NISTIR 6505

Curing of High-Performance Concrete:

Phase I Study

Nicholas J. Carino

Kenneth W. Meeks

National Institute of Standards and Technology

Technology Administration, U. S. Department of Commerce

NISTIR 6505

Curing of High-Performance Concrete:

Phase I Study

Nicholas J. Carino

Structures Division

Building and Fire Research Laboratory

Kenneth W. Meeks

Tri-State University

April 2001

U.S. Department of Commerce

Donald L. Evans, Secretary

National Institute of Standards and Technology

Karen H. Brown, Acting Director

iii

ABSTRACT

This report summarizes the results of an exploratory study on the effects of curing duration on

the variation of mortar strength with distance from the drying surface. The study was sponsored

by the Partnership for High-Performance Concrete Technology, which is a coordinated research

effort by the Building and Fire Research Laboratory to provide the bases for reliable application

of high-performance concrete in construction. In this study, a novel, notched cylindrical test

specimen was used to measure tensile strength at different depths. Two mortar mixtures with w/c

of 0.30 and 0.45 were used; the former was assumed to be representative of the paste system in a

high-performance concrete. Specimens were moist cured for (1, 3, or 7) d and then exposed to air

at 25 °C and 50 % or 70 % RH. The cylinders were sealed to simulate one-dimensional drying in

a large member. Tensile strengths were measured at 28 d. Relationships between tensile strength

and depth were compared with those of specimens continuously moist cured. The data tended to

show that 1 d of moist curing might be sufficient to ensure adequate strength development at a

depth of 25 mm from the exposed surface. The phenomenon of increasing strength with drying

may have confounded the results, and recommendations for additional studies are provided.

Keywords: Building technology; curing; high-performance concrete; mortar; statistical analysis;

tensile strength
iv
v

TABLE OF CONTENTS

ABSTRACT................................................................................................................................. iii

1. BACKGROUND....................................................................................................................... 1

1.1 Motivation for Study.................................................................................................... 1

1.2 Curing Requirements in the ACI Building Code ......................................................... 2

1.3 Applicability of Curing Practices to High-Performance Concrete............................... 3

1.4 Duration of Curing Period............................................................................................ 5

2. EXPERIMENTAL PROGRAM............................................................................................. 9

2.1 Objective and Scope..................................................................................................... 9

2.2 Procedure ..................................................................................................................... 9

3. RESULTS................................................................................................................................ 15

3.1 Continuously Moist-Cured Specimens ...................................................................... 15

3.1.1 w/c = 0.30......................................................................................................... 15

3.1.2 w/c = 0.45......................................................................................................... 17

3.2 1 Day Moist-Cured Specimens ................................................................................. 19

3.2.1 w/c = 0.30......................................................................................................... 21

3.2.2 w/c = 0.45......................................................................................................... 22

3.3 3 Day Moist-Cured Specimens ................................................................................. 23

3.3.1 w/c = 0.30......................................................................................................... 24

3.3.2 w/c = 0.45......................................................................................................... 27

3.4 7 Day Moist-Cured Specimens ................................................................................. 27

3.4.1 w/c = 0.30......................................................................................................... 27

3.4.2 w/c = 0.45......................................................................................................... 29

4. SUMMARY............................................................................................................................. 31

5. REFERENCES........................................................................................................................ 35
vi
1

1. BACKGROUND

1.1 Motivation for Study

In 1990, the National Institute of Standards and Technology (NIST) organized an

international workshop to:

• Identify ongoing and planned research programs on high-performance concrete;

• Identify potential applications where high-performance concrete could be used on a

routine basis;

• Identify technical barriers to widespread use of high-performance concrete;

• Identify institutional barriers and deficiencies in standards which hinder the use of highperformance concrete;

• Develop a listing of critical research to overcome the technical barriers and provide a

sound basis for the needed standards.

The workshop, co-sponsored by the American Concrete Institute, was attended by prominent

international experts in various aspects of concrete technology. The workshop proceedings

(Carino and Clifton 1990) adopted the following definition of high-performance concrete:

Concrete having desired properties and uniformity which cannot be obtained routinely

using only conventional constituents and normal mixing, placing, and curing practice. As

examples, these properties may include:

• Ease of placement and compaction without segregation

• Enhanced long-term mechanical properties

• High early-age strength

• High toughness

• Volume stability

• Long life in severe environments

The above definition was modified and adopted in 1998 as the ACI definition of highperformance concrete, as follows (Russell 1999):

Concrete meeting special combinations of performance and uniformity requirements that

cannot always be achieved routinely using conventional constituents and normal mixing,

placing, and curing practices.

Examples of desired characteristics were included in a “commentary” to the ACI definition.

The above definitions have been criticized as being too broad and not practical for specification

purposes. Consequently, others (Zia et al. 1991; Goodspeed et al. 1996) have defined different

classes of high-performance concrete with specific properties. In general, the majority of highperformance concretes used in North America can be characterized as concretes with watercementitious materials ratios (w/cm) lower than about 0.4. The term “water-cementitious

materials ratio” is used instead of “water-cement ratio” because other cementitious materials

(pozzolans or ground slag) besides portland cement are typically used to produce highperformance concrete. Thus high-performance concrete typically has high compressive strength

and high resistance to fluid penetration.
2

The proceedings of the NIST/ACI workshop provided an outline of primary and secondary

research needs within the following general areas:

• Materials and proportioning

• Processing and curing

• Mechanical properties and test methods

• Durability and test methods

• Structural performance and design

• Standards and acceptance criteria

The outline of research needs has provided a roadmap for a multi-faceted, long-term research

program on high-performance concrete at the National Institute of Standards and Technology.

The following research needs related specifically to curing were identified (Carino and Clifton

1990):

• Evaluate the effectiveness of moist curing considering the degree of hydration as a

function of time;

• Seek an understanding of interactions between ambient exposure conditions, mixture

rheology, and needed evaporation control measures;

• Develop a more comprehensive understanding of the effects of internal curing

temperature, and develop guidelines for curing high-performance concrete based on

sound technical knowledge.

One of the first curing-related studies in the NIST program established the applicability of

the maturity method to high-performance concrete (Carino et al. 1992). The study that is

summarized in this report represents the initial experimental effort to establish the basis for the

duration of the moist curing period for high-performance concrete. Prior to initiating the

experimental program, the authors prepared a report on the state-of-the-art related to curing of

high-performance concrete (Meeks and Carino 1999). That report covered the following topics:

• Review of the characteristics of high-performance concrete;

• Review of the physical and chemical properties of cement paste related to curing;

• Historical review of the ACI building code requirements for curing;

• Review of other curing recommendations, standards, and criteria;

• Review of recent research on curing requirements;

• Recommended research needs.

1.2 Curing Requirements in the ACI Building Code

A review of the predecessors to ACI 318-95 revealed that the general requirements for curing

of concrete have changed very little since the first standard regulations were proposed in 1909

(Meeks and Carino 1999). The basic requirement has been to cure concrete made with normal

portland cement for a period of at least 7 d and to cure high-early-strength concrete for at least

3 d.

Tests reported by Price (1951) indicated that normal strength concrete that is moist cured for

7 d and then stored in air would attain approximately the same 28 d strength as if it had been

continuously moist cured. These tests provide validation of the 7-day criterion in the ACI Code.
3

Since high-early-strength concrete will gain strength more rapidly, the Code permits a 3-day

curing period.

In the 1971 Code, a requirement was added to maintain the concrete temperature above 10 °C

during the curing period. This addition is to ensure that sufficient strength development will

occur during the prescribed minimum curing periods. In addition, a new provision was added for

checking the adequacy of curing procedures based on strength tests of field-cured cylinders. Both

requirements were carried over to the 1995 version of ACI 318.

