NBSIR 80-2163

Nondestructive Evaluation

Methods for Quality Acceptance

of Hardened Concrete in

Structures

James R. Clifton

Erik D. Anderson

Center for Building Technology

National Bureau of Standards

U.S. Department of Commerce

Washington, DC 20234

January 1981

°repared for - (• f - _

J.S. Army Construction Engineering

1UU Research Laboratory

, U56 Champaign, Illinois

30-2163

1981

NBSIR 80-2163

NONDESTRUCTIVE EVALUATION

METHODS FOR QUALITY ACCEPTANCE

OF HARDENED CONCRETE IN

STRUCTURES

National Bureau of Standards

Library, E-01 Admin. Bldg.

FEB 2 7 1981

not clcc- - Quod

• no. 86’ 20,3

&ZI

James R. Clifton

Erik D. Anderson

Center for Building Technology

National Bureau of Standards

U.S. Department of Commerce

Washington, DC 20234

January 1981

Prepared for U.S. Army Construction Engineering

Research Laboratory

Champaign, Illinois

U.S. DEPARTMENT OF COMMERCE, Philip M. Klutznick, Secretary

Jordan J. Baruch, Assistant Secretary for Productivity. Technology, and Innovation

NATIONAL BUREAU OF STANDARDS. Ernest Ambler, Director
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l !Q . "SnI A 10- J ,yv*<lt,

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ABSTRACT

Nondestructive evaluation methods which can be used in quality acceptance

programs for hardened concrete have been critically reviewed and are described

in this report. Methods have been identified which provide information on

the strength, uniformity, thickness, air content, stiffness, finish, density

of concrete as well as the location and condition of steel reinforcement. Both

commonly used methods and potentially useful test methods are covered. In

addition, the feasibility of combining two or more test methods for improving

the prediction of the strength or quality of concrete is explored.

Key Words: Concrete; construction; nondestructive evaluation; quality

assurance; steel reinforcing bars; test methods.

iii
TABLE OF CONTENTS

Page

1. INTRODUCTION 1

2. SURVEY OF NONDESTRUCTIVE EVALUATION (NDE) METHODS FOR CONCRETE 2

3. COMMONLY USED NONDESTRUCTIVE EVALUATION METHODS 7

3.1 Probe Penetration Method 7 3.1.1 Probe Equipment and Use 7 3.1.2 Applications 9

3.1.3 Advantages and Limitations 12

3.2 Rebound Method 13

3.2.1 Description of Method 13

3.2.2 Applications 15

3.2.3 Advantages and Limitations 16

3.3 Cast-In-Place Pullout 17

3.3.1 Test Method 17

3.3.2 Reliability of Method 19

3.3.3 Advantages and Limitations 19

3.4 Ultrasonic Pulse Velocity Method 21

3.4.1 Principle of Method 21

3.4.2 Assessment of the Uniformity and Quality of Concrete ... 23

3.5 Gamma Radiography 27

3.5.1 Principle and Applications 27

3.5.2 Limitations 28

3.6 Neutron Moisture Meters 29

3.6.1 Principle and Applications 29

3.6.2 Limitations 29

3.7 Cover Meters 30

3.7.1 Principle and Applications 30

3.7.2 Limitations 33

3.8 Electric Potential Measurements 33

3.8.1 Principle 33

3.8.2 Limitations 35

iv
TABLE OF CONTENTS (CONTINUED)

Page

4.

APPLICATION OF OTHER NDE METHODS FOR CONCRETE 35

4.1 Ultrasonic Pulse Echo

4.2 Microwaves

4.3 Infrared Thermography

4.4 Maturity Concept .... 4.5 Acoustic Emission ...

36

36

37

38

40

5. COMBINATIONS OF NONDESTRUCTIVE EVALUATION METHODS

f ,.. 40

5.1 Combination of Ultrasonic Pulse Velocity and Rebound

Hammer Methods 41

5.2 Other Possible NDE Combinations , 42

6. SUMMARY r . 44

7. REFERENCES 45

v
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•, • * - *
NONDESTRUCTIVE EVALUATION METHODS FOR

QUALITY ACCEPTANCE OF HARDENED CONCRETE

1 . INTRODUCTION

Quality-^ and uniformity of hardened concrete in a newly constructed building

depends on a series of processes including the selection of concrete

ingredients, design of the concrete mix, batching of and mixing the concrete

ingredients, placing and consolidating plastic concrete, and curing operations.

Specification codes, standard guidelines, and standard test methods form the

basis for decision making in these processes. For example, concrete

ingredients can be specified by citing the appropriate American Society for

Testing and Materials (ASTM) standards. In addition, standard test methods

atre available for checking the characteristic^ of plastic concrete prior

to placement. However, the quality and uniformity of hardened concrete also

are affected by construction practices, workmanship, and curing conditions.

It is the role of concrete inspectors to determine if the building contractor

is adhering to acceptable practices and to the design specifications. Often,

an inspector only can form subjective conclusions which are highly dependent

on his level of training and experience. The present testing and inspection

approach to quality control of concrete does not always adequately ensure

that the hardened concrete in the structure is of the desired quality or

uniformity. An effective quality control program also should include

evaluation of hardened concrete and reinforcement at the time of acceptance

of the building.

-/ In this paper "quality of concrete" refers to the levels of the chemical

and physical properties of concrete needed to meet the designed perfor- mance and service life.

1
Quality control testing of hardened concrete, i.e., quality assurance testing,

is gaining recognition as being an important phase of quality acceptance

programs. Because of the importance of such programs, an international

symposium was held in 1979 [1] which addressed the "Quality Control of

Concrete Structures." The nondestructive evaluation (NDE) approach is becoming

the foundation for quality assurance testing of hardened concrete. NDE

methods which can be used in quality acceptance programs are critically

reviewed and described in this report. Quality acceptance programs based on

NDE methods have been reported to be cost effective [2] and reliable [2, 3].

The on-job time for concrete inspectors can be reduced. In addition, poor

quality or nonuniform concrete can be readily detected and the contractor

required to replace it before a building is accepted. This can increase the

service life of the building and also reduce maintenance costs.

Another application of NDE is in selecting sites for coring. Cores are often

taken if hardened concrete appears to be substantially below the specified

quality or strength. Taking and testing numerous cores from a concrete

structure is undesirable for several reasons including the time consumed,

the expense, and the damage to the structure. NDE can be used to identify

those areas that should be cored, thereby reducing the number of cores needed

to quantitatively evaluate the concrete.

2. SURVEY OF NONDESTRUCTIVE EVALUATION METHODS FOR CONCRETE

The selection of NDE methods for characterizing concrete should be based on

considerations of its important functional properties. In quality acceptance

testing of hardened concrete, properties and factors of concrete which might

be determined include strength, uniformity, thickness, air content, stiffness,

2
finish, density, and location and condition of steel reinforcment . NDE

methods which can be used to obtain information on these factors are given

in table 1. The Recommended Test Methods in table 1 are commonly used and

their limitations are understood, while the reliability and feasibility of

Possible Test Methods are still being assessed.

Operation, principles, and applications of commonly used NDE methods are

outlined in table 2. The test equipment for all the methods is sufficiently

portable so that the concrete can be tested in the field. In addition,

results can normally be obtained and evaluated within one or two days. With

the exception of gamma radiography and the neutron moisture meter, the equipment can be operated by most concrete testers or inspectors. These radiation

techniques only can be used by personnel licensed by the Nuclear Regulatory

Commission. The methods outlined in table 2 are described in more detail in

section 3. The ASTM has recognized the need for standards and specifications

covering nondestructive evaluation methods for a variety of materials and

applications. ASTM standards for NDE methods for concrete and for taking

test cores from concrete are listed in table 3.

