, w-c ratio, and aggregate

content, type, and size.

2.2.1 Curing Temperature

The work by Elvery and Ibrahim (16) is probably the most extensive recent

investigation into the variables influencing the relationship between pulse

velocity and early- age compressive strength. The experiments were performed

using 100 mm cubes. The cubes were placed in curing cabinets immediately

after casting to allow them to reach the specified temperature as quickly as possible

To compare the influence of different variables upon the pulse

velocity-compressive strength relationship, the test data were divided into

four ranges, and separate statistical analyses were made for each range. The

four ranges were delineated at the following compressive strengths: range A,

0.05 to 0.30 MPa; range B, 0.30 to 2.00 MPa; range C, 2.00 to 15.00 MPa; range

D, 15 to 60 MPa.

Elvery and Ibrahim concluded that curing temperature does not influence

the relationship between pulse velocity and compressive strength in ranges C

and D, but that temperature did have some influence in ranges A and B.

Sturrup, Vecchio, and Caratin (33) presented pulse velocity- strength

curves for concretes cured at 10, 21, and 32 °C. They concluded that curing

temperature does affect the pulse velocity-compressive strength relationship,

but they did not generalize about how it affects the relationship.

The results of these studies seem to show that the influence of curing

temperature upon the pulse velocity-compressive strength relationship is

uncertain.

2.2.2 Water-Cement Ratio

The relationship between pulse velocity and compressive strength has been

found to be independent of w-c ratio (8,9,16,21). Elvery and Ibrahim (16)

reported that changes in w-c ratio had even less influence on the pulse

velocity- compressive strength relationship than changes in curing

temperature. Jones (21) pointed out that an increase in w-c ratio increases

the number of water voids present in concrete. These voids decrease both

7
compressive strength and pulse velocity, and the relationship between the two

apparently is unchanged.

2.2.3 Content, Type, and Maximum Size of Coarse Aggregate

It is clear from the results of previous work that an increase in

aggregate content produces a higher pulse velocity for a given compressive

strength (6,16,19,21). Since normal weight coarse aggregate has a higher

elastic modulus than cement paste (for moderate w-c ratios) , increasing the

aggregate content of concrete would be expected to increase the pulse

velocity. This is discussed further in Section 4.3.

Changes in pulse velocity-compressive strength relationships caused by

changes in aggregate type have been reported (21,33). An aggregate with a higher elastic modulus can be expected to lead to a higher pulse velocity for

a given aggregate content.

Many researchers have reported that concretes made with a larger maximum

aggregate size exhibit a lower compressive strength for a given pulse velocity

(6,8,9,33). Bullock and Whitehurst (6) demonstrated this for concretes tested

at an age of 28 days. This effect, however, was attributed largely to an

increased percentage of coarse aggregate in a properly proportioned concrete

when a larger size aggregate is used, leading to a higher velocity. Byfors

(8) reported that this effect is greater at earlier ages than later ages.

2 .SUMMARY

It is apparent that one difficulty encountered in using the ultrasonic

pulse velocity technique to monitor the setting behavior of concrete is that

it is difficult to make measurements at very early ages. It appears that a certain degree of hydration is needed before successful measurements can be

made. There does appear to be some correlation between the development of

pulse velocity in concrete and the results of setting time tests performed on

cement pastes.

The influence of changes in curing temperatures on the relationship

between pulse velocity and compressive strength is uncertain. The pulse

velocity- compressive strength relationship has been found to be independent of

w-c ratio, and an increase in aggregate content was found to lead to higher

pulse velocity for a given compressive strength. Lastly, an aggregate with a higher elastic modulus will lead to higher pulse velocity for a given

aggregate content.
Wove

Velocity,

ft

per

sec

i

Elapsed Time Attar Addition of Water ts Mix

Figure 2.1 Development of pulse velocity with time (38).

9
CHAPTER 3

BACKGROUND

3 . 0 INTRODUCTION

This chapter presents background information needed for the remainder of

the report. The experimental technique for making impact- echo measurements is

explained in Section 3,1; adaptation of the method for use on early age

concrete is discussed in Chapter 4. The setting time and strength experiments

were performed using cylindrical concrete specimens. In order to interpret

the impact-echo measurements, the vibration of a cylinder caused by impact has

to be understood; this is explained in Section 3.2. In Section 3.2 it will be

seen that one of the factors that determines the velocity of stress waves

within a solid is the elastic modulus. Therefore .Section 3.3 discusses the

development of elastic modulus in cement paste.

3.1 IMPACT -ECHO TECHNIQUE

As stated in Chapter 1, the test technique used in this study involves

introducing transient stress waves into a cylindrical specimen by mechanical

impact. Multiple reflections of these waves within the boundaries of the

cylinder were monitored and used to calculate wave velocity. Changes in this

wave velocity with age were monitored and related to setting and strength

gain.

The impact sources were small steel spheres ranging in diameter from 5.56

to 12.7 mm (0.22 to 0.50 in.). To make a measurement, a sphere was dropped

from a height of 200 mm (7.9 in.) onto the top surface of the cylinder. The

stress waves introduced into the cylinder by the impact are discussed in

Section 3 . 2 . 1 . 1

The resulting vibrations were monitored with a receiving transducer, also

located on the top surface of the cylinder. The transducer used in the study

is shown in Figure 3.1. It is a broadband displacement transducer that

responds to normal surface displacement (29). The transducer was originally

developed for acoustic emission testing of metals, so a thin lead strip was

inserted between the piezoelectric element and the cylinder surface to

complete the transducer circuit.

The time -history of the surface displacement detected by the transducer

is called a displacement waveform. Figure 3.2 (a) shows a typical

displacement waveform obtained from a 152 mm diameter by 152 mm deep hardened

concrete cylinder. The waveform is a digital record comprised of 1024 points

recorded at a sampling frequency of 500 kHz (sampling interval of 2 us). Thus

the duration of the recorded response is 1024 x 2 us - 0 . 002048 seconds

The waveform in Figure 3.2 (a) is composed of the surface displacement

plus a low frequency oscillation attributed to the resonance of the

transducer. The effects of transducer resonance can be reduced by using a high pass filter, which strongly attenuates that part of a signal below a specific threshold frequency. A filter was used In some instances in the

setting time and strength studies, and is discussed further in those

respective chapters

Multiple reflections of stress waves from the boundaries of the cylinder

give the displacement waveform shown in Figure 3.2 (a) a periodic nature. The

10

j
periodicity of the reflections contained in the waveform was revealed by

transforming the time -domain waveform into the frequency domain using the

principle of the Fourier transform (32). The principle behind the

transformation is that the displacement waveform can be constructed by the

superposition of sine curves of varying amplitudes and frequencies, with

appropriate phase shifts . The transformation of the waveform in Figure 3 .

(a) results in the frequency spectrum shown in Figure 3.2 (b)

The

frequencies of the large amplitude sine curves in the displacement waveform

appear as large amplitude peaks, in the frequency spectrum. Thus, peaks in the

frequency spectrum reveal the frequencies of the multiple reflections within

the cylinder. As will be explained in Section 3.2.3, the peak with the lowest

frequency in the spectrum (above the peak attributed to transducer resonance)

and the specimen height were used to compute the velocity of stress waves

within the specimen. The transformation from the time domain to the frequency

domain was made using the Fast Fourier Transform (FFT) technique (32)

In digital signal analysis, the frequency interval in the frequency

spectrum, Af (difference between adjacent frequency values), is

Af - fs / N (3.1)

where fs is the sampling frequency. N is the number of points in the record,

which is dependent on the data acquisition device. Thus, as the sampling

frequency increases (providing greater resolution in the displacement

waveform) , for a fixed value of N, the frequency interval in the frequency

spectrum increases . For the example in Figure 3.2, the frequency interval is

500 kHz/1024 - 0.488 kHz.

A unity gain amplifier which was connected close to the transducer was

used. This amplifier allowed the signal produced by the transducer to be

measured without reducing its voltage.

The experimental set-up is shown in Figure 3.3. Data acquisition and

processing, such as the FFT, were done using a digital processing oscilloscope

with a maximum sampling frequency of 500 kHz. The oscilloscope was equipped

with two disk drives for permanent data storage, and was interfaced with a plotter so that displacement waveforms and frequency spectra could be plotted.

3.2 VIBRATION OF A CYLINDER CAUSED BY IMPACT

Interpretation of the impact- echo tests on cylindrical concrete specimens

requires an understanding of the response of a cylinder to impact. An

unsupported cylinder has stress free surfaces and would be able to vibrate

freely in space. Clearly, this condition can not be realized in the

laboratory, because the cylinder must be supported in some manner. As a result, certain boundary displacements are constrained and surface stresses

can arise. However, support can be provided in such a way that effects on the

resulting vibrations are negligible, and vibrations can thus be considered to

approximate those of a free cylinder.

To understand the response of a cylinder during an impact-echo test

requires an understanding of impact. Thus, Section 3.2.1 discusses the

details of impact, examining the stress waves generated by impact, as well as

11
the force -time function, contact time, and frequency content of impact between

a sphere and a solid. To help understand the response of a cylinder to

impact, previous experimental work studying the mode shapes and frequencies of

vibration of solid cylinders under the action of constant driving frequencies

is reviewed in Section 3.2.2. Section 3.2.3 reports the results of an

experimental investigation into the vibration of a cylinder during the

impact- echo test, examining the effects of the frequency content of the impact

on the resulting vibrations

3.2.1 Elastic Impact on a Solid

3. 2. 1.1 Stress Waves Generated by Impact

Three types of waves are generated by impact on the surface of an elastic

solid; a surface wave, a dilatational wave, and a distortional wave. The

surface wave , also known as the Rayleigh or R-wave , propagates along the

surface of the solid, whereas the dilatational and distortional waves

propagate into the solid. Dilatational waves are also known as primary or P-waves, and distortional waves are also known as secondary, shear or

S -waves. The shape of the wavefronts of each wave type depends upon the

impact source. For the point source impact considered in this study, the Pand S -waves radiate from the impact point into the solid along spherical

wavefronts, while the R-wave propagates along the surface in a circular

pattern. The wavefronts associated with each wave type caused by point impact

are shown schematically in Figure 3.4.

Each type of wave can be characterized by the direction of particle

motion of the medium through which the wave is propagating, relative to the

direction of wave propagation. In P-waves, particle motion is parallel to the

direction of wave propagation, as illustrated in Figure 3.5 (a). P-waves are

associated with either tensile or compressive stresses parallel to the

propagation direction. S -waves are characterized by particle motion

transverse to the direction of wave propagation, as illustrated in Figure 3.5

(b)S-waves are associated with shearing stresses, and thus cannot propagate

through a medium with zero shear rigidity. In R-waves, the particles move in

an elliptical pattern as the wave propagates along the surface of the solid.

This is illustrated in Figure 3.5 (c)

The velocity of propagation of stress waves through an infinite elastic

medium depends upon the elastic modulus of the material E, Poisson's ratio u, and the material density p . The P-wave velocity can vary depending on the

dimensions of the solid relative to the wavelength of the propagating wave.

In an infinite elastic solid, the P-wave velocity Cp is given (34) by the

equation

For rod- like structures free to expand laterally, the stress component

( 1 + v ) ( 1 - 2 v )

E ( 1 - u ) (3.2)

12
perpendicular to the axis of the rod becomes negligible, and the P-wave

velocity approaches what is termed the longitudinal rod velocity (34)

(3.3)

The S-wave velocity in an infinite elastic medium, Cs , is given (34) by the

equation

(3.4)

The shear rigidity G can be written as

E

G -

2 ( 1 + u ) (3.5)

Thus, the S-wave velocity is given by the equation

V-¥ 2 P ( 1 + v ) (3.6)

The ratio of S- to P-wave velocities can be expressed in terms of Poisson's

ratio as JTTTT7T

f 2 ( 1 - u ) (3.7)

The R-wave velocity, Cr , is approximated (37) by the equation

0.87 + 1.12 u

1 + u (3.8)

In early age concrete, the values of E and v are changing continuously

with time. Hence Cp, Cs , and Cr , and their relative values are also changing

with time. However, as stated in the introduction, the purpose of this

13
section is to provide a basic understanding of the vibration of a cylinder

caused by impact. Attention is thus focused on a hardened concrete cylinder.

It has been shown that the principles of elastic wave propagation can be

applied to hardened concrete (31)

The P-wave generated by impact on the surface of a concrete cylinder

initially propagates as a compression wave. When this wave is incident on a free surface, reflection occurs. The stresses associated with a reflected

P-wave change sign, i.e., a P-wave propagating as a compression wave is

reflected as a tension wave, and a tension wave is reflected as a compression

wave. In addition to the reflected P-wave, an incident P-wave also generates

an S-wave by a process called mode conversion. Similarly, an S-wave incident

on a free surface is reflected as an S-wave and a mode converted P-wave. A

more complete discussion of the reflection and mode conversion of stress waves

can be found in References (23) and (24).

3. 2. 1.2 Force-Time Function and Contact Time of the Elastic Impact of a

The impact of interest is that of steel spheres , ranging in diameter from

5.56 to 12.7 mm, dropped onto the surface of a cylindrical concrete specimen.

The elastic impact of a sphere on a solid can be described using the

Hertz theory of elastic impact. In this theory, the force -time function of

the impact is a half- cycle sine curve (17)

Fmax is the maximum force exerted during the impact, and tc is the contact

time of the impact. This force time function is shown in Figure 3.6.

Using Hertz elastic theory, it has been shown that for E and for

hardened concrete with a P-wave velocity of 4000 m/s (36 x 10^ N/m^ (5.2 x 10^

psi) and 0.2), the contact time of a steel sphere dropped onto a concrete

specimen is (31)

where R is the radius of the sphere and h is the drop height, both in meters.

Thus the contact time is directly proportional to the sphere diameter, and

relatively insensitive to changes in drop height. Experimental contact times

may be longer than predicted by eq. (3.10) because of inelastic behavior in

the concrete. The inelastic behavior can result from local crushing of the

cement paste at the point of impact.

Sphere on a Plate

0 < t < t c (3.9)

tc - 0.00858 R / ( h ) 0 - 1 (3.10)

14
3 . 2 . 1 . 3 Frequency Content of Impact

Using the Fourier transformation, the force-time function in Figure 3.6

is equivalent to the summation of continuous sinusoidally varying force- time

functions with different frequencies’. Thus a range of frequencies is imparted

into an object by impact. The frequency content of this force -time function

is revealed by examination of its frequency spectrum as shown in Figure 3.7

(31). The amplitude and frequency axes of the spectrum are expressed in terms

of the contact time. It is seen that there are "zeroes" in the frequency

spectrum, occurring at frequencies of 1.5/(tc ), 2.5/(tc ), etc. The most

significant contributions of energy imparted by the impact are at frequencies

below the first zero in the spectrum. Using Figure 3.7, the frequency content

for a given contact time can be determined. For example, for a 60 us contact

time, the first zero in the frequency spectrum occurs at 1. 5/(60 us) - 25

kHz. It can be seen that as the contact time becomes shorter (smaller

sphere), the range of frequencies containing significant energy increases. As

will be shown in Section 3.2.3, the frequency content of the impact determines

the modes of vibration that can be excited.

3.2.2 Mode Shapes of Vibration of a Free Cylinder

When a cylinder is subjected to impact it will vibrate. As it vibrates,

each frequency of vibration has associated with it a deformed shape, or mode

shape. The response of the cylinder to impact is the superposition of each of

these mode shapes. Theoretically, a cylinder of given dimensions has an

infinite number of mode shapes. An understanding of the modes of vibration of

a cylinder is helpful in interpreting the results of the impact-echo tests.

An experimental study of the mode shapes and natural frequencies of

vibration of solid, isotropic, elastic cylinders was performed by McMahon

(25)

McMahon was able to experimentally determine the frequency and

approximate mode shapes of 20 of the lowest 22 modes of vibration of solid

aluminum and steel cylinders. McMahon studied 3- and 4-in. diameter cylinders

with thickness to radius ratios (T/r) ranging from 0.201 to 3.29 for aluminum,

and 0.205 to 2.48 for steel. In the discussions to follow, the T/r ratio is

referred to as the aspect ratio. The cylinders were placed on a 2

in. diameter cylindrical ceramic shell and driven at various frequencies. In

order to determine the approximate mode shapes of vibration, McMahon sprinkled

fine sand on the top surface of the cylinder, and statically charged lucite

shavings on the vertical surface. When the cylinder was driven at a natural

frequency, the sand and lucite particles moved to the locations of nodes which

are locations of minimum velocity.

Figure 3.8, from McMahon, shows the approximate form of vibration of 20

of the first 22 modes of vibration. Solid lines in the plan sketches of the

top surfaces indicate the approximate locations of nodal lines , and arrows

indicate the directions traced by sand particles during the vibration. The

dashed lines in the cross-sections indicate the approximate mode shapes.

Modes are grouped by their circumferential order n, o’r degree of symmetry with

respect to rotation about the axis of the cylinder. The first mode of

vibration was identified by McMahon as the flexural mode, and the second mode

as the longitudinal or rod mode.

15
Figure 3.9, adapted from McMahon, shows the experimentally determined

frequencies of the first six modes of circumferential order 0 (symmetric about

the axis of the cylinder) for various aspect ratios of the aluminum and steel

cylinders. Frequencies are expressed as a dimensionless frequency parameter

ur/C, where C is the longitudinal rod velocity. From this figure it is seen

that the fundamental mode (lowest frequency mode) of vibration changes with

the aspect ratio. For a small aspect ratio, i.e.

a thin disk, the flexural

mode (mode 1) is the fundamental mode of vibration. Above an aspect ratio of

approximately 1.5, the rod mode (mode 2) becomes the fundamental mode.

It is also seen from Figure 3.9 that, for any given aspect ratio, the

values of the frequency parameters determined for the aluminum and steel

cylinders are about the same. Thus the frequency parameters are not greatly

affected by changes in Poisson's ratio. This was confirmed in theoretical

work by Hutchinson (18). Thus the frequency parameters, u>r/C, of the first

four axially symmetric modes of a hardened concrete cylinder, for an aspect

ratio of 2, were estimated from Figure 3.9. This was done by determining the

frequency parameters for aluminum (u -0.344) and steel (u-0.293) cylinders with

an aspect ratio of 2, and making a linear extrapolation to the concrete

cylinder based on an assumed Poisson's ratio of 0.20. The results of this

extrapolation are presented in Table 3.1. It is noted that for an aspect

ratio of 2, the rod mode (mode 2) is the fundamental mode of vibration. The

frequency ratios of the first four axially symmetric modes, also presented in

Table 3.1, are therefore expressed in terms of the frequency of the rod mode.

These ratios will be used to help interpret the results of the experimental

study presented in the next section.

3.2.3 Experimental Study of the Vibration of a Free Cylinder Caused by Impact

In the experimental studies by McMahon, the cylinders were subjected to

excitation by a continuously applied driving frequency, and as a result, a single mode of vibration was observed. However, in the vibration of a cylinder caused by impact, several modes are excited at once. This is because

energy is imparted to the cylinder over a range of frequencies and it is

possible to simultaneously excite more than one mode of vibration. The

resulting cylinder vibration is the superposition of each of these modes. As

the duration of the impact becomes shorter, the energy imparted to the

cylinder is distributed over a wider range of frequencies, and therefore

higher modes of vibration are expected in the response.

To confirm the expected influence of contact time, the vibration of a cylinder caused by impact was studied experimentally. A 152 -mm diameter by

152 -mm thick (aspect ratio - 2) hardened concrete cylinder was used in the

study. As in the McMahon study, the cylinder was not completely "free", but

instead rested on a rubber mesh, isolating the cylinder from the supporting

table

Impact was generated by dropping steel spheres onto the center of the top

surface of the specimen. The resulting surface displacements were monitored

with the displacement transducer positioned at half the radius.

Two spheres with diameters of 12.7 and 5.56 mm were used. The contact

times for these two spheres, calculated from eq. (3.10) are approximately 60

and 30 us, respectively. The frequency content for these two contact times,

based on the half-cycle sine curve force-time function (Figure 3.4), are shown

16
in Figure 3.10. The first zero frequencies in the spectra occur at 25 and 50

kHz for 60 and 30 us impacts, respectively.

The waveform shown in Figure 3.11 (a) is the experimentally recorded top

surface displacement caused by the impact of the 12.7 mm ball (tc — 60 us).

The record consists of 1024 data points, recorded at a sampling frequency of

500 kHz. The time interval between peaks in the waveform, At, is interpreted

as the time between successive arrivals of the P-wave at the top surface of

the cylinder, as it is multiply reflected between the top and bottom surfaces

of the cylinder. The mode shape associated with the multiple P-wave arrivals

is the rod mode identified by McMahon, which as noted earlier is the

fundamental mode for the given aspect ratio. The path length traveled by the

multiply reflected P-wave between successive arrivals at the top surface is

2T. Hence the P-wave velocity in the concrete cylinder can be calculated as

2 T

At

(3.11)

The periodicity of the multiply reflected P-wave can be obtained by computing

the frequency spectrum of the waveform. Figure 3.11 (b) is the frequency

spectrum of the displacement waveform shown in Figure 3.11 (a). The dominant

peak in the spectrum at 12.695 kHz is the frequency fp of successive arrivals

of the multiply reflected P-wave. Hence the time interval between successive

arrivals is

(3.12)

Substituting eq. (3.12) into eq. (3.11) leads to

2Tf, (3=13)

From Table 3.1 the frequency parameter cj r/C for this lowest mode of vibration

was estimated to equal 1.48. Again it is noted that this parameter is

approximate, based on an assumed Poisson's ratio for concrete and

extrapolation from McMahon's experimental results. Substituting « — 2 t f and

T/2 - r, and solving for C, the rod velocity, leads to C - 2.12 T f, as compared to Cp - 2 T fp, given by eq. (3.13). The reason for this difference

is unclear.

In this study eq. (3.13) was used to convert frequency to wave velocity.

From eq. (3.13), the P-wave velocity in the concrete cylinder was calculated

as Cp - 2 x 0.152 m x 12.695 kHz - 3860 m/s. This velocity was checked by

determining the velocity through the specimen using the pulse velocity method

(ASTM C 597) (3)

The P-wave velocity determined by the pulse velocity method

was 4300 m/s, which is about 11 % higher than the velocity value measured by

17
the impact-echo method. This difference between the P-wave velocities

measured by the two methods was expected, and has been demonstrated by others

(27,31). The reason for this difference remains unknown.

However, it is c.lear that the frequency of the fundamental mode of

vibration that was excited by the impact can be used to monitor the

development of P-wave velocity.

Also present in the spectrum in Figure 3.11 (b) is a peak at 18.555 kHz

due to a higher mode of vibration, and a peak at 0.977 kHz due to the natural

vibration of the transducer. The frequency ratio 18.555/12.695 - 1.46 agrees

closely with the 1.48 ratio of the third and second modes estimated from

McMahon's results (Table 3.1). Other smaller amplitude frequency peaks may be

due to other modes excited by the impact.

Figure 3.12 (a) is the top surface displacement caused by the impact of

the 5.56 mm ball. This waveform is more difficult to interpret in the time

domain than the waveform caused by the 12.7 mm ball. This is because the

smaller ball, having a shorter contact time, imparted significant energy at

higher frequencies into the specimen, thus exciting additional higher modes of

vibration. The superposition of these higher modes resulted in the increased

number of perturbations in the waveform. The corresponding frequency spectrum

shown in Figure 3.12 (b) reveals the frequencies of the higher modes excited

by the impact.

A comparison of the frequency spectra reveals that both the 60- and 30 -us contact times excited modes of vibration with frequencies of 12.695 and 18.555

kHz. Further, Figure 3.12 (b) has a large amplitude peak at 24.414 kHz which

is absent from Figure 3.11 (b) . The 24.414 kHz mode of vibration could not be

excited by the 60 -us impact because the frequency content of the impact is

zero at 25 kHz (refer to Figure 3.10). Thus, as expected, the frequency

content of the impact determines which modes can be excited, and the resulting

vibration of the cylinder is the superposition of each of these modes. Long

duration impacts, containing a narrow range of low frequencies, excite only

lower modes of vibration. In contrast, short duration impacts, containing a broader range of higher frequencies, excite additional higher modes of

vibration.

Lastly, the frequency of the fundamental mode of vibration, as indicated

by the computed frequency spectra, remained unchanged, regardless of the

duration of the impact. In the setting time and strength development studies

reported in later chapters, the results of impact-echo tests were analyzed in

the frequency domain. The specimens used in those studies had an aspect ratio

greater than 1.5. Thus the lowest frequency peak in the frequency spectrum

(excluding peaks attributed to transducer resonance) was used to compute the

P-wave velocity according to eq. (3.13).