The ACI Code, however, makes no distinction between strength and durability

considerations with regard to curing requirements. Since ACI 318 deals primarily with structural

safety, the provisions are intended primarily to ensure adequate structural capacity. The only

explicit mention of durability in relation to curing is contained in the provisions (originally added

in 1971) dealing with accelerated curing.

The ACI Code also does not address curing requirements for concretes made with other

cementitious materials besides portland cement. Since the nature of the cementitious system

affects early-age strength development characteristics, this omission may be a major deficiency

in the current Code.

1.3 Applicability of Curing Practices to High-Performance Concrete

Carino and Meeks (1999) concluded that current curing practices and standards are based on

studies related primarily to strength development characteristics of conventional (ordinary)

concretes. Most high-performance concretes, however, are fundamentally different from

conventional concrete, because they typically have a low water-cementitious materials ratio

(w/cm) and one or more admixtures. In addition, supplementary cementitious materials, such as

silica fume, fly ash, and ground slag, are commonly used in practicable mixtures to achieve high

strength, low permeability, reduced temperature rise, and economy. High range water-reducing

admixtures are used typically to provide workability. Since the composition of high-performance

concrete differs from conventional mixtures, early-age characteristics of the hydrating paste will

also differ. Therefore, existing curing practices may not be optimal for high-performance

concrete. A better understanding is needed of the role of an external supply of moisture and of

the adequacy of membrane-forming compounds when a low w/cm is involved.

The effects of self-desiccation are also important considerations in high-performance

concretes with low w/cm. Self-desiccation refers to the process by which concrete dries itself

from the inside. Moisture in the paste is consumed by the hydration reactions, and the internal

relative humidity may decrease to the point where there is not enough remaining free water to

sustain hydration. Consequently, hydration will terminate at an early age if additional moisture is

not provided. To prevent early-age self-desiccation, water that is consumed by hydration needs

to be replaced by the ingress of external moisture. Therefore, the common practice of sealing

concrete with a membrane-forming compound may not be an appropriate curing practice for low

w/cm concrete. However, for how long is moist curing effective? As hydration proceeds,

capillary pores in the paste become discontinuous, thereby hindering the ingress of additional
4

water into the concrete. When this state is reached, additional moist curing may be of little, or

no, benefit, because the water may not be able to penetrate to the interior quickly enough to

maintain saturation of the capillaries and sustain hydration. Current curing requirements, based

on research on conventional concrete, do not consider these factors.

One of the most controversial topics that emerged from the literature review by Meeks and

Carino (1999) concerns the sensitivity of various properties of high-performance concrete to

different curing conditions. Some researchers have reported that high-performance concrete is

more sensitive to the details of curing than normal concrete; whereas, others have found the

opposite to be true, at least for some properties. These differences may be attributed to the

different experimental procedures that have been used. For example, Hasni et al. (1994) reported

that the use of silica fume makes high-performance concrete more sensitive to different curing

methods when considering both strength and durability properties. In addition, they reported that

high-performance concrete with silica fume is more sensitive to different curing methods than is

normal concrete for characteristics such as compressive and flexural strength, depth of

carbonation, and microcracking. Comparison of high-performance concrete without silica fume

with normal concrete showed that normal concrete was more sensitive to the curing method for

these same properties. With respect to resistance to penetration of chloride ions, results showed

that high-performance concretes with and without silica fume, as well as normal concrete, were

insensitive to the curing method.

Results of work in Norway that was summarized by Gjørv (1991) generally agree with the

findings by Hasni et al. (1994). Gjørv reported that the use of silica fume makes concrete more

sensitive to proper curing compared with normal concrete. Silica-fume concrete is more

vulnerable to plastic shrinkage cracking than normal concrete, which necessitates good, earlyage, curing practices to control this tendency. Another reason cited by Gjørv for why silica-fume

concrete is more sensitive to proper curing is related to the effects of drying on strength

properties. Good curing practices must be used to prevent early drying, which can reduce tensile

and flexural strengths of silica-fume concrete more than for normal concrete.

Torii and Kawamura (1994) also reported on the effects of curing on mechanical and

durability-related properties of concrete, and some of their results do not agree with those

summarized in the previous paragraphs. Their results indicated that the detrimental effects of

poor curing practices on pore structure are more significant in normal-strength concrete than in

high-strength concrete with silica fume. In their studies, high-strength concrete in which 8 % of

the mass of cement was replaced by silica fume apparently developed a dense pore structure at

early ages regardless of curing method. This independence of the curing method is attributed to

the use of a low w/cm (0.30) and the rapid early-age pozzolanic reactions of the silica fume.

Tests for resistance to chloride ion penetration and carbonation depth also showed that highstrength concrete, both with and without silica fume, was less affected by poor curing conditions

than normal concrete. This can be attributed to the fact that concrete with a low w/cm may attain

a low porosity paste at a lower degree of hydration than concrete with a higher w/cm.

Comparisons between high-strength concretes, with and without silica fume, revealed that the

silica-fume concrete was less affected by changes in curing method, when considering resistance

to chloride ion penetration and carbonation. Carino and Meeks (1999) conclude that additional
5

studies are needed to reconcile these conflicting conclusions regarding the sensitivity of low

w/cm concrete to the curing method.

1.4 Duration of Curing Period

Hilsdorf and co-workers (Hilsdorf and Burieke 1992; Hilsdorf 1995) have presented

informative work on concrete curing. Their efforts include experimental and theoretical studies

in the search for rational curing requirements. Although their work was not directed specifically

to high-performance concrete, the underlying approaches are applicable to all types of concrete.

According to Hilsdorf and Burieke (1992), concretes can be distinguished by their curing

sensitivity, which refers to the curing duration needed to reach some specified level of durability

or strength. The long-term properties of concrete with low curing sensitivity would not be

affected significantly by the duration of the curing period. Curing sensitivity is affected by the

characteristics of the cementitious materials, mixture proportions, and the environment to which

the concrete is exposed after curing has been terminated. The latter factor affects the rate of

moisture loss from exposed surfaces. The w/cm of a particular concrete has a significant

influence on the curing sensitivity. Concretes with low w/cm will gain strength faster and

become impermeable sooner than those with higher w/cm. This is an important characteristic

since it may mean that curing duration can be reduced in accordance with the w/cm.

Based on the above considerations, Hilsdorf summarized the four factors that must be

considered in establishing minimum curing durations (Hilsdorf 1995):

• Curing sensitivity of the concrete as influenced primarily by the cementitious system;

• Concrete temperature as it affects the rate of hydration (and, therefore, rate of strength

development and reduction in porosity);

• Ambient conditions during and after curing as these affect the rate of strength

development and severity of drying of the surface layer;

• Exposure conditions of the structure in service as these affect the required “skin”

properties for adequate service life.

To establish minimum curing durations, Hilsdorf emphasized that attainment of compressive

strength is not the only criterion that must be considered; other possible criteria include the

following:

• Depth of carbonation

• Permeability

• Maturity or degree of hydration

The depth of carbonation must be controlled to ensure that the reinforcing steel is surrounded

by an alkaline environment and remains in a passive state. The minimum duration of curing for

adequate resistance to carbonation depends on the depth of cover, the desired service life, the

relationship between time and depth of carbonation, and the relationship between concrete

permeability and carbonation. Given this information, additional relationships between

permeability, water-cement ratio, and time can be used to estimate the minimum duration of

curing (see Meeks and Carino [1999] for a summary). It should be noted that carbonation is not a
6

pervasive problem in North America compared with other regions. This can be attributed, in

general, to the deeper cover over reinforcing steel and overall better quality of the concrete in

North America1.

The permeability criterion is a more general form of the carbonation criterion. In this case,

the minimum curing duration is based on achieving a certain level of impermeability as

measured by a specific test method. One difficulty in using the permeability criterion is the

selection of the critical level of impermeability because there is insufficient knowledge of the

relationships between measured permeability values and long-term durability.