The Possible Test Methods listed in table 1 may become recognized methods

after their reliabilities and feasibilities have been demonstrated. Principles and potential applications of these methods are discussed in section 4.

The feasibility of combining two or more test methods for predicting the

strength or quality of concrete is being explored with some success. This

subject is covered in section 5.

3
TABLE 1. NDE METHODS FOR QUALITY ASSURANCE TESTING OF HARDENED CONCRETE

CONCRETE PROPERTIES RECOMMENDED TEST METHODS POSSIBLE TEST METHODS

Strength Windsor Probe

Schmidt Rebound Hammer

Cast-In-Place Pullouts

Maturity Concept

Cores

General Quality

and Uniformity

Windsor Probe

Schmidt Rebound Hammer

Ultrasonic Pulse

Gamma Radiography

Cores

Ultrasonic Pulse Echo

Microwaves

Thickness Covermeter

Microwaves (Radar)

Cores

Gamma Radiography

Ultrasonic Pulse

Air Content Cores Neutron Density Gage

Moduli of

elasticity

Dead Weight Loading

with Acoustic

Emission

Surface

Properties

Visual

Schmidt Rebound Hammer

Cores

Microwaves

Ultrasonic Pulse Echo

Density Gamma Radiography

Cores

Neutron Density Gage

Rebar Size and

Location

Covermeter

Gamma Radiography

Microwaves (Radar)

Ultrasonic Pulse Echo

Corrosion State of

Reinforcing Steel

Visual

Electrical Potential

Measurement

4
TABLE 2. SURVEY OF COMMONLY USED METHODS FOR CONCRETE INSPECTION

Method Principle Main Applications Equipment Cost User Expertise Advantages Llaltatlons

1. Windsor Probe Probe fired Into concrete

and depth of penetration

la measured. Surface and subsurface hardness

measured.

Estimations of compressive

strength, uniformity

and quality of concrete.

$900 plus

cost of probes.

Low, can be operated by ordinary

field personnel.

Equipment is simple

end durable. Good

for determining

quality of concrete.

Slightly damages

small area. Does

not give precise prediction of strength.

2. Rebound Hammer Spring loaded plunger

strikes surface of concrete and rebound distance is given in R-values. Surface hardness is measured.

Estimation of compressive

strength, uniformity and quality of concrete.

$250 to $600 Low, can be readily operated by ordinary

field personnel

Inexpensive. Large amount of data can

be quickly obtained. Good for determining

uniformity of concrete.

Results affected by condition of concrete

surface* Does not give precise prediction

of strength.

3. Cast-in-P lace Pullout

Force required to pull

out steel rod with

enlarged head cast in concrete, is measured. Pullout forces produce

tensile and shear stresses in concrete.

Estimation of compressive and tensile strengths

of concrete.

$1000 to $4000 Low, can be used by field concrete

testors and inspectors

Only NDE method which directly measures in place strength of concrete. Appears

to give good prediction of concrete

strength.

Pullout devices must

be inserted during

construction. Cone of concrete may be pulled

out, necessitating

minor repairs.

4. Ultrasonic Pulse

Based on measuring the transit time of a wave propagating through concrete

Estimation of

the quality and uniformity of

concrete.

$4000 to $5000 Moderate, user

must understand method and recognize poten- tial problems.

Excellent for determining the quality and uniformity of concrete

Does not provide estimate

of strength. Skill required in analysis of results. Moisture variations can affect results.

5. Gamma Radiography

Gamma radiation attenuates

when passing through concrete. Extent of atten- uation controlled by density

and thickness of concrete.

Locating cracks, honeycombing,

voids, and reinforcement

in concrete.

around $5000 Must be operated by trained and licensed

personnel

Portable and rela- tively inexpensive

compared to X-ray

radiography.

Radiation intensity

cannot be adjusted.

Long exposure times can be required. Dangerous radiation.

6. Neutron

Moisture Meters

Fast neutrons are slowed

by interactions with hydrogen atoms. Back- scattered slowed neutrons

are measured which gives amount of hydrogen atoms present in a material.

Moisture content and possibly

density of concrete.

$3500 to $6300 Must be operated by trained and licensed

personnel

Portable moisture

estimates can be made of in-place

concrete.

Only measures moisture

content of surface

layers (50 tom)

Dangerous radiation.

7. Covermeter Presence of steel affects

the magnetic field of a probe. Closer probe is

to steel, the greater

the effect.

Determination

of presence, location, and depth of rebars

in concrete.

$800 to $1500 Moderate. Easy

to operate.

Need training

to interpret

results

Portable equipment,

good results if concrete is lightly

reinforced.

Difficult to interpret

if concrete is heavily reinforced or if wire

mesh is present.

8. Electrical Potential

Measurements

Electrical potential of steel reinforcement

measured. Potential

indicates probability

of corrosion.

Determining

condition of steel rebars

in concrete.

$1000 to $2000 Moderate. User must be able

to recognize

possible

problems

Portable equipment.

Field measurements

readily made. Appears to give reliable information.

Does not provide information on rate of corrosion.
TABLE 3. ASTM STANDARDS FOR CONCRETE NDE METHODS

Nondes tructive

Method ASTM Standard and Designation

Rebound Hammer Test for rebound number of hardened concrete:

C805

Penetration Probe Test for penetration resistance of hardened

concrete: C803

Ultrasonic Pulse

Velocity

Test for pulse velocity through concrete: C94

Cores Drilled cores and sawed beams of concrete,

obtaining and testing: C42

Nuclear Moisture

Meter

Standard test method for moisture content of

soil and soil-aggregate in place by nuclear

methods (shallow depth): D3017

Cast-in-Place

Pullouts

Tentative test method for pullout strength

of hardened concrete: C900-78T

Visual Standard recommended practice for examination

of hardened concrete in construction: C823

6
3. COMMONLY USED NONDESTRUCTIVE EVALUATION METHODS

NDE methods which are commonly used and generally accepted for the evaluation of concrete are described herein.

3.1 Probe Penetration Method

The probe penetration method is based on measuring the depth to which a

steel probe penetrates concrete when driven by a powder charge. Actually

the exposed length of the probe is measured rather than the depth of

penetration. This method is useful for assessing the quality and uniformity of concrete in situ, and for delineating areas of poor quality or

deteriorated concrete in structures.

Probe results also have been used to estimate the compressive strength of

concrete by using correlation graphs. The graphs are constructed by

plotting the exposed lengths of probes versus experimentally measured

compressive strengths.

3.1.1 Probe Equipment and Use

2 / The Windsor Probe-' is the most commonly selected and possibly the only

commercially available apparatus for measuring the penetration resistance

of concrete. It consists of a special driving gun (figure 1) into which

is inserted a high-strength steel probe that is driven into the concrete

by firing of the powder charge (a calibrated 32 caliber blank cartridge).

A series of three measurements is made in each area using the spacer

2 / - Precise identification of NDE equipment is necessary to clearly describe

the testing procedure. In no case does such identification imply

recommendation or endorsement by the National Bureau of Standards.

7
Figure 1

.

Windsor Probe gun, probe and blank cartridge.

jj

8
plate shown In figure 2. The length of a probe extending from the surface

of concrete can be measured using a simple device as shown in figure 3.

Operating procedures for the Windsor Probe are given by the manufacturer. In

addition, testing procedures are given in ASTM Standard C803 [4]. The probe

can be easily operated by concrete inspectors and is readily portable.