3.3 DEVELOPMENT OF ELASTIC MODULUS IN CEMENT PASTE

In Section 3.2 it was seen that stress wave velocities within a solid

depend upon the elastic modulus. The elastic modulus of early-age concrete

increases with age as the paste matures. The discussion presented in this

section gives a possible explanation for how the elastic modulus in cement

paste increases as the paste matures.

Upon initial mixing, cement paste consists of solid particles (cement

grains) suspended in a fluid (mixing water)

This is shown schematically in

18
Figure 3.13 (a). The relative volumes of cement and water are proportional to

the areas shown in the figure. Shown on the surface of each cement grain is

an initial coating of hydration product that formed when the cement and water

first came into contact. The hydration products formed are cement gel and

calcium hydroxide. Cement gel is the name given to the hydrates,

predominantly calcium silicate hydrate, formed during the hydration process.

The cement gel is composed of solid particles and interstitial spaces called

gel pores (28) . For the purposes of this discussion, only the gel products

are represented in Figure 3.13 (a).

The fresh cement paste remains in the state shown in Figure 3.13 (a) for

a period of time called the induction or dormant period. This period

generally lasts from 1 to 3 hours. The end of the induction period is marked

by a rapid increase in hydration. Several mechanisms for the end of the

induction period have been proposed and are summarized by Bye (7)

Figure 3.13 (b) shows the post- induction period structure of the cement

paste. As can be seen in the figure, an initial skeletal framework has been

established by the gel. As hydration proceeds, the volume of gel product

increases, filling in the water- filled spaces. However, the volume occupied

by the gel is less than the initial volume occupied by the unhydrated cement

and mixing water. Thus some of the water -filled space will empty if

additional water is not supplied externally. In this example, additional

water is supplied externally, so the voids remain water -filled.

Figure 3.13 (c) shows the paste structure at a later stage of hydration.

Both the volume of gel product and the degree of interlocking have increased.

The remaining volume of water- filled voids create small cavities called

capillary pores.

Lastly, Figure 3.13 (d) shows the structure of the cement paste at an

advanced stage of hydration. The gel products occupy most of the initial

paste volume. Shown in the figure are small remains of unhydrated cement and

the capillary pores mentioned above.

It is seen from the above discussion that two changes take place in the

structure of the hydrating cement paste. First, the volume of voids in the

paste decreases. Second, the degree of interlocking between gel product

increases. These two changes are the result of gel growth in the volume

established by the initial cement and water mixture.

Prior to the end of the induction period, stress waves in the cement

paste propagate through a fluid- solid mixture. Once an initial skeletal

framework has been established, the stress waves propagate through a porous

saturated solid composed of unreacted cement and hydration product, the

relative proportions of each changing as the paste matures. The propagation

of stress waves in fluid- solid mixtures and porous saturated solids is

discussed in Reference (30).

For the purposes of this study, the cement paste is considered from a macroscopic viewpoint. It is treated as an elastic solid with an elastic

modulus that increases as the paste matures. Hydration reduces the porosity

of the paste and increases the degree of interlocking between hydration

products (Figures 3.13 (b) and (c)), thus increasing the elastic modulus of

the paste.

Consider as a second example a cement paste with a higher w-c ratio,

shown in Figures 3.14 (a) through (d) . The percentage of cement hydrated in

each figure is the same as shown for the lower w-c ratio mixture in Figures

3.13 (a) through (d)

Comparison of these figures reveals that at any degree

19
of hydration a greater degree of interlocking exists between the cement grains

of the low w-c ratio paste as compared to the high w-c ratio paste. This is

because the cement grains of the low w-c ratio paste are initially closer to

each other than are the grains of the high w-c ratio paste. Thus at any

degree of hydration, the low w-c paste will probably exhibit a higher elastic

modulus

Further, if hydration proceeds at the same rate in both the low and high

w-c ratio pastes, the elastic modulus will probably develop at a faster rate

in the low w-c ratio paste, and attain a higher long-term value.

It was seen in Section 3.2 that the P-wave velocity also depends upon

Poisson's ratio. In fresh concrete, it may approach a value of 0.5 (fluid).

As the concrete matures, it decreases to a value of about 0.20 for mature,

hardened concrete

3 . 4 SUMMARY

This chapter presented background information needed for the remainder of the

report. First, the experimental set-up and general procedure used to make

impact-echo velocity measurements was described. Next, the stress waves

generated by impact on the surface of a solid were reviewed, and the

force -time function, contact time, and frequency content of impact between a sphere and a plate were discussed.

The vibration of a cylinder caused by impact was explained. It was shown

that the modes of vibration excited by the impact depended upon the frequency

content of the impact, which in turn depended upon the contact time. Further,

it was shown that the frequency of the fundamental mode of vibration remained

the same, regardless of the contact time.

It was shown that for a cylinder with an aspect ratio of 2, the

fundamental mode of vibration is the rod mode. The P-wave velocity can be

obtained from the cylinder height T and the measured frequency of the rod mode

fp. For this study, the P-wave velocity was computed using:

C

P

2 T f

p

(3.13)

Lastly, the development of elastic modulus in maturing cement paste was

discussed. The increase in elastic modulus as hydration proceeded was

attributed to the reduction of porosity of the paste, and to the increased

degree of interlocking between hydration products

20
Table 3.1 Frequency parameters for a cylinder (aspect ratio — 2)

i ALUMINUM STEEL CONCRETE*

For Concrete

wi / w2

1 1.81 1.78 1.73 1.169

2 1.45 1.46 1.48 1

3 2.22 2.21 2.19 1.480

4 1.94 1.91 1.86 1.257

*Estimated from McMahon [25] for Poisson's ratio - 0.2

21
DISPLACEMENT

Output

to preamp

Figure 3.2 Typical impact-echo measurement: a) surface displacement

waveform; and, b) frequency spectrum

22
CYLINDER

Figure 3.3 Schematic of experimental set-up

Figure 3.4 Wavefronts of P-, S- and R-vaves caused by poin

Direction of Wave Propagation

Direction of

Particle Motion

t

I

P-Wave S-Wave

(a) (b)

R-Wave

(c)

Figure 3.5 Characterization of P-, S-, and R-waves: a)

b) S-wave; and, c) R-wave

t impact

P-wave;

23
AMPLITUDE/CONTACT

TIME

AMPLITUDE

Figure 3®6 Force-time function of the elastic impact of a sphere

on a solid (31)

1 1 “— ! 1 r 0 2 4

FREQUENCY x CONTACT TIME

Figure 3*7 Frequency content of the force-time function shown

in Figure 3.6 (31)

24
FREQUENCY

PARAMETER

-

WO/C

Figure 3.8 Approximate form of vibration of 20 of the first 22

modes of vibration of solid cylinders (25)

0 l 2 SO I 2 HCIQHT / RAOIUS Figure 3.9 Frequencies of the first

circumferential order 0 (25)

25

six modes of vibration
AMPUTUDE/CONTACT

TIME
DISPLACEMENT

DISPLACEMENT

Figure 3.11 Response of a hardened concrete cylinder to impact

by a 12.7 mm ball (

c =60 us): a) surface displacement waveform;

and, b) frequency spectrum

Figure 3.12 Response of a hardened concrete cylinder to . impact

by a 5.56 mm ball Ct

c - 30 us): a) surface displacement waveform;

and, b) frequency spectrum

27
UNHYDRATED CEMENT

Figure 3.13 Structure of cement paste at various stages of

hydration: a) after initial mixing; b) post-induction period structure;

c) even later stage of hydration; and, d) advanced stage of hydration

Figure 3.14 Structure of cement paste at various stages of hydration

- higher w-c ratio: a) after initial mixing; b) postwinduc t ion period

structure; c) even later stage of hydration; and, d) advanced stage

of hydration

28
CHAPTER 4

MEASUREMENT OF SETTING TIME

4 .

INTRODUCTION

The correlation between the development of P-wave velocity in concrete

and setting time is presented in this chapter. The technique used to make the

velocity measurements on early- age concrete is discussed in section 4.1.

Results from initial experiments studying the relationship between P-wave

velocity and penetration resistance on prepared mortars are discussed in

section 4.2. In section 4.3, a simple model is used to explain how the P-wave

velocity in concrete may be affected by the velocities in the paste and

aggregate phases, and the relative proportions of each. Section 4.4 presents

the results of velocity measurements on concrete specimens made with different

w-c ratios, and with and without set-controlling admixtures. Lastly, two

approaches for using the impact-echo method to measure the setting time of

concrete are presented in section 4.5.

4.1 DEVELOPMENT OF MEASUREMENT TECHNIQUE

The basic principle of the impact-echo technique was discussed in section

3.1. This section discusses how the method was adapted for use on early- age

concrete to monitor the development of P-wave velocity. The following

discussion is applicable to both the concrete and prepared mortar specimens.

4.1.1 Experimental Set-up

Figure 4.1 is a schematic representation of the experimental set-up used

to monitor the development of P-wave velocity in early-age concrete.

Cylindrical specimens 152 mm (6.0 in.) in diameter by 160 mm (6.3 in.) deep

were used in the study. Since the development of velocity had to be monitored

at the earliest possible age it was necessary to make velocity measurements

while the concrete was still fluid- like. It was not practical to attempt to

generate stress waves by impact directly on fresh concrete, nor could the

receiving transducer be placed directly on the fresh concrete surface.

Therefore, 9. 5 -mm (0.37 in.) thick polycarbonate plates, the dimensions of

which are given in Figure 4.2, were placed onto the surface of the fresh

concrete to act as impact and receiver surfaces . The impact plate was placed

at the center of the concrete surface, and the receiver plate placed at half

the radius of the cylinder (see Figure 4.1).

The small steel spheres used to create impact were dropped from a height

of 200 mm onto the impact surface. A 200 mm long plastic tube resting on the

impact surface was used to control the drop height. The transducer rested on

the receiver surface, with a thin lead strip inserted between the

piezoelectric tip of the transducer and the polycarbonate plate to complete

the electrical circuit of the transducer.

In order to make reliable measurements, good acoustic coupling had to be

maintained between the concrete and the impact and receiver surfaces. Bleed

water migrating to the surface of the specimen helped to maintain this

coupling. For this reason, the fresh concrete was placed in the mold to a level 10 mm below the top rim of the mold, allowing room for the bleed water

29
to accumulate. Only a small amount of surface water was needed. For very wet

mixtures exhibiting excessive bleeding, excess water was drawn from the

surface to keep the transducer dry. As hydration proceeded, the cement paste

began to stiffen and adhere to the impact and receiver surfaces . This

adhesion was beneficial, leading to improved acoustic coupling between the

specimen and polycarbonate surfaces. As the bleed water evaporated,

additional water was added to the top surface to maintain good coupling. Tie

specimen was covered with a plastic sheet when not being tested. The presence

of small amounts of water on the surfaces of the specimens did not alter the

P-wave velocities.

Impact and receiver plates of three different thicknesses were

investigated: 3.2, 6.4, and 9.5 mm. For these thicknesses, no noticeable

differences were observed in either the measured displacement waveforms or the

frequency spectra. The 9.5 mm plate was selected and used throughout the

testing, because it provided the greatest clearance between the receiving

transducer and the specimen surface, which helped keep the transducer dry.

4.1.2 Signal Interpretation

A typical surface displacement waveform obtained from a specimen at an

age of about 7.3 hours is shown in Figure 4.3 (a). This displacement waveform

is easily interpreted in the time domain. The time interval between peaks in

the waveform caused by P-wave arrivals is 270 us. Thus, the P-wave velocity

Cp in the specimen calculated using eq. (3.11) is 1185 m/s. Displacement

waveforms at earlier ages such as the one shown in Figure 4.3 (b) at 3.1 hours

are not as easily interpreted because the signal is dominated by the resonance

of the transducer (see section 4.1.4), and the P-wave arrivals are difficult

to discern.

When the displacement waveforms shown in Figures 4.3 (a) and (b) are

transformed into the frequency domain, as shown in Figures 4.4 (a) and (b) , interpretation is greatly simplified. The frequency peak corresponding to the

multiply reflected P-wave is identified in the figures. Using the resonant

frequencies obtained from each figure, the P-wave velocity can be calculated

according to eq. (3.13). For the spectrum shown in Figure 4.4 (a), C

p - 1188

m/s, which is the same result obtained above using eq. (3.11). Tor the

spectrum shown in Figure 4.4 (b) , Cp - 313 m/s. Throughout the testing,

results were analyzed in the frequency domain, using eq. (3.13) to compute

velocities

The frequency peak attributed to resonance of the receiving

plate- transducer assembly was reduced through the use of filters, as will be

explained section 4.1.4. Other lower amplitude peaks present in the frequency

spectra are due to higher modes of vibration excited by the impact.

4.1.3 Sampling Frequency and Voltage Range

The displacement waveforms shown in Figures 4.3 (a) and (b) are each

composed of 1024 data points, recorded at a sampling frequency of 100 kHz

(sampling interval of 10 us). Thus, the total duration of the record is 10240

us. From eq. (3.1), the frequency interval in the frequency spectrum is 0.098

kHz, which allowed the P-wave velocity to be computed to the nearest 30 m/s.

Refining the displacement waveforms by increasing the sampling frequency to

200 kHz (sampling interval of 5 us) would increase the frequency interval to

30
0.195 KHz, which in turn would allow the velocities to be computed to the

nearest 60 m/s. Therefore, the refinement to which the velocity can be

computed is determined by the sampling frequency. The 10 us sampling rate was

selected and used throughout the testing.

The displacement waveforms shown in Figures 4.3 (a) and (b) are plots of

signal voltage versus time. The signal voltage is directly related to the

amplitude of the displacements detected by the receiving transducer, large

amplitude displacements appearing as large voltage signals. With increasing

specimen age, the measured displacements in the specimens decreased.

Therefore, the sensitivity of the measurement had to be increased to detect

these smaller voltage signals. This was accomplished by reducing the voltage

range of the oscilloscope. For this study, voltage ranges from . 2.0 V to t

0.1 V were used.

4.1.4 Transducer Resonance

The effect of the low frequency transducer assembly resonance is clearly

seen as the dominant low frequency oscillation present in the surface

displacement waveform in Figure 4.3 (b) , and as the low frequency peak in the

corresponding frequency spectrum in Figure 4.4 (b) . The superposition of

receiver resonance on the specimen response was one of the factors that made

interpretation of displacement waveforms difficult at very early ages

In the early age measurements , the peak in the frequency spectra

attributed to resonance of the receiving plate -transducer assembly remained

essentially constant, ranging from 0.293 kHz to 0.488 kHz. At later ages,

this frequency peak increased slightly, varying between 0.391 and 0.684 kHz.

One way to reduce the effects of transducer resonance on displacement

waveforms and frequency spectra is to filter the signal. In this study

several filters were tried, and a variable high pass filter was found to give

the best results. Using this filter, the frequency below which the signal was

attenuated could be varied from 0 to 1.0 kHz.

For the 160 mm deep specimen used in the setting time tests, the effect

of transducer resonance appeared as a low frequency peak in the frequency

spectrum that was distinct from the peak corresponding to specimen resonance.

This low frequency peak was ignored when interpreting the results

4.2 PREPARED MORTAR TESTS

4.2.1 Objective and Scope

The objective of the initial phase of the study was to determine if a correlation existed between the development of P-wave velocity in prepared

mortars, measured by the impact-echo technique, and initial and final setting

as defined by ASTM C 403 (Standard Test Method for Time of Setting of Concrete

Mixtures by Penetration Resistance) . Three different mixture proportions were

studied as described in Table 4.1. A total of nine experiments were

conducted.

31
4.2.2

Materials

The prepared mortars were made using Type I portland cement, natural

sand, and tap water. The sand was passed through a #4 sieve to remove any

large particles that otherwise might have interfered with the penetration

needles. Fineness modulus, specific gravity, absorption, and periodic

moisture contents were determined according to applicable ASTM Standard Test

Methods (C 136, C 128, C 566) (3). Material properties for the sand used in

this part of the study are summarized in Table 4.2.

4.2.3 Specimen Preparation

The mortars were mixed in a 1.5 cubic foot capacity revolving drum

mixer. The mixing procedure was as follows:

1. Thoroughly wet the mixing drum and allow it to drip dry in an

inverted position for 1 minute.

2. Add sand and cement and dry mix for 30 seconds.

3. Add mixing water over a 1 minute period while drum is revolving.

4. Continue mixing for 1.5 minutes, cover the mixer opening, and allow

mortar to rest for 1.5 minutes: then mix again for an additional 1.5

minutes

After mixing, the mortar was placed into three cylindrical molds

measuring 152 mm in diameter by 170 mm deep, according to the procedure

prescribed in ASTM C 403. Mortar was placed to a depth of 160 mm in each

mold. Two specimens were used for the penetration resistance tests: they were

tested as prescribed in ASTM C 403, with successive penetration resistance

tests made on alternate specimens. The impact surface, receiver surface, lead

coupling strip and receiver were placed onto the third specimen approximately

1 hour after initial contact between cement and water.

The three specimens were the same size, and were exposed to the same

temperature and humidity conditions. Hence, all specimens were assumed to be

at the same stage of hydration at any given time.

Replicate tests were performed on different days. The temperatures of

the specimens were not monitored in these initial experiments.

4.2.4 Results and Observations

In the following discussions, the four low w-c ratio (0.40) tests are

referred to as ML1 through ML4, and the five high w-c ratio (0.55) tests as MH1 through MH5

Figure 4.5 (a) shows the development of P-wave velocity for test ML2.

Figure 4.5 (b) shows the corresponding results of the penetration resistance

measurements. Superposed on Figure 4.5 (b) is a horizontal line at a penetration resistance of 500 psi. On each of these graphs, curves through

the data were drawn by hand, and the "age" represents elapsed time after

initial contact between cement and water.

Impact- echo velocity determinations were attempted at an age as early as 1.25 hours. As can be seen in Figure 4.5 (a), there was considerable scatter

in the early measurements. In several of the earliest readings, it was

impossible to determine which frequency in the spectrum corresponded to the

32
resonant frequency of the specimen. In these instances, • no result was

plotted. From this figure, it is seen that the early velocity measurements

are scattered about a constant value up to an age of approximately 3^/2 to 4

hours. After this period, the scatter in the measurements diminished, and the

development of velocity in each specimen could be followed. At the start of

the well-defined velocity gain curve, the velocity measurements in the test

ML2 ranged from 350 m/s to 400 m/s.

It is not clear whether these earliest scattered measurements have

physical meaning, or result from the experimental set-up. This initial period

of scatter was also observed in the tests made on concrete specimens described

in section 4.4. The significance of the scattered measurements was not

investigated as part of this study.

Different high pass filters were being evaluated during the ML series

tests. A set technique was adopted beginning with, test MH4. This technique

involved using a variable high pass filter, and is the technique used

throughout the remainder of the setting time experiments . The filter was

introduced after the resonant response of the cylinder had been clearly

identified, and was used to filter frequencies lower than approximately one

half the resonant frequency of the cylinder.

Figures 4.6 (a) and (b) show, respectively, the results of velocity and

penetration resistance measurements on tests MH4 and MH5. Examination of

Figure 4.6 (a) reveals that the well-defined velocity gain curve starts at an

age between 3 and 4 hours. The corresponding P-wave velocities were

approximately 250 m/s. During this same period, initial penetration

resistance measurements ranged from 50 psi to 160 psi (Figure 4.6 (b)). A

penetration resistance of 500 psi was achieved at approximately 4.8 hours in

both tests. The corresponding P-wave velocity at 4.8 hours was approximately

375 m/s.

Figures 4.7 (a) and (b) show, respectively, the results of velocity and

penetration resistance measurements of all four replicate ML test specimens.

Similarly, Figures 4.8 (a) and (b) show results of replicates MH1 through

MH3. From these figures, and Figures 4.6 (a) and (b) , a consistent ranking of

the penetration resistance and velocity curves is seen. In general, the test

that showed the earliest time of initial setting measured by penetration

resistance also showed the earliest development of P-wave velocity. Thus, the

impact- echo technique appears to be sensitive to the same factors that affect

the setting behavior as monitored by penetration resistance.

There is no apparent relationship between the development of P-wave

velocity and final setting as defined by a penetration resistance of 4000

psi. In Figures 4.6 (a), 4.7 (a), and 4.8 (a) it is seen that at the time

corresponding to a penetration resistance of 4000 psi, the velocity curves

continue to increase

It is useful to re-plot representative results from the low and high w-c

ratio series together on the same graph. Figures 4.9 (a) and (b) show the

velocity and penetration resistance results of series ML1, ML2 , MH4, and MH5

The results of the impact -echo measurements revealed a difference in the rate

of velocity increase between the low and high w-c ratio mortars. This

difference was reflected in the slopes of the rising portions of the velocity

curves, with the lower w-c ratio mortar exhibiting a greater slope than the

higher w-c ratio mortar. The following discussion gives a possible

explanation for this behavior.

33
As discussed in section 3.3, the cement grains of the lower w-c ratio

paste were closer to each other compared with the higher w-c ratio paste. If

hydration were proceeding at the same rate in both pastes, then the elastic

modulus of the low w-c paste would probably develop faster, as a result of a greater degree of interlocking between hydration products and a lower porosity

at any stage of hydration (refer to Figures 3.13 and 3.14).

A penetration resistance of 500 psi was reached at 3.9 and 3.6 hours for

series ML1 and ML2 respectively, and at 4.8 hours for series MH4 and MH5

This difference in initial setting times (approximately 1 hour) between the

low and high w-c ratio series was also reflected in the velocity

measurements . As can be seen by the time separation between the corresponding

velocity curves shown in Figure 4.9 (a), there is approximately a 1 hour

difference in the velocity curves at the start of velocity gain. Thus, a correlation appears to exist between the time a penetration resistance of 500

psi was reached, and the time the P-wave velocity began to increase rapidly.

4.3 SERIES MODEL

Wave propagation in homogeneous elastic solids was discussed in Chapter

3. However, concrete is non-homogeneous because of large differences between

the elastic properties of the aggregate and paste phases. The traditional

series model can used to gain an understanding of how the velocity in the

concrete may be affected by the velocities in the paste and aggregate phases

separately. The model is described in section 4.3.1, followed by the results

of an experiment performed to evaluate its validity in section 4.3.2.

4.3.1 Description of the Series Model

The series model is a simple approach for predicting the properties of a composite material based on the properties of the constituents. In the

following discussion, the two constituent phases used to represent the

concrete are the paste and the aggregate. The basis for choosing the series

model is as follows. As a stress wave travels through the concrete, a single

ray will alternately propagate through regions of paste and regions of

aggregate. This is shown in Figure 4.10 (a). The path crossed by the ray can

be replaced by an idealized series of paste and aggregate regions as shown in

Figure 4.10 (b)

This, in turn, can be replaced by the two-phase model shown

in Figure 4.10 (c)

hence the series model. Assuming the concrete is

uniformly heterogeneous , each ray in the stress wave propagates through a similar two-phase model, and the entire stress wave can be represented as propagating through the series model shown in Figure 4.11.

The time required for a P-wave to travel through the concrete, At' , is

taken as the summation of times required to travel through each phase

individually, or

(4.1)

34
where Cp C

, CpP, and Cpa are the velocities in the concrete, paste phase, and

aggregate phase, respectively. The total path length L is the summation of

the individual path lengths LP in the paste phase and L

a in the aggregate

phase. For a unit cube, L - 1, and LP and L

a can be replaced by their

fractional volumes VP and V

a respectively. Equation 4.1 can then be written

as

1

(4.2)

For a given concrete mixture, the quantities VP, V

a

, and Cpa are constant.

Only CpP, the velocity through the paste, varies as a function of time and

temperature. Therefore, if the parameters VP, V

a

, and Cpa are known, and the

development of CpP is also known, the velocity in the concrete, CpC

, can be

calculated from tne model.

Knudsen (22) has shown that, for concrete cured at a constant

temperature, properties that are directly related to the extent of cement

hydration can be represented by a hyperbolic expression of the form

^ ( K ( t - tQ ) ) A- (4.3)

( 1 + K ( t - tQ ) )

where A is the property that varies with the degree of hydration, evaluated at

a time t, and Ah is the long term or limiting value of A. K is a constant

related to the initial slope of the hyperbola, and tQ is an induction period

before the property A begins to develop. A similar expression has been used

to represent the strength gain of concrete as a function of maturity (11)

which is the cumulative time- temperature history of the concrete. In this

study it is assumed that velocity gain also obeys a hyperbolic relationship.