In the degree of hydration or maturity criterion, the minimum duration of curing is based on

the concrete reaching a specified degree of hydration or maturity. Once the required degree of

hydration is defined, empirical relationships between time, temperature, and degree of hydration

(or maturity) can be used to estimate the minimum curing duration. The empirical relationships

are expected to be affected by the characteristics of the cementitious system in the concrete. As

is the case with the permeability criterion, there is insufficient knowledge to relate the minimum

degree of hydration (or maturity) at the end of the curing period with long-term performance.

A compressive strength criterion may involve one of two approaches:

1. R1-Concept: The concrete is cured until it attains a specified minimum strength. As an

example, a suggested minimum strength is the strength after 7 d of moist curing that would

be obtained by a reference concrete with a water-cement ratio of 0.6 and made with the same

materials as the concrete to be cured (Hilsdorf 1995). A water-cement ratio of 0.6

corresponds closely to the highest value for which capillary pores can become segmented

with good curing.

2. R2-Concept: The concrete is cured until the in-place compressive strength reaches a

prescribed fraction of the 28 d specified compressive strength so that at 28 d the concrete at a

prescribed depth will attain the specified strength.

The R1-Concept offers the advantage that the use of mixtures with low water-cement ratios

or having rapid early strength development can reduce the curing period. This criterion may be

applicable when durability is of concern, because it has been established that, for a given

concrete, there is a “reasonably reliable” correlation between compressive strength and other

durability-related characteristics (Hilsdorf and Burieke 1992; Ho and Lewis 1988).

In the R2-Concept, the curing duration is independent of the water-cement ratio, but it would

depend on the rate of strength development. The R2-Concept is appropriate when structural

strength is of concern. The basic notion is that the concrete should be cured long enough so that

the in-place strength at some depth below the surface attains the specified strength used to design

the structure. This is illustrated schematically in Figure 1, where the solid curve represents

strength development of the concrete under standard curing and the dashed curve represents inplace strength development at some prescribed distance from the exposed surface. When curing

is terminated, drying of the surface occurs and hydration ceases when the moisture content in the

surface layer falls below a critical value. However, it will take time for the drying front to

penetrate into the concrete. As result, the interior concrete continues to gain strength after curing

1 V. M. Malhotra, Personal communication.
7

is terminated. When the drying front reaches the prescribed depth, two things happen: (1) the

strength increases due to drying and (2) the rate of hydration is reduced. Later, the concrete at the

prescribed depth dries below a critical level and strength development ceases. The objective is to

ensure that the two strength development curves cross at an age of 28 d or later.

The question that has to be answered to implement the R2-Concept is as follows: What

fraction of the standard-cured strength has to be attained at the end of the curing period to ensure

that the design strength is attained in the interior of the member? ACI Committee 308 (1998)

specifies that the strength at the end of the curing period should be at least 0.7 of the design

strength. Hilsdorf (1995) notes that this value is based on data obtained in the early 1950s, and

those results may not be applicable to modern concretes. Hilsdorf suggests that a value of 0.7

may be conservative, and that research is needed to understand the dynamics of internal drying

and strength development after curing is terminated for different types of modern concretes.

Hilsdorf (1995) suggested that the curing period should be long enough so that at 28 d (or

other applicable age) the concrete strength at the depth of the first layer of reinforcement will

equal the design strength. The rationale for this requirement is to ensure that the bond strength

(or development length) of the reinforcing steel will attain the value assumed in the structural

design. Hilsdorf used analytical models to estimate the required curing duration. Diffusion theory

was used to model the drying of the concrete from the exposed surface. It was assumed that the

rate of hydration was not affected until the moisture content dropped below the value that is in

equilibrium with a relative humidity of 90 %. The calculations were carried out for a concrete

with a 28 d strength of 40 MPa, for cements with different hardening rates, and for different

values of ambient relative humidity (ambient temperature was 20 °C). The cover depth was taken

conservatively as 25 mm. The results of the calculations are summarized in Figure 2.

The vertical axis in Figure 2 represents the ratio of the strength when curing is terminated to

the standard-cured strength at 28 d. The horizontal axis represents the 28 d strength at a depth of

Age

Moist

Curing

f28

fc

Standard Curing

End of 28 days

Curing

Air

Drying

Strength

Below

Surface

Figure 1–Schematic of strength development below surface for standard curing and for

moist curing followed by air drying
8

25 mm expressed as a fraction of the 28 d standard-cured strength. The effects of different

cement types were minor (see Hilsdorf 1995), and so the results of the calculations are shown as

three curves, each representing a different ambient relative humidity. The objective is to provide

sufficient curing so that the strength at 28 d at a depth of 25 mm equals the standard-cured 28 d

strength, that is, the value on the horizontal axis of Fig. 2 should equal 1. For example, for an

ambient relative humidity of 60 %, curing may be terminated when the concrete has attained 0.6

of the standard-cured, 28 d strength. If the ambient relative humidity is 50 %, curing has to be

maintained until 0.85 of the standard-cured strength is attained. On the other hand, if the ambient

relative humidity is 80 %, only about 0.4 of the standard-cured strength has to be attained. The

time required to achieve these fractional strengths at a specific temperature can be estimated

from the strength development characteristics of the cement.

In summary, Hilsdorf and co-workers presented a rational approach to establish the curing

duration. A key factor affecting this duration is the controlling criterion for adequate long-term

performance. Hilsdorf’s studies showed that, in most cases, the critical curing duration was

controlled by compressive strength criteria (Hilsdorf and Burieke 1992; Hilsdorf 1995). This is

an important finding because it tends to affirm that strength-based criteria may be the most

practical approaches to evaluate the adequacy of curing, possibly even when durability is a

primary concern. If preliminary testing of the specific concrete mixture to be used in

construction results in a reliable correlation between strength and durability, in-place strength

measurements would be a suitable method for assessing the adequacy of curing in the field.

Figure 2–Relationships between strength ratio at depth of 25 mm and ratio of strength at

end of curing period (based on figure provided by H.K. Hilsdorf)

0

0.2

0.4

0.6

0.8

1

0 0.2 0.4 0.6 0.8 1

RH = 80%

RH = 60%

RH = 50%

Fraction of 28 d Strength

at End of Curing

Fraction of Standard–Cured 28 d Strength at

25 mm
9

2. EXPERIMENTAL PROGRAM

2.1 Objective and Scope

The exploratory study summarized in this report examines the influence of the duration of

moist curing on the variation of strength with distance from the drying surface. The strength at

an age of 28 d was used as the basis for comparison. To simplify testing, mortar was used instead

of concrete and only portland cement was used as the cementitious material. While these

simplifications may limit the direct applicability of the results, it was felt that correct trends

would be revealed.

Two mortar mixtures with water-cement ratios of 0.30 and 0.45 were used; the former is

intended to be representative of the hydration and drying behavior of a high-performance

concrete with a low w/cm. Three moist curing periods were used: (1, 3, or 7) d. At the end of the

moist-curing period, the specimens were sealed and allowed to dry at 25 °C at either 50 % or

70 % relative humidity (RH). Reference specimens were continuously moist cured by storing

them in a limewater bath.

Tensile strength was measured at 28 d as a function of distance from the drying surface using

cylindrical test specimens with circular notches cast at various depths. The notches created

reduced cross sections that forced failures to occur at predetermined distances from the drying

surface. The estimated average tensile strength at a depth of 25 mm was used as the basis for

evaluating the influence of the different curing procedures. The objective was to determine the

minimum duration of moist curing so that the 28 d strength at 25 mm was not lower than the case

of continuous moist curing.

Six curing treatments, in addition to continuous moist curing, were investigated for each

water-cement ratio. Four notch depths were used for each treatment. Three replicate specimens

were tested for each notch depth. For the continuously moist-cured specimens, two runs

(batches) were used to establish the reproducibility of the results. Additional details of the

experimental program may be found in the doctoral dissertation of the second author (Meeks

1997).