The manufacturer supplies a set of five calibration curves, each curve corresponding to a specific Moh's hardness for the coarse aggregate used in the

concrete, by which probe measurements can be converted to compressive strength

values. However, several investigators [5, 6] have observed that use of the

manufacturer's calibration curves often results in grossly incorrect estimates

of the compressive strength of concretes. Therefore, it is recommended that

the Windsor Probe should be calibrated by the individual user and should

be recalibrated whenever the type of aggregate or mix design is changed.

3.1.2 Applications

The Windsor Probe can be used for assessing the quality and uniformity of

concrete because physical differences in a concrete will affect its resistance

to penetration. A probe will penetrate deeper as the density, subsurface

hardness, and strength of the concrete decreases. Areas of poor concrete can

be mapped by making a series of penetration tests.

The Windsor Probe has been used to estimate the compressive strength of

concrete. However, the relationship between the depth of penetration of the

probe and the compressive strength can only be obtained empirically because

penetration of the probe depends on a complex mixture of tensile, shear,

9

i

I

Figure 3. Device for measuring length of probe extending from

surface of concrete.

11

1
frictional, and compressive forces [5]. The estimation of compressive

strengths with the Windsor Probe, therefore, must be made using a correlation

diagram with appropriate confidence limits.

The probe technique appears to be gaining acceptance as a practical NDE

method for estimating the compressive strength of concrete. Improved

correlations between probe results and in place strength can be obtained by

keeping the curing conditions of the test specimens close to those of the

in-place concrete and by making sure that the test concrete is representative of the mass concrete. If the Windsor Probe is calibrated using

concrete specimens taken from an early construction stage, the calibration

chart could be used to estimate the strength of concrete placed during later

stages (assuming that the concrete design is the same).

3.1.3 Advantages and Limitations

The Windsor Probe equipment is simple, durable, requires little maintenance,

and can be used by inspectors in the field with little training. Care must

be exercised, however, because a projectile is fired and safety glasses

should be worn. The gun can only be fired when the gun is pushed against a

special spacer plate.

The Windsor Probe primarily measures surface and subsurface hardness and does

not yield precise measurements of the in situ strength of concrete. However,

useful estimates of the compressive strength of concrete may be obtained if

the probe is properly calibrated. The probe test is very useful in assessing

the general quality and relative strengths of concrete.

12
The Windsor Probe test does damage the concrete, leaving a hole of about

5/16 in. (8 mm) in diameter for the depth of the probe and, also, may cause

minor cracking and some surface spalling, necessitating minor repairs.

3.2 Rebound Hammer Method

The rebound hammer method is similar to the penetration probe method in that

it measures surface hardness. The rebound method is based on the rebound

theories of Shore [7]. He developed the Shore Scleroscope method in which

the height of rebound of a steel hammer dropped on a metal test specimen is

measured. The only commercially available instrument based on the rebound

principle for testing concrete is the Schmidt Rebound Hammer [8]

.

The Schmidt Rebound Hammer has gained wide acceptance by researchers and

concrete inspectors and is one of the most universally used nondestructive

test methods for determining the in situ quality of concrete and for deciding

when forms may be removed. Provisional [6] standards have been drafted in

Poland and Romania for the Rebound Hammer. The British Standards Institution has issued Building Standards 4408 which covers nondestructive test

methods for concrete and includes the rebound hammer method in part 4 of the

Standard [6]

.

Recently, ASTM issued a Standard C805, Test for Rebound Number

of Hardened Concrete, which gives procedures for the use of the rebound

hammer

.

3.2.1 Description of Method

The Schmidt Rebound Hammer consists of a steel weight and a tension spring in

a tubular frame (figure 4). When the plunger of the hammer is pushed against

the surface of the concrete, the steel weight is retracted against the force

13
14
of the spring. When the head is completely retracted, the spring is

automatically released, the weight is driven against the plunger and it

rebounds. The rebound distance is indicated by a pointer on a scale that

is usually graduated from zero to 100, and the rebound readings are termed

R-values. The determination of the R-values is outlined in the manual

supplied by the manufacturer. R-values give an indication of the hardness

of the concrete with valves increasing with the hardness of the concrete.

Each hammer is furnished with a calibration chart supplied by the manufacturer, showing the relationship between compressive strength of the concrete

and rebound readings based on data from tests conducted by the Swiss Federal

Materials Testing and Experimental Institute. However, users should not

place too much confidence on the calibration chart and should develop their

own for each concrete mix.

3.2.2 Applications

Numerous investigators [9-11] have shown that there is some correlation

between compressive strength of concrete and the hammer rebound number.

There is, however, extensive disagreement (e.g. references [12] and [13])

concerning the accuracy of the strength estimates from rebound measurements.

Mitchel and Hoagland [14] found that the coefficient of variation for compressive strength, for a wide variety of specimens from the same concrete,

averaged 18.8 percent. Arni [5] found that the rebound hammer gave a less

reliable estimate of compressive strength than the Windsor Probe.

Several investigators [13, 15] have attempted to establish correlations

between the flexural strength of concrete and the hammer rebound number.

15
Relationships similar to those obtained for compressive strengths were

obtained, except that the statistical variations were even greater.

Mitchel and Hoagland [14] attempted to correlate rebound numbers with the

modulus of elasticity of the concrete specimens. They concluded that no

valid correlations could be made. Peterson and Stoll [9] and Klieger [16]

have developed some empirical relations between the dynamic modulus of

elasticity and hammer rebound.

The rebound hammer, like the Windsor Probe, is very useful in assessing the

general quality of concrete and for locating areas in which poor quality concrete is located. A large number of measurements can be rapidly taken so

that large exposed areas of concrete can be mapped within a few hours.

3.2.3 Advantages and Limitations of the Rebound Hammer

The Schmidt Rebound Hammer is a simple and quick method for the

nondestructive testing of concrete in situ. The equipment is inexpensive,

costing less than $1000, and can be operated by field personnel with a limited

amount of instruction.

The Schmidt Rebound Hammer, however, has recognized limitations. The rebound

measurements on in situ concrete are affected by [13, 17, 18]:

(1) Smoothness of the concrete surface

(2) Moisture content of concrete

(3) Type of coarse aggregate

(4) Size, shape and rigidity of specimen, e.g., a thin wall or beam

(5) Carbonation of the concrete surface.

16
The rebound method is largely an imprecise test and it does not provide a

reliable estimation of the strength of concrete. The rebound hammer, however, is very useful in the rapid assessment of the relative quality of

concrete.

3.3 Cast-In-Place Pullout

The pullout test measures the force required to pull out a steel insert,

having an enlarged end, which has been cast in the concrete (figure 5). The

concrete is subject to a complex state of stresses by the pullout force, and

a cone of of concrete is removed at failure. The pullout force is usually

related to the compressive strength of the concrete, with the ratio of pullout strength (force divided by surface area of the conic frustrum) to

compressive strength being in the range of 0.1 to 0.3 [18]. Correlation

graphs are used to relate relate pullout force to compressive strength. The

pull-out technique is the only nondestructive method which directly measures

a strength property of concrete in place (the measured strength is thought

to be a combination of tensile and shear strengths) . There are several

commercially available test apparatuses for measuring the pullout resistance

of concrete [19, 20], with prices varying from $1000 to $5000.

3.3.1 Test Method

ASTM has recently issued a tentative test method, C900-78T, which describes

in detail the pullout assembly and gives allowable dimensions. The pullout

insert is cast-in-place during placing of fresh concrete, and it is either

pulled completely out of the concrete, or pulled until maximum load is

reached, with a manually operated hollow tension ram exerting pressure through

17
DRAW

ROD

Figure 5. Schematic of cast-in-place pullout cl -vice

18

PULLOUT

TESTER
the steel reaction ring. Components of one possible test apparatus for pulling

out the insert are shown in figure 6. Testing and calculation procedures

are also given in ASTM C900-78T.