However, because there is an initial velocity during the induction period the

relationship is as follows:

( CpP -C0 )(K(t-t0 ))

C

p

P - + CQ (4.4)

( 1 + K ( t - tQ ) )

where CpP is the limiting paste velocity, and CQ the initial paste velocity at

time tQ , when velocity gain starts. Using this hyperbolic expression, the

development of velocity in the paste is as shown schematically in Figure 4.12

(a). The P-wave velocity in the aggregate remains constant with time, as is

shown in Figure 4.12 (b)

Thus, the resulting velocity in concrete for

various aggregate contents, as predicted by eq. (4.2), is as shown in Figure

4.12 (c).

35
4.3.2 Experimental Evaluation of Series Model

The ability of the series model to predict the P-wave velocity in

concrete was evaluated experimentally by monitoring the development of P-wave

velocity in cement paste, and in concretes with controlled volumes of

aggregate. Ordinary Type I portland cement was used in the experiment. The

aggregate was a rounded gravel, ranging in size from 12.7 to 4.75 mm. The

cement paste was mixed by hand first, to which controlled amounts of aggregate

were then added. To minimize absorption of water from the paste, the

aggregate was soaked in water for a period of 24 hours and surface dried with

paper towels and an air stream just prior to use.

Three 152 -mm diameter by 160-ram deep cylindrical specimens were made.

One specimen was made with cement paste only. The remaining two specimens

were made with approximately 40 and 60 percent aggregate by volume. The

cylinders were kept in a water bath at room temperature in an attempt to keep

the temperature of each as similar as possible. The temperature of each

cylinder was monitored with a thermocouple placed in its center. The earliest

velocity measurements were attempted at an age of 2.5 hours, and velocities

were monitored to an age of 195 hours. Each cylinder was removed from the

water bath while making velocity measurements and returned to the bath

immediately afterwards

Despite efforts to maintain each specimen at approximately the same

temperature, the paste cylinder experienced a much greater temperature

increase than either of the concrete specimens. Figure 4.13 shows the

temperature of each specimen for the first 100 hours of the experiment.

Because the initial temperature increase in the paste cylinder was much

greater than either of the concrete specimens, it was at a more advanced stage

of hydration at any given time. Results are therefore presented in terms of

maturity instead of time. Maturity M can be expressed in equation form as

M - ( Tt

- Tq ) dt (4.5)

where Tt is the temperature at any time t, and T0 is a datum temperature. In

this experiment, TQ is taken as 5 °C, a value typical of Type I portland

cements cured at about 20 °C (11) . The approximate maturity of each specimen

was calculated in discrete time increments, using the time and temperature

data recorded in the experiment, according to the equation

M - £ ( Ta

- To ) At (4.6)

where Ta

is the average temperature during the time interval At. At lower

maturities, when the temperatures of the specimens were changing rapidly with

age, the time interval At was 15 minutes. At higher maturities, when the

temperature of each specimen remained relatively constant with age, At was

36
taken as 2 hours. Results are reported with time expressed in hours and

temperature in °C, hence M has the units °C* hours.

The relationship between paste velocity and maturity is obtained by

substituting maturity for age in eq. (4.4):

( CHP -C0 )K(M-M0 ) C

p

P - + C0 (4.7) 1+K(M-M0 )

where M0 is the maturity of the paste cylinder at the start of velocity gain.

To evaluate the series model, eq. (4.7) had to be fitted to the

velocity-maturity data from the paste specimen. This was done as follows.

First, the velocity- time results for each specimen were transformed into

velocity-maturity results, using eq. (4.6) and a datum temperature of 5 °C.

Next, the limiting paste velocity is determined. At higher maturities,

M s ( M - M0 ) (4.8)

and eq. (4.7) can be rearranged into the linear equation

1 - 111. + (4.9)

( C

p

P - C Q ) K ( CpP - CQ ) M ( C£P - C0 )

The y-intercept of eq. (4.9) is (CpP - C Q ) . In this experiment, C 0 was taken

as 625 m/s because the measured data appeared to follow a hyperbolic

relationship beyond this point. The best- fit of eq. (4.9) to the

velocity-maturity results from the paste specimen was determined by linear

regression. The regression was performed beginning with the results at

highest maturity, and incrementally including one additional earlier

measurement until the best- fit linear curve was obtained. The criterion used

to determine how many data points to include was to use the number of points

that produced the highest correlation coefficient. The results of the

analysis are shown in Figure 4.14 (a). The best fit linear curve was obtained

using 10 data points, resulting in a value of 2470 m/s for (CpP - C0

)

Lastly, to determine K and M0 , eq. (4.7) was rearranged into a second

linear equation

cp

P - C o

(( CgP - C0 ) - ( C

p

P - CQ ))

KM- K M0 (4.10)

The slope of the line is K and the x- intercept is M0

. As stated earlier, K is

related to the initial slope of the hyperbola. Thus, a second linear

37
regression was performed using eq. (4.10), beginning with the data at lowest

maturity, and incrementally including one additional later measurement until

the best-fit linear curve was obtained. Again, the criterion was to use the

number of points that produced the highest correlation coefficient. The

results of the analysis are shown in Figure 4.14 (b) . The best- fit linear

curve was obtained using 21 data points, resulting in values for K and M0 of

0. 005838 (°C -hours) and 157 °C a hours, respectively.

Once this relationship for the velocity in the paste specimen as a function of maturity was established, eq. (4.2), the series model, was used to

predict the velocities in the concretes containing 40 and 60 percent aggregate

by volume. In computing the predicted velocities in the concretes, the

velocity in the aggregate was taken as 4850 m/s. This velocity is within the

range of velocities (4300 to 5000 m/s) measured experimentally through large

aggregate particles using the through transmission pulse velocity technique

(3).

Figure 4.15 is a plot of the experimentally measured velocity in each

specimen versus maturity. Superposed on the paste specimen results is the

velocity predicted by eq. (4.7), using the values of (CpP - C 0 ) , K, and M0

arrived at as described above. Superposed on the concrete specimens results

are the velocities predicted by the series model (eq. 4.2). From Figure 4.15

it is seen that eq. (4.7) represents the velocity development in the paste

well. Further, from Figure 4.15 it is seen that the series model gives a good

prediction of the P-wave velocity in the concrete. Thus, the series model

appears to adequately describe the influence of aggregate content on the

velocity in the concrete. This information is useful in explaining some of

the observations presented in the sections that follow.

4.4 CONCRETE AND SIEVED MORTAR TESTS

4.4.1 Objective and Scope

As stated in Chapter 1, one of the main objectives of this research

project was to evaluate the usefulness of the impact- echo technique as a means

of measuring the setting time of concrete. Results of impact-echo velocity

tests performed on concrete are reported in this section. The results of

these measurements are compared to initial setting times measured by

penetration resistance.

Four different concrete mixtures were examined:

1. low w-c ratio concrete (approximately 0.39 after aggregate

moisture corrections)

2. low w-c ratio concrete with an accelerator (0.39)

3. low w-c ratio concrete with a retarder (0.39)

4. high w-c ratio concrete (0.78)

A total of twelve experiments (three replicate tests of each mixture) were

performed. Replicate tests were performed on different days.

For each test, four cylindrical specimens were prepared; one concrete

specimen and three sieved mortar specimens. The concrete and sieved mortar

was placed to a depth of 160 mm in each mold. The concrete specimen and one

38
sieved mortar specimen were used to monitor the development of P-wave

velocities. The remaining two sieved mortar

specimens were used to monitor the development of penetration resistance.

4.4.2 Materials

The concretes were made using Type I portland cement, rounded gravel with

nominal maximum size of 19 mm (0.75 in.), sand with a fineness modulus of 2.12

and tap water. Fineness modulus, specific gravity, absorption, and periodic

moisture contents were determined according to applicable ASTM Standard Test

Methods (C 127, C 128, C 136, C 566) (3). Mixture proportions and material

properties for the aggregates used in this part of the study are summarized in

Tables 4.3 and 4.4 respectively. When a set-controlling admixture was used,

the amount of mixing water was adjusted to account for the water content of

the admixture. The accelerator used in the experiments conformed to ASTM C

494 (Standard Specification for Chemical Admixtures for Concrete) (3), Type

C. The retarder used conformed to ASTM C 494, Types B and D. Admixtures were

used at the dosages recommended by the manufacturer. The amount of each

admixture used is listed in Table 4.3.

4.4.3 Specimen Preparation

The concretes were mixed in a 1.5 cubic foot capacity revolving drum

mixer. The mixing procedure was as follows:

1. Thoroughly wet the mixing drum and allow it to dry in an inverted

position for 1 minute.

2. Add coarse aggregate, fine aggregate, and cement and dry mix for 30

seconds

3. Add mixing water over a two minute period while drum is revolving.

4. Continue mixing for an additional 3 minutes.

5. Perform slump test; ASTM C 143 (3) (results of the slump tests are

reported in Table 4.5).

6. Briefly remix, cover the mixer opening, and allow the concrete to

rest for 20 minutes elapsed time after initial contact between

cement and water.

The concrete was allowed to rest for 20 minutes prior to the sieving operation

to allow absorption of mixing water by the coarse and fine aggregate to take

place. This was done to ensure similar w-c ratios in the paste phases of the

concrete and the mortar sieved from the concrete.

Just prior to sieving, a portion of the fresh concrete was set aside to

make the concrete specimen. The remainder was passed through a #4 sieve to

remove large aggregate, as prescribed in ASTM C 403. A vibrator was used to

aid in the sieving operation.

4.4.4 Results and Observations

As stated in section 4.4.1, four different concrete mixtures were

examined: low w-c ratio concrete (approximately 0.39 after aggregate moisture

corrections), high w-c ratio concrete (approximately 0.78 after aggregate

moisture corrections), accelerated concrete, and retarded concrete. In the

39
following discussions, these four concrete mixtures are referred to as CL, CH,

CA, and CR respectively. The CL, CA, and CR concretes all have the same w-c

ratio (0.39); the only difference between each series was the use of the

admixtures. The data from the impact-echo and penetration resistance tests

performed on the CL, CH, CA, and CR series are presented in Appendix A.

4. 4. 4.1 Velocity Measurements on Concrete

In this section, the results of impact- echo velocity measurements on the

low w-c ratio concrete series (CL) are discussed. Observations about the

scatter in the velocity measurements, strength of the signals, and higher

modes of vibration, are applicable to all the series tested (CL, CH, CA, and

CR) . Figure 4.16 (a) shows the results of penetration resistance measurements

on test CL2, one of the CL series replicates. From this figure, times of

initial and final setting, as defined by ASTM C 403, were determined to be 4.4

and 5.6 hours, respectively. Figure 4.16 (b) shows the results of the

velocity measurements made on the concrete and sieved mortar from test CL2.

Superposed on Figure 4.16 (b) as dashed lines are the times of initial and

final setting determined from the penetration resistance test.

As was found in the prepared mortar tests, there was considerable scatter

in the earliest velocity measurements in both the concrete and sieved mortar

specimens. Again this scatter appeared to be about a constant value. After

this initial period of scatter, the velocity measurements became more clearly

defined, and the development of velocity could be followed.

Figures 4.17 (a) through (1) show the frequency spectra for some of the

impact-echo tests made on the concrete specimen shown plotted in Figure 4.16

(b) . The frequency range shown in each spectrum is from 0 to 15 kHz. Noted

on each frequency spectrum is the frequency peak used to compute the velocity

plotted in Figure 4.16 (b)

As can be seen in Figures 4.17 (a) and (b) , early frequency spectra were

characterized by large amplitude low frequency peaks . As the concrete

matured, the peak attributed to the resonance of the receiver became clearly

visible as a single low frequency, high amplitude peak (Figures 4.17 (c) and

(d)). As can be seen in Figure 4.16 (b)

the velocity measurements are still

scattered at this age (2.73 hours). Beginning at an age of 2.88 hours (Figure

4.17 (e)), the peak at 0.781 kHz attributed to the resonant response of the

specimen could be clearly identified. Using eq. (3.13), the corresponding

P-wave velocity was calculated as 250 m/s.

At the time of this earliest well-defined measurement the resonant

response of the cylinder was weak; that is, the magnitude of the peak in the

frequency spectrum due to the fundamental mode of vibration of the specimen

was small relative to the peak attributed to transducer resonance. The fact

that the earliest reliable measurements were associated with smaller amplitude

frequency peaks (relative to the peak caused by the transducer) aided in

identifying the reliable measurements.

Once the peak attributed to the resonant response of the specimen was

clearly identified, the development of velocity could be easily followed. In

Figure 4.16 (b) it is seen that at ages later than 2.88 hours, the scatter in

the velocity development curve diminishes. Further, as hydration proceeded,

the response of the cylinder became stronger. Higher modes of vibration

40
became more evident in the frequency spectra, as can be seen by the smaller

amplitude, higher frequency peaks beginning in Figure 4.17 (i).

Figure 4.16 (b) shows that at any given time, the concrete exhibited a higher velocity than the sieved mortar. This is easily explained by .the

series model. Because the coarse aggregate had been removed from the sieved

mortar, it had a lower aggregate -paste ratio than the concrete. If the paste

phases were similar in both the concrete and sieved mortar at any given time

(neglecting temperature differences discussed in section 4. 4. 4. 4), then the

concrete, containing a higher proportion of aggregate, would be expected to

exhibit a higher velocity. Hence, the vertical separation between the

concrete and sieved mortar velocity curves reflects the difference in

aggregate content between the two specimens.

From Figures 4.16 (a) and (b) it is seen that at the time of initial

setting (as defined by a 500 psi penetration resistance) , the velocity

development curves are already well-defined for both the concrete and sieved

mortar. Further, as in the prepared mortar tests, it is seen that at the time

of final setting (as defined by a 4000 psi penetration resistance) there are

no discontinuities in the velocity curves.

Clearly, an advantage of the impact- echo method is that the development

of P-wave velocity can be monitored for a longer period of time than can

penetration resistance. It is impractical to continue penetration resistance

tests much beyond a penetration resistance of 4000 psi. The material is

beyond the stiffening or setting period and is gaining strength. However,

impact-echo velocity measurements can still be made at these later ages, into

the region of strength gain. This is discussed in detail in Chapter 5.

The results of all three replicate CL series tests are shown plotted

together in Figures 4.18 (a) and (b) . It is seen from these figures that in

test CL3 , setting began earlier than in tests CL1 and CL2 . This earlier

setting time was reflected by similar shifts in both the velocity and

penetration resistance curves. Thus, as in the prepared mortar tests, the

velocity measurement was influenced by the same factors that altered the

setting time measured by the penetration resistance test. The test that

showed the earliest time of initial setting as defined by penetration

resistance also showed the earliest gain in P-wave velocity.

For replicate tests CL1 and CL3 the earliest well-defined velocity

measurements were 281 m/s at 3.17 hours, and 313 m/s at 2.87 hours,

respectively. As stated earlier, the times of these earliest reliable

velocity measurements mark the end of the period during which there is

considerable scatter in the measurements. At the time the measurements become

clearly defined, the P-wave velocity begins to increase rapidly with age. The

differences in the values of the initial velocity readings in each of the CL

replicates are not significant. This is because of the velocity tests were

made at discrete time intervals, whereas the velocity was increasing

continuously (approximately 275 m/s per hour at the time of the earliest

measurements). Further, as pointed out in section 4.1.3, the accuracy to

which the velocity could be computed was limited by the sampling interval.

It is noted that the values of the earliest well-defined velocity

measurements are less than the velocity of sound in air (343 m/s) . The reason

for this is unclear. It may be the result of inelastic or viscoelastic

behavior in the very weak concrete, because the stress waves introduced by

impact are transmitted through what is probably a sparsely interconnected

microstructure

41
A comparison was made of the time the P-wave velocity began to develop in

each CL series concrete specimen with the time of initial setting (penetration

resistance of 500 psi) of the corresponding sieved mortar specimen. For

simplicity, the beginning of velocity gain in each concrete specimen was taken

as the time a velocity of 300 m/s was achieved. Clearly, this is within the

range of the earliest reliable measurements obtained experimentally. The

times corresponding to velocities of 300 m/s in the concrete specimens were

obtained by interpolation on curves hand drawn through the data points. A

similar interpolation was made to determine the initial setting times

corresponding to penetration resistances of 500 psi in the sieved mortars.

Initial setting occurred at 4.4 hours for tests CL1 and CL2 , and at 3.9

hours for test CL3 . The beginning of velocity development in the concrete

specimens (300 m/s) occurred at 3.3, 3.1, and 2.9 hours for tests CL1,.CL2,

and CL3, respectively. Thus the difference between the time of initial

setting and the beginning of P-wave velocity development in the concrete was

approximately the same for each test (approximately 1.1, 1.3, and 1.0 hours

for tests CL1, CL2, and CL3 respectively). Thus, on the average, the velocity

in the concrete began to increase dramatically about 1.1 hours before initial

setting of the corresponding sieved mortar.

A comparison was also made of the magnitude of the velocity in each

concrete specimen at the time of initial setting in the corresponding sieved

mortar specimen. By interpolation on the concrete velocity curves, velocities

of 590, 620, and 590 m/s were obtained at the times of initial setting for

tests CL1, CL2, and CL3, respectively. Thus, on the average, the velocity in

each CL concrete specimen was approximately 600 m/s at the time of initial

setting of the corresponding sieved mortar.

4. 4. 4. 2 Influence of Set-Controlling Admixtures

Figures 4.19 (a) and (b) show the results of the penetration resistance

and velocity measurements for the three replicate accelerated concrete tests.

Initial setting occurred at 3.3 hours for test CAl, and at 3.6 hours for tests

CA2 and CA3. As in the low w-c ratio concrete tests, the velocity in the

accelerated concrete was about 300 m/s at the onset of velocity development.

The velocity in each accelerated concrete specimen reached 300 m/s at 2.4,

2.7, and 2.6 hours for tests CAl, CA2 , and CA3, respectively. Thus, as was

found in the CL series, there was a consistent difference (approximately 0.9

hours for tests CAl and CA2, and 1.0 hours for test CA3) between the time of

initial setting, and the onset of velocity development in the accelerated

concrete specimens

The same analysis of the results of the three replicate retarded concrete

tests, shown plotted in Figures 4.20 (a) and (b) , lead to similar results.

Initial setting occurred at 6.6, 6.3, and 6.4 hours for tests CR1, CR2, and

CR3 , respectively. The velocity in each corresponding concrete specimen

reached 300 m/s at 5.1, 4.8 and 5.0 hours respectively. Thus the difference

between the time of initial setting, and the onset of velocity development was

the same for each test (approximately 1.5 hours for tests CR1 and CR2 , and 1.4

hours for test CR3)

From the above results, it is seen that the time interval between the

onset of P-wave velocity development and initial setting is shorter in the

accelerated concrete series as compared to the retarded concrete series. This

shorter time separation is explained by Figures 4.21 (a) and (b) , which show

42
the results of penetration resistance and velocity measurements from tests CA3

and CR1. In these figures, the times of initial setting are indicated with

dashed lines, and the times that 300 m/s velocities were reached in the

concrete specimens are indicated with solid lines. From Figure 4.21 (b) it is

seen that there was a more rapid rate of velocity increase in test CA3 as compared to test CR1. This is especially evident immediately after the

beginning of velocity development in each test. Also, there is a slightly

greater rate of increase of penetration resistance in the accelerated mortar

as compared to the retarded mortar. At the onset of velocity development, the

penetration resistances for tests CA3 and CR1 were about the same low value.

Thus, because of the slower rate of increase in CA3, a longer time is needed

to reach the 500 psi penetration resistance. As a result, for test CR1, a longer period of time elapsed between when the velocity in the concrete

reached 300 m/s, and the penetration resistance in. the sieved mortar reached

500 psi, as compared to CA3.

Again, it is useful to compare the velocity in each concrete specimen at

the time of initial setting of the accompanying sieved mortar specimen. By

interpolation on the velocity curves shown in Figure 4.19 (a), the velocities

in the accelerated concrete specimens at the time of initial setting were the

same; 700 m/s for test CAl, and 720 m/s for tests CA2 and CA3 . By

interpolation on the velocity curves shown in Figures 4.20 (a), the velocities

in the retarded concrete specimens at the time of initial setting were

approximately the same; 650, 770, and 680 m/s, for tests CR1, CR2 , and CR3

respectively.

4. 4. 4.Influence of Changes in Water-Cement Ratio

Figures 4.22 (a) and (b) show the results of penetration resistance and

velocity measurements for the three replicate high w-c ratio (0.78) concrete

tests. Initial setting occurred at 5.6, 6.0, and 5.7 hours, for tests CHI,

CH2 , and CH3 , respectively. As in the low w-c ratio tests, the velocity in

the high w-c ratio concrete at the onset of velocity development was about 300

m/s. The velocity in each corresponding concrete specimen reached 300 m/s at

4.3, 4.8, and 4.5 hours, respectively. Thus, as was found in the CL, CA, and

CR series, there was a consistent difference (approximately 1.3 hours for test

CHI, and 1.2 hours for tests CH2 and CH3) between the C 403 -time of initial

setting and the onset of velocity development.

By interpolation on the velocity curves in Figure 4.22 (b)

the velocity

in each high w-c ratio concrete specimen at the time of initial setting was

approximately the same; 520, 510, and 480 m/s, for tests CHI, CH2 , and CH3,

respectively. These velocities are lower than the velocities in the CL, CA,

and CR concrete specimens at their respective initial setting times.

Figures 4.23 (a) and (b) show comparison of representative results from

the low and high w-c ratio tests (tests CHI and CL2). As was observed in the

prepared mortar tests, the rate of velocity increase was different for the

different w-c ratio mixtures. The initial setting time, and the time a P-wave

velocity of 300 m/s was reached for each test are shown as dashed and solid

lines, respectively, in the figures.

The time interval between the onset of velocity development in the

concrete specimen and the time of initial setting of the sieved mortar was the

same for both tests; 1.2 hours. This consistent 1.2 hour time difference is

somewhat surprising, since the rates of velocity and penetration resistance

43
increase in the CH and CL mixtures were different. In section 4.4.4.

it was

shown that because these rates differed between the accelerated and retarded

concretes, the time interval between the onset of velocity development and

initial setting also differed. Accordingly, it might have been expected that

the time interval between the onset of velocity development and initial

setting would have been longer in test CHI as compared to test CL2

This apparent contradiction may be explained as follows. Most of the

adjustment in mixture proportions to produce the high w-c ratio concrete was

made by a volumetric replacement of some cement with fine aggregate. Thus the

aggregate contents of the CH series concretes and sieved mortars were more

similar to each other, than were the concretes and sieved mortars of the CL

series, assuming the sieving operation was equally effective in both series.

No attempt was made to determine the fractions of paste and aggregate in the

sieved mortars. However, examination of Figure 4.23 (b) reveals a smaller

vertical separation between the velocity curves for the CHI concrete and

sieved mortar, as compared to the CL2 concrete and sieved mortar. Recalling

that this vertical separation may result from a difference in aggregate

content further suggests that the aggregate contents pf the CH series

concretes and sieved mortars were more similar to each other than were those

of the CL series. The effect of aggregate content on penetration resistance

was not studied as a part of this investigation. However, it is reasonable to

expect that the presence of more fine aggregate may have led to a higher

penetration resistance for a given cement paste strength. Thus the initial

setting time for the CH series sieved mortars would tend to be lower. This

lower initial setting time would have reduced the time interval between the

onset of velocity development in the concrete and the time of initial setting

in the sieved mortar.

4. 4. 4.

Specimen Temperatures

The temperatures of the concrete and sieved mortar specimens were

monitored using thermocouples positioned at the center of the specimen. Using

a data logger, the specimen temperatures were automatically sampled once every

30 seconds , and average temperatures computed and recorded every 15 minutes

Three sieved mortar specimens were prepared for each test (one for velocity

measurements, and two for penetration resistance tests), with the temperature

monitored in one mortar specimen considered as representative of all three.

Figures 4.24 (a), (b)

and (c) show the temperatures of the concrete and

sieved mortar specimens recorded from tests CL3, CA3 , and CR1, respectively.

Superposed on each figure as dashed lines are the initial setting times, and

as solid lines the times the P-wave velocity in each concrete specimen reached

300 m/s.

The ratio of paste to aggregate was higher in each sieved mortar specimen

than its corresponding concrete specimen. As a result, the sieved mortar

specimens, containing a higher proportion of cement paste, exhibited a greater

temperature rise than the concrete specimens.

The significance of this temperature difference at any given age depended

upon the cumulative time- temperature (maturity) difference. For the tests

shown in Figures 4.24 (a), (b) , and (©)

the difference in temperature between

the concrete and sieved mortar specimens was not significant at the C 403 -time

of initial setting. Thus, up to this time, the paste phases of the concrete

44
and sieved mortar specimens were probably at approximately the same degree of

hydration. This was typical for all CL, CA, and CR tests.