2.2 Procedure

Table 1 lists the mixture proportions of the two mortars used to prepare the cylindrical

specimens. The two mixtures were proportioned so that they had approximately the same volume

fraction of paste. The water in the high-range water reducer was included as part of the mixing

water. The sand was graded silica sand that conformed to ASTM C 778. The portland cement

was a sample of cement 116 issued by the ASTM Cement and Concrete Reference Laboratory

(CCRL) in its proficiency sample program. Table 2 lists the degree of hydration versus age for

curing of CCRL cement 116 under saturated conditions. These data were obtained from the

difference in mass measurements at 105 °C and 950 °C (Bentz 1997). Note that for the
10

w/c = 0.30 paste, the long-term degree of hydration is about 70 % because there is insufficient

capillary porosity to accommodate the reaction products corresponding to complete hydration.

As shown in Figure 3, the molds for the cylindrical mortar specimens were made from plastic

pipe with a nominal inside diameter of 50 mm. Lengths of pipe, approximately 125 mm long,

were attached to acrylic plastic bases using silicone sealant. The sealant provided a watertight

seal and allowed easy removal when the specimens had to be taken out of the molds. Figure 3(b)

shows the 12 molds for a test series ready to be filled with mortar. Each mold included a 5 mm

thick ring with an inside diameter of about 40 mm. The ring created a reduced cross sectional

area of about 64 % of the full cross section. The rings were positioned at depths of (10, 20, 30,

and 40) mm from the top surface. The rings were held in place by friction, and care was taken

when filling the molds to avoid disturbing their position.

The mortars were mixed in accordance with ASTM C 305. Each mold was filled in three

layers, and consolidated with a rubber tamper plus a vibrating table to reduce the number of large

air voids (see Meeks 1997 for additional details). The top surface was smoothed with a trowel.

Table 1Mass Proportions of Mortar Mixtures

w/c = 0.30 w/c = 0.45

Water 0.278 0.450

Cement 1.000 1.000

Sand 2.207 2.750

HRWR 0.036 0.006

Table 2Degree of Hydration versus Age for CCRL Cement 116

Cured Under Saturated Conditions at 25 oC (Bentz 1997)

Degree of Hydration, %

Age, d w/c = 0.30 Age, d w/c = 0.45

0.3 13.7 0.3 15.4

1.0* 39.1 1.0* 41.7

2.0 50.9 2.0 51.9

3.0* 56.3 3.0* 58.4

7.0* 62.0 7.0* 71.7

14.0 65.5 14.0 78.8

28.0 68.9 28.0 83.1

28.2 69.9 28.0 80.1

54.0 69.9 56.0 82.7

90.0 68.7 91.0 87.5

*Duration of moist curing used in this study
11

At approximately 75 min from the start of mixing, the specimens were submerged carefully into

a lime-saturated water bath at (23 ±2) °C.

Figure 4 summarizes the experimental procedure. At the end of the designated period of

moist curing, the molds were removed and the bottoms and sides of the cylinders were covered

with plastic “duct” tape so that subsequent drying would occur only from the top surface. The

Rings

Figure 3Molds used to prepare notched, cylindrical tensile strength specimens

(a) Cross section of mold filled with mortar

b) Set of 12 molds for one specimen series c) Top view showing rings in place

50 mm

≈125 mm

Varies

40 mm

PVC pipe

Acrylic plastic

base

Silicone sealant

5 mm

Plastic ring
12

masses of the sealed specimens were measured, and the specimens were placed in the

environmental chambers. Figure 5 shows the taped specimens in the chamber set at 70 % RH.

The masses of the drying specimens were measured periodically.

At an age of 27 d the cylinders were prepared for tensile testing. The tape was removed from

the cylinder bottoms, and the ends were sand blasted to expose clean surfaces. For the

continuously moist-cured cylinders, the specimens were removed from the water bath and the

sides were sealed with tape prior to sand blasting. Steel disks, 12.5 mm thick and with a central

threaded hole, were bonded to the ends of the cylinders using a high-strength, structural grade

epoxy. Short Teflon rods were screwed into the disks before bonding to keep the holes free of

(c) Tensile test

Epoxy

Steel disc

Fracture top

or bottom

(a) Moist curing

Water bath

(b) Air drying

Environmental

chamber

Plastic

tape

Figure 4Summary of procedures for curing, drying, and testing of mortar cylinders
13

epoxy. Figure 6(a) shows a series of specimens with the discs applied to their bottoms. A jig was

used to ensure that the disks were aligned properly with respect to the cylinders (Meeks 1997).

After the epoxy hardened, discs were attached to the other ends. The epoxy was allowed to cure

overnight. Figure 6(b) shows the cylinders with both discs attached and ready to be tested.

On the 28th day, the specimens were tested in uniaxial tension. A hydraulic, servo-controlled

testing machine, as shown in Figure 7(a), applied the tensile load. The load was transferred to the

disks through a “hook and eye” linkage to reduce bending effects. Figure 7(b) shows this linkage

Figure 5Specimens covered with plastic tape stored in environmental chamber

Figure 6(a) Specimens with steel discs on bottom ends; jig shown between the cylinders

was used for proper alignment of discs; (b) specimens ready to be tested
14

system. Load was applied by constant movement of the machine cross head so that the applied

stress rate in the linear portion of the tests was (35 ± 15) kPa/s. Figure 7(c) shows a specimen

after completion of the tensile test. In this case the ring is 10 mm from the top surface, but the

failure surface occurred at the lower surface of the ring. Therefore, the failure surface is at 15

mm.

After testing was completed, the disks were removed carefully with a hammer and chisel, and

the top portion of the cylinders were split in half using the splitting-tension loading method. The

exposed surfaces were examined visually to estimate the depths of the drying fronts (indicated by

the lighter shade). The drying front was more difficult to observe in the w/c = 0.45 specimens

because of their generally lighter shade compared with the w/c = 0.30 specimens.

Figure 7–(a) Overall view of tensile test; (b) close up view of specimen showing linkage

system; (c) specimen after test, in this case failure occurred at bottom of ring

(a) (b)

Ring (c)
15

3. RESULTS

The individual results of the tensile tests are reported in tables and shown graphically using

scatter plots. Commercial statistical analysis software was used to establish relationships

between the measured tensile strengths and the depth of the failure surface, and to determine

confidence intervals for the estimated average strength at a depth of 25 mm. The results for the

continuously moist-cured specimens are used as the bases for comparisons.

3.1 Continuously Moist-Cured Specimens

Tables 3 and 4 list the measured tensile strengths. There were two runs (batches) for the

w/c = 0.30 and w/c = 0.45 specimens that were cured under water continuously for 27 d. The two

runs were used to establish the details of the testing procedure and the batch-to-batch

repeatability. The results for the two runs are indicated in the top and lower halves of the tables.

In all cases except one (see Table 4), the failure plane was at the reduced cross section nearer to

the top surface of the cylinder. The last column in each table gives the coefficient of variation of

the replicate test results. With few exceptions, the coefficient of variation is about 10 % or less.

This is not unreasonable for a direct tensile strength test that is more sensitive to testing errors

than the typical compressive strength tests. Some individual test results were identified as

outliers as is explained below.

Figures 8(a) and 9(a) show the individual test results plotted as a function of the nominal

depth of the failure surface. Contrary to expectation, there appears to be a tendency for

increasing strength with distance from the top surface. To examine whether this trend is

statistically significant, it was first necessary to establish whether the data for the two runs could

be combined for the purpose of regression analysis. A linear fit was assumed and an F-test was

used to compare the sum of the squares of the residuals for a separate regression for each run

with the residuals for a single regression for both runs grouped together. See Carino et al. (1983)

for the procedure to carry out this F-test. For each w/c, the results of the F-tests indicated

insufficient evidence to reject the null hypothesis (no difference between the lines). Thus for

each w/c, one line was fitted to the combined data from the two runs.