Because the pullout insert is usually cast-in-place during placing of the

fresh concrete, these tests must be planned in advance. Alternatively,

hardened concrete can be drilled to receive the pullout insert. This necessitates drilling through the bottom or backside of a concrete slab, for example,

to the proper depth and width to permit the insertion of the enlarged head;

a smaller hole, sufficient to permit insertion of the steel shaft, is drilled

through the remaining portion of the concrete slab; the insert is placed

through the bottom or backside; and the test carried out.

Techniques have been developed so that the inserts can be embedded deep in

concrete, thereby permitting testing of the interior concrete.

3.3.2 Reliability of Method

Malhotra [18] and Richards [19] have shown that the pullout method is a reliable method for estimating the compressive strength of concrete. Malhotra

found that the coefficients of variation for pullout test results was in the

same range as obtained from testing standard cylinders in compression.

Correlation coefficients of 0.97 to 0.99 for normal weight concrete have

been obtained from curve fitting of pullout and compression test results

[19].

3.3.3 Advantages and Limitations

The pullout technique is the only nondestructive test method w tich directly

measures a strength function of concrete in place.

19
Figure 6. Components of pullout test apparatus

20
The major disadvantage of the pullout test is that a cone of concrete is

sometimes pulled out necessitating minor repairs. However, if the pullout

forces are quickly released when failure is just initiated, the pullout

assembly and concrete cone will not be torn loose, and no repairs will be

required. Another disadvantage is the need to plan where inserts are to be

placed and to make provisions for their placement prior to placing concrete.

3.4 Ultrasonic Pulse Velocity Method

Several types of nondestructive test methods have been developed based on wave

propagation principles such as the measurement of acoustic pulse velocities,

seismic velocities, and ultrasonic pulse velocities. These often are

collectively termed sonic tests [21]. The ultrasonic pulse velocity is by

far the most widely accepted vibrational method for field use and is one

of the most universally used nondestructive test methods for assessing the

quality of concrete.

3.4.1 Principle of Method

The ultrasonic pulse velocity method is based on measuring the travel time of

an ultrasonic pulse passing through concrete. The pulse generated by an electroacoustic transducer is picked up by a receiver transducer and amplified.

The time of travel of the pulse is measured electronically. A commercial

instrument for measuring the ultrasonic pulse velocity of concrete is shown

in figure 7.

When a mechanical pulse is applied to concrete by an electroacoustic

transducer, waves are induced in the concrete. Usually longitudinal waves

are used in testing concrete. These waves are transmitted by particles

21
Figure 7. Ultrasonic pulse velocity equipment

22
vibrating parallel to the direction of propagation. The velocity of the wave

is controlled by the elastic properties and the density of the material being

tested. It is virtually independent of the geometry of the concrete object

being tested.

If a longitudinal wave encounters a discontinuity such as a crack or void,

it may "bend," i.e., be diffracted around the discontinuity. This will

increase the internal distance the wave must pass between the transmitting

and pickup transducers, and consequently its travel time. Therefore, for a

given separation of the transducers, and for a given concrete, the travel

time of a longitudinal wave will be less in a region where the concrete is

sound than where the concrete is more porous or contains cracks. In addition, the velocity of the longitudinal wave will be effected by changes in

density and plastic properties of concrete.

At least three ultrasonic pulse velocity units are commercially available [18,

21] including the Ultrasonic Concrete Tester, the Soniscope, and Pundit. The

Ultrasonic Concrete Tester has a testing range of about 50 feet (15.6 m)

,

whereas both the Soniscope and Pundit can be used to test concrete having a

thickness up to about 75 feet (30 m) . Their respective operating frequencies

are 150 kHz, 50 kHz, and 50 kHz.

3.4.2 Assessment of the Uniformity and Quality of Concrete

Ultrasonic pulse velocity methods are considered to be the best NDE methods

for determining the uniformity of in situ concrete. For example, velocity

measurements have been successfully used to detect deteriorated regions in

concrete bridges and the uniformity of concrete in walls [21]. In general,

23
if substantial variations in pulse velocities are found in a structure,

* without any apparent reason (such as intentional changes in materials,

% concrete mix, or construction procedures) , this indicates that unsound

concrete is present [21].

A general rating which has been proposed to assess the relative quality of

concrete is presented in table 4. These criteria should be used with caution

*• as differences in the qualities of concrete cannot be as sharply delineated

r as indicated in table 4. In addition, velocity is affected by the density of

£ the aggregate and the amount of aggregate in concrete. A crude assessment

of the quality of similar types of concrete can be made , however , using these

criteria. For example, if one concrete has a pulse velocity above 15,000

ft/sec (4570 m/sec) while another concrete with a similar composition has a

velocity below 10,000 ft/sec (3050 m/sec), then there is clearly a significant

difference in their qualities.

Table 4. Pulse Velocities in Concrete [22]

Pulse Velocity

ft. per sec (m. per sec)

Above 15,000

12.000-

15,000

10.000-

12,000

7,000-10,000

(4570)

(3660-4570)

(3050-4570)

(2130-3050)

General Condition

Excellent

Good

Questionable

Poor

Below 7,000 (2130) Very Poor
3.4.3 Estimation of Strength Properties

Numerous investigations have attempted to correlate compressive and flexural

strengths of concrete with pulse velocity. Some correlations have been

obtained in laboratory studies, provided that mix proportions, the cement,

types of aggregate, and curing conditions were not varied [6]

.

If these

factors were varied, however, no usable correlations were obtained. For

example, Parker [23] made a comparison of pulse velocities and compressive

strengths for concretes made from only one type of aggregate but containing

different cements from different sources and a variety of admixtures. Hi

s

analysis of the total data indicated that at the 95 percent confidence level

o the estimated strength of 4440 psi (30.7 MN/m ), concrete ranged from about

2100 to 6000 psi (14.5 to 41.8 MN/m^). Obviously, the ultrasonic pulse method

cannot be used to obtain any reliable estimates of the compressive or flexural

strengths of concrete in a structure when its composition is unknown.

Probably the best concluding remarks regarding strength prediction from wave

propagation methods are those stated by Jones [24]:

"In spite of some of the promising results of the early

investigations, it must be concluded that no general

relation has been found between the dynamic modulus

of elasticity and its flexural or compressive strength."

(This statement still holds if one substitutes "pulse

velocity" for "dynamic modulus of elasticity.")

25
3.4.4 Extraneous Effects on Measurements

The measurement of the pulse velocity of concrete is affected by several

factors which are not intrinsic properties of concrete, and, therefore, are

not a function of the quality or strength of concrete [18]:

(1) smoothness of concrete at transducer contact area. Good

acoustical contact between the transducers and concrete is

required. In addition, a coupling agent, such as an oil or

a jelly must be used.

(2) Concrete temperatures outside of the range between 41 and 86°F

(5 and 30°C) affect the measured pulse velocity. Below this

temperature range, the velocity is increased, and above the

velocity is decreased.

(3) Moisture condition of concrete. Pulse velocity generally

increases as the moisture content of concrete increases, while

compressive strength decreases as moisture increases.

(4) Presence of reinforcing steel. The pulse velocity in steel is

1.2 to 1.9 times the velocity in concrete. Measurements made

near steel reinforcing bars, therefore, may not be representative

of the concrete. If possible, measurements should be made perpendicular to the main axis of the bars. If measurements must be

made parallel to the main axis of the steel bars, crude correction factors are available.

26
3.5 Gamma Radiography

Gamma radiography is similar to X-ray radiography except that gamma rays are

generated by a radioactive isotope, while X-rays are electronically generated.