Figure 4.24 (d) shows the temperatures of the concrete and sieved mortar

specimens recorded from test CHI. Again, superposed as dashed and solid

lines, are the time of initial setting and the time the compression wave

velocity reached 300 m/s in the concrete specimen. As can be seen, the

concrete and sieved mortar specimens of test CHI exhibited a lower temperature

rise than the specimens of tests CL3, CA3, and CR1. This is because the high

w-c ratio mixture contained less cement. Further, the largest temperature

difference between the concrete and sieved mortar specimens in the high w-c

ratio mixture was only 2 °C. Hence the temperature differences between the

concrete and sieved mortar specimens were probably insignificant, and paste

phases of both the concrete and sieved mortar specimens were at approximately

the same degree of hydration throughout the duration of the test. This was

typical of all CH series tests.

4.5 DEFINING SETTING TIME USING IMPACT-ECHO VELOCITY MEASUREMENTS

In section 4.4 it was observed that for a given concrete series, there

was a consistent difference between the time of initial setting in sieved

mortar specimens and the onset or beginning of velocity development in the

concrete specimens. The onset of P-wave velocity development was taken as the

time a velocity of 300 m/s was reached in each concrete specimen. It was also

observed in section 4.4 that for a given concrete series, the velocities in

the concrete specimens at the times of initial setting in the corresponding

mortar specimens were approximately the same in each replicate test. These

two observations suggest the following two approaches to defining the setting

time of concrete using impact-echo velocity measurements:

1. Defining setting at the time corresponding to the onset or beginning

of P-wave velocity development in the concrete.

2. Defining setting at the time a specified P-wave velocity is reached

in the concrete.

These two approaches are shown schematically in Figure 4.25 as "l" and

"2" respectively. This second approach is analogous to the way initial and

final setting are defined by penetration resistance in ASTM C403.

The following sections explore each of these approaches in greater

detail, examining the basis and implications of each.

4.5.1 Defining Setting Time at the Onset of P-Wave Velocity Development

Table 4.6 lists the C 403 -time of initial setting for the mortar

specimens, and the time at which a 300 m/s velocity was reached in the

corresponding concrete specimens. Figure 4.26 is a plot of these two setting

times for each test. From this figure it is seen that variations in initial

setting times, measured by penetration resistance, were reflected by

proportional variations in the times the velocities began to increase in each

concrete specimen. The straight line superposed on the data was determined by

linear regression, and the correlation coefficient is 0.9956. Thus there is a strong correlation between the time of initial setting and the onset of

45
velocity development. Therefore, by defining setting time as the onset of

velocity development, it appears that different mixtures will show

proportional changes in setting times measured by impact- echo and penetration

resistance

Defining the setting time of concrete as the time of the start P-wave

velocity development is appealing, because setting is thus based on an actual

physical change occurring in the cement paste, rather than on some arbitrary

strength or stiffness that is changing continuously. Setting will mark the

time the elastic modulus of the paste begins to increase dramatically. This

dramatic increase is probably due to the interlocking of the hydrating cement

grains in the developing microstructure, and may indicate the end of the

induction period. A change in aggregate content should not alter the setting

time measured in this way, because the time at which the elastic modulus

begins to increase rapidly is determined by changes occurring in the paste.

The disadvantage to this approach is that the onset of velocity

development is sometimes difficult to identify because the earliest

measurements are weak. Further, the concrete is still weak at this age; at

the onset of velocity development in the concrete, the penetration resistance

in the accompanying sieved mortar is about 80 psi. Thus setting time defined

by this approach may not have much practical significance insofar as being

related to specific construction operations such as slipforming or finishing

operations

Defining the setting time at the onset of velocity development can be

useful in the laboratory. The entire velocity development curve can be

constructed, and the influence of different variables upon the time of setting

and subsequent rate of velocity increase can be compared.

4.5.2 Defining Setting Time at a Specified P-Wave Velocity

A second approach to defining the setting time of concrete based on

impact-echo velocity measurements is to use the time a specified P-wave

velocity is reached. This approach to defining setting has several

implications which are discussed in the following paragraphs.

For the CL, CA, and CR series (same mixture proportions) , it is logical

to assume, based on the series model, that at a given P-wave velocity in each

concrete, the paste phase in each has the same elastic modulus. Thus, for a given mixture proportion, initial setting time based on a specified velocity

should indicate when the paste has reached a given stage of hydration.

For two concrete mixtures with the same aggregate fractions but different

w-c ratios, it is logical to assume that for the same P-wave velocity in both

concretes, the paste phases have the same elastic modulus. However, to

achieve the same elastic modulus in the pastes, the cement grains of the

higher w-c ratio mixture would probably have to be at a more advanced stage of

hydration. This is because the cement grains are further apart in a high w-c

ratio mixture. If elastic modulus is dependent upon the degree and stiffness

of interlocking between hydration products and the volume of voids, the higher

w-c ratio mixture would therefore require a greater degree of hydration to

achieve a given elastic modulus. In short, although the pastes may have the

same elastic modulus at any given concrete velocity’, the hydration will

probably be more advanced in the higher w-c ratio concrete (see Figures 3.13

and 3 . 14)

46
Lastly, for two concrete mixtures of the same w-c ratio, but different

aggregate fractions, a given P-wave velocity in both concretes would indicate

the paste phases of each had different elastic moduli. The paste in the

concrete with the lower aggregate content would have a higher elastic modulus

(or probably a greater degree of hydration)

From the discussion above, it is seen that by selecting a target velocity

to define the setting time of concrete, the paste phases of different concrete

mixtures could be at different stages of hydration or have different elastic

moduli. Setting time would be defined in terms of the composite elastic

modulus of the material. If, however, setting was defined by a specific

target velocity for a particular concrete mixture, then the paste phases of

different batches of that mixture would have the same elastic modulus and

degree of hydration at that velocity.

Table 4.6 also lists the times a target velocity of 670 m/s was achieved

in each concrete specimen. This target velocity was taken as the average

velocity of the CL, CA, and CR concrete specimens at the C 403 -time of initial

setting. Thus a target velocity of 670 m/s makes the two setting times

approximately the same for the CL, CA, and CR series. Figure 4.27 is a plot

of these two setting times. Because the setting times were made to be

approximately the same, a line through the origin with a slope equal to 1

is drawn on the figure. As can be seen, the points from the CH series fall

below this line. Thus by defining setting time by this second approach,

concretes with different mixture proportions may not have proportional setting

times measured by impact-echo and penetration resistance.

Defining the setting time of concrete at a specific target velocity may

have more practical uses. A relationship can be established for a given

concrete mixture between a specific P-wave target velocity and certain

construction operations. Further, target velocities are easier to identify

than the onset of velocity development.

The two approaches to using impact -echo velocity measurements to define

the setting time of concrete could be used in combination. Initial setting

time could be defined as the time of onset of P-wave velocity development, and

final setting at some later target velocity. Then the time interval between

initial and final setting might be used as an indication of the rate of

hardening of the given mixture.

4 . 6 SUMMARY

The results of tests aimed at demonstrating the feasibility of using the

impact -echo method to determine the setting time of concrete were presented in

this chapter. The development of P-wave velocity

was monitored and related to setting time measured by penetration resistance.

It was shown that very early- age velocity measurements were scattered,

and that the scatter appeared to be about a constant velocity value. At the

age the velocity measurements became well-defined, the velocity began to

increase dramatically and the development of velocity could be easily

followed.

It was demonstrated that there is a strong correlation between the C

403 -time of initial setting and the onset of P-wave velocity development in

concrete. This was demonstrated in two ways. First, for a given series, a consistent difference was found between the time the P-wave velocity began to

47
increase rapidly and the time of initial setting of the corresponding sieved

mortar. Second, for a given series, the velocity in each concrete specimen

was the same at the time of initial setting in its corresponding sieved

mortar. It is also noted that the onset of velocity development occurred

earlier than initial setting of the sieved mortar.

The impact -echo method was shown to be sensitive to changes in setting

times caused by set-controlling admixtures. Variations in initial setting

time, measured by penetration resistance, were reflected by proportional

variations in the times the velocities began to increase dramatically in each

concrete specimen.

Lastly, it was shown that the influence of aggregate content on the

compression wave velocity in concrete could be described by a simple series

model. The time of transit of a compression wave through the concrete could

be taken equal to the summation of the time of transit through each phase

individually.

Based on the observations summarized above , two approaches were presented

for using the impact-echo method to define the setting time of concrete:

1. Defining setting time at the onset of velocity development;

2. Defining setting at the time a specified velocity value is

reached.

The first approach defines setting based upon changes occurring in the

cement paste. The second approach defines setting in terms of a composite

elastic modulus for the concrete.

48
Table 4.1 Mixture proportions (by weight) for prepared mortar tests

Table

Test Sand Cement Water

ML1 , ML2, ML3 , ML4 2.4 1.0 0.40

MHI , MH2 , MH3 3.0 1.0 0.55

MH4, MH5 2.7 1.0 0.55

4.2 Material properties for prepared mortar tests

Cement

Type I (ASTM C 150)

Natural Sand:

Fineness Modulus 2.34

Bulk Specific Gravity (SSD Basis) 2.63

Absorption (SSD Basis) 1.4 %

Range of Moisture Contents 0.65 to 0.50 %

49
Table 4.3 Mixture proportions for concrete-sieved mortar tests

Test Series Water Cement Coarse Fine Admixture

(gm) (gm)

Aggregate

(gm)

Aggregate

(gm) (mL)

CL 425 1000 2091 1091

CA 423* 1000 2091 1091 2.0

CR 423* 1000 2091 1091 2.6

CH 795 1000 2426 3558

* Quantity of mixing water adjusted for water content of admixture

Table 4.4 Material properties for concrete-sieved mortar tests

Cement

Type I (ASTM C 150)

Natural Sand:

Fineness Modulus

Bulk Specific Gravity (SSD Basis)

Absorption (SSD Basis)

Range of Moisture Contents

2.12

2.65

1.6 %

0.50 to 0.40 %

Coarse Aggregate (Gravel)

Bulk Specific Gravity (SSD Basis)

Absorption (SSD Basis)

Range of Moisture Contents

2.56

0.7 %

0.60 to 0.13 %

50
Table 4.5 Results of slump tests

Test

Slump

(mm)

CL1 140

CL2 - 150

CL3 180

CAl 140

CA2 165

CA3 120

CR1 190

CR2 190

CR3 190

CHI 95

CH2 70

CH3 65

Table 4.6 Comparison of setting times

Test

Initial

Setting

Time (C 403)

(hours)

Time to

Reach

300 m/s

(hours)

Time to

Reach

670 m/s

(hours)

CL1 4.4 3.3 4.7

CL2 4.4 3.1 4.6

CL3 3.9 2.9 4.1

CAl 3.2 2.4 3.2

CA2 3.6 2.7 3.5

CA3 3.6 2.6 3.5

CR1 6.6 5.1 6.7

CR2 6.3 4.8 6.1

CR3 6.4 5.0 6.4

CHI 5.6 4.3 6.3

CH2 6.0 4.8 6*. 8

CH3 5.7 4.5 6.7

51
Figure 4.1 Schematic representation of experimental set-up used

to monitor the development of P-wave velocity in early-age concrete

45

9.5

All dimensions

in millimeters

\ “77^ ^

r

i i

i i

i

i

1 25

y 1

/ % / \

# % If vZZ~l

Impact Plate Receiver Plate

Figure 4.2 Impact and receiver plates

52
DISPLACEMENT

H

Z

LU

2

LU

O

<

—J Q.

CO

Figure 4.3 Typical surface displacement waveforms at ages of:

a) 7.3 hours; and, b) 3.1 hours

Figure 4.4 Frequency spectra of the waveforms shown in: a) Figure

4.3 (a); and, b) Figure 4.3 (b)

53
PENETRATION

RESISTANCE

(psl)

VELOCITY

(km/a)

(Thousands)

AGE (hours)

Figure 4.5 Results of prepared mortar test ML2: a) P-wave velcc.A

vs age; and, b) penetration resistance vs age

54
PENETRATION

RESISTANCE

(psl)

VELOCITY

(km/s)

(Thousands)

Figure 4.6 Results of prepared mortar replicates MH4 and MH^: a)

P-wave velocity vs age; and, b) penetration resistance vs age

55
AGE (hours)

Figure 4.7 Results of prepared mortar replicates ML1 through

ML4: a) P-wave velocity vs age; and, b) penetration resistance vs age

56
PENETRATION

RESISTANCE

(psl)

VELOCITY

(km/s)

(Thousands)

2.2

AGE (hours)

AGE (hours)

Figure ^-.8 Results of prepared mortar replicates MH1 through

MH3 : a) P-wave velocity vs age; and, b) penetration resistance vs

age

57
m

o.

UiO

Z

__ n ^ c

t/1 o

uj n K

=_ ° g-C O H

<

Q£Uiz

UJ(L

AGE (hours)

Figure 4.9 Results of prepared mortar tests ML1 , ML2 , MH4, and

MH5: a) P-wave velocity vs age; and, b) penetration resistance vs age

58
(a) (b)

Figure 4.10 Description of the

propagating through the concrete; b)

series of paste and aggregate regions

model

series model: a) Single ray

idealization of the path as a

; and, c) equivilant two-phase

Figure 4.11

aggregate phases

Two-phase series

, used to model the

model, consisting

concrete

of paste and

59
c

a

P

Constant

Figure 4.12 Gain in velocity in concrete witt

velocity in the cement paste; b) constant velocity

and, c) gain in velocity in concrete predicted by Eq.

aggregate contents

c 0 (a)

(b)

(c)

age: a) gain in

in the aggregate;

4.2 for different

60
TEMPERATURE

(C)

32

Figure 4.13 Temperatures of the cement paste, 40 % aggregate,

and 60 % aggregate specimens vs age
/

(Cp

p

-

C©)

(1/km/i)

0.00054

Figure 4.14 Linear regressions to determine ( C^P - C

Q

) , K, and

M

q

: a) Equation 4.9; and, b) Equation 4.10

62
VELOCITY

(

km/a )

4

3.5

3

2.5

2

1.5

1

0.5

i i i i i i i i i i i i i i i r 0 0.4 0.8 1.2 1.6 2 2.4 2.8

(Thousands)

MATURITY ( C hrs )

P-wave velocity vs maturity for the cement

and 60 % aggregate specimens

3.2

Figure 4.15

40 % aggregate*

paste

,
VELOCITY

(km/s)

PENETRATION

RESISTANCE

(pal)

(Thousands)

test CL2 (low w-c racio mixture): a)

and, b) P-wave velocity vs age

Figure 4.16 Results from

penetration resistance vs age;

64
LU

Q

(c) 2.42 Hours (d) 2.73 Hours

3

L_ 1.074

i | .0.977

L

Z/1.074

LU

Q

D

CL

<

FREQUENCY (kHz) FREQUENCY (kHz)

Figure 4.17 Frequency spectra from test CL2 at ages of: a) 1.7 8

hours; b) 2.10 hours; c) 2.42 hours; d) 2.73 hours; e) 2.88 hours;

f) 3.00 hours;

65
AMPLITUDE

AMPLITUDE

AMPLITUDE.

3.7 1 1

0 150 15

FREQUENCY (kHz) FREQUENCY (kHz)

Figure 4.17 (continued) Frequency spectra from test CL2 at age c£: g)

3.13 hours; h) 3.45 hours; i) 4.07 hours; j) 4.60 hours; k) 6.00 hours;

and, 1) 7.50 hours

66
VELOCITY

(km/s)

PENETRATION

RESISTANCE

(psl)

(Thousands)

5

AGE (hours)

Figure 4.18 Results of replicate tests cf a low w-c ratio mixtu'.-

( tests CL1 , CL2, and CL3) : a) penetration resistance vs age; ana,

b) P-wave velocity vs age

67
AGE (hours)

Figure 4,19 Results of replicate tests of an accelerated mixture

(tests CA1 , CA2 , and CA3) : a) penetration resistance vs age; and,

b) P-vave velocity vs age

.

68
VELOCITY

(km/s)

PENETRATION

RESISTANCE

(psl)

(Thousands)

5

(a)

AGE (hours)

Figure 4.20 Results of replica te’ tests of a retarded mixture (tests

CR1 , CR2 , and CR3) : a) penetration resistance vs age; and, b) F-vave

velocity vs age

69
VELOCITY

(km/a)

PENETRATION

RESISTANCE

(pal)

(Thousands)

2

1 .9

1 .8

1 .7

1 .6

1 .5

1 .4

1 .3

1 .2

1 .1

1 0,9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0

1 3 5 7 9 11

AGE (hours)

Figure 4*21 Results of tests CA3 and CR1 : a) penetration resistance

vs age; and, b) P-wave velocity vs age

70

I
VELOCITY

(km/s)

PENETRATION

RESISTANCE

(psl)

(Thousands)

AGE (hours)

Figure 4.22 Results cf replicate tests of a high v-c ratio nix ture

(tests oHi ) Ch2 , a Clg uHj> ) i a.

,

p erie Li & c l on. r e s i s - ai-c e vs j cuw j b) P-wave velocity vs age

71
VELOCITY

(km/a)

5

*>a

Uia

z

n S?

VI o

US n K

§1

<

K

UiZ

UICL

Figure 4,23 Results of tests CHI and CL2: a) penetration resistance

vs age; and, b) P-wave velocity vs age

72
TEMPERATURE

(

C

)

TEMPERATURE

(

C

)

42

Figure 4*24 Specimen temperature vs age: a) test CL3 ; b)

CA3

73

test
TEMPERATURE

(

C

)

TEMPERATURE

(

C

)

Figure 4.24 (continued) Specimen Temperature vs age: c) test CRi

and, d) test CHI

74
Velocity

Figure 4.25

measurements

Two approaches for using

for defining the setting time of c impac t-echooncrete

vel oc i ty

75
INITIAL

SETTING

TIME

BY

P.R.

(hours)

INITIAL

SETTING

TIME

BY

R.R.

(hours)

7

Figure 4.26 Comparison of setting times - penetration res: initial setting time vs start of velocity gain

Figure 4.27 Comparison of setting times - penetration resistance

initial setting time vs target velocity of 67C ml s 76
CHAPTER 5

MEASUREMENT OF COMPRESSIVE STRENGTH

5 . 0 INTRODUCTION

As stated in Chapter 1, this research project had two main objectives.

The first of these objectives, to evaluate the impact-echo technique as a means of measuring the setting time of concrete, was explained in Chapter 4.

The second objective, to evaluate the impact-echo technique as a means of

estimating the compressive strength of concrete, is discussed in this

chapter. The findings presented are a first step towards an in-place strength

estimation test based upon the measurement of P-wave velocity by the

impact -echo method.

The general methodology for in-place strength estimation during

construction is as follows (1). First, a relationship is established between

concrete strength and the results of a specific in-place test. This is

usually done in the laboratory for a given concrete mixture. Tests are

performed at different maturity values and the resulting paired data are used

to develop an empirical relationship, such as the one shown in Figure 5.1.

Once established, this relationship is used to estimate the strength of the

concrete in the structure, using the results of in-place tests performed on

the structure.

In this study, the relationship between cylinder compressive strength and

P-wave velocity was studied. The scope of the study is discussed further in

section 5.1, followed by a description of the experimental set-up and testing

procedures in section 5.2. Section 5.3 describes how the velocity and

strength measurements were made. The anticipated relationship between

strength and velocity is discussed in section 5.4, followed by the

experimental results in section 5.5.

5.1 OBJECTIVE AND SCOPE

In order to reliably use the impact-echo technique as a means of

estimating in-place strength, it is important to understand how the

relationship between strength and impact -echo velocity is influenced by

factors that are commonly encountered in construction practice. Such factors

might include variations in curing temperatures, w-c ratio, and aggregate

content, to name a few. Thus, as an integral part of evaluating the

applicability of the impact- echo technique as a means of estimating

compressive strength, the influence of each of these factors upon the

strength- velocity relationship was examined. A comparison was made of the

strength-velocity relationships for concretes made with the same mixture

proportions, but water-cured at three different temperatures, approximately

10, 20, and 35 °C. A comparison was also made of the strength-velocity

relationships for two concrete mixtures with the same aggregate content, cured

at room temperature (approximately 20 °C)

but having different w-c ratios

(0.50 and 0.57). Lastly, a comparison was made of two concrete mixtures with

the same w-c ratio, cured at room temperature, but with different agregate

contents (67 and 72 percent by volume)

77
5.2

EXPERIMENTAL SET-UP AND PROCEDURES

5.2.1 Materials

The concrete specimens were made using Type I portland cement, graded

rounded gravel ranging in size from 31.5 to 4.75 mm (1.25 to 0.187 in.),

natural sand with a fineness modulus of 2.12, and tap water. Fineness

modulus, specific gravity, absorption, and periodic moisture contents were

determined according to the applicable ASTM Standard Teat Methods (C 127, C

128, C 136, C 566) (3). Material properties are summarized in Table 4.4 (the

materials used in the strength tests were the same as those used in the

concrete and sieved mortar setting time tests)

5.2.2 Mixture Proportions

Three different mixture proportions were used in the study. The mixture

proportions for each are given in Table 5.1. The basis of comparison used

throughout the study was a 0.57 w-c ratio concrete, with a volumetric

aggregate fraction of 0.67 (mixture 1), cured at 20 °C. To investigate the

influence of changes in curing temperature, a total of four batches were made

using this mixture. In the discussion to follow, the specimens cured at 20,

35, and 10 °C will be referred to as room-, hot-, and cold-cured specimens,

respectively. In order to detect differences between the specimens cured at

different temperatures that may actually have been caused by batch- to -batch

variations, room-cured cylinders were also cast from the batches used to cast

hot- and cold-cured cylinders. It was found that the strength-velocity

relationships for the room-cured cylinders taken from all four batches were

the same, i.e., there were no batch to batch differences between concretes.

Hence the observations made in section 5.5.2 can be attributed solely to

changes in curing temperatures

The investigation of the influence of changes in w-c ratio was made using

a second concrete mixture (mixture 2) with the same volumetric fraction of

aggregate as the base mixture (0.67), but with a lower w-c ratio (0.50 as compared to 0.57). Two batches of this lower w-c ratio concrete were made and

specimens were cured at room temperature.

Lastly, the investigation of the influence of changes in aggregate

content was made using a concrete mixture (mixture 3) with the same w-c ratio

as the base mixture (0.57), but containing a 5 percent increase in the volume

fraction of aggregate. Two batches of this higher aggregate content mixture

were made and specimens were cured at room temperature.

Table 5.2 presents a summary of the 8 batches, listing the number of

specimens cast from each batch, the number of specimens cured at each

temperature, and the aggregate content, w-c ratio, and slump of each batch.

5.2.3 Specimen Preparation

The concretes were mixed in a 1.5 cubic foot capacity revolving drum

mixer. The mixing procedure was as follows:

1. Thoroughly wet the mixing drum and allow it to drip dry in an

inverted position for 1 minute.

78
2 . Add the coarse aggregate, fine aggregate, and cement, and dry mix

for about 30 seconds

3. Gradually add the mixing water while the drum is revolving. Cover

. the drum opening and continue mixing for approximately 8 minutes

4. Perform the slump test (C 143) (3), then briefly re-mix the

concrete

The specimens were cast in 102 mm (4.0 in.) diameter by 203 mm (8.0 in.)

deep cylindrical plastic molds. Concrete was added to each mold in two equal

layers , each layer rodded 15 times with a 9 . 5 mm diameter rod with a rounded

tip. The fresh concrete surfaces were then leveled off with a trowel.

Immediately after casting, the fresh concrete specimens were placed in

lime -saturated curing baths at the specified temperatures. The water in each

curing bath was raised to a level approximately 25 mm above the top surfaces

of the specimens. Care was taken to ensure that the top surfaces were

undisturbed and remained plane with the bottom surfaces.

Three curing baths were used, and were maintained at approximately 10,

20, and 35 °C. The temperatures of the room- and cold-curing baths were

monitored using thermocouples. The temperature of the hot-curing bath was

monitored using a glass thermometer. The room- curing bath remained relatively

constant throughout the experiment, ranging from 18 to 23 °C. The hot-curing

bath consisted of a metal tub fitted with a lid, that was placed in an oven

maintained at approximately 35 °C. As a result, the temperature of the

hot -curing bath also remained relatively constant throughout the duration of

the experiment, ranging from 34 to 37 °C. The cold-curing bath consisted of a plastic tub, covered with insulation board, and kept outdoors in the winter

climate. During the early part of the experiment, the temperature of the

cold-curing bath ranged mostly from 5 to 12 °C. During one period at later

ages (compressive strengths of approximately 21 MPa), the temperature of the

cold curing bath fell as low as 0 C.