The presence of outliers was examined by using normal probability plots of the residuals for

the best-fit lines to all the data points. Figure 8(b) and 9(b) show the normal probability plots of

the residuals when all points for each w/c were considered in fitting straight lines. If the residuals

are normally distributed, which is a fundamental assumption of regression analysis, they should

plot approximately along a straight line in a normal probability plot.

3.1.1 w/c = 0.30—As shown in Figure 8(b), when all data points are used, there are three

points with large negative residuals that deviate from the straight line that would be defined by

the remaining residuals. These three points were, therefore, considered as outliers and

disregarded. Figure 8(a) shows the best-fit line to the combined data with outliers removed. The

equation of the line is:
16

Y (MPa) = 3.55 MPa + 0.053 MPa/mm X (1)

The residual standard deviation of the fit is 0.36 MPa. The slope of the line is 0.053 MPa/mm,

and this value was found to be statistically significant (p < 0.0001). The probability “p” in this

case represents the likelihood that the calculated slope is due to chance. Thus a low value of “p”

means it is unlikely that the calculated slope is due to chance. As is the custom, a probability

value of less than 0.05 is adopted as an indicator of statistical significance. Figure 8(a) also

shows, as dashed lines, the 95 % confidence limits for the average strength as a function of

depth. These confidence limits are used in subsequent data analyses as the basis for comparing

the results for the various curing and drying regimens. Based on the straight-line relationship, the

estimated average tensile strength at a depth of 25 mm is 4.88 MPa, with a 95 % confidence

interval of 4.72 MPa to 5.05 MPa.

Table 3—Tensile Strength Results for Continuously Moist-Cured

Cylinders, w/c = 0.30

Run Depth,

mm

Strength,

MPa

Mean,

MPa

S.D.,

MPa

C.V.

1 10 3.68

4.17 3.85 0.28 0.07

3.70

20 4.93

4.75 4.39 0.78 0.18

3.49† (4.84)‡ (0.13) (0.03)

30 5.05

5.82 5.46 0.39 0.07

5.51

40 5.01

5.99 5.57 0.50 0.09

5.71

2 10 4.03

4.53 4.16 0.32 0.08

3.93

20 4.62

4.32 4.60 0.27 0.06

4.85

30 4.82

3.32† 4.40 0.95 0.21

5.07 (4.95) (0.18) (0.04)

40 5.16

4.28† 5.17 0.89 0.17

6.06 (5.61) (0.64) (0.11)

†Outliers

‡Values in parentheses are with outliers removed
17

3.1.2 w/c = 0.45—The normal probability plot of the residuals in Figure 9(b) shows that,

when a straight line is fitted to all of the w/c = 0.45 data, the residuals fall approximately on a

straight line with the exception of two points. These two points were considered, therefore, as

outliers and were disregarded. Figure 9(a) shows the best-fit straight line to the combined data

with the two outliers removed. The equation of the line is:

Y (MPa) = 2.53 MPa + 0.016 MPa/mm X (2)

The residual standard deviation of the best-fit line is 0.29 MPa The value of the slope is

0.016 MPa/mm, and this value was found to be statistically significant (p = 0.0008). The

estimated average strength at a depth of 25 mm is 2.94 MPa, with a 95 % confidence interval of

2.81 MPa to 3.08 MPa.

Table 4—Tensile Strength Results for Continuously Moist-Cured

Cylinders, w/c = 0.45

Run Depth,

mm

Strength,

MPa

Mean,

MPa

S.D.,

MPa

C.V.

10 2.19

1.67† 2.31 0.71 0.31

3.07 (2.63)‡ (0.62) (.24)

25 2.99

2.67 2.83 0.16 0.06

2.82

40(45) 3.13

3.59 3.18 0.39 0.12

2.81

55 3.80

3.52 3.48 0.34 0.10

1

3.12

10 2.89

2.70 2.86 0.15 0.05

3.00

20 2.69

2.64 2.70 0.06 0.02

2.76

30 3.59

2.20† 2.94 0.70 0.24

3.02 (3.31) (0.40) (0.12)

40 3.38

2.87 3.17 0.27 0.08

2

3.25

†Outliers

‡Values in parentheses are with outliers removed
18

In summary, the results for the specimens that were continuously moist cured indicate

increasing tensile strength with distance from the top surface of the cylinders. This may be

related to two factors:

• The presence of bleed water would tend to reduce the strength of the upper layer because this

bleed water is mixed within the mortar during consolidation of the upper layers.

• The lower layers are more densely compacted than the upper layer.

Figure 8(a) Tensile strength versus depth for continuously moist-cured specimens with

w/c = 0.30; (b) normal probability plot of residuals for straight line fit using all data

0

1

2

3

4

5

6

7

0 10 20 30 40 50 60

Run #1

Run #2

Combined

95 % Confidence limits

Tensile Strength, MPa

Depth, mm

Outlier Outlier

Outlier

w/c = 0.30, moist cured (a)

-2

-1.5

-1

-0.5

0

0.5

1

.01 .1 1 5 10 2030 50 7080 90 95 99 99.9 99.99

Residual, MPa

Percent

w/c = 0.30, moist cured

All data

(b)
19

Such variation of strength with depth in concrete members is well known (Bartlett and

MacGregor 1999).

3.2 1 Day Moist-Cured Specimens

Table 5 lists the tensile strengths for specimens that were moist cured for 1 day. The depths

within parentheses indicate that those cylinders had failure planes at the bottom of the rings (see

Figure 9(a) Tensile strength versus depth for continuously moist-cured specimens with

w/c = 0.45; (b) normal probability plot of residuals for straight line fit using all data

0

1

2

3

4

5

6

7

0 10 20 30 40 50 60

Run #1

Run #2

Combined

95 % Confidence limits

Tensile Strength, MPa

Depth, mm

Outlier

Outlier

w/c = 0.45, moist cured (a)

-1

-0.5

0

0.5

1

.01 .1 1 5 10 2030 50 7080 90 95 99 99.9 99.99

Residual, MPa

Percent

w/c = 0.45, moist cured

All data

(b)
20

Figure 4), and these are shown as depths of (15, 25, 35, and 45) mm. Missing data points are a

result of failures near the bond line between the mortar and steel disk. Regression analysis was

used to examine the variation of tensile strength with depth for each condition (water-cement

ratio and relative humidity after moist curing).

Table 5—Tensile Strength Results for Cylinders Moist Cured for 1 Day

w/c Rel. Hum., % Depth, mm Strength, MPa Mean, MPa S.D., MPa C.V.

10 4.60

10 5.13 4.87 0.37 0.08

20 4.78

20 5.27 5.03 0.35 0.07

30 (35) 4.47

30 (35) 4.95 4.66 0.25 0.05

30 (35) 4.57

40 4.07

40 (45) 4.85 4.58 0.44 0.10

50

40 (45) 4.81

10 4.53

10 4.60 4.70 0.23 0.05

10 4.96

20 (25) 3.27†

20 (25) 4.28 4.14 0.80 0.19

20 (25) 4.86 (4.57)‡ (0.08) (0.02)

30 (35) 5.10

30 (35) 5.18 5.04 0.18 0.04

30 4.83

40 3.35†

40 (45) 4.39 4.01 0.57 0.14

0.30

70

40 4.28 (4.34) (0.08) (0.02)

10 3.07

10 3.59 3.15 0.40 0.13

10 2.80

20 2.44

20 3.34 3.13 0.61 0.20

20 3.61

30 3.10

30 3.24 3.32 0.27 0.08

30 3.63

40 3.27

40 (45) 3.04 3.02 0.26 0.09

50

40 2.75

10 2.89

10 3.60 3.08 0.46 0.15

10 2.75

20 3.25

20 (25) 3.67 3.36 0.27 0.08

20 3.16

30 3.21

30 3.64 3.46 0.22 0.06

30 (35) 3.53

40 2.89

40 3.25 3.06 0.18 0.06

0.45

70

40 3.05

†Outlier

‡Values in parentheses are with outliers removed
21

3.2.1 w/c = 0.30—Figure 10(a) shows the strength versus depth results for w/c = 0.30 and

drying at 50 % RH. Regression analysis indicated that the slope was not statistically significant