In addition, the gamma source is stored in a portable lead-shielded wheeled

container permitting field inspections while most high energy X-ray equipment

is less portable.

3.5.1 Principle and Application

Gamma radiation is attenuated when passing through materials. The extent of

attenuation is dependent on the density and thickness of material, and on the

energy of the gamma rays. In gamma radiography differences in radiation

attenuation produced by variations in the density and thickness of a material

are recorded on photographic film. For example, when reinforced concrete is

radiographically inspected, steel reinforcement attenuates the radiation more

than concrete and appears as a lighter area in the film. Voids and cracks in

the concrete will appear as darker areas on the film due to little attenuation of the incident radiation.

In practice, gamma rays generated by a suitable radioactive isotope are

allowed to pass through concrete, with the emerging radiation being recorded

on X-ray film held in a light-tight cassette. Commonly used gamma sources

are listed in table 5. Note that the relative penetration abilities of the

gamma rays are controlled by their energies.

27
Table 5. Gamma Sources [26]

Radioactive

Source

Gamma

Energy

(MeV)

Half-life

( t l/2>

Optimum Working

Thickness of

Concrete (mm) Dose Rate^

Iridium 192 0.296

and

0.613

70 days 30-200

Cesium 137 0.66 33 years 100-300 0.39

Cobalt 60 1.17

and

l._2

5.3 years 150-450 1.35

^Roentgens per hour per curie at 1 meter. On curie is equal to 3.7 x 10^

disintegrations per second.

Some of the applications for gamma radiography in inspecting concrete include

locating reinforcement and splices, determining if voids, cracks, or honeycombing are present and determining if ungrouted tendons are present in

post-tensioned concrete [18, 25].

3.5.2 Limitations

Because exposure to gamma rays is injurious to the body tissues and the effect

may be accumulative, the use of gamma-producing isotopes is closely controlled

by the Nuclear Regulatory Commission. This means gamma radiography work

should only be performed by licensed and qualified NDE inspectors using

approved equipment. Therefore, such inspection work is normally contracted

to a firm specializing in gamma radiographic NDE.

28
Unlike X-rays, the energy of gamma radiation cannot be increased (except

by changing the radioactive isotope). The usual sources require exposure

times of hours when inspecting concrete slabs or beams over 450 ram thick.

3.6 Neutron Moisture Meters

Nuclear meters have been used to nondestructively detect moisture in roofing

materials, soils, and concrete [18, 27]. It is conceivable that this method

could be used to gain information concerning the density of concrete.

3.6.1 Principle and Applications

Fast neutrons are slowed by collision with the nuclei of atoms. Hydrogen

atoms are the most effective moderators in slowing the neutrons because their

nuclear mass is nearly equal to that of neutrons.

Collision of neutrons with heavier nuclei do not reduce the energy of fast

neutrons as much. During the collisions, some of the slowed neutrons are

reflected back to the surface near the source of the fast neutrons. These

backscattered slow neutrons can be counted using a slow neutron detector such

as a scintillator crystal or a boron trifluoride type detector. The number

of slow neutrons being counted gives a measure of the hydrogen content of a

material. In the case of concrete, water and hydroxyl groups are the main

source of hydrogen. Therefore, if calibrated against concrete of known moisture content, a neutron moisture meter can give an indirect measure of the

moisure content of the concrete.

Several commercial neutron moisture meters are available. The most commonly

used sources of fast neutrons are mixtures of americium-beryllium and

radiumberyllium. Both americium and radium are alpha particle emitters and

29
and their alpha particles interact with beryllium, with fast neutrons being

emitted as a result of the nuclear reactions.

In addition to neutron sources, most commercial nuclear moisture meters also

have gamma sources. The gamma rays are used to determine the density of

materials.

3.6.2 Limitations

Similar to gamma radiography, a license must be obtained from the Nuclear

Regulatory Commission to use the radioactive isotopes in the neutron

source of the neutron moisture meters.

Neutron moisture meters give reliable information of the moisture content of

only the surface layers of concrete, about the first two inches (50 mm). In

addition, nonuniformities in the surface moisture contents of concrete can

lead to erroneous results.

3.7 Cover Meters

Cover meters are portable, battery-operated magnetic devices that are primarily

used to estimate the depth that reinforcing steel (bars and tendons) is

embedded in concrete and to locate its position. In addition, some information

can be obtained regarding the dimensions of the reinforcement [18].

3.7.1 Principle and Applications

Cover meters are magnetic devices that are based on the eddy current principle.

A typical meter is shown in figure 8. A magnetic field is induced between the

two faces of the probe, which houses a magnetic core, by an alternating current

passing through a coil. If the magnetic tield passes through concrete containing

reinforcement, eddy currents will be produced in the reinforcement. Those

30
31
eddy currents will in turn induce a current in the probe which slightly changes

the inductance of the probe. The magnitude of this change in inductance is

measured by a meter. For a given probe, the magnitude of the induced current

is largely controlled by the distance between the steel reinforcement and the

given probe, the magnitude of the induced current is largely controlled by

the distance between the steel reinforcement and the probe.

The relationship between the induced current and the distance of the reinforcement is nonlinear, largely because the magnetic flux intensity of a magnetic

material decreases with the square of the distance. In addition, the magnetic

permeability of the concrete, even though it is low, will have some affect

on the reading. Therefore, the calibrated scales on the meters of commercial

equipment are nonlinear. Also, a meter must be readjusted if a different probe

is attached.

The probe is highly directional, i.e., a sharp maximum is induced current is

observed when the long axis of the probe and reinforcement are aligned and

when the probe is directly above the reinforcement.

The commercial cover meter shown in figure 8 can measure concrete cover over

reinforcement up to 8 inches (200 mm). Through the use of spacers of known

thickness, the size of reinforcing between 3/8 to 2 inches (6 to 31 mm) can

be estimated.

Another possible application of the cover meter is to estimate the thickness

on both sides. If a steel plate is aligned on one side with the probe on the

other side, the measured induced current will give an indication of the

thickness of the slab.

32
3.7.2 Limitations

Cover meters are most useful when reinforced concrete has only one layer of

widely separated reinforcing bars. In highly reinforced concrete, the presence of secondary reinforcement makes the satisfactory determination of the

depth of concrete cover extremely difficult. Furthermore, reinforcing bars

running parallel to that being measured influence the induced current if the

distance between bars is less than two or three times the cover distance [18].

3.8 Electrical Potential Measurements

Information on the corrosion state of steel reinforcing bars in concrete can

be obtained from measuring the electrical potential of the rebars using a

standard reference electrode and a voltmeter. If the steel has a certain

electrical potential then the probability of the occurrence of active corrosion

is high.

3.8.1 Principle

The electrical potentials of steel reinforcement are measured by making an

electrical connection from a voltmeter to the reinforcement and a second

electrical contact from the voltmeter to a reference cell making physical

contact with the surface of the concrete (figure 9). Dry concrete must first

be moistened before making electrical measurements. A saturated copper-copper

sulfate electrode is usually used as the reference cell. The electrical

potential of the reinforcement below the reference cell is measured.

If the electrical potential of the steel reinforcement is more negative than

-0.35 volts versus the copper sulfate electrode, active corrosion is probably

taking place. Values in the range of -0.30 to -0.35 volts seem to suggest

33
VOLTMETER

Figure

9.

Measurement

of

electrical

potential

of

steel

reinforcing

bars

in

concrete
that corrosive conditions are developing within the concrete, while values

less negative than -0.30 volts indicate that the steel is probably passive,

i.e., not corroding [28, 29].