To facilitate stripping the specimens from the molds, each mold was

lightly oiled prior to casting. In addition, each mold had a 6-mm diameter

hole drilled in the center of its bottom, which was then covered with a masking tape. To strip the specimens from the molds, pressurized air was

blown through this hole, forcing the cylinder from the mold. For very low

strength specimens (2 to 7 MPa) , the perimeter of the bottom surfaces of the

specimens were damaged slightly by this operation. However, as described in

the next paragraph, the specimens were capped prior to testing, filling in any

damaged areas that otherwise might have reduced the measured compressive

strength.

Prior to making the velocity measurement and subsequent compressive

strength test, each cylinder was capped with a high strength sulfur mortar, as prescribed in ASTM C 617 (Standard Method of Capping Cylindrical Concrete

Specimens) (3). It was difficult to cap the very low strength cylinders (2

MPa) because the capping material adhered more strongly to the capping plate

than to the specimen.

5 . 3 SPECIMEN TESTING

For each concrete cylinder, the velocity was measured, then the

compressive strength was determined. Hence each data record is the result of

79
a velocity measurement and a compression test on the same cylinder. The

following sections discuss how the velocity and strength tests were made.

5.3.1 Velocity Measurement

The technique used to make the impact- echo measurements on the capped

cylinders was similar to that used in the setting time study described in

section 4.1. In the strength study, the capped surfaces of the specimens

provided a smooth surface upon which to make the velocity measurements,

without the need for any surface preparation. Stress waves were generated by

the impact of a small steel sphere dropped from a height of approximately 0.2

m onto the center of the top surface of the capped specimen. The receiving

transducer, located at half the radius, detected the resulting surface

displacements. A thin lead strip was inserted between the piezoelectric tip

of the transducer and the sulfur cap to complete the electrical circuit of the

transducer.

In the measurements at the lowest maturities, the velocities through the

concrete specimens were approximately 2100 m/s. The corresponding compressive

strengths were approximately 2 MPa. For the specimens cured at room

temperature, this was approximately 12 hours after mixing.

A typical displacement waveform and accompanying frequency spectrum

obtained from a specimen with a compressive strength of 1.87 MPa are shown in

Figures 5.2 (a) and (b) . It is possible to interpret this signal in either

the time or frequency domain, as was explained in section 4.1.2. The time

interval between peaks in the displacement waveform, At, can be used to

calculate compression wave velocity according to eq. (3.11). From Figure 5.2

(a), At was found to be 190 us. Using the nominal cylinder height T — 0.203

m, the P-wave velocity was calculated as Cp = 2 x 0.203 m / 190 us - 2140

m/s. Alternately, the 5.176 kHz peak in the frequency spectrum due to the

fundamental response of the specimen can be used to calculate the velocity

according to eq. (3.13). This leads to Cp = 2 x 0.203 m x 5.176 kHz = 2100

m/s

Throughout the testing, results were analyzed in the frequency domain.

Velocities were computed according to Eq 3.13. A 100 kHz sampling frequency

(10 us sampling interval) was used to make the measurements, resulting in a 0.098 kHz frequency interval in the frequency spectra. Hence, for the 0.203 m

specimen height used, the velocity could be computed to the nearest 40 m/s.

Note that this is only 2 percent of the earliest velocity measurements made

(2100 m/s)

Capping each specimen prior to making the velocity measurement caused a slight decrease in the measured velocity. This was attributed to the added

thickness of the capping material. In terms of obtaining a true measure of

the velocity through the concrete, it would have been better to cap the

specimens after making the velocity measurement. However, this was not

practical from a procedural viewpoint, because at the time of the earliest

measurements, the velocity in the specimens was changing rapidly with time.

Capping these early- age specimens was especially time consuming, thus the

velocity would have changed appreciably before the strength could be

determined. It was therefore decided to cap the cylinders prior to making the

velocity measurements , introducing a shift (approximately 190 m/s) in the

strength-velocity curve to a lower velocity value.

80
5.3.2 Compressive Strength Determination

Immediately after the velocity measurements were made, the cylinders were

tested in compression as prescribed in ASTM C 39 (Standard Test Method for

Compressive Strength of Cylindrical Concrete Specimens) (3) . Each cylinder

was loaded at a rate of approximately 0.15 MPa per second, and the maximum

load sustained recorded to the nearest 0.45 kN. In computing the failure

compressive stress of each specimen, fc , the cross-sectional area was taken as 0.008 m^, based on the nominal diameter of the cylinder.

5.4 ANTICIPATED COMPRESSIVE STRENGTH - VELOCITY RELATIONSHIP

It has been shown empirically in work by Pauw (26) that, for mature

concrete, the modulus of elasticity of concrete Ec is proportional to the

square root of compressive strength, or

Ec (5.1)

Further, it has been shown from the physics of wave propagation that P-wave

velocity through an elastic solid is proportional to the square root of the

elastic modulus E, or

(5.2)

Assuming eq. (5.1) also applies to immature concrete, and from Eqs . 5.1 and

5.2, taking E as Ec , it is seen that

or,

cp

a (5.3)

fc <* ( C

p

) 4

(5.4)

Thus it is expected that there may exist an approximately fourth power

relationship between compressive strength and compression wave velocity. This

fourth power relationship is shown schematically in Figure 5.3.

5.5 EXPERIMENTAL RESULTS

This section discusses the experimental results. Section 5.5.1 examines

the general relationship between compressive strength and velocity, and

describes two empirical curves used to represent this relationship. The

discussion is presented using the data from the room-cured cylinders.

Sections 5.5.2, 5.5.3, and 5.5.4 discuss, respectively, the influence of

81
changes in curing temperature, w-c ratio, and aggregate content, using the

room- cured cylinders as a reference for comparison. The strength-velocity data from all batches are

presented in Appendix B.

5.5.1 Regression Model: Room-Cured Specimens

Figure 5.4 shows the experimental results for 33 concrete cylinders from

mixture 1 cured at room temperature. In agreement with the discussion in

section 5.4, the data appears to follow a power relationship. At lower

velocities, the velocity is increasing rapidly relative to the strength. At

higher velocities, the opposite is true; that is, relatively large gains in

strength are accompanied by only small increases in velocity. It is also

noted that there is little scatter in the data, thus there was minor

batch- to -batch variability.

Two empirical equations were used to represent the strength velocity data

shown in Figure 5.4. One equation used was the power relationship

b

fc - a (C

p

) (5.5)

To determine the constants a and b, eq. (5.5) was transformed into a linear

equation by taking its logarithm:

log fc - log a + b log C

p

(5.6)

The intercept (log a) and slope (b) were then determined by linear regression

in log space. Figure 5.5 (a) shows the best-fit line (predicted line) of

eq. (5.6) to the room temperature data, calculated using the method of least

squares. The best- fit values of a and b were 0.063 and 4.44, respectively,

and the standard error (in real space) was 2.18 MPa. It can be seen in Figure

5.5 (a) that in the center region of the graph the predicted line is

consistently above the experimental data. Further, above a log velocity of

0.56, the predicted line falls below the experimental data. Thus, the simple

power relationship does not appear to fit the data. This lack of fit is

further demonstrated in Figure 5.5 (b) , which is a plot of the residuals

(difference between experimental and predicted values) from Figure 5.5 (a) as

a function of the log of velocity. In the calculation of the best fit line,

each data point was assumed to have equal importance. In addition,

least- squares regression analysis assumes that the residuals (errors) have a normal distribution, with an average error of 0 (5). Thus, a predicted curve

that fits the data will have residuals randomly scattered above and below the

zero-residual line. Clearly, the residuals are not randomly scattered in

Figure 5.5 (b) . Hence, a simple power function does not appear to be an

accurate description of the relationship between strength and velocity.

When transforming from log space back into real space, the residuals at

higher velocities become larger relative to the residuals at lower

velocities. This can be seen by comparing Figure 5.5 (c)which is a plot of

the residuals in real space, with Figure 5.5 (b) . Consequently, an equation

82
that fits the data will have a residual plot in real space that displays a fan- like pattern. The residuals will be randomly distributed about the

zero-residual line, and increase in magnitude with increasing velocity.

Residual plots in real space will be used to help interpret the results

presented in later sections of this chapter.

Lastly, Figure 5.5 (d) shows the best-fit power curve (eq. 5.5)

superposed onto the room- cured data. As was explained in the preceding

development, it can be seen that in the range of velocities from 2900 to 3500

m/s, the power curve over-predicts strength, whereas above 3700 m/s, it

under -predicts strength.

A second equation used to represent the strength-velocity data was

(5.7)

Again, the equation was transformed into a linear equation by taking its

logarithm:

log fc - log a + b log (5.8)

The constants a and b were determined by linear regression for incrementing

values of n. The criterion for determining n was to increment n by 50 until

the standard error remained unchanged in the third decimal place. The

resulting best-fit values of a, b, and n were 0.079, 306.2 and 200,

respectively, and the standard error in real space was 1.26 MPa.

Figures 5.6 (a), (b) , (c)and (d) show, respectively, the linear fit in

log space, the residuals in log space, the residuals in real space, and the

fit in real space of eq. (5.7) to the room- cured cylinder data. Comparison of

these figures with Figures 5.5 (a) through (d) reveals that eq. (5.7) agrees

more closely with the experimental data than does eq. (5.5). As seen in

Figure 5.6 (d)

in the range from 2900 to 3500 m/s, the experimental data fall

close to the best- fit line; above 3700 m/s, it passes through the data, rather

than underestimating it as was the case with the simple power function given

by eq. (5.5). The residuals in Figure 5.6 (b) show a random distribution

about the zero-residual line, and in Figure 5.6 (c) , the residuals in real

space form the fan- like pattern described earlier. Lastly, as can be seen

from Table 5.3, the standard errors obtained from Eqs . 5.5 and 5.7

respectively were 2.182 MPa and 1.261 MPa. Thus eq. (5.7) represents the

strength-velocity data better than eq. (5.5). Therefore, eq. (5.7) was used

to evaluate the results presented in the remainder of this chapter. It is

noted that there may be other empirical functions that fit the data better

than eq. (5.7). However, it is felt that eq. (5.7) fits the data reasonably

well, and is satisfactory for the purpose of this study.

83
5.5.2 Curing Temperature Variation

5. 5. 2.1 Hot-Cured Specimens

The earliest tests of the hot-cured specimens were made 6 hours after

initial mixing. The velocities through the concrete specimens were

approximately 2080 m/s, and the corresponding strengths approximately 1.98

MPa.

Figure 5.7 (a) is a plot of the strength-velocity data for the hot-cured

specimens. Superposed on the data is the best- fit curve (eq. 5.7) for the

room-cured specimens, discussed in the previous section. Figure 5.7 (b) is a plot of the residuals of the hot -cured data about the room- cured curve in real

space. From these figures it is seen that up to velocities of approximately

2800 m/s, the hot-cured data agrees well with the room-cured prediction

curve. At velocities above 2800 m/s, the room-cured curve over-predicts. the

strength. For this concrete mixture, a velocity of 2800 m/s corresponds to a strength of about 6.1 MPa. Thus increasing the curing temperature from 20 to

35 °C alters the strength-velocity relationship at higher velocities. Above

2800 m/s, the hot-cured specimens exhibit lower strengths relative to the

room- cured specimens for a given velocity.

5. 5. 2.

Cold-Cured Specimens

The earliest test of a cold- cured specimen was made approximately 24

hours after initial mixing. The velocity in the concrete was 2000 m/s, and

the corresponding strength was 1.59 MPa.

Figure 5.8 (a) shows the strength-velocity data for the cold-cured

specimens. Superposed on the data is the best- fit curve to the room-cured

specimens. Figure 5.8 (b) is a plot of the residuals in real space. As can

be seen in the figures, good agreement was found between the cold-cured

cylinders and the room-cured prediction curve. The residuals shown in Figure

5.8 (b) are randomly scattered about the best- fit curve along the entire

length of the curve. Thus a decrease in the curing temperature from 20 to 10

°C did not alter the strength-velocity relationship.

5.5.3. Water-Cement Ratio Variation

Figure 5.9 (a) shows the strength-velocity data from the low w-c ratio

cylinders, with the prediction curve of the room-cured (high w-c ratio)

specimens superposed. Figure 5.9 (b) is a plot of the residuals of the low

w-c ratio data about this prediction curve. It can be seen from these two

figures that up to velocities of approximately 4000 m/s (strengths of 33 MPa)

there is good agreement between the low and high w-c ratio results. Thus the

change in w-c ratio did not alter the strength-velocity relationship for

velocities below 4000 m/s. It is noted that 33 MPa is approximately the

28 -day strength of the high w-c ratio mixture. Residuals were computed for

the low w-c ratio mixture by extrapolating the best- fit high w-c ratio curve

above 33 MPa.

The low w-c ratio concrete reached any given strength-velocity point at

an earlier age than the high w-c ratio concrete. Thus even though the

strength- age or strength-maturity relationships for the two concretes

differed, the strength velocity relationships were the same below a velocity

84
of 4000 m/s. At velocities greater than 4000 m/s (strengths greater than 33

MPa) , the prediction curve from the high w-c ratio concrete underestimates the

strength of the low w-c ratio mixture.

5.5.4 Aggregate Content Variation

Figure 5.10 (a) shows the strength -velocity data from the high aggregate

content concrete. Again, superposed on the figure is the best- fit curve of

the room-cured (low aggregate content) specimens. Figure 5.10 (b) is a plot

of the residuals of Figure 5.10 (a). It is seen that a change in aggregate

content alters the strength-velocity relationship. Figure 5.10 (a) shows that

an increase in aggregate content shifts the strength-velocity data to the

right, resulting in a higher velocity at any given strength. This shift is

evident along the entire length of the curve. This result is not surprising.

In section 4.3 it was shown that the velocity through the concrete depends

upon the velocities through the paste and aggregate phases, and the relative

proportions of each. Increasing the aggregate content increases the

proportion of higher elastic modulus material, thus increasing velocity.

Increasing the aggregate content also increases the area of paste-aggregate

interfaces, which may also influence strength. The 28-day strengths of both

the low and high aggregate content mixtures were the same. Thus it appears

that the velocity rather than the strength was influenced by the increase in

aggregate content, resulting in a higher velocity for a given strength.

5.6 DISCUSSION AND SUMMARY

It was observed that at lower velocities, the velocity was increasing

rapidly relative to strength, and at higher velocities, the opposite was

true. It follows that the impact-echo method would therefore be a more

sensitive strength estimator at lower velocities, because small changes in

strength would be revealed by relatively large changes in velocity.

Figure 5.11 shows the velocity and strength data from the room-cured

cylinders plotted versus the time after initial mixing. From this figure it

is seen that most of the velocity gain occurred during the first day. The

velocities at 1, 3, and 7 days were approximately 3200, 3600, and 3800 m/s,

respectively. The corresponding strengths were approximately 10.4, 19.2, and

26.4 MPa, respectively. In Figure 5.6 (d)

the measurements that were made at

ages of 1, 3, and 7 days are identified. It is seen from Figure 5.6 (d) that

for the given concrete mixture, cured at room temperature, the sensitive

region of the strength-velocity relationship lasts for about 3 days (up to

velocities of approximately 3600 m/s). The 28-day strength of the concrete

was found to be 33.6 MPa, hence the strength at 3600 m/s represents

approximately 60 percent of the 28 -day strength. Clearly, this is within the

range typically required for safe removal of formwork, or the transfer of

prestressing.

It was shown that a decrease in curing temperature from 20 to 10 °C did

not affect the strength-velocity relationship. It also was shown that at

higher velocities, an increase in curing temperature from 20 to 30 *°C did

affect the strength-velocity relationship, resulting in a lower strength at a given velocity for the hot-cured specimens.

85
The reason for the lower strength at a given velocity for the hot-cured

specimens might be as follows. The rate of hydration increases with an

increase in curing temperature. It has been suggested by Verbeck and Helmuth

that at higher temperatures, the rate of hydration may be too fast to allow

the products of hydration to diffuse and precipitate uniformly throughout the

interstitial space between cement grains (36). As a result, hydration product

will concentrate close to the cement grains. This has two consequences.

First, complete hydration may be prevented, because water cannot diffuse

through the layer of hydration product to the unreacted cement. Second, the

porosity of the cement paste will be non-uniform; the paste in the regions

near the cement grains will be less porous than the paste further away from

the grains. Verbeck and Helmuth suggested that even if hydration were

complete, the non-uniform porosity would lead to a lower strength, because

strength would be limited by the more porous portion of the paste. P-wave

velocity may also be dependent upon porosity, but not upon the distribution of

porosity. Thus, at any velocity, both the room- and hot-cured specimens may

have the same porosity (same degree of hydration). However, the non-uniform

distribution of porosity in the hot-cured specimens may lead to a lower

strength at that velocity.

As a consequence, it may not be possible to employ accelerated curing to

develop the strength-velocity correlation for a given batch, which could then

be used to predict the strength of the concrete in the structure from that

batch.

A significant change in w-c ratio was shown to influence the

strength-velocity relationship only at higher strengths and velocities (33

MPa, 4000 m/s). This, however, is probably above the useful range anyway,

because of the insensitivity of velocity to changes in strength at high

velocities. Based on these results, small changes in w-c ratio due to normal

batching variations would not be expected to alter the strength-velocity

relationship, and the relationship derived from laboratory testing would be

applicable

Both the low and high w-c ratio mixtures were cured at room temperature.

Thus the distribution of porosity was probably about the same in both

mixtures, and the porosity influenced both the strength and velocity in the

same manner in both mixtures

The strength-velocity relationship was greatly influenced by a 5 percent

increase in aggregate content. It was suggested that the velocity rather than

the strength was influenced by the increase in aggregate content. The higher

aggregate content mixture exhibited higher velocities at any given strength.

It is therefore apparent that the correlation must be established for a given

concrete mixture, and that control must be exercised over batching operations

for the correlation to be useful.
Table 5.1 Mixture proportions for compressive strength tests

Mixture Water Cement Coarse Aggregate Fine

Number 31.5 to 12.5 to Aggregate

12 . 5 mm 4.75 mm

(gm) (gm) (gm) (gm) (gm)

1 570 1000 801 2185 1984

2 500 1000 743 2008 1754

3 574 1000 974 2632 2287

Table 5,.2 Summary of strength test program

Volumetric Number of Specimens

Batch Mixture Aggregate w- c Slump Number of cured at

Number Fraction Ratio (mm) Specimens 10 C 20 C 35 C

1 1 0.67 0.57 195 15 15

2 1

It It 200 18 4 14

3 1

It tt 180 20 15 5

4 1

tt tt 180 19 9 10

5 2 0.67 0.50 90 14 14

6 2 0.67 0.50 90 13 13

7 3 0.72 0.57 25 14 14

8 3 0.72 0.57 25 13 13

87
DISPLACEMENT

Cylinder

strength

Figure 5.1 Methodology cf in-place strength estimation (*)

Figure 5.2 Typical velocity measurement: a) surface displacem

waveform; and, b) frequency spectrum

88
STRENGTH

(

MPa )

Figure 5 »3 Anti r i p « t e d f ourrn-povr i compressive strength arc P-vave velocity

40 -r ROOM-CURED

35 -

r - a

30 -

25 -

20 -

15 -

10 -

i

D

§

5 -

0

1.8

"T"

Figure 5.4

atP

m —i

1 1 1 1 1 1 1 r

~ 2.2 2.6 3 3.4 3.8

VELOCITY ( km/a ) Room-cured specimens: strength vs velocity

4.2

89
RESIDUALS

OF

LOG

LOG

STRENGTH

1.6

LOG VELOCITY

LOG VELOCITY

Figure 5.5 Results of regression analysis using Eq . 5.5: a) linear

fit of Eq . 5.5; b) residuals in log space

90
STRENGTH

(

MPa )

REDIDUALS

(

MPa )

7

6 -

5 -

4 -

3 -

2 -

1 -

0 -

-1 - -2 - -3 - -4 - -5 -

1.8 2.2 2.6 3 3.4 3.8 4.2

VELOCITY ( km/s ) 40

3b

30

25

20

15

1 0

5

0

1.8 2.2 2.6 3 3.4 3.8 4.2

VELOCITY ( km/s )

Figure 3.5 (continued/ Results of regression analysis using £q . 3-5: cJ

residuals in real space; and, d) best-fit of Eq • 5.5

ROOM-CURED (c)

,

%

CD

1 1 1 T I T 1 r

91
RESIDUALS

OF

LOG

LOG

STRENGTH

1.6

' 1 .5

1 .4

1 .3

1 .2

1.1

1 0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.004 0.005 0.006 0.007 0.008 0.009

LOG VELOCITY

0.09

0.08

0.07

0.06

0.05

0.04

0.03

0.02

0.01

0 -0.01 -0.02

-0.03

-0.04

-0.05

-0.06

0.004 0.005 0.006 0.007 0.008 0.009

LOG VELOCITY

Figure 5.6 Results of regression araiysi? using Eq . 5-7: a; lire.afit of Eq. 5.7; b) residuals in log space

° o

D

o

c o

o

D

i 1 r

92
STRENGTH

(

MPa )

REDIDUALS

(

MPa )

6

5 - 4 -

3 -

2 -

1 -

0 -

-1 - -2 - -3 - -4 - -5 - -6 -

1.8 2.2 2.6 3 3.4 3.8 4.2

40

35

30

25

20

1 5

10

5

0

1.8 2.2 2.6 3 3.4 3.8 4.2

VELOCITY ( km/s ) Figure 5.6 (continued) Results of regression analysis using Eq . 5.7: c)

residuals in real space; and, d; best-fit of Eq . 5.7

VELOCITY ( km/s )

ROOM-CURED (c)

% n CP TT Trr

cr

93
RESIDUALS

(

MPa )

STRENGTH

(

MPa )

40

35 - HOT-CURED

ROOM-CURED

(a)

30 -

25 -

20 -

15 -

10 -

5 -

0 -1.8 2.2

—I 1 1— 1 1 1 1 1 2.6 3 3.4 3.8 4.2

VELOCITY ( km/s )

4

3 -

2 -

1 -

0 --

•1 - 2 - 3 -

4

•5

*67 H

-8

(b)

-t—±- + +

+ HOT-CURED

+ + + +

+ +

1 .8 2.2 2.6 3.4 3.8 4.2

VELOCITY ( km/3 ) Figure 5*7 Hot-cured specimens: a) strength vs velocity; and,

b) residuals in real space about best-fit of Eq . 5.7 to room-cured

results

94
RESIDUALS

(

MPa )

STRENGTH

(

MPa )

40

7

6

5

4

3

2

10

-1

-2

-3

-4

-5

1.8 2.2 2.6 3 3.4 3.8 4.2

VELOCITY ( km/3 ) Figure 5.8

b) residuals

results

Cold-cured specimens: a) strength vs velocity; and,

in real space about best-fit of Eq . 5.7 to room-cured

95
RESIDUALS

(

MPa )

STRENGTH

(

MPa )

40’

35

30

25

20

15

10

5

0

9

8

7

6

5

4

3

2

10

-»1 -2

-3

1.8 2.2 2.6 3 3.4 3.8 4.2

VELOCITY ( km/a )

Figure 5.9 Lew w-c ratic: a) strength vs velocity; and, b residuals

in real space about best-fit of Eq. 5.7 to high w-c ratio results

96
RESIDUALS

(

MPa )

STRENGTH

(

MPa )

1.8 2.2 2.6 3 3.4 3.8 4.2

VELOCITY ( km/s ) Figure 5.10 High aggregate content: a) strength vs velocity; and.

b) residuals in reel spac^ about best-rit of Ec . 5.7 to low aggregate

content results

97
VELOCITY

(

km/a )

36

34

32

30

2e

26

24

22

20

18

1 6

1 4

1 2

10

8

6

4

2

0

0 200 400 600

AGE ( hours )

Figure 5.11 Room-cured specimens: velocity and strength vs age

4

3.8

3.6

3.4

3.2

3

2.8 H

2.6

2.4

2.2

2 H

1 A

B

-# +

-H- s*

*

CD 9

+

VELOCITY

+ STRENGTH

98

STRENGTH

(

MPa )
CHAPTER 6

SUMMARY, CONCLUSIONS, AND FUTURE RESEARCH

6 .

INTRODUCTION

The objective of this research was to experimentally evaluate the

usefulness of the impact -echo technique as a means of measuring the setting

time and strength of concrete. The work was presented as the first step

towards the development of reliable in-place setting time measurement and

strength estimation tests based on the impact-echo method.

6 . 1 SUMMARY

6.1.1 Setting Time Study

A strong correlation was found between the onset of P-wave velocity

development in concrete and the ASTM C 403 -time of initial setting of mortar

sieved from the concrete. This correlation was shown in two ways. First, for

a given series, a consistent difference was found between the time the P-wave

velocity began to increase rapidly and the time of initial setting in the

sieved mortar. Second, for a given series, the velocity in each concrete

specimen was at the same value at the time of initial setting of the

corresponding sieved mortar.