(p = 0.28). The best-fit line is shown using long dashes. A normal probability plot of the

residuals about the best-fit line indicated no outlying points. Since the slope of the line is not

statistically significant, an overall average strength was computed. This overall average strength

for all depths is 4.75 MPa, which is shown as a solid line in Figure 10(a). The standard error

(standard deviation divided by the square root of the number of points) of the overall average is

0.11 MPa. Therefore, the 95 % confidence interval for the overall average strength is 4.51 MPa

to 4.99 MPa, which is shown in Figure 10(a) by the error bars for the point (open circle) at

25 mm. Also shown in Figure 10(a) is the 95 % confidence interval for the estimated average

Figure 10Tensile strength versus depth for w/c = 0.30 with 1 day of moist curing followed

by: (a) drying at 50 % RH and (b) drying at 70 % RH

0

1

2

3

4

5

6

7

0 10 20 30 40 50

Linear fit

Average

Estimate at 25 mm

95 % Conf. Int. Moist

Tensile Strength, MPa

Depth From Drying Surface, mm

w/c = 0.3, 1 d, 50 % RH (a)

0

1

2

3

4

5

6

7

0 10 20 30 40 50

Linear fit

Average

Estimate at 25 mm

95 % Conf. Int. Moist

Tensile Strength, MPa

Depth From Drying Surface, mm

w/c=0.30, 1 d, 70% RH

Outlier Outlier

(b)
22

strength as a function of depth that was obtained for the continuously moist-cured specimens

(Figure 8(a)). It is clear that there is a difference between the strength-depth relationships of the

continuously moist-cured specimens and those moist cured for 1 d. The average strength,

however, of 4.75 MPa for the specimens with the 1 d of moist curing is within the 95 %

confidence interval of the estimated strength at 25 mm obtained from the continuously moist

cured specimens.

Figure 10(b) shows the results for the w/c = 0.30 specimens exposed to drying at 70 % RH

after 1 d of moist curing. The slope of the best-fit line was also not statistically significant

(p = 0.83). A normal probability plot of the residuals revealed that two points had large

deviations from the straight line defined by the rest of the residuals. These points are indicated as

“outliers” in Figure 10(b), and were not considered in the final analysis. The overall average of

the remaining strengths is 4.70 MPa with a standard error of 0.11 MPa. Therefore, the 95 %

confidence interval of the overall average strength is 4.46 MPa to 4.94 MPa, which overlaps the

95 % confidence interval of 4.7 MPa to 5.05 MPa for the average strength at 25 mm for the

continuously moist-cured specimens.

Comparison of the 95 % confidence intervals of the average strengths for the specimens that

were allowed to dry at 50 % RH with those allowed to dry at 70 % RH shows that they are nearly

the same. Thus there was no statistically significant difference in the results for the two drying

conditions.

3.2.2 w/c = 0.45—Figures 11(a) and (b) show the 28 d strengths for the w/c = 0.45

specimens stored at 50 % and 70 % RH, respectively, after 1 d of moist curing. The results are

similar to those for the w/c = 0.30 specimens. Regression analyses indicated that the slopes were

not statistically significant (p = 0.82 for 50% RH and p = 0.74 for 70 % RH). In addition, there

were no obvious outliers. For drying at 50 % RH, the overall average strength is 3.16 MPa, the

standard error is 0.11 MPa, and the 95 % confidence interval for the average strength is

2.92 MPa to 3.38 MPa. For drying at 70 % RH, the corresponding values are 3.24 MPa,

0.09 MPa, and 3.04 MPa to 3.44 MPa. Since the confidence intervals for the average strengths

overlap, there is no statistically significant difference between the results for the two drying

conditions. Figures 11(a) and 11(b) also show the 95 % confidence intervals for the average

strength versus depth relationship obtained from the continuously moist-cured specimens (Figure

9(a)). While the 1-day moist-cured specimens tend to have a higher average strength than the

estimated strength of 2.94 MPa at 25 mm for the continuously moist-cured specimens, the

overlapping 95 % confidence intervals shown in Figures 11(a) and 11(b) indicate that the

differences are not statistically significant.

In summary, the results for the specimens that were moist cured for 1 d prior to exposure to

drying indicate that sufficient 28 d strength was attained at a depth of 25 mm in comparison with

the specimens that were continuously moist cured. An unexpected result is that the measured

strengths at a depth of 10 mm tended to be greater than the corresponding strengths in the

continuously moist-cured specimens. It was expected that the short period of moist curing

followed by drying would hinder hydration and result in lower strengths compared with

continuous moist curing. However, it is well known that cementitious materials become stronger
23

as they dry out (Bartlett and MacGregor 1994). Thus it is possible that the increase in strength

due to drying compensated for any strength reduction due to reduced hydration. Verification of

this hypothesis would require testing specimens at later ages than used in this exploratory study.

3.3 3 Day Moist-Cured Specimens

Table 6 lists the 28 d tensile strengths for the specimens that were moist cured for 3 d. The

notation is the same as in the previous tables. The missing value at 30 mm for the w/c = 0.30,

Figure 11Tensile strength versus depth for w/c = 0.45 with 1 day of moist curing followed

by: (a) drying at 50 % RH and (b) drying at 70 % RH

0

1

2

3

4

5

6

7

0 10 20 30 40 50

Linear fit

Average

Estimate at 25 mm

95 % Conf. Int. Moist

Tensile Strength, MPa

Depth, mm

w/c = 0.45, 1 d, 50% RH (a)

0

1

2

3

4

5

6

7

0 10 20 30 40 50

Linear fit

Average strength

Estimate at 25 mm

95 % Conf. Int. Moist

Tensile Strength, MPa

Depth, mm

w/c = 0.45, 1 d, 70 % RH (b)
24

50 % RH condition is due to a testing error.

3.3.1 w/c = 0.30—Figure 12(a) shows the individual test results for the w/c = 0.30 specimens

that were stored at 50 % RH. The results show more scatter than the previous results for the 1-

day moist cured specimens. The low strength of 2.79 MPa for the 40 mm depth was considered

Table 6—Tensile Strength Results for Cylinders Moist Cured for 3 Days

w/c Rel. Hum., % Depth, mm Strength, MPa Mean, MPa S.D., MPa C.V.

10 4.03

10 (15) 5.14 4.83 0.70 0.15

10 (15) 5.33

20 (25) 4.15

20 (25) 5.67 5.05 0.80 0.16

20 (25) 4.17

30 3.93

30 4.97 4.45 0.74 0.17

40 4.75

40 2.79† 3.70 0.99 0.27

50

40 (45) 3.56 (4.16)‡ (0.84) (0.20)

10 (15) 5.27

10 5.35 5.31 0.04 0.01

10 5.30

20 (25) 5.44

20 (25) 5.27 5.26 0.18 0.03

20 (25) 5.08

30 (35) 4.92

30 (35) 4.94 5.14 0.36 0.07

30 5.55

40 (45) 5.04

40 4.17† 4.81 0.56 0.12

0.3

70

40 (45) 5.22 (5.13) (0.13) (0.02)

10 3.65

10 3.74 3.68 0.06 0.01

15 3.64

20 3.13

20 3.41 3.28 0.14 0.04

20 3.29

35 2.95

35 3.02 3.10 0.20 0.06

35 3.32

45 3.17

40 3.46 3.30 0.15 0.05

50

45 3.26

10 3.94

10 3.86 3.74 0.27 0.07

15 3.43

20 3.30

20 3.79 3.60 0.27 0.07

20 3.72

35 3.37

35 3.24 3.32 0.07 0.02

35 3.34

45 3.49

40 3.43 3.33 0.23 0.07

0.45

70

45 3.06

†Outlier

‡Values in parentheses are with outliers removed
25

to be an outlier and was excluded from the analysis. That specimen was found to have a large air

void within the failed cross section (Meeks 1997). Regression analysis showed that the slope was

not statistically significant (p = 0.39). The overall average tensile strength is 4.57 MPa with a

standard error of 0.22 MPa. The 95 % confidence interval for the average strength is 4.07 MPa to

5.07 MPa. Figure 12(a) shows that the confidence interval for the overall mean overlaps the

confidence interval for the estimated strength at 25 mm based on the continuously moist-cured

specimens.