An electrical potential diagram of a concrete slab can be rapidly constructed

in which areas of similar potentials are outlined. This can be used to

identify those areas where the reinforcement may be corroding.

3.8.2 Limitations

Measurements of the electrical potential of steel reinforcement provide

information concerning the probability for the occurrence of corrosion but

do not indicate either the rate or the extent of corrosion.

A direct electrical connection must be made to the reinforcing steel. If the

steel is not exposed, then some concrete covering must be removed.

4. APPLICATION OF OTHER NDE METHODS FOR CONCRETE

The feasibilities of applying other NDE methods to the inspection of concrete

are currently being assessed. Methods which appear to be the most promising

are described herein. All of these methods need further development before

their usefulness or reliability for inspecting concrete can be determined.

4.1 Ultrasonic Pulse Echo

Ultrasonic pulse echo is similar to the ultrasonic pulse velocity technique

(section 3.4) in that vibrational waves are induced in concrete. However, in

the echo method waves which are reflected off of discontinuities (e.g. , cracks

and voids) and from interfaces (e.g., interface between concrete and air)

are recorded. Both the transmitting and receiving transducers are contained

33
in the same probe, thus, only those waves which are reflected back at nearly

180° to the incident waves are detected.

Echo techniques have been extensively used to identify and locate discontinuities and defects in metals and welds. The echo technique is one of the

most versatile and accepted NDE methods for metals. However, its application

to concrete has been slow, largely because the extensive pore system, presence of cracking, and the heterogeneous nature of concrete result in multiple

reflections which both significantly attenuate the reflected waves and complicate the interpretation of the observations. Recent results suggest that

the echo method could be used to measure the thickness of concrete pavements.

Thornton and his colleagues at the U.S. Army Engineering Waterways Experiment

Station (Vicksburg, Mississippi) are exploring the use of ultrasonic pulse

echo to assess the condition of concrete piles [30],

4.2 Microwaves

Microwaves (or radar waves) are a form of electromagnetic radiation which

have frequencies between 300 MHz and 300 GHz* corresponding to wavelengths of

one meter to one millimeter. Microwaves are usually generated in special

vacuum tubes called klystrons and transported in a circuit by waveguides.

Diodes are commonly used to detect microwaves.

The use of microwaves to estimate the moisture contents of roofing materials

[27] and concrete [18] has been explored. Boot and Watson reported [31] that

the microwave technique only estimated the moisture content of concrete to

* M = mega = 10^

G = giga = 10 9

36
within 12 to 30 percent of its mean value. Its low accuracy was largely

attributed to the heterogeneity of concrete, and the internal scattering and

diffraction it caused.

Other possible applications of microwaves that are being explored, by the

Boulder, Colorado laboratories of the National Bureau of Standards, are to

locate voids and cracks in concrete, to measure the thickness of concrete

slabs and to provide information on the quality of concrete finishing.

4.3 Infrared Thermography

Infrared thermography measures surface temperature and infrared NDE is based

on the principle that heat flow in a material is altered by the presence of

some type of anomaly. Changes in heat flow cause localized temperature

differences on the material surface above the anomaly.

The temperature and temperature distribution at the surface of an object

controls the infrared radiation being emitted by it. The relationship

between temperature (T) of an object and the intensity of its infrared

radiation (W) is given by the Stefan Boltzmann Law,

W = e oT4

(1)

where

e is the emissivity

a is the Stefan Boltzmann constant (5.67 x 10 ^ watt/cm^/ °K4

)

.

The infrared radiation emitted by an object is normally measured using a

radiometer which collects and focuses the radiation on an infrared detector.

The temperature profile (i.e. , temperature distribution) of an object is

usually displayed on a video screen.

37
Infrared thermography has been successfully used to detect heat loss through

buildings and roofs [27]. Clemena [32] has reported that it can also be used

to detect deteriorated regions in bridge decks. However, infrared thermography

was recently performed on a series of concrete specimens with varyng degrees of

freeze-thaw damage at the Nationl Bureau of Standards and the results were

inconclusive. Clearly, additional work needs to be performed to develop the

emthod for application to concrete to assess its reliability for detecting

deteriorated concrete.

4.4 Maturity Concept

The maturity concept has been proposed as a method for predicting the early age

strength development of hardening concrete. It relates the combined effect of

temperature and hydration time of concrete to its strength [33, 34]. Maturity

(M) is usually expressed as:

M = r(T-To)At (2)

where T is temperature of the concrete, T

Q

is the highest temperature below

which the strength of hardening concrete does not change, and At is the

increment of time for each temperatur. One proposed relationship for the

» prediction of compressive strength (f

c

) from maturity has taken the form:

F

c = A + B log M

where A and B are regression constants.

Lew and Reichard [35] have recently studied the compressive strength and maturity

relationship for concrete and developed the following nonlinear regression

relationship

38
t K (3)

f -

C V

1 + Ka [log(M - 30)] b

where K, a and b are numerical constants and 30° F-days is the maturity below

which concrete has no measurable strength. They found that the compressive

strength-maturity relationship is dependent on the type of cement and on

the water- to-cement ratio.

The maturity concept appears to be a promising method for predicting the early

age compressive strength of concrete. Such information will be useful in

deciding when formwork can be safely removed. Conceivably, the mechanical

properties of mature concrete also could be estimated.

One commercially available instrument which automatically computes the

maturity of concrete is produced in Denmark [36]. This instrument is called

"Maturity Computer" and costs around $2000. The Maturity Computer has been

largely used in the production of precast concrete. Its applicability has

not been evaluated in the United States. Alternatively, a continuous recording thermometer will suffice for obtaining the necessary data to make

maturity calculations.

4.5 Acoustic Emission

Acoustic emissions are elastic stress waves created by localized but real

movements in a material under stress [18]

.

Acoustic emissions are created by

several processes, e.g., when concrete cracks or when the reinforcement disbonds from concrete. The intensity or energy of acoustic emissions and their

rate of development gives information on the extent of cracking of concrete

and can be used to detect impending failures.

39
Commercial instruments are available to detect acoustic emission and to monitor

crack development. In the simplest case, only one transducer is required,

which is attached to the object being tested. Acoustic emission instruments

are sophisticated and expensive (above $10,000). Their use has been largely

confined to the laboratory. However, concrete pressure tanks are currently

being checked for crack development when pressurized using acoustic emission

techniques. In this case several transducers are placed around suspicious

regions so that the location of cracks can be determined. Acoustic emission

possibly can be combined with dead weight load tests on concrete structures

to assess their stiffness and structural integrity. Further work is required,

however, before relationships between acoustic emission and structural integrity are developed.

5. COMBINATIONS OF NONDESTRUCTIVE EVALUATION METHODS

While no single NDE method is entirely satisfactory for predicting the

strength or quality of concrete, combinations of methods which respond to

different factors may give more definite information.

To predict the compressive strength of in situ concrete more accurately,

two or even three different NDE are performed, and their results combined.

The most popular combination has been the ultrasonic pulse velocity method

in conjunction with the rebound hammer [37]. Other common combinations are

the ultrasonic pulse velocity method and the measurement of the damping constant of concrete [38], and the ultrasonic pulse velocity and pulse attenuation methods [39]. The latter two combinations are essentially laboratory

research techniques and therefore will not be discussed further.

40
The results of two methods can be combined in a linear equation of the form:

f'c = A(NDE

l

) + B(NDE

2

) + C (4)

? Where f c is the estimated comprehensive strength from the combined method,

NDE^ and NDE

2 are the results of the individual methods, and A, B, and C are

empirically determined constants.

5.1 Combination of Ultrasonic Pulse Velocity and Rebound Hammer Methods

This combination of nondestructive tests has been used in Europe, primarily,

with the most exhaustive studies being carried out by Facaoaru [40,-43].