The impact-echo method was also shown to be sensitve to the influences of

set-controlling admixtures on setting times. Variations in the C 403 -time of

initial setting due to admixtures were reflected by proportional variations in

the times the velocities began to increase in each specimen.

The influence of aggregate content on the P-wave velocity in concrete was

described using a simple series model.

Two approaches for using the impact- echo method to define and measure the

setting time of concrete were presented. The first approach is to define

setting time at the onset of P-wave velocity development. This approach is

appealing because setting is thus based on an actual physical change occurring

in the cement paste, rather than an arbitrary strength or stiffness value that

is changing continuously. Setting time defined in this way would be

independent of aggregate content, because the onset of rapid velocity

development depends upon changes occurring in the paste. The second approach

is to define setting time at the time a specified P-wave velocity is reached.

In this case setting is based on the composite elastic modulus of the

concrete. By selecting a target velocity to define setting, the paste phases

of different concrete mixtures could be at different stages of hydration, or have different elastic moduli at setting.

6.1.2 Compressive Strength Study

The relationship between cylinder compressive strength and P-wave

velocity was studied. It was found that at lower velocities, the velocity was

increasing rapidly relative to strength, and at later ages the opposite was

true

A decrease in curing temperature from 20 to 10 °C did not affect the

strength-velocity relationship. An increase in curing temperature from 20 to

99
35 °C did affect the relationship at higher velocities. Above a velocity of

about 2800 m/s, the hot-cured specimens exhibited a lower strength at any

given velocity. A significant change in w-c ratio was found to affect the

strength velocity relationship only at higher velocities, and an increase in

aggregate content led to a higher velocity at any given strength.

6 . 2 CONCLUSIONS

The work presented in this report clearly demonstrates that the

impact- echo method is a feasible means of measuring the setting time and

estimating the compressive strength of concrete.

Throughout this report, the results of the setting time and compressive

strength studies were presented separately. However, some of the highest

velocities measured in the setting time study were approximately 2000 m/s,

and, as noted earlier, this is about where the lowest velocity measurements

were made in the strength study. Thus the impact- echo method is a feasible

means of monitoring the early-age properties of concrete from the time of

initial casting to well into the region of strength gain.

Measurements can be attempted while the concrete is still fluid- like, so the setting behavior can be monitored. Although there is considerable scatter

in data from the earliest attempts, the measurements soon become well-defined

and the development of velocity can be easily followed. Two approaches were

presented in this report for using these early measurements to define the

setting time.

Velocity measurements can continue to be made as the concrete passes

through the setting period and into the region of strength gain, and these

later measurements can be correlated to compressive strength. Thus the same

test can be used to measure the setting time and to estimate the strength of

concrete

It was seen that the velocity increases rapidly relative to the strength

at lower velocities. Thus the method is a more sensitive strength estimator

at lower velocities. For the concrete mixtures in this study, the sensitive

region of the strength velocity relationship extended to velocities of about

3600 m/s, or compressive strengths of 19 MPa. This was seen to be about 60 %

of the 28-day strength. Reliable estimates of strengths within this range

could be used to improve construction safety and economy.

6.3 PRACTICAL APPLICATIONS

In its current state, the impact-echo technique described in this report

can be used as a means of monitoring the setting time of concrete in the

laboratory. The method can be used to evaluate admixture performance in

concrete and may be incorporated into performance acceptance standards.

Further, the technique may be useful as a means of non- destructively

monitoring changes in the elastic modulus of concrete cylinders caused by

deterioration due to freeze-thaw or alkali-aggregate reactivity.

The most important practical applications for the impact-echo method will

come when it is developed into a reliable field technique for measuring the

setting times and estimating the strength of concrete in-place. Impact-echo

velocity measurements may have many uses in the concrete construction

100
industry. For example, measurements may be useful as a means of determining

formwork removal times, the start and termination of curing operations,

transfer of prestressing, rate of slipforming, rate of concrete placement and

finishing times. Further, impact-echo velocity measurements may be useful as

a means of quality control when set- controlling admixtures are being used, or

as a means of detecting flash or false setting.

6.4 FUTURE RESEARCH

The experiments described in this report were performed on small

cylindrical mortar and concrete specimens. Only a limited number of variables

were examined. Future work should study additional parameters such as air

content, other admixtures, cement types, fly ash, and different aggregates, to

gain more experience with the technique.

The next step towards a reliable field technique is to perform

impact-echo tests on actual structural elements such as slabs, walls, columns,

and beams. Measurements on these structures may be more complicated than

measurements performed on cylinders for two reasons. First, complicated modes

of vibration may be excited by the impact, making determination of the P-wave

velocity more difficult. Second, the energy imparted by the impact will no longer be confined to a small volume of concrete as in a cylinder. Thus

different impact sources may have to be evaluated for testing actual

structural elements. Both plain and reinforced concrete structures need to be

examined to study the possible influence of reinforcing steel, and the

possible influence of different types and stiffnesses of formwork needs to be

evaluated.

It was explained in section 3. 2. 1.1 that the P-wave velocity is higher in

large plate structures than in thin rod structures such as cylinders. The

magnitude of this difference in early-age concrete, and how it changes as the

concrete matures, needs to be examined. This is necessary if

strength-velocity relationships established using cylinders in the laboratory

are to be used to predict strength in the field.

In addition to measuring the setting time of concrete and estimating

strength, the technique presented in this thesis may have applications in

other related areas as well. It was also explained in section 3. 2. 1.1 that

P-, S-, and R-waves travel with different velocities, and that the relative

velocities of each can be expressed in terms of Poisson's ratio (eqs. 3.7 and

3.8). Thus, information about the relative velocities of each wave type may

provide a means of measuring Poisson's ratio of concrete at a very early age.

Lastly, early- age velocity measurements may be used in conjunction with the

electron microscope to study the structure of hydrating cement paste.

101
CHAPTER 7

REFERENCES

1. American Concrete Institute, Committee 228, "In-place Methods for

Determination of Strength of Concrete," in preparation.

2. American Society for Testing and Materials, 1985 Annual Book of

Standards, Philadelphia, Vol. 4.01, Cement; Lime; Gypsum.

3. American Society for Testing and Materials, 1985 Annual Book of

Standards, Philadelphia, Vol. 4.02, Concrete and Mineral Aggregates.

4. Arndt, W. J., Discussion of "Dynamic Testing of the Soniscope Apparatus,"

Proceedings, 29th Annual Meeting Highway Research Board,

1949, pp. 181-183.

5. Bethea, R. M. , Duran, B. S., and Boullion, T. L. , Statistical Methods for

Engineers and Scientists . 2nd ed. , Marcel Dekker, Inc., New York, 1985,

698 pp.

6. Bullock, R.E., and Whitehurst, E. A., "Effect of Certain Variables on

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Effects of Concrete Characteristics on the Pulse Velocity - A Symposium,

1959, pp. 37-41.

7 . Bye , G . C . , Portland Cement Composition. Production and Properties , Pergamon Press, Oxford, 1983, pp. 94-98.

8. Byfors , J., Plain Concrete at Early Ages . Swedish Cement and Concrete

Research Institute, Stockholm, 1980, 464 pp

9. Byfors, J., "Pulse Velocity Measurements for Indication of the

Compressive Strength at Early Ages," International Conference on Concrete

of Early Ages, Vol. 1, April, 1982, pp. 117-122.

10. Carino, N.J., and Sansalone, M. , "Pulse-Echo Method for Flaw Detection in

Concrete," National Bureau of Standards Technical Note 1199, July, 1984,

33 pp.

11. Carino, N. J., "The Maturity Method: Theory and Application," Cement,

Concrete, and Aggregates, Vol. 6, No. 2, 1984, pp . 61-73.

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Early Age Properties of Concrete," International Conference on Concrete

of Early Ages, Vol. 1, April, 1982, pp . 129-135.

13. Cheesman, W. J., "Dynamic Testing of Concrete with the Soniscope

Apparatus," Proceedings, 29th Annual Meeting Highway Research Board,

1949, pp. 176-181.

102
14. Cheesman, W. J., Discussion of "Use of the Soniscope for Measuring the

Setting Time of Concrete," Proceedings of the American Society for

Testing Materials, Vol. 51, 1951, pp. 1176-1180.

15. Cheesman, W. J., "Soniscope Studies of Concrete During the Setting

Period," Hydro Research News, Vol. 3, No. 3, July- Sept. , 1951, pp . 8-11.

16. Elvery, R. H. , and Ibrahim, L. A. M. , "Ultrasonic Assessment of Concrete

Strengths at Early Ages," Magazine of Concrete Research, Vol. 28, No. 97,

Dec., 1976, pp. 181-190.

17. Goldsmith, W. , Impact: The Theory and Physical Behavior of Colliding

Solids . Edward Arnold Press, Ltd., 1965, pp. 24-50.

18. Hutchinson, J.R., "Vibrations of Solid Cylinders," Journal of Applied

Mechanics, ASME, Vol. 47, Dec., 1980, pp. 901-907.

19. Jones, R. , "The Non- Destructive Testing of Concrete," Magazine of

Concrete Research, Vol. 2, 1949, pp. 67-76.

20. Jones, R. , "A Non- Destructive Method of Testing Concrete During

Hardening," Concrete & Constructional Engineering, April, 1949, pp.

127-129.

21. Jones, R. , "Testing of Concrete by Ultrasonic-Pulse Technique,"

Proceedings, 32nd Annual Meeting, Highway Research Board, 1953, pp.

258-275.

22. Knudsen, T. , "On Particle Size Distribution in Cement Hydration,"

Proceedings of the 7th International Congress on the Chemistry of Cement,

Vol. 2, Paris, 1980, pp. 170-175.

23. Kolsky, H. , Stress Waves in Solids . Dover Publications, Inc., New York,

1963.

24. Krautkramer, J., and Krautkramer, H. , Ultrasonic Testing of Materials . 3rd ed. , Springer-Verlag, Heidelberg, 1983, 667 pp.

25. McMahon, G. W. , "Experimental Study of the Vibrations of Solid,

Isotropic, Elastic Cylinders," The Journal of the Acoustical Society of

America, Vol. 36, No. 1, Jan., 1964, pp . 85-92.

26. Pauw, A., "Static Modulus of Elasticity of Concrete as Affected by

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Dec. , 1960, pp. 679-687.

27. Philleo, R. E. , "Comparison of Results of Three Methods for Determining

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103
28. Powers, T. C., "Structure and Physical Properties of Hardened Portland

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29. Proctor, T. M. , Jr., "Some Details on the NBS Conical Transducer,"

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Soils and Foundations . Prentice-Hall, Inc., Englewood Cliffs, 1970,

Chapter 5

31. Sansalone, M. , "Flaw Detection in Concrete Using Transient Stress Waves,"

PhD Thesis, Cornell University, June 1986, 220 pp.

32. Stearns, S. D. , Digital Signal Analysis . Hayden Book Co., Inc., Rochelle

Park, 1975, 280 pp.

33. Sturrup, V.R. , Vecchio, F. J., and Caratin, H. , "Pulse Velocity as a Measure of Concrete Compressive Strength," Non-Destructive Testing . ACI

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Vol. 51, 1951, pp. 1166-1176.

104
APPENDIX A Setting Time Data

Elapsed Elapsed Elapsed

Time Frequency Cp Time Frequency Cp Time Frequency C P (hours) (kHz) ( km/ s) ( hours) (kHz) (km/ s) ( hours) (kHz) (km/ s

CL 1 CONCRETE 4.28 1 . 856 0.594 6.42 4.834 1 .547

2.12 0.684 0.219 4.60 2.148 0.687 6. 92 5.371 1.719

2.42 0.488 0.156 5.00 2.588 0.828 7.42 5.859 1 . 87 5 2.67 0.733 0.235 5.50 3.125 1.000 7. 92 6.299 2.016

2.92 0.684 0.219 6.00 3.711 1.188 9.02 7.178 2 . 2 97 3.17 0. 87 9 0.281 6.50 4.297 1.375

3.42 1.074 0.344 7.00 4.883 1.563 CL3 SIEVED MORTAR

3.67 1.270 0.406 7.50 5.322 1.703 2.08 0.684 0.219

3 . 87 1.367 0.437 8.00 5.811 1.860 2.28 0.586 0.188

4.17 1.660 0.531 9.00 6.641 2.125 2.52 1.026 0.328

4.38 1 . 856 0.594 2.78 0.684 0.219

4.58 2.051 0.656 CL 2 SIEVED MORTAR 2. 95 0.586 0.188

4.95 2.441 0.781 2.02 0.684 0.219 3.12 0. 87 9 0.281

5.45 3.027 0. 96 9 2.17 0. 87 9 0.281 3.12 0.684 0.219

6.05 3 . 80 9 1.219 2.33 0.684 0.219 3.28 0.7 81 0.250

6. 83 4.688 1.500 2.50 0.781 0.250 3.28 0. 97 7 0.313

7.33 5.225 1.672 2.67 1.026 0.328 3.47 0. 87 9 0.281

7. 83 5.762 1 . 844 2.82 0. 87 9 0.281 3.47 1.367 0.437

8.33 6.152 1 . 96 9 2.95 0.488 0.156 3.63 0. 977 0.313

9.33 7.031 2.250 3.07 0.5 86 0.188 3. 82 1.074 0.344

3.07 1.074 0.344 3.97 1.172 0.375

CL 1 SIEVED MORTAR 3.22 0.5 86 0.188 4.07 1.270 0.406

2.05 0.684 0.219 3.38 0.684 0.219 4.17 1.367 0.437

2.33 0.586 0.188 3.38 0. 87 9 0.281 4.33 1.465 0.469

2.58 0. 83 0 0.266 3.75 0.781 0.250 4.50 1.660 0.531

2. 83 1 .074 0.344 3 . 93 0. 87 9 0.281 4.75 1 . 856 0.594

3.08 0.488 0.156 4.17 1.172 0.375 5.00 2.148 0.6 87 3.33 0.5 86 0.188 4.33 1.270 0.406 5.50 2.637 0.844

3 .58 0.684 0.219 4.45 1.270 0.406 6.17 3.41 8 1 .094

3.75 0. 87 9 0.281 4.53 1.367 0.437 6 . 50 3 . 809 1.219

4.00 0 . 97 7 0.313 4.70 1.465 0.469 7.00 4.3 95 1.406

4.25 1.074 0.344 4.92 1 .660 0.531 7 . 50 4. 93 2 1 . 57 8 4.50 1 .172 0.375 5.17 1 . 856 0.594 8.00 5.469 1.7 50

4.75 1.367 0.437 5.42 2.051 0.656 9.08 6.836 2.188

5.12 1.563 0.500 5.92 2.539 0. 812

5.72 2.051 0.656 6.42 3.027 0. 969 CHI CONCRETE

5.92 2.344 0.750 6. 92 3. 516 1.125 2.10 0.586 0.188

6.75 3.223 1.031 7.42 4.053 1 .297 3.17 0 . 92 8 0.2 97 7.25 3.711 1.188 7.92 4.5 90 1.46 9 3.50 0. 87 9 0.281

7.75 4.199 1.344 8.92 5.664 1 . 812 3.75 0. 97 7 0.313

8.25 4.688 1 . 500 4.00 0.781 0.250

9.25 5.811 1 = 860 CL3 CONCRETE 4.25 0. 977 0.313

1 .60 0.586 0.188 4.42 1.074 0.344

CL 2 CONCRETE 1.75 0. 83 0 0.266 4.58 1.074 0.344

1.58 0.684 0.219 2.00 0.87 9 0.281 4. 83 1.172 0.375

1.78 0.684 0.219 2.20 0. 977 0.313 5.10 1.367 0.437

1 . 92 0.684 0.219 2.45 1.074 0.344 5.33 1.465 0.46 9 2.10 0.684 0.219 2.70 0. 87 9 0.281 5.58 1.612 0.516

2.25 1.026 0.328 2.70 0.586 0.188 5 . 83 1.758 0.563

2.42 1.074 0.344 2. 87 0. 97 7 0.313 6.18 2.051 0.656

2.55 0. 830 0.266 3.03 1 .172 0.375 6.58 2.246 0.719

2.73 1.026 0.328 3.20 1.270 0.406 7.17 2.637 0.844

2.88 0.7 81 0.250 3.38 1.367 0.43 7 7.67 2 . 97 9 0.953

3.00 0. 87 9 0.281 3.57 1.563 0.500 8.17 3.320 1.062

3.13 0. 977 0.313 3.73 1.660 0.531 8.67 3.613 1.156

3.27 1.123 0.359 3. 90 1 . 856 0.5 94

3.33 1.172 0.375 4.23 2.246 0.719 CHI SIEVED MORTAR

3.45 1.270 0.406 4.58 2.637 0.844 2.25 0.684 0.219

3.67 1.367 0.437 4.92 3.027 0. 96 9 3.25 0. 87 9 0.281

3. 87 1.465 0.46 9 5.42 3.711 * 1.188 3.58 0 . 87 9 0.281

4.07 1.660 0.531 6.08 4.444 1.422 3 . 83 0 . 7 81 0.250

105
Elapsed Elapsed Elapsed

Time Frequency Cp Time Frequency Cp Time Frequency Cp

( hours) (kHz) ( km/ s) ( hours) (kHz) ( km/ s) ( hours) (kHz) ( km/ s

4 » 0 8 0.6 84 0.219 6.00 1.367 0.437 5.00 0. 87 9 0. 2 81 4.30 0.781 0.250 6.50 1 . 563 0.500 5.25 1.074 0.344

4.47 0.781 0.250 7.00 1,758 0.563 5.42 1.172 0.375

4.67 0. 87 9 0.281 7.50 2.051 0.656 5.58 1.319 0.422

4. 92 0. 977 0.313 8.00 2.246 0.719 5.77 1.465 0.469

5.17 1.172 0.375 8.78 2.637 0.844 5.95 1 . 563 0.500

5.42 1.270 0.406 6.20 1.758 0.563

5.67 1.367 0.437 CH3 CONCRETE 6.38 1.905 0.610

5.92 1.563 0.500 2.70 0.586 0.1 88 6.65 2.1 97 0.703

6.25 1.660 0.531 2.95 0.5 86 0.188 7.02 2. 53 9 0. 8i2

6.67 1 . 953 0.625 3.17 0.586 0.188 7.30 2. 881 0.922

7.25 2.246 0.719 3.33 0.684 0.219 7.63 3.272 1.047

7.75 2.4 90 0.797 3.55 0.781 0.250 8.00 3.711 1.188

8.25 2.734 0. 87 5 3.75 0.7 81 0.250 8.50 4.395 1 . 406

8.75 3.027 0. 96 9 3 o 92 0. 87 9 0.281 9.00 4.981 1.594

4,08 1.026 0.328 9.50 5.51 8 1.766

Cl 2 CONCRETE 4 o 2 5 0. 879 0.281 10.00 6.055 1 . 93 8 2.50 0.468 0.156 4.43 0.87 9 0.281 11.00 6.984 2.235

2.75 0.684 0.219 4,58 0. 97 7 0.313 12.55 8.203 2.625

3.00 0.5 86 0.188 4.77 1.074 0.344

3.25 0.684 0.219 5.05 1.172 0.375 CR1 SIEVED MORTAR

3.42 0.684 0.219 5.30 1.270 0.406 4.07 0. 83 0 0.266

3.58 0.781 0.250 5.58 1.465 0.469 4.35 0 c 87 9 0.281

3.75 0.87 9 0.281 5.92 1.612 0.516 4.62 0. 977 0.313

3 . 92 0.586 0.188 6.42 1.953 0.625 4. 87 0.5 86 0.188

4.08 0.6 84 0.219 6 . 92 2.246 0.719 5.07 0.684 0.219

4.25 0.733 0.235 7 . 42 2.539 0.812 5.33 0.7 81 0.250

4.42 0.781 0.250 7 . 92 2.832 0. 906 5.48 0.7 81 0.250

4.58 0. 87 9 0.281 8.58 3.223 1.031 5.65 0. 87 9 0.281

4.75 0. 97 7 0.313 9.63 3.858 1.235 5. 83 0 . 977 0.313

4. 92 1.026 0.328 6.10 1.074 0.344

5.08 1.074 0.344 CH3 SIEVED MORTAR

0.250

6.30 1.172 0.37 5 5.33 1.270 0.406 2.80 0.7 81 6.48 1.367 0.437

5.58 1 .367 0.437 3.02 0.684 0.219 6.73 1.465 0.469

5.85 1.465 0.469 3.23 0.7 81 0.250 7.08 1.758 0.563

6.08 1.612 0.516 3.42 0.488 0.156 7.38 2.002 0.641

6.58 1 . 90 5 0.610 3.62 0.488 0.156 7.72 2.295 0.734

7.08 2.246 0.719 3 . 82 0.586 0.188 8.10 2.637 0. 844

7.58 2.53 9 0.812 3 . 98 0. 5 86 0.188 8.58 3.174 1.016

8.08 2.83 2 0. 906 4.15 0.6 84 0.219 9.08 3.711 1.188

8. 90 3.223 1.031 4.32 0.684 0.219 9.58 4.199 1.344

4.48 0.684 0.219 10.10 4.785 1.531

CH2 SIEVED MORTAR 4.65 0.7 81 0.250 11.08 5.859 1 . 87 5 2.58 0. 87 9 0.281 4. 83 0. 87 9 0.281 12.65 7.764 2.484

2. 83 0.684 0.219 5.12 1.026 0.328

3.08 0.781 0.250 5.37 1.172 0.375 CR2 CONCRETE

3.33 0. 87 9 0.281 5.67 1.270 0.406 3.58 0. 5 86 0.188

3.50 0. 928 0.297 6.00 1.465 0.46 9 3.85 0. 830 0.266

3.67 0.684 0.219 6 . 50 1.660 0.531 4.12 0.488 0.156

3.83 0.781 0.250 7.00 1 . 953 0.625 4.37 0. 5 86 0.188

4.00 0.781 0.250 7.50 2.246 0.719 4.62 0.7 81 0.250

4.17 0. 87 9 0.281 8.00 2.441 0.781 4.80 0. 87 9 0.281

4.33 0. 97 7 0.313 8.68 2.7 83 0.891 5.03 1.172 0.375

4.50 0.781 0.250 9.70 3.320 1.062 5.32 1.465 0.46 9 4.67 0.87 9 0. 281 5.55 1 . 563 0.500

4.83 0.928 0.2 97 CR1 CONCRETE 5. 92 1 . 953 0.625

5.00 0. 97 7 0.313 4.00 0.7 81 0.250 6.25 2.295 0.734

5.17 0. 977 0.313 4.25 0.7 81 0.250 6.63 2.734 0. 875

5.25 1.074 0.344 4.50 0.488 0.156 7.17 3.418 1.094

5.50 1.172 0.375 4„$0 1.172 0.375 7.67 4.102 1.313

5.75 1.270 0.406 4.75 0.684 0.219 8.17 4.785 1.531

106
Elapsed Elapsed Elapsed

Time Frequency Cp Time Frequency Cp Time Frequency Cp

( hour s) (kHz) ( km/ a

)

( hours) (kHz)’ (km/ s) ( hours) (kHz) (km/ s

)

9.28 5.762 1 . 844 7.42 2.148 0.687 CA2 SIEVED MORTAR

10.25 6.738 2.156 7. 83 2.539 0. 812 1 . 93 0.5 86 0.188

8.33 2. 930 0. 93 8 2.27 0.684 0.219

CR2 SIEVED MORTAR 9.40 4.004 1 .281 2.27 0. 87 9 0.281

3.67 0.5 86 0.188 10.03 4.5 90 1 .46 9 2.50 0.684 0.219

3. 92 0.83 0 0.266 12. 87 7.617 2.437 2.72 0.5 86 0.188

4.18 1 .172 0.375 3.23 1.074 0.344

4.43 0.684 0.219 CA1 CONCRETE 3.48 1.270 0.406

4.68 0.684 0.219 1.63 0.5 86 0.188 3.73 1.563 0.500

5.10 0.781 0.250 1.97 0.733 0.235 3.98 1.758 0.563

5.38 0. 87 9 0.281 2.37 0. 97 7 0.313 4.23 2.051 0.656

5.62 0. 97 7 0.313 2.57 1.172 0.375 4.75 2.734 0. 87 5 5.85 1.074 0.344 2.73 1.367 0.437 5.23 3.418 1.094