Figure 12(b) shows the individual test results for the specimens stored at 70 % RH. The data

are less scattered. The low strength of 4.17 MPa for the 40 mm depth was considered to be an

Figure 12Tensile strength versus depth for w/c = 0.30 with 3 d of moist curing followed

by: (a) drying at 50 % RH and (b) drying at 70 % RH

0

1

2

3

4

5

6

7

0 10 20 30 40 50

Linear fit

Average Strength

Estimate at 25 mm

95 % Conf. Int. Moist

Tensile Strength, MPa

Depth From Drying Surface, mm

w/c = 0.30, 3 d, 50 % RH (a)

Outlier

0

1

2

3

4

5

6

7

0 10 20 30 40 50

Linear fit

Average Strength, MPa

Estimate at 25 mm

95 % Conf. Int. Moist

Tensile Strength, MPa

Depth From Drying Surface, mm

w/c = 0.30, 3 d, 70 % RH (b)

Outlier
26

outlier and excluded from the analysis. It also had a large air void within the failure cross section

(Meeks 1997). Again, the slope is not statistically significant (p = 0.16), and the overall average

tensile strength is 5.22 MPa with a standard error of 0.06 MPa. The 95 % confidence interval for

the average strength is 5.08 MPa to 5.35 MPa, which does not overlap the interval for the

estimated average strength at 25 mm obtained from the continuously moist-cured specimens.

Thus these results show that the w/c = 0.30 specimens exposed to 70 % RH after 3 d of moist

curing had higher 28 d strength at 25 mm than the specimens that were moist cured continuously.

Figure 13Tensile strength versus depth for w/c = 0.45 with 3 d of moist curing followed

by: (a) drying at 50 % RH and (b) drying at 70 % RH

0

1

2

3

4

5

6

7

0 10 20 30 40 50

Linear fit

Estimate at 25 mm

95 % Conf. Int. Moist

Tensile Strength, MPa

Depth From Drying Surface, mm

w/c = 0.45, 3 d, 50 % RH (a)

0

1

2

3

4

5

6

7

0 10 20 30 40 50

Linear fit

Estimate at 25 mm

95 % Conf. Int. Moist

Tensile Strength, MPa

Depth From Drying Surface, mm

w/c = 0.45, 3 d, 70 % RH (b)
27

Statistical comparison of the overall average tensile strength for the specimens stored at 50 %

RH with those stored at 70 % RH indicated a statistically significant difference (p = 0.008). Thus

the specimens allowed to dry at 50 % RH were, on average, weaker.

3.3.2 w/c = 0.45—Figure 13(a) shows the 28 d tensile strengths for the w/c = 0.45 specimens

stored at 50 % RH. In contrast with the previous results, regression analysis indicated that that

the slope was statistically significant (p = 0.021). The slope, however, was negative and strength

decreased with depth. From the best-fit line, the estimated average strength at 25 mm is

3.36 MPa, with a 95 % confidence interval of 3.23 MPa to 3.49 MPa. As seen in Figure 13(a),

the 28 d average strength at 25 mm is higher for the specimens allowed to dry after 3 d of curing

than for the continuously moist-cured specimens.

Figure 13(b) shows similar results for the specimens allowed to dry at 70 % RH. The

negative slope is statistically significant (p = 0.012), and the estimated average strength at

25 mm is 3.52 MPa with a 95 % confidence interval of 3.39 MPa to 3.65 MPa.

The confidence intervals of the estimated strength at 25 mm for the two drying conditions

overlap. Thus there is no statistically significant difference due to relative humidity.

Examination of Figures 13(a) and 13(b) shows that the statistically significant negative

slopes are due primarily to the slightly higher strengths at 10 mm compared with the strengths at

the other depths. These higher strengths are attributed, again, to the increase in strength due to

drying.

3.4 7 Day Moist-Cured Specimens

Table 7 lists the 28 d strengths of the specimens that were moist cured for 7 d prior to drying.

There are no missing data points, and no points were identified as outliers.

3.4.1 w/c = 0.30—Figure 14(a) shows the results for the w/c = 0.30 specimens dried at 50 %

RH. Linear regression indicated that the slope was not statistically significant (p = 0.65). The

overall average tensile strength is 5.13 MPa, the standard error is 0.12 MPa, and the 95 %

confidence interval is 4.87 MPa to 5.40 MPa. This interval overlaps the confidence interval of

the estimated strength at 25 mm for the continuously moist-cured specimens. Thus the strength at

25 mm for the 7-day cured specimens is similar to that of the continuously moist cured

specimens.

Figure 14(b) shows the results for the specimens dried at 70 % RH. The behavior differed

from the other cases. The strengths of the specimens with reduced sections at 10-mm were lower

than similar w/c = 0.30 specimens subjected to drying. These strengths, however, are in good

agreement with those obtained from the continuously moist cured specimens. Therefore, the

lower strength could be due to the “top-to-bottom” effect if the drying front did not penetrate

beyond 10 mm. There is, however, some uncertainty about the strengths of the two specimens

that failed at 10 mm (Meeks 1997). When they were originally tested, failure occurred near the

steel-epoxy interface. New steel discs were bonded to the ends of the specimens and the
28

specimens were re-tested. Some damage could have occurred during the first loading. Thus it is

not possible to state with certainty whether the strength at 10 mm is lower in this case compared

with the other w/c = 0.30 specimens exposed to drying. If the measured strengths at 10 mm are

assumed to be valid values, the slope of the best-fit line is statistically significant (p = 0.007).

The estimated average strength at 25 mm is 4.61 MPa with a 95 % confidence interval of

Table 7—Tensile Strength Results for Cylinders Moist Cured for 7 Days

w/c Rel. Hum., % Depth, mm Strength, MPa Mean, MPa S.D., MPa C.V.

10 (15) 4.85

10 (15) 5.22 5.03 0.19 0.04

10 (15) 5.01

20 5.64

20 4.73 5.37 0.55 0.10

20 (25) 5.73

30 5.54

30 (35) 5.67 5.23 0.65 0.12

30 (35) 4.48

40 5.02

40 (45) 4.92 4.91 0.11 0.02

50

40 4.80

10 (15) 4.31

10 3.92 4.07 0.21 0.05

10 3.98

20 (25) 4.99

20 (25) 4.66 5.00 0.34 0.07

20 (25) 5.34

30 (35) 4.63

30 (35) 4.71 4.64 0.07 0.02

30 (35) 4.57

40 5.17

40 4.95 5.05 0.11 0.02

0.3

70

40 5.04

10 (15) 3.72

10 (15) 3.65 3.61 0.13 0.04

10 (15) 3.47

20 (25) 3.02

20 (25) 2.82 3.00 0.17 0.06

20 (25) 3.15

30 (35) 3.38

30 3.37 3.39 0.02 0.01

30 (35) 3.41

40 2.88

40 3.01 3.03 0.16 0.05

50

40 3.19

10 (15) 3.70

10 3.36 3.63 0.24 0.07

10 3.83

20 3.13

20 3.09 3.02 0.15 0.05

20 2.85

30 (35) 3.49

30 3.21 3.29 0.17 0.05

30 3.18

40 (45) 3.11

40 (45) 3.71 3.37 0.31 0.09

0.45

70

40 3.28
29

4.39 MPa to 4.81 MPa. Figure 14(b) shows that this interval overlaps the interval for the

estimated average strength based on the continuously moist-cured specimens.