In this combined approach ultrasonic pulse velocity measurements are made

on in situ concrete, and the rebound number is measured with the Schmidt

Rebound Hammer. The pulse velocity and rebound number are substituted into

a previously derived regression equation to predict compressive strength

[38] . It is believed that the multiple regression equation should give a

more accurate estimate of compressive strength than given by either of the

individual measurements.

Facaoaru [40] has developed calibration charts for standard concrete mixes

from which the compressive strengths can be estimated when the pulse velocities and rebound numbers are known. Correction factors have also been

developed to be used in the case of nonstandard concrete mixes. This

combined method has been used often in Romania to estimate the compressive

strength of in situ concrete, with improved accuracy [40,42]. Based on his

experiences, Facaoaru contends that by using the combined method, the

following accuracy in predictions of compressive strengths can be obtained:

41
(1) When composition is known and test specimens or cores are

available for calibration purposes, accuracy is within 10 to 15

percent

(2) When only the composition of the concrete is known, accuracy is

within 15 to 20 percent.

(3) When neither the composition is known nor test specimens or cores

are available, accuracy is within 20 to 30 percent.

This suggests that, for case (3), the combined method gives no better prediction of the compressive strength than can be obtained by measuring only the

ultrasonic pulse velocity or only the rebound number; in case (2), the improvement is marginal. Therefore, only when the concrete is well characterized is

this combined method better than the individual nondestructive methods.

5.2 Other Possible NDE Combinations

Possible combinations of NDE techniques for estimating the compressive strength

of concrete are listed in table 6. The first combination series, i.e., with

the maturity concept being the basic method, is mostly applicable to formwork

removal decisions. The remaining series are more applicable to determining

the uniformity and strength of mature concrete.

Note that the ultrasonic pulse velocity, the rebound hammer, and the probe do

not give a direct measure of the strength properties of concrete, whereas the

pullout method does. Therefore, combination series No. 2 in table 6 should

give a better estimation of strength than series No. 3. However, less

planning is required with the methods in series No. 3 and a large amount of

data can be rapidly obtained.

42
Table

6.

Possible

Combinations

of

Nondestructive

Test

Methods

<44 r—

44

O 60s

re (0 3 Jd re u

•H "d

U

44 "0 o re

54 <u Jd re »6M! re re CO re 3 H 44 54 re re re re re re re re a 3 3 cO 0) a' 3 3 3 3 3 3 o re S 0) d d i—

d d pH C d h CO ri T3 3 •T3 T3 3 •a -a

p-i 3 60 re <4 54 T3

Uu T3 54

H s d cO cO O cO cO O cO cO

4-' •H <— jd S rC Jd s rC Jd d V 4-) 3 u o 0) (l) 0) O a) 3 o 3 3

J4

a

«s rH o o •H o 3 •H 0 O

54 i-i

u •r! CO cO 44 cO 44 CO CO

O o (U re 4-1 4-1 re 44 44 re 44 44

Od

u

d l-l

u cO >4 54 CO i-t 54 0 o d 3 3 3 rH 3 3 rH 3 3 H cj> w H CO CO W CO CO W CO CO

>4 rH 54 pH 54 3 3 3 3 3 s id., S a 0

60 0 0 0 d cO o 3 u 3

•H jd •H 44 Jd •i4 4J jd

44

d •H r* •H

O re

u X) o O •3 o a -d

CO 3 d re

O

d re o d

H o O 3 3 3 rH 3 3 3 i—

3 3 3 Jd i—

Jd

O 54 3 o Jd 54 <UO Jd

•U 44 rHO Jd 44 > Jd

O 44 > -3

O

d 3 3 54 3 pH 3 54 pH 3 54 M a CU P4

pc! 3> p3 CM td» Pd P-i

d3! oe

Jd re

4-1 •H

60d3 44

54 iH

•X) Jd 4J re 3 4-> re d

44 60 3 O d 54 T3 •h re 3 3

T3 T3 54 3 re <44 3 O •W jd 3 •H

54 Jd re re i-4

P-. 44 3 T3 3 3 T3 TJ 3

re

X > c O

d 3 3 0 •H

•H

O re

a

4J -H re 3 O 54

5-> re 3 rH •H 3 3 3 54 •H 4-1 44 a w a- re re 3 o 0 d 3 • 63

54 o 3 r-4 re a, Jd u H w 3 3

54 Jd O3 d3O

4J 33 a O re 3 Jd 3 1—*1 O 44 44 pH

-T3

d •H 3 3 o c U 0 T3 Jd 3 o O O

44 •H pH 3 3 >> d 3 O

44 o > d

O

•H 3 re 3 *H

C) 54 o 3 3 54 44 •H 3 1—

54 re 3 re

co 4J tH 44 pH <44 3 3 3 3 rH 3 3 r4

PP

X p4 3) pu P4

W 1 * . » i-4 CNI CO r-l| CM
6. SUMMARY

Nondestructive evaluation of hardened concrete is becoming an important part

of quality acceptance of completed buildings. Quality acceptance programs

based on nondestructive evaluation methods (NDE) have been found to be cost

effective, reliable, and may increase the service life of buildings while

reducing maintenance costs.

In quality acceptance testing of hardened concrete, information which should

be obtained includes strength, general quality and uniformity, thickness, air

content, stiffness, finish, density and location and condition of steel reinforcement. Nondestructive evaluation methods which can be used to obtain

such information have been critically reviewed in this report. Both commonly

used test methods and potentially useful test methods have been discussed.

In addition, the feasibility of combining two or more test methods for

improving the prediction of the strength or quality of concrete has been

explored.

44
7. REFERENCES

1. Quality Control of Concrete Structures, June 17-21, Stockholm, Sweden,

RILEM Symposium (Swedish Cement and Concrete Research Institute, 1979).

2. B. Pohl, Combined Nondestructive Testing Methods to Assess the Strength

of In Situ Concrete for a Silo, pp. 151-158, Reference 1.

3. U. Bellander, NDT Testing Methods for Estimating Compressive Strength

in Finished Structures - Evaluation of Accuracy and Testing System,

pp. 37-46, Reference 1.

4. Test for Penetration Resistance of Hardened Concrete, ASTM Designation

C803.

5. H. T. Arni, Impact and Penetration Resistance Tests of Portland Cement

Concrete, Federal Highway Administration Report No. FHWA-RD-73-5 (1972).

6. J. R. Clifton, Nondestructive Test to Determine Concrete Strength - A

Status Report, NBSIR 75-729 (National Bureau of Standards, 1975).

7. A. T. Shore, Properties of Hardness in Metals and Materials, Proceedings

ASTM, Vol . XI, pg. 733 (1911).

8. E. Schmidt, A Nondestructive Concrete Test, Concrete, Vol. 59 (No. 8),

pg. 34 (1951).

9. P . H. Peterson and V. W. Stoll, Relation of Rebound Hammer Test Results

to Sonic Modulus and Compressive-Strength Data, Proceedings, of Highway

Research Board, Vol. 34, pg. 387, (1955).

10. C. Boundy and G. Hondrus, Rapid Field Assessment of Strength of Concrete

by Accelerated Curing and Schmidt Rebound Hammer, Journal of American

Concrete Institute, Vol. 61 (No. 1), pg. 1 (1964).

11. D. J. Victor, Evaluation of Hardened Field Concrete with Rebound Hammer,

Indian Concrete Journal, Vol. 37 (No. 11), pg. 407 (1963).

12. G. W. Greene, Test Hammer Provides New Method of Evaluating Hardened

Concrete, Journal of American Concrete Institute, Vol. 26 (No. 3),

pg. 249, (1954).