6.10 1.270 0.406 2.98 1.758 0.563 5.73 4.004 1 .281

6.33 1.367 0.437 3.23 2.148 0.6 87 6.23 4.593 1.470

6.58 1.563 0.500 3.50 2.637 0.844 6.75 5.273 1 .687

6.92 1 . 856 0.594 3.75 3.125 1.000 7.72 6.250 2.000

7.25 2.148 0.6 87 4.15 3. 906 1.250 8.75 7.324 2.344

7.75 2.637 0.844 4.65 4.834 1 . 547

8.25 3.125 1.000 5.05 5.469 1.750 CA3 CONCRETE

9.35 4.2 97 1 .375 6.10 6. 836 2.188 1.62 0.684 0.219

10.33 5.420 1.734 7.98 8.496 2.719 1.95 0.586 0.188

1 . 95 0.781 0.250

CR3 CONCRETE CA1 SIEVED MORTAR 2.25 0.5 86 0.188

3.50 0.684 0.219 1.72 0.5 86 0.188 2.50 0. 87 9 0.281

3.75 0.586 0.188 2.05 0.781 0.250 2.75 1.172 0.375

4.00 0.684 0.219 2.28 0. 87 9 0.2 81 3.00 1.46 5 0.469

4.25 0.684 0.219 2.42 0.5 86 0.188 3.25 1 .758 0.563

4.43 0.7 81 0.250 2.63 0.781 0.250 3.50 2.148 0.6 87 4.58 0. 87 9 0.281 2. 82 0.87 9 0.281 3.75 2.588 0.828

4.78 0. 83 0 0.266 3.05 1.172 0.375 4.00 3.027 0. 96 9 5.00 0.928 0.2 97 3.30 1.416 0.453 4.25 3.418 1.094

5.25 1.172 0.375 3.57 1.758 0.563 4.75 4.297 1.375

5.50 1.270 0.406 3. 83 2.051 0.656 5.25 5.030 1.610

5.75 1.465 0.469 4.22 2.637 0.844 6.25 6.201 1 . 984

6.00 1.660 0.531 4.73 3.418 1.094 7.27 7.227 2.313

6.25 1 . 90 5 0.610 5.15 4.004 1 .281 8.27 8.008 2.563

6.58 2.246 0.719 6.12 5.322 1.703

6.92 2.638 0.844 8.00 7.617 2.437 CA3 SIEVED MORTAR

7.33 3.125 1.000 1.70 0.488 0.156

7.75 3.613 1.156 CA2 CONCRETE 1.70 0.684 0.219

8.25 4.151 1.328 1.53 0. 5 86 0.188 2.03 0.5 86 0.188

9.32 5.371 1.719 1 . 87 0.5 86 0.188 2.55 0.684 0.219

10.02 5. 957 1 . 906 2.18 0.5 86 0.188 2.82 0.684 0.219

12.78 8.203 2.625 2.42 0.781 0.250 3.07 0.781 0.250

2.67 1.026 0.328 3.07 0.97 7 0.313

CR3 SIEVED MORTAR 2.92 1.270 0.406 3.13 0. 97 7 0.313

3.60 0.7 81 0.250 3.17 1.612 0.516 3.33 1.172 0.375

3. 83 0.684 0.219 3.42 1 . 953 0.625 3.57 1.465 0.469

4.08 0. 87 9 0.281 3.67 2.344 0.750 3. 83 1.660 0.531

4.32 0. 87 9 0.281 3.92 2.734 0.87 5 4.07 1 . 953 0.625

4.52 0.87 9 0.281 4.17 3.125 1.000 4.32 2.246 0.719

4.67 0. 586 0.188 4.67 3.955 1.266 4.85 2.930 0. 93 8 5.07 0.684 0.219 5.17 4.688 1 . 500 5.32 3.613 1.156

5.32 0.87 9 0.281 5.67 5.37 1 1.719 6.30 4. 834 1.547

5.57 0. 977 0.313 6.17 5.957 1 . 906 7.33 6.104 1 . 953

5.92 1.074 0.344 6.67 6.494 2.078 8.28 7.373 2.359

6.08 1.172 0.375 7.67 7.324 2.344

6.32 1.319 0.422 8.68 8.057 2.578

6.63 1.514 0.484

7.00 1.758 0.563

107
Elapsed Penetration Elapsed Penetration Elapsed Penetration

Time Resistance Time Resistance Time Resistance

(hours) ( ps i) ( hours) ( psi) ( hours) (psi)

CL 1 CH2 6. 82 1220

3.00 36 3.50 30 7.08 1 840

3.50 76 4.02 64 7.52 3120

3.83 192 4.47 98 7.77 4200

4.08 220 4. 97 144

4.1 8 480 5.47 264 CR3

4.35 390 5.93 460 5.02 42

4.42 5 90 6.22 680 5.47 96

4.67 600 6.45 780 6.00 248

4. 83 12 40 7.17 1720 6.55 580

5.17 1540 7.66 2440 6.75 7 80

5.50 3360 8.17 3200 7.03 1440

5.75 4400 8.62 3680 7.50 2480

8. 80 4000 7 . 87 4440

CL 2

3.50 88 CH3 CA1

3.77 140 3.50 36 2.32 42

4.00 244 4.00 66 2.58 76

4.25 350 4.50 122 2. 93 248

4.50 620 5.00 208 3.10 360

4.75 820 5.25 328 3.37 650

5.08 1580 5.53 420 3.62 1200

5.58 3680 5. 83 520 3 . 87 2240

5.67 4600 6.17 920 4.17 2640

7.08 2160 4.37 5000

CL3 7 .52 2 920

3.25 110 7.75 2 920 CA2

3 . 50 1 80 7 . 83 3080 2.3 8 22

3.75 364 8.05 4480 2.75 7 8

4.00 690 3.02 152

4.25 880 CR1 3.28 268

4.58 1740 4.98 30 3.53 460

5.03 4320 5.75 106 3.78 670

6.27 264 4e05 1280

CHI 6.50 490 4.33 2120

3.93 88 6.77 620 4.58 3400

4.22 88 6.98 1000 4.80 4160

4.50 172 7.25 1400

4.75 1 80 7.75 2720 C A3

5.00 300 8.05 4080 2.38 30

5.25 330 2.78 130

5.50 480 CR2 3.03 148

5.75 590 5.12 64 3.28 316

6.08 1040 5.50 134 3.52 520

6.52 1760 6.00 316 3.77 690

6. 83 2200 6.42 510 4.02 1200

7 .20 3160 6.78 1210 4.35 3080

7 . 50 3400 4.67 4000

7.75 3200

7.7 8 4000

108
:

!

APPENDIX B - Strength Data

Preq u.ency Cp f c Frequency Cp f c Batch (kHz) ( km/ s ) ( MPa) Batch (kHz) ( km / a ) ( MP »

ROOM- CURED SPECIMENS

1 4.981 2.022 1 . 87 3 8.496 3.449 13.30

1 5.469 2.220 2.03 3 8.155 3.311 13.28

1 6.152 2.498 3.29 3 8. 936 3.628 19.37

1 6.201 2.518 3.29 3 9.082 3.6 87 20.52

1 6.885 2.795 5.87 3 9.180 3.727 26.06

1 7.031 2.855 5.60 3 9.277 3.766 26.45

1 7.471 3.033 7.35 3 9.815 3.985 32.26

1 7.422 3.013 7.08 3 9.717 3.945 31.17

1 7 . 910 3.211 10.26

1 8.008 3.251 10.59 LOW W-C RATIO SPECIMENS

1 8.545 3.46 9 15.09 5 5.176 2.101 1 . 87

1 8.447 3.42 9 13.72 5 5.225 2.121 1 . 87

1 8.789 3.568 15.91 5 6.885 2.795 5.38

1 8.691 3.529 16.30 5 6.738 2.736 5.32

1 9.277 3.766 25.90 5 8.155 3.311 11.08

2 5.762 2.339 3.35 5 8.301 3.370 12.07

2 6.934 2.815 5.16 5 9.1 80 3.727 21.62

2 8.300 3.370 11.74 5 9.180 3.727 21 . 95

2 9.473 3.846 30.12 5 9.668 3.925 29.96

3 4. 883 1 . 982 1 . 87 5 9.668 3. 925 29.08

3 6. 930 2.814 6.31 5 9.766 3.96 5 35.66

3 7.324 2. 97 4 7.08 5 9.863 4.004 35.23

3 8.691 3.529 16.63 5 10.059 4.084 39. 94

3 9.473 3. 846 28.92 5 9. 863 4.004 40. 93

4 8.203 3.330 11.58 6 6.055 2.458 3.35

4 8. 838 3.588 19.09 6 5. 90 8 2.399 3.13

4 8. 887 3.608 1 9.31 6 7.520 3.053 7.41

4 9.473 3.846 26.50 6 7.178 2. 914 7.24

4 9.277 3.766 26.28 6 8.984 3.648 18.71

4 9.619 3. 90 5 30.64 6 8. 887 3.608 18.55

4 9.570 3.885 30.73 6 9.375 3. 806 25.90

4 9.863 4.004 33.14 6 9.375 3. 806 24.64

4 9. 961 4.044 34.02 6 9.815 3.985 32.43

6 9.570 3.885 31.60

HOT-CURED SPECIMENS 6 9.766 3 . 96 5 37.42

2 4. 93 2 2.002 1.70 6 9.912 4.024 36. 82

2 5.127 2.082 1 . 98 6 10.059 4.084 39.67

2 5.51 8 2.240 2.19

2 5. 90 8 2.399 3.35 HIGH AGGREGATE CONTENT SPECIMENS

2 6.543 2.656 4.06 7 6.250 2.538 3.51

2 6.738 2.736 4.55 7 6.201 2.518 3.40

2 7.324 2. 97 4 6.36 7 7.324 2.974 5.27

2 7.031 2. 855 6.09 7 7.227 2. 93 4 5.38

2 7. 910 3.211 9.05 7 8.203 3.330 9.77

2 8.057 3.271 9.27 7 8.203 3.330 9. 82

2 8.008 3.251 10. 97 7 9.375 3.806 19.04

2 8.643 3.509 15.03 7 9.277 3.766 1 9.26

2 9.082 3.6 87 19.48 7 9.570 3.885 25.40

2 9.668 3. 92 5 26.01 7 9.473 3.846 25.84

4 8.496 3.449 12.78 7 9. 81 5 3.985 30.73

4 8.398 3.410 12.78 7 9. 961 4.044 31.00

4 9.033 3.667 17.23 7 10.018 4.067 35.12

4 8. 984 3.648 17.72 7 10.156 4.123 33.31

4 9.180 3.727 22.72 8 6.055 2.458 2.80

4 9.277 3.766 2 2. 83 8 6.738 2.736 4.17

4 9.522 3. 866 25.84 8 6.738 2.736 4.12

4 9.47 3 3.846 25.84 8 7. 813 3.172 6. 97

4 9.61 9 3. 90 5 27.32 8 7.715 3.132 6.97

4 9.668 3. 92 5 28. 81 8 8.740 3.548 12.78

8 8.594 3.489 12.56

COLD- CURED SPECIMENS 8 9.375 3. 806 20.63

3 4. 883 1 . 982 1.59

8 9.229 3.747 20.90

3 5.859 2.37 9 2.74

8 9.766 3.96 5 28.20

3 6.006 2.43 8 2.58

8 9. 815 3.985 29.74

3 6. 836 2.77 5 4.61

8 10.205 4.1*3 3 3.' ! 3 6.592 2.676 4. 83

8 10.254 4.163 32.

3 7 . 520 3.053 8.1 8

3 7.373 2. 993 7 . 9 f

109
NBS-114A (rev. 2 «ac) U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions

4. TITLE AND SUBTITLE

1. PUBLICATION OR

REPORT NO.

2. Performing Organ. Report No. 3. Publication Date

NBSIR 87-3575 JULY 1987

Measurement of the Setting Time and Strength of Concrete

by the Impact-Echo Method

5. AUTHOR(S)

Stephen P. Pessiki and Nicholas J. Carino

6. PERFORMING ORGANIZATION (If joint or other than N BS, see instructions) 7„ Contract/Grant No. NATIONAL BUREAU OF STANDARDS -

UeS, DEPARTMENT OF COMMERCE

GAITHERSBURG, MD 20899

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State, ZIP)

- -*

,,

i

. Type of Report & Period Covered

National Bureau of Standards

Department of Commerce

Gaithersburg, MD 20899

10. SUPPLEMENTARY NOTES

[~ ' Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most significant in formation. If document includes a si gnifi cant bibliography or literature survey „ mention it here)

Tests were performed to evaluate the feasibility of using the impact-echo method to

determine setting time and monitor strength development of concrete. In the impactecho method, the test object is subjected to point impact and the surface displacement

adjacent to the impact point is monitored. From the measured displacement waveform

and the thickness of the object, the P-wave velocity is determined. Changes in the Pwave velocity with time reveal information about the development of mechanical

properties. Setting time tests were made on concrete mixtures of two water-cement

ratios and with and without set-controlling admixtures. A strong correlation was

found between the time of initial setting of mortars sieved from the concrete, as determined by penetration resistance ( ASTM C 403), and the time when the P-wave

velocity began to increase. Two approaches for using the impact-echo method to define

the setting time of concrete are presented. Tests were performed to examine the

relationship between P-wave velocity, as determined by the impact-echo method, and the

compressive strength of concrete. At early ages (up to about 3 days at standard

temperature) , the relationship was independent of curing temperature and water cement

ratio. It is concluded that the impact-echo method is a promising technique for

nondestruc tively monitoring the development of mechanical properties in concrete from

initial setting to ages of several days.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key

concrete; compressive strength; early-age; impact-echo method;

testing; mortar; setting time; wave propagation.

words by semicolon s) nondestructive

13. availability 14. NO. OF

PRINTED PAGES

fxl Unlimited

1

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Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

20402. 15. Price

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Order From National Technical Information Service (NTIS), Springfield, VA. 22161

$18.95

u SCOmm-OC 8043-PS0


A 1 1 1 0 2 b1b750

| puBUCATIONS

NBSIR 87-3576

The ABC’s of Standards-Related

Activities in the United States

Maureen A. Breitenberg

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Standards Code and Information

Office of Product Standards Policy

Gaithersburg, MD 20899

May 1987

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

Research Information Center

National Bureau of Standards

NBSIR 87-3576 Gaithersburg, Maryland 20899

K S TO S A

/\! The ABC’S OF STANDARDS-RELATED

ACTIVITIES IN THE UNITED STATES

WO.%1'35

iOiQn I I o ( 0 Cl L- • <j

~

Maureen A. Breitenberg

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Standards Code and Information

Office of Product Standards Policy

Gaithersburg, MD 20899

May 1987

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Foreword

The Standards Code and Information Program in the NBS Office of Product Standards

Policy from time to time develops and publishes standards-related documents as a service to producers and users of standards and certification systems, both in

the government and in the private sector. This report has been designed as an introduction to standardization, certif ication, and laboratory accreditation for

those not fully familiar with these fields and their interrelationships. We hope

that this material will be informative and will also serve to stimulate wider and

more profitable applications of standardization. In addition, the interested

reader may wish to take further advantage of other available publications and

services provided by this office, some of which are described in the appendix.

Donald R. Mackay

Manager, Standards Code and Information

Office of Product Standards Policy

iii
,

9-.
Table of Contents

Page

Foreword .....i

Acknowledgements .........iii

Abstract . . . . . i v Introduction 1 Background ......1

Historical Notes on Standardization 2

Types of Standards 3

Private Standards Organizations in the U.S. 5 Standards Development Procedures ......7

Benefits and Problems of Standardization.......... 7 Certification ....8

Laboratory Accreditation 10

Summary .......11

Appendix 12

Footnotes IB

v

Acknowledgements

I would like to thank Tom Kelsey, International Trade Administration; dipl. Ing.

Alfonse F. Donko, Deputy Managing Director, Osterreichisches Normungsinstitut

(ON), and many others for their careful review of and comments on this document.

I would especially like to thank Walter Leight, OPSP, who oversaw the preparation

of many of the initial drafts of this document.

Maureen A. Breitenberg

Standards Code and Information

Abstract

This report provides an introduction to voluntary standardization, product

certification and laboratory accreditation for a reader who is not fully familiar

with these topics. It highlights some of the more important aspects of these

fields; furnishes the reader with both historical and current information on these topics; describes the importance and impact of the development and use of

standards; and serves as background for using available documents and services.

Key Words: Certification; inspection; laboratory accreditation; standardization;

standards; testing

ix

Introduction

"The inch is a standard of measurement.

Money is a standard of exchange.

Words are standards of communication.

Traffic lights are safety standards.

Octane numbers of gasoline are quality standards.

'No more than 1% shrinkage' is a performance standard." iy

As the above indicates, standardization has a major impact on our lives, yet most

people know little about the process or about the standards themselves. They know

that camera film marked ISO 100 is likely to give good results in a camera with

the film speed set at 100, but few understand that the ISO 100 marking on the

package means that the film conforms to a standard established by the

International Organization for Standardization (ISO), an international

organization that writes standards. Few people question that three-holed

notebook paper will align with the three rings in most notebooks, yet such

confidence would not be possible without standards. While driving we are on the

lookout for hexagonal, not round or square-shaped stop signs, just as we know

that inverted triangles indicate where traffic should yield. These are just a few of the thousands of standards that impact on our lives.

Because standards have such an impact, it is important to have some familiarity

with what they are and how they are developed and used. This paper is designed

to be an introduction to some of the more significant aspects of standards

development, product certification, and laboratory accreditation. It will also

discuss some of the benefits and problems associated with these processes. The

interested reader is encouraged to increase his knowledge of the field by taking

advantage of other available publications and services described in the appendix.

Background

A standard was defined by the National Standards Policy Advisory Committee as:

"A prescribed set of rules, conditions, or requirements concerning

definitions of terms; classification of components; specification of materials,

performance, or operations; delineation of procedures; or measurement of quantity

and quality in describing materials, products, systems, services, or practices."

2 / Though often unrecognized, standards can help to assure health and safety and to

increase the quality of life. Standards are vital tools of industry and

commerce. They often provide the basis for buyer-seller transactions, hence they

have tremendous impact on companies and nations, and even on the economic fabric

of the world market.

In the United States alone, approximately 30,000 current voluntary standards have

been developed by more than 400 organizations. These do not include a much

greater number of procurement specifications (developed and used by Federal,

State, and local procurement authorities), as well as mandatory codes, rules and

regulations containing standards developed and adopted at Federal,

1
State, and local levels. In addition, numerous foreign national, regional and

international organizations produce standards of interest and importance to U.S.

manufacturers and exporters.

There are numerous international organizations that produce standards. The

International Organization for Standardization (ISO) probably produces the

largest number of International Standards, having issued approximately 6,000

standards. ISO's work is carried out through some 2,300 technical bodies in

which more than 20,000 experts from all over the world participate annually in

the development of ISO standards.

The international General Agreement on Tariffs and Trade (GATT) has as one of its

major components the Agreement on Technical Barriers to Trade (usually referred

to as the Standards Code). The framers of the Standards Code recognized that

standards and standards-related activities can seriously hinder the free fluw of

goods in international commerce. The Code established, for the first time, some

requirements for the procedures by which standards are developed, adopted, and

used and for the systems which determine conformity with such standards.

The Trade Agreements Act of 1979 implemented the Standards Code in the United

States. Federal agencies are required under the Act to:

"-Not engage in standards activities that are prepared, adopted or applied to

create, or have the effect of creating, unnecessary obstacles to the foreign

trade of the United States;

-Ensure that imported products are treated no less favorably than domestic

products

-Use international standards, if appropriate, as a base for developing new standards

-Develop standards based on performance rather than design criteria, if

appropriate; and

- Allow foreign suppliers access to their certification systems on the same basis

as access is permitted to domestic suppliers." 3/

Historical Notes on Standardization

The history of standardization is both fascinating and demonstrative of the scope

and variety of such activities. A predecessor of the American National Standards

Institute (ANSI) noted that one of the first known attempts at standardization in

the Western world occurred in 1120. King Henry I of England ordered that the

ell, the ancient yard, should be the exact length of his forearm, and that it

should be used as the standard unit of length in his kingdom. 4y

That history also notes that, in 1689, the Boston city fathers recognized the

need for standardization when they passed a law making it a civic crime to

manufacture bricks in any size other than 9x4x4. The city had just been

destroyed by fire, and the city fathers decided that standards would assure

rebuilding in the most economic and fastest way possible. 5/

2
Eli Whitney is sometimes referred to as "the Father of Standardization" in the area of interchangeability, having originated and implemented the concept of mass production in the United States in 1780. He was awarded a contract to produce

10,000 muskets by then Vice-President Thomas Jefferson. Though standardized

parts had been successfully used in other parts of the world, Whitney brought the

concept to this country when he divided the manufacturing process into individual

steps and put different groups to work on each step of the process. All parts of

the same type were copied from a model musket and were made to be

interchangeable. Subsequently, when he appeared before Congress with a collection of assorted parts and proceeded to assemble ten working muskets by

selecting the required parts at random. Congress was convinced of the benefits of mass production made possible by standardization. 6/

Standards are known to have existed as early as 7000 B.C. when cylindrical stones

were used as units of weight in Egypt. However, the great blaze in downtown

Baltimore in February 1904 and other, similar catastrophes provided tragic and

undeniable evidence of the importance of standards. While the fire in Baltimore

burned, fire engines from as far away as New York rushed to the scene only to

discover that their hoses would not fit Baltimore hydrants. Those "alien" fire

engines were useless! The inferno burned for more than thirty hours, destroying

1526 buildings covering more than seventy city blocks. All electric light,

telephone, telegraph, and power facilities were also razed. TJ * n contrast, 23

years later, help from 20 neighboring towns saved Fall River, Massachusetts from

destruction since hydrants and hose couplings had been standardized in these

communities. 8/

As late as 1927, a color-blind motorist had as good (or as bad) a chance as anyone else when trying to interpret traffic signals. Purple, orange, green,

blue, yellow, and red lights greeted him as he drove from state to state. In some states, green meant "Go," in others "Stop." Red, not yellow, lights meant

caution in New York City. In 1927 a national code for colors was established

through the work of the American Association of State Highway Officials, the

National Bureau of Standards and the National Safety Council. 9/ Imagine the

chaos that would occur during rush hour in any major U.S. city today if newcomers

and tourists did not know what traffic signals meant!

Probably the most significant standard ever developed in the United States,

however, was the railroads' standard track gage. This standard, now used in

Great Britain, the U.S., Canada and much of continental Europe, enables railroad

rolling stock to cross the country. 10 /

It was the Second World War, however, that brought the urgency of extending

domestic standardization to the international level. Allied supplies and

facilities were severely strained because of the incompatibility of tools,

replacement parts, and equipment. The War highlighted the need for standards

aimed at reducing inventories and increasing compatibility.

Types of Standards

Standards may be classified in numerous ways, some of which are described here.

ISO Draft Guide 2 differentiates eight types based on purpose, 11/ A basic

standard has a broad ranging effect in a particular field, such as a standard for

metal which affects a range of products from cars down to screws. Terminology

3
standards define words permitting representatives of an industry or parties to a transaction to use a common, clearly understood language. Testing standards

define the test methods to be used to assess the performance or other

characteristics of a product. Product standards establish qualities or requirements for a product (or related group of products) to assure that it will

serve its purpose effectively. Process standards specify requirements to be met

by a process, such as an assembly line's operation, in order to function

effectively. Service standards , such as for servicing or repairing a car,

establish requirements to be met in order to achieve the designated purpose

effectively. Interface standards , such as the point of connection between a telephone and a computer terminal, are concerned with the compatibility of

products. The last type provides a listing of data requirements for a product or service for which values need to be obtained.

Standards may also be classified by the intended user group. These

classifications range from company standards , meant for use by a single

industrial organization, to international standards . International standards are developed and promulgated by international governmental and non-governmental

organizations, such as the North Atlantic Treaty Organization's (NATO's) Military

Agency for Standardization (governmental) and the ISO (non-governmental).

International standards may be voluntary or mandatory in nature. A harmonized

standard , on the other hand, can be either an attempt by a country to make its

standard compatible with an international, regional or other standard or it can

be an agreement by two or more nations on the content and application of a standard, the latter of which tends to be mandatory. Harmonized standards may

also be identical in content to other standards. There are still other

classifications such as industry standards , developed and promulgated by an industry for materials and products related to that industry; and mi 1 itary or government standards , such as those designed to be used by the Department of

Defense or by the Federal Government. These should not be confused with Federal

and Military Specifications , used by the Federal Supply Services in the General

Services Administration and by the Department of Defense respectively.

Specifications are a set of conditions and requirements that provide a detailed

description of a procedure, process, material, product, or service for use primarily in procurement and manufacturing. 12 / Another distinction among standards is the manner in which they specify

requirements. Those standards that describe how a product is supposed to

function are called performance standards. In contrast, design standards define

characteristics or how the product is to be built. For example, a performance

standard for water pipe might set requirements for the pressure per square inch

that a pipe must withstand, along with a test method to determine if a specimen

meets the requirement. On the other hand, the specification that a pipe be made

of a given gage of copper would characterize a design standard. The distinction,

however, between these two types of standards is not always clear cut. It is

possible to include two different requirements within the same standard, one of

which is stated in terms of performance and the other in terms of design. For

example, in a standard for copper pipe, requirements for the pipe can be

specified in terms of its performance (being able to withstand a given amount of

pressure), but the same standard may require that the pipe's flanges or couplings

meet specific design requirements.

4
Design standards may be appropriate, as in testing methods where the need for

comparability may outweigh other considerations. In general, however,

performance standards, though usually more difficult to write and enforce, tend

to be less restrictive than design standards, and more likely to encourage

innovation. For that reason, signatories to the Standards Code are encouraged to

write technical regulations and standards in terms of performance, rather than

design, character istics

Still another classification scheme distinguishes between voluntary standards , which by themselves impose no obligations regarding use, and mandatory

standards . A mandatory standard is generally published as part of a code, rule

or regulation by a regulatory government body and imposes an obligation on specified parties to conform to it. However, the distinction between these two

categories may be lost when voluntary consensus standards are referenced in

government regulations, effectively making them "mandatory" standards. Voluntary

consensus standards may also become " quasi -mandatory " due to conditions in the

marketplace. For example, the health care industry is sensitive to the need to

have available the safest products to ensure patient safety and to protect

manufacturers, vendors and health care providers against lawsuits. Informed

buyers of health care products will frequently insist that products meet all

appropriate voluntary consensus standards. If they wish to compete effectively,

manufacturers of such products are obliged to conform to such standards.

It is clear, then, that standards cover a broad range of types and serve a wide

variety of purposes.

Private Standards Organizations in the O.S.

The need for safe and economical structures, such as roads and bridges, led to

the founding of the International Association for Testing and Materials in 1896.

Its mission was to develop standardized test methods. Two years later, the

American Section of this organization was formed and became the forerunner of the

American Society for Testing and Materials, now known as ASTM. Since becoming an independent organization in 1902, ASTM has continued to grow and now produces the

largest number of non-governmental, voluntary standards in the United States.

In 1918, ASTM was one of five private, technical society originators of the

American Engineering Standards Committee, later to be known as the American

Standards Association (ASA), and subsequently as the American National Standards

Institute (ANSI). ANSI today serves as the coordinator of voluntary standards

activities in the United States and as the agency that approves standards as American National Standards. ANSI is also the coordinator and manager of U.S.

participation in the work of two non-governmental, international standards

organizations, ISO and the International Electrotechnical Commission (IEC).

Another of the major private standards organizations, the American Society of

Mechanical Engineering (ASME), was founded in 1880 and first issued the ASME

Boiler Code in 1914. Today that Code is mandatory not only in the United States,

but in many other countries throughout the world. In 1952, a forerunner of ANSI

stated: "Probably no other single standard in America has done more for national

safety than the ASME Boiler Code." 13 / The ASME Boiler Code may be the most

widely used voluntary standard in the world.

5
The founding of the Society of Automotive Engineers (SAE) in 1910 led to the

pioneering efforts of the American automotive industry to achieve substantial

inter-company technical standardization. Most drivers now take these efforts for

granted when choosing motor oils by SAE designations (such as 10W-40) without

being aware of the full significance and background of the detailed standards

development process.

Most consumers also take for granted the familiar UL mark on a range of products

from electrical appliances to fire extinguishers. The Underwriters Laboratories

(UL), founded in 1894, is not only a major standards writer, but also operates

non-profit testing laboratories whose mission is to investigate products and

materials with respect to hazards that might affect life or property and to list

those items which appear to pose no significant hazards.

The work of other major standards organizations, although equally vital, tends to

be less well known outside the standards community. For example, the National

Fire Protection Association (NFPA) has for more than three quarters of a century

produced the National Electrical Code, used in building construction, and many

other standards affecting our safety from fires and other hazards. We accept

without conscious thought the safety of aircraft unaware of the standards

produced by the Aerospace Industries Association of America (AIA) for guidance

and control systems and many other items. The Association of American Railroads'

(AAR) standards similarly affect our railroads. Even the quality and size of

paper is standardized through the work of the Technical Association of the Pulp

and Paper Industry (TAPPI).

In all, more than 400 organizations develop voluntary standards of many different

types for a broad range of services, products, and tests. Some organizations,

such as ANSI and ASTM, are primarily concerned with standards. Others are trade

associations interested in all matters affecting their members. The Electronic

Industries Association, for example, has been a standards developer in the areas

of electrical and electronic products and components since 1926.

Many professional and technical organizations are also standards developers. The

Institute of Electrical and Electronics Engineers (IEEE), which traces back to

1884, maintains more than BOO standards with 800 more under development. IEEE is

responsible for the National Electrical Safety Code, widely used by governments

and regulatory agencies for electric supply and communications installations.

Still other standards developers are primarily research and testing bodies, such

as the National Sanitation Foundation (NSF), which develops standards for

products from a health and sanitation perspective. The Factory Mutual Research

Corporation (FM), another standards developer, is a "product listing" type of

organization, as is UL.

In addition, building code organizations, such as the Building Officials and Code

Administrators International (BOCA), the International Conference of Building

Officials (ICBO), and the Southern Building Code Congress International (SBCCI),

are involved in standards development. These organizations are composed of

building, construction, zoning, and inspection officials; they have developed

model building codes adopted by thousands of State and local governments.

6
The broad range of organizations participating in standards development reflects

the impact standards have on a vast spectrum of interests and disciplines.

Standards Development Procedures

Two of the most widely used procedures for assuring consensus in the development

of standards are the committee and the canvass methods.

Committee Method . Committee standards are subject to wide review and

consideration by all interested parties. The requirements of this process vary

among organizations. In some organizations, consensus may be defined as an agreement of at least 51% of the participants. Other organizations may also

include requirements for due process, appeals procedures, the mandatory

consideration of negative votes or comments, and for "committee balance." Balance

is achieved when all parties having an interest in the outcome of a standard have

an opportunity to participate and where no single interest can dominate the

outcome. Standards organizations differ widely in the emphasis placed on each of

these requirements. Organizations which emphasize all four factors, in addition

to the achievement of substantial agreement among participants, produce standards

that are more likely to be adopted and used.

Canvass method The "canvass" method is frequently used by an organization that

has prepared a standard under its own internal procedures. To gain greater

stature and acceptance of the drafted standard, the developer may then submit it

to balloting by a set of organizations representing a variety of interests, such

as manufacturers, consumers, government, and others. Any objections or comments

from organizations on the "canvass list" must be addressed and satisfactorily

resolved. Changes in a proposed standard, as well as any unresolved objections

and the developing organization's rationale for its response, must be resubmitted

to the "canvass list." It is crucial that all interested groups be included on the list. Two problems sometimes arise: the response level may be low and

consumers and others on the "canvass list" may have difficulty commenting on a standard, because they did not participate in the initial drafting and may not

understand the reasons for or implications of particular provisions.

Benefits and Problems of Standardization

On the whole, the benefits of standardization far outweigh the difficulties and

potential for abuse. Standards promote understanding between buyer and seller

and make possible mutually beneficial commercial transactions. Product

attributes cannot always be evaluated by individual purchasers by inspection or even from prior experience. However, a product's conformance to accepted

standards readily provides an efficient method of conveying complex information

on the product's suitability. Architects use standards in a shorthand manner

when drafting plans for buildings; purchasing agents can also use standards as an easy way of communicating their needs to potential suppliers. In a host of

situations standards are or may be used to replace large quantities of complex

information.

Standards underlie mass production methods and processes. They promote more

effective and organized social interaction, such as the example of the

standardized colors for traffic lights and many other widely accepted

conventions. Standards are essential in efforts to improve product safety and a to clean up the environment. Standardized and interchangeable parts can reduce

inventory requirements and facilitate product repairs. They can also promote

7
fair competition by facilitating the comparison of prices of standardized

commodities

In general , standards permit society to make more effective use of its resources

and allow more effective communication among all parties to particular

activities, transactions, or processes. Indeed, standards are crucial to every

form of scientific and industrial process. Without standards, the quality of

life would be significantly reduced.

No system, particularly one as complex and diverse as the O.S. voluntary

standards system, is without problems. In a recent case of great significance,

the United States Supreme Court on May 17, 1982, rendered its decision in favor

of Hydrolevel, a manufacturer of low-water fuel cutoff devices, in the case of

American Society of Mechanical Engineers (ASME) v. Hydrolevel. It found ASME

liable for conspiring to restrain trade since two subcommittee officers, serving

as volunteers but acting in the name of ASME, issued a misinterpretation of a standard and produced an adverse effect on the competitiveness of the plaintiff.

Similarly, the Federal Trade Commission held hearings on standards and

certification and uncovered "substantiated complaints of individual standards and

certification actions that have, in fact, unreasonably restrained trade or deceived or otherwise injured consumers." 14 /

In part, problems result from the sometimes substantial costs of participation in

standards development, making it difficult (if not impossible) for small firms

and non-industry representatives to be active in the process. The standards

themselves may cause problems if highly technical in nature. It is frequently

difficult, if not impossible, to get qualified consumer representatives to

participate actively. This seriously complicates the attempts to achieve

balanced representation by all interests concerned.

Other problems may occur when a standard undergoes review and revision. Unless

the original writers of the standard participate in its revision, the reviewers

may not be able to understand how the document was prepared, what was eliminated

from consideration, and the reasons or assumptions underlying decisions and the

resultant provisions. Problems can also occur in the application of specific

provisions if the intent behind them is unclear. Rationale statements , which

sometimes accompany a standard, are specifically designed to define the purpose

and scope of the standard, to explain the criteria used in developing its

requirements and to provide all other relevant information at the disposal of the

developers. However, the use of rationale statements is not yet extensive. 15 / Certification

"The first time a craftsman claimed that his product met a commonly accepted

standard, the most basic form of certification came into being." 16 / Today

product certification schemes range from the simple to the complex. The

hallmarking of precious metals was an early form of certification. Many early

attempts, most unsuccessful, were also made to certify weights and measures to

provide a uniform basis for the exchange of goods. Now there are over 100

private organizations and over 60 Federal programs in the United States which

certify products ranging from electrical cords to kitchen cabinets. In addition,

many certification programs are operated at the state and local level. Consumers

see evidence of the extensiveness of certification when they note the

Underwriters' Laboratory (UL) certification mark on many products ranging from

coffee potsto fire extinguishers; the U. S. Department of Agriculture

8
(USDA) mark on meats, poultry and other agricultural products; and the

International Wool Secretariat's Woolmark and Woolmarkblend on wool or wood blend

textile goods. These are only a few of the many certification programs which are conducted in the United States.

Product certification is intended to confirm that a particular product conforms

to one or more specified standards, thus providing the user with explicit or implicit information about the characteristics and/or performance of the product.

Certification is a method for increasing a buyer's confidence in a product and

for furnishing product information.

In the U.S., if a manufacturer or supplier attests to the fact that his product

meets one or more standards, the process is called self-certification . This

process is also known as a manufacturer's declaration in other parts of the

world. The manufacturer's capability, integrity, and reputation determine the

degree of confidence that can be placed in self-certification.

Third-party certification is the term applied to the process by which an organization, independent of either the manufacturer or supplier, assesses the

product's conformance to one or more standards. A manufacturer's overall quality

control program may also be examined as part of the certif ication process. A

quality control program is a series of activities designed to assure that quality

is being maintained at all phases of production. There are hundreds of

third-party certification programs in the United States operated by Federal,

State, and local governments and by many private organizations. Third-party

certification programs differ greatly from one another, and the degree of

confidence in the resultant certification depends on the program's type and

comprehensiveness

The methods used in third-party certification programs can be classified as follows

o Type-testing/initial Inspection - This assures that the manufacturer's design

specifications can produce a product that conforms to a particular standard.

Products from a production run are not inspected or tested, and there is no information on whether products from a production run also consistently meet the

specification.

o Audit-Testing - In this procedure, test samples are selected at random from the

marketplace. Extensive testing is usually required to provide adequate assurance

that products meet the desired standard.

o Surveillance of the Manufacturing Process - Assessment of a manufacturer’s

production and control processes can, at relatively low cost, provide assurance

that the manufacturer's quality control procedures are adequate.

o Field Investigations - Alleged failures of products under use conditions are investigated to determine the cause of failure and to suggest appropriate

corrective action.

9
o Batch-testing - A sample of products is selected from a production batch and

tested for conformance to the standard. If the sampling procedure and the sample

size are adequate, batch-testing makes it likely that all products in that batch

conform to the standard. It does not, however, ensure that a specific untested

product in the batch will meet the standard nor does it furnish information on the quality of products produced in earlier or subsequent batches. Batch testing

is used in many certification programs for building products, such as those for

energy conservation.

o 100 Percent Testing - Each individual product is tested to determine if it

meets the designated standard. If the testing procedures are adequate, the

procedure provides the highest possible level of assurance that the product

conforms to a particular standard. It is also usually the most expensive method

and can be applied only where the test has no adverse effect on the product. 17 / Many programs apply two or more of these methods in their certification process.

The choice of methods depends on the needs of both the buyer and the seller and

the nature of the product. The methods chosen can greatly affect both the cost

of the program and the level of confidence that can be ascribed to it. ANSI and

ISO have each developed criteria to evaluate certification programs. ANSI has

also developed a program to accredit certification schemes which meet its

criteria, but only two programs have been accredited to date.

Laboratory Accreditation

Laboratory accreditation is a process for evaluating testing facilities and

designating those laboratories judged competent to perform specific tests using

standard test methods, where available. The National Voluntary Laboratory

Accreditation Program (NVLAP) in the National Bureau of Standards, Department of

Commerce, and the American Association for Laboratory Accreditation (AALA) are the two largest accreditation agencies in the United States. There are many

other Federal, State and local government programs, as well as many private

sector laboratory accreditation programs. Some of these include: the Department

of Defense's (DOD) programs for accrediting laboratories which test products

which will later be sold to DOD; the State of Massachusetts programs for

accrediting concrete testing laboratories and laboratories which test solid fuel

burning appliances; and the National Kitchen Cabinet Association's (NKCA)

accreditation program for laboratories which test kitchen cabinets as part of the

NKCA's certification program. 18 /

It should be emphasized that laboratory accreditation assesses the capability o f a laboratory to conduct testing, cjenerally using standard test methods. The

accreditation process should not b(2 confused with certification nor with the

validation of a certification, which is "an action by a third party to assure

that the producer (or certifier) is adhering to the requirements of a given

certification program." 19 / Laboratory accreditation neither reviews nor assesses products, nor does it ch€»ck the tests conducted on specific products or product batches. In addition, laboratories may be accredited to conduct tests

(such as EPA's accreditation program for laboratories testing drinking water) in

fields where no certification program exists.

Laboratory accreditation, however,, can affect the quality of certification

programs by requiring evidence thal: a certifying laboratory has competent

personnel, adequate equipment, and sufficient knowledge of the testing

10
procedures for which accreditation is sought. Also, laboratory accreditation is

assuming increased importance in trade. As countries seek acceptance of their

test data by trading partners, they must assure that the data comes from

competent laboratories. Laboratory accreditation can help to provide that

assurance

Summary

Standardization, product certif ication , and laboratory accreditation are closely

linked. In many developing countries, all three activities are conducted by the

same organization. Certification programs are communication tools designed to

reduce the cost of exchanging information between buyer and seller. The quality

of the information conveyed depends on both the competence of the testing

laboratory selected and the adequacy and appropriateness of the standards against

which the product is to be evaluated. Certification can result in widespread

consumer deception if performance characteristics or test methods contained in

the standard are insufficient to assure adequate product performance or if the

testing laboratory is incompetent or has biases which affect the reporting of

test results.

Considering the number of standards in existence and the variety of fields

covered by private sector standards development and certification organizations,

the United States has one of the most developed and complex standardization and

certification systems in the world. Furthermore, the number of Federal, State,

and local government standardization and certification activities and the large

volume of standards, regulations, and procurement specifications that these

agencies have developed, result in an immense impact of standardization and

related activities on almost every aspect of life in the United States. Not only

are considerable resources invested in this country in such activities every

year, but purchasers (consumers) depend on standards and certification to ensure

that products purchased are safe and perform satisfactorily. Recognition of the

impact of standards and certification on trade, as evidenced by the Standards

Code, is also increasing. Society depends on standardization and related

activities for its existence.
APPENDIX

Information Available froa

The Standards Code and Inf creation (SCI) Program

National Bureau of Standards

Administration Building, Room A629

Gaithersburg, MD 20899

(301) 975-4040

o Directory of International and Regional Organizations Conducting

Standards-Related Activities (NBS SP 649)

Directory contains information on 272 international and regional organizations

which conduct standardization, certification, laboratory accreditation, or other

standards-related activities. Volume describes their work in these areas, as well as the scope of each organization, national affiliations of members, U.S.

participants, restrictions on membership, and the availability of any standards

in English.

o Standards Activities of Organizations in the United States (NBS SP 681)

The directory summarizes the standardization activities of more than 750

organizations in the United States, including Federal and State agencies and

approximately 420 private sector groups that develop standards. It also contains

listings of State procurement offices, sources of standards documents and

information, a subject index and related listings that cover acronyms and

initials, defunct bodies and organizations with name changes.

o Private Sector Product Certification Programs in the United States (NBS SP 703)

This directory presents information from 109 private sector organizations in

the United States which engage in product certification activities. Entries

describe the type and purpose of each organization, the nature of the activity,

products certified, standards used, certification requirements, availability and

cost of services, and other relevant details.

o Federal Government Certification Programs for Products and Services

(NBS SP 714)

This directory presents information on 61 U.S. Government certification

programs for products and services. Entries describe the scope and nature of

each certification program, testing and inspection practices, standards used,

methods of identification and enforcement, reciprocal recognition or acceptance

of certification, and other relevant details.

0 KWIC Index (Computer Output Microform (COM) produced)

The KWIC Index contains the titles of more than 25,000 U.S. voluntary product

and engineering standards. A standard can be located by means of any significant

or key word in the title. Key words are arranged alphabetically. A standard with

five key words, for example, would therefore be listed in five different places.

To purchase microfiche copies of the 1985 revision of the Index, contact the

National Technical Information Service, 5285 Port Royal Road, Springfield, VA

22161; (703) 487-4600. Use order no. PB86-154408; cost is $13.50 for purchasers

in the U.S.

12
o tbt news

This newsletter provides information on government programs and available

services established in support of the GATT Agreement on Technical Barriers to

Trade (Standards Code), tbt news reports on the latest notifications of proposed

foreign regulations; bilateral consultations with major U.S. trade partners;

programs of interest to U.S. exporters; and availability of standards and

certification information. Subscription is free upon reguest.

o Technical Barriers to Trade

This booklet explains the basic rules of the international Agreement on Technical Barriers to Trade negotiated during the Tokyo Round of the Multilateral

Trade Negotiations (MTN), and describes Title IV of the U.S. Trade Agreements Act

of 1979 which implements the United States' obligations under the Agreement. The

Agreement, popularly known as the Standards Code, was designed to eliminate the

use of standards and certification systems as barriers to trade. The booklet

describes the functions of the Departments of Commerce and Agriculture, the

Office of the U.S. Trade Representative, and the State Department in carrying out

the U.S.'s responsibilities.

o "GATT Standards Code Activities"

This brochure gives a brief description of NBS ' activities in support of the

Standards Code. These activities include operating the U.S. GATT inquiry point

for information on standards and certif ication systems; notifying the GATT

Secretariat of proposed U.S. regulations; assisting U.S. industry with

trade-related standards problems; responding to inquiries on foreign and U.S.

proposed regulations; and preparing reports on the Standard Code.

0 R e PQrt to the United States Congress on the Agreement on Technical Barriers to

Trade - "Standards Code"

This 2nd triennial report describes the programs and activities established to

implement the Standards Code in the United States by the four responsible U.S.

government agencies: Office of the U.S. Trade Representative; Department of

Commerce (National Bureau of Standards, International Trade Administration);

Department of Agriculture and Department of State.

o Free handout material on the Office of Product Standards Policy's (OPSP), the

National Center for Standards and Certification Information's (NCSCI) and GATT

activities, and standards-related information such as: Government sources of

specifications and standards, use of the KWIC index, foreign and international

standards bodies, U.S. standards organizations. State purchasing offices, NCSCI

fact sheet and its certification rules activity, and OPSP publications list

(bibliography )

In addition to general inquiry services, the following assistance is also

available

o GATT Hotline

A telephone hotline provides current information received from the GATT

Secretariat in Geneva, Switzerland, on proposed foreign regulations which may

significantly affect trade. The recorded message is updated weekly and gives the

product, country, closing date for comments (if any) and Technical Barriers to

Trade (TBT) notification number. The hotline number is (301) 975-4041 (not

tol 1 -f ree )

13
o Assistance to U.S. and foreign exporters -- Current regulations and

certification information for the manufacture of products in the U.S. for export

are obtained from foreign countries. To aid foreign exporters, NCSC1 provides

directory information on State offices prepared to respond to queries concerning

conditions to be met by goods for sale in their state, as well as standards and

certification information for export to the U.S.

14
FOOTNOTES

1/ USA Standards Institute, "What is ... a Standard? the USA Standards

Institute? a USA Standard?," USA Standards Institute, New York, New York, p. 1.

2y National Policy on Standards for the United States and a Recommended

Implementation Plan , National Standards Policy Advisory Committee, Washington,

D.C. , December, 1978. p. 6.

3y International Trade Administration, The Tokyo Round Agreements: Technical

Barriers to Trade - Volume 4 , Dept, of Commerce, Washington, D.C., September

1981.

4/ American Standards Association, "Through History with Standards" in Rowen

Glie (ed.). Speaking of Standards , Cahner Books, Boston, MA, 1972, p. 38.

5/ Ibid., p. 42.

6/ Ibid., p. 44.

7/ Rexmond C. Cochrane, Measures for Progress: A History of the National Bureau

of Standards, National Bureau of Standards, U.S. Department of Commerce,

Washington, D.C., pp. 82-86, 1974.

8/ American Standards Association, p. 60.

9/ Ibid., p. 68.

10/ Ibid., p. 60.

11 / International Organization for Standardization, draft revision of ISO Guide

2, "General terms and their definitions concerning standardization, certif ication

and testing laboratory accreditation," July 1985.

12 / W. E. Andrus, Jr., Draft NBS Glossary of Terms for Product Standardization,

Product Certification and Laboratory Accreditation , U.S. national Bureau of

Standards, Dept, of Commerce, 1974.

13/ American Standards Association, p. 48.

14 / Bureau of Consumer Protection, Standards and Certification: Final Staff

Report - April 1983 , Federal Trade Commission, Washington, D.C., April, 1983, p.

2 .

15 / David A. Swankin, Rationale Statements for Voluntary Standards — Issues , Technigues, and Conseguences , National Bureau of Standards, Dept, of Commerce,

Gaithersburg, MD, November, 1981.

16 / International Organization for Standardization, Certification: Principles

and Practice , International Organization for Standardization, Geneva,

Switzerland, 1980, p. 7.

15
17 / Douglas B. Thomas, NVLAP Glossary of Terms for Laboratory Accreditation,

Product Certification and Standardization , U.S. National Bureau of Standards,

Washington, D.C. 1980,

18 / Charles W. Hyer, Principal Aspects of U.S. Laboratory Accreditation

Programs , National Bureau of Standards, Gaithersburg, MD 20899, October 1984.

19/ Ibid. 21.

16
NBS-114A (REV. 2«8C) U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

NBSIR 87-3576

2. Performing Organ. Report No. 3. Publication Date May 1987

4. TITLE AND SUBTITLE

The ABC's of Standards Related Activities in the United States

5. AUTHOR(S)

Maureen A. Breitenberg

G. PERFORMING ORGANIZATION (If joint or other than N BS. see instructions)

NATIONAL BUREAU OF STANDARDS

U.S. DEPARTMENT OF COMMERCE

GAITHERSBURG, MD 20899

7. Contract/Grant No.

8 . Type of Report & Period Covered

Final

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State. ZIP)

10.

SUPPLEMENTARY NOTES

' J Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11.

ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bi bliography or literature survey, mention it here)

This report provides an introduction to voluntary standardization, product

certification and laboratory accreditation for a reader who is not fully

familiar with these topics. It highlights some of the more important aspects

of these fields; furnishes the reader with both historical and current

information on these topics; describes the importance and impact of the

development and use of standards; and serves as background for using available

documents and services.

12.

KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

certification; inspection; laboratory accreditation; standardization; standards;

testing

13. AVAILABILITY

|X | Unlimited

| | For Official Distribution. Do Not Release to NTIS

| Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

20402.

lX~l Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

23

15. Price

$ 9.95

U SCOMM-DC 6043-P80