13. Discussion of Reference 12, Journal of American Concrete Institute,

Vol. 27 (No. 4), pg. 256, (1955).

14. L. J. Mitchel and G. G. Hoagland, Investiga cion of the Impact- Type

Concrete Test Hammer, Highway Research Board Bulletin 305, pg. 14,

(1961).

15. C. H. Williams, Investigation of the Schmidt Concrete Test Hammer,

Miscellaneous Report No. 6-267, U. S. Army Engineer Waterways

Experiment Station, (Vicksburg, Mississippi, 1958).

45
16. Discussion of Reference 9 by P . Klieger, Proceedings of Highway

Research Board, Vol. 34, pg. 392, (1955).

17. G. A. Erickson, Investigation of the Impact-type Concrete Test Hammer,

Model II, Concrete Laboratory Report C-928, Division of Engineering

Laboratories, Dept, of the Interior (Denver, 1959).

18. V. M. Malhotra, Testing Hardened Concrete: Nondestructive Methods,

American Concrete Institute Monograph No. 9, (Detroit 1976).

19. 0. Richards, Pullout Strength of Concrete, in Reproducibility and

Accuracy of Mechanical Testing, ASTM STP 626, pp. 32-40 (1977).

20. P. Kierkegaard-Hansen, Lok-strength, Saertry K of Nordisk Betong, 3,

pp. 1-10, 1975.

21. E. A. Whitehurst, Evaluation of Concrete Properties from Sonic Tests,

American Concrete Institute Monograph No. 2, (Detroit 1966).

22. J. R. Leslie and W. J. Cheesman, An Ultrasonic Method of Studying

Deterioration and Cracking in Concrete Structures, Journal of American

Concrete Institute, Vo. 46 (No. 1), pp. 17-36 (1949).

23. W. E. Parker, Pulse Velocity Testing of Concrete, Proceedings ASTM,

Vol. 53, pg. 1033 (1953).

24. R. Jones, Nondestructive Testing of Concrete, Cambridge University

Press, (Oxford, 1962).

25. J. A. Forrester, Gamma Radiography of Concrete, Session No. 2,

Proceedings of Symposium on Nondestructive Testing of Concrete and

Timber, pp. 9-13, Institute of Civil Engineers (London, 1969).

26. Radiography Fundamentals 1, in Home Study and Extension Course on Nondestructive Testing, by American Society for Metals and American

Society for Nondestructive Testing.

27. H. Busching, R. Mathey, W. Rossiter, and W. Cullen, Effects of Moisture

in Built-up Roofing — A State-of-the-Art Literature Survey, National

Bureau of Standards Technical Note 965 (1978).

28. K. C. Clear and R. E. Hay, Time-to-Corrosion of Reinforcing Steel in

Concrete Slabs, Vol. 1, Effect of Mix Design and Construction Parameters

,

Federal Highway Administration Report No. FHWA-RD073-32 (Washington, D.C.,

1973).

29. R. F. Stratfull and V. VanMatre, Corrosion Autopsy of a Structurally

Unsound Bridge Deck, Materials and Research Department Report

CA-IIY -MR-51 16-8-72-41 , Division of Highways (State of California,

1972).

46
30. H. T. Thornton, Development of Procedures for Nondestructive Testing

of Concrete, Report 1, Present Practices, U.S. Army Engineer Waterway

Station, (Vicksburg, Mississippi, 1977); Available from NTIS, No. ADA046113

.

31. A. Boot and A. Watson, Applications of Centimetric Radiowaves in

Nondestructive Testing, in Application of Advanced and Nuclear Physics

to Testing Materials, ASTM STP-373, pp. 3-24 (1965).

32. G. Clemena, Application of Infrared Thermography in Highway Maintenance Problems, Virginia Highway and Transportation Research Council

Report VHTRC 79-RP6 (1978).

33. S. G. Bergstrom, Curing Temperature, Age and Strength of Concrete,

Magazine of Concrete Research, Vol. 5 (No. 14), pg. 61 (1953).

34. J. M. Plowman, Maturity and the Strength of Concrete Research,

Magazine of Concrete Research Vol. 8 (No. 22), pg. 13 (1956).

35. H. S. Lew and T. W. Reichard, Prediction of Strength of Concrete from

Maturity in Accelerated Strength Testing, American Concrete Institute

SP 56 (Detroit 1978).

36. Maturity Computer, Thiim Ltd. Engineering, Herlev, Denmark.

37. R. Jones and I. Facaoaru, Analysis of Answers to a Questionnaire on

the Ultrasonic Pulse Technique, RILEM Materials and Structures,

Vol. 1 (No. 5), pg. 457 (1968).

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Testing Methods to Derive the Compressive Strength of Concrete, Report

No. B1-68-61/IHI .8 , Insitut TNO Voor Bouwmaterialen en Bouwconstructies

,

(Delft, Netherlands, 1968).

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and Damping Constant, Journal of American Concrete Institute, Vol. 64

(No. 10), pg. 678 (1967).

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Symposium on Nondestructive Testing of Concrete and Timber, Institution of Civil Engineers, (London, 1969).

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Determination by Nondestructive Combined Methods, RILEM Report,

Aachen, 41 (1966).

42. I. Facaoaru, I. Dumitrescu, and Gh. Stamate, New Developments and

Experience in Applying Combined Nondestructive Methods for Testing

Concrete, RILEM Report, Varna, 26 (1968).

47
43. I. Facaoaru, Chairman’s Report of the RILEM Committee on Nondestructive

Testing of Concrete, RILEM, Materials and Structures, Vol. 2 (No. 10),

pg. 253 (1969).

48
NBS-I I4A iKtV. W* / c

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO.

NBSIR 80-2163

10*

4. TITLE AND SUBTITLE

Nondestructive Evaluation Methods for Quality Acceptance

of Hardened Concrete

5. Publication Date January 1981

: I. Performing Organization Code

7. AUTHOR(S)

James R. Clifton and Erik D. Anderson

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, DC 20234

11. Contract/Grant No.

12. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, city, State. ZIP)

Construction Engineering

Research Laboratory

P. 0. Box 4005

Champai gn ,--JHine4-s

' 6 -18-20

' PLEME

“

13. Type of Report & Period Covered

14. Sponsoring Agency Code

15. SUP NTARY NOTES

| | Document describes a computer program; SF-185, FIPS Software Summary, is attached.

IS. ABSTRACT (A 200-word or leas factual summary of moat significant information . If document includes a significant bibliography or

literature survey, mention it here,) Nondestructive test methods which can be used In quality acceptance programs for

hardened concrete have been critically reviewed and are described in this report.

Methods have been identified which provide information on the strength, quality

and uniformity, thickness, air content, stiffness, finish, density of concrete as well as the location and condition of steel reinforcement. Both commonly used

methods and possible test methods are covered. In addition, the feasibility of

combining two or more test methods for improving the prediction of the strength or quality of concrete is explored.

17, KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name

;

separated by semicolons)

Concrete; construction; nondestructive evaluation, quality assurance, steel

reinforcing bars; test methods

18. AVAILABILITY Unlimited

| | For Official Distribution. Do Not Release to NTIS

£

Order From Sup. of Doc U.S. Government Printing Office, Washington, DC

20402 , SD Stock No, SN 03-003-

r~l Order From National Technical Information Service (NTIS), Springfield, ^VA. 22161

19. SECURITY CLASS

(THIS REPORT)

UNCLASSIFIED

21. NO. OF

PRINTED PAGES

52

20. SECURITY CLASS

(TH S PAGE)

UNCLASSIFIED

2?. Price

$ 8.00

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