3.4.2 w/c = 0.45—Figure 15(a) shows the 28 d tensile strength for the w/c = 0.45 specimens

exposed to drying at 50% RH after 7 d of moist curing. The best-fit line has a slope that is on the

borderline of being statistically significant (p = 0.058). If the line is assumed to represent the

variation of strength with depth, the estimated strength at 25 mm is 3.31 MPa with a 95 %

confidence interval of 3.14 MPa to 3.48 MPa. As seen in Figure 15(a), this confidence interval

lies slightly above the interval for the estimated average strength at 25 mm for the continuously

moist-cured specimens. Note also that the three measured strengths at 25 mm are in good

agreement with the confidence interval of the moist-cured specimens.

Figure 14Tensile strength versus depth for w/c = 0.30 with 7 d of moist curing followed

by: (a) drying at 50 % RH and (b) drying at 70 % RH

0

1

2

3

4

5

6

7

0 10 20 30 40 50

Linear fit

Average strength

Estimate at 25 mm

95 % Conf. Int. Moist

Tensile Strength, MPa

Depth From Drying Surface, mm

w/c = 0.30, 7d, 50 % RH (a)

0

1

2

3

4

5

6

7

0 10 20 30 40 50

Linear fit

Estimate at 25 mm

95 % Conf. Int. Moist

Tensile Strength, MPa

Depth From Drying Surface, mm

w/c = 0.30, 7 d, 70 % RH (b)
30

Finally, Figure 15(b) shows the results for the specimens exposed to drying at 70 % RH. The

slope of the best-fit line was not statistically significant (p = 0.75). The overall average strength

is 3.33 MPa, the standard error is 0.09 MPa, and the 95 % confidence interval is 3.14 MPa to

3.52 MPa. This confidence interval is also slightly higher than the interval for the strength at

25 mm for continuous moist curing.

The similarity in the confidence intervals for the estimated average strength at 25 mm for the

two drying conditions shows that relative humidity did not have a significant effect.

Figure 15Tensile strength versus depth for w/c = 0.45 with 7 d of moist curing followed

by: (a) drying at 50 % RH and (b) drying at 70 % RH

0

1

2

3

4

5

6

7

0 10 20 30 40 50

Linear fit

Estimate at 25 mm

95 % Conf. Int. Moist

Tensile Strength, MPa

Depth From Drying Surface, mm

w/c = 0.45, 7 d, 50 % RH (a)

0

1

2

3

4

5

6

7

0 10 20 30 40 50

Linear fit

Average strength

Estimate at 25 mm

95 % Conf. Lim. Moist

Tensile Strength, MPa

Depth From Drying Surface, mm

w/c = 0.45, 7 d, 70 % RH (b)
31

4. SUMMARY

This exploratory study examined the influence of the duration of moist curing on the

variation of 28 d strength with distance from the drying surface. The objective was to determine

the duration of moist curing so that the strength at 25 mm from the exposed surface would not be

less than the corresponding strength for the case of continuous moist curing (reference

condition). Table 8 summarizes the 95 % confidence intervals for the estimated, average 28 d

strength at 25 mm from the exposed surface. The last column of Table 8 indicates the nature of

the measured relationships between strength and depth. A “+” indicates that the strength

increased with depth, a “-” indicates that strength decreased with depth, and a “0” indicates no

relationship between strength and depth.

Figure 16 is a graphical representation of the confidence intervals in Table 8. The horizontal

dashed lines are the 95 % confidence intervals for the estimated strength at 25 mm obtained from

the continuously moist-cured specimens. The results show that for the various treatments in no

case was the strength at 25 mm significantly lower (from a statistical viewpoint) than for the case

of continuous moist curing. In fact in some cases, the strength was greater than for the reference

Table 8—Summary of Estimated 95 % Confidence Intervals of Average Strength at

25 mm

w/c Moist Curing, d RH, % Average Strength,

MPa

Strength vs. Depth†

Continuous 4.88 ±0.17 +

1 50 4.75 ±0.24 0

70 4.70 ±0.24 0

3 50 4.57 ±0.50 0

70 5.22 ± 0.14 0

50 5.13 ±0.27 0

0.30

7

70 4.61 ±0.21 +

Continuous 2.94 ±0.14 +

1 50 3.16 ±0.23 0

70 3.24 ±0.20 0

3 50 3.36 ±0.13 -

70 3.52 ±0.13 -

50 3.31 ±0.17 -

0.45

7

70 3.33 ±0.19 0

† “0” indicates no relationship between strength and distance from the drying surface,

“+” indicates strength increased with depth, and “-” indicates that strength decreased with depth
32

condition. These results were unexpected, especially for the 1-day duration of moist curing. It

was anticipated that a short duration of moist curing would result in rapid drying and a decrease

in the hydration rate. As explained in the previous section, the increase in strength due to the

physical process of drying may have compensated for any reduction due to a decrease in the

degree of hydration. Figure 16 also shows that the relative humidity during the drying period did

not have a consistent and statistically significant effect.

Additional insight into the reason for the observed behavior summarized in Figure 16 is

given in Table 9. As mentioned, some of the specimens were split after tensile testing to visually

observe the depth of the drying front. While these measurements are approximate, the results

appear to show that, for the testing period of 28 d used in this study, the drying front never

penetrated to a distance of 25 mm.

Based on these tests of mortar specimens, it could be concluded that only 1 d of moist curing

is sufficient to ensure adequate strength development at 25 mm from the exposed surface for the

water-cement ratios and drying conditions that were used. These results may explain why

extended curing has typically not been considered a major concern with regard to structural

capacity of reinforced concrete.

Table 9—Observed Approximate Depth of Drying Front (Meeks 1997)

Moist w/c = 0.30 w/c = 0.45

Curing, d 50 % RH 70 % RH 50 % RH 70 % RH

1 14 mm 11 mm NM 14 mm

3 10 mm 8 mm NM 12 mm

7 NM* 6 mm 14 mm 12 mm

*NM = not measured

Figure 16The 95 % confidence intervals of the estimated average strength at a depth of

25 mm versus the duration of moist curing prior to exposure to drying

1

2

3

4

5

6

012345678

Continuous moist curing

50 % RH

70 % RH

Estimated Average Tensile Strength

at 25 mm, MPa

Time of Moist Curing, days
33

Since the study focused on the effects of duration of moist curing on strength, the results

should not be extrapolated to mean that short curing periods are also adequate for durability. The

mass losses of the mortar specimens during drying provide some insight into the effects of curing

duration on potential durability. Parrot (1992, 1996) demonstrated that moisture loss during the

first 4 d after termination of moist curing was a good indicator of subsequent durability

properties, such as water sorptivity. In this study, the moisture loss at 5 d after the end of moist

curing is used as a basis for comparison. The 5 d mass loss is used because this was the common

drying time for which all specimens had a moisture loss measurement (Meeks 1997). Figure

17(a) shows the 5 d mass loss for all the test specimens. Mass loss is expressed in terms of

reduction in mass per unit area of exposed surface. One of the data points was identified as an

Figure 17Moisture loss after 5 d of drying versus the duration of the moist curing period:

(a) all data and (b) 95 % confidence intervals of the average mass loss

0

0.5

1

1.5

2

2.5

012345678

w/c = 0.30, 50 % RH

w/c = 0.30, 70 % RH

w/c = 0.45, 50 % RH

w/c = 0.45, 70 % RH

Moisture Loss, 5d of drying, kg/m2

Moist Curing Time, days

(a)

Outlier

0

0.5

1

1.5

2

012345678

w/c = 0.30, 50 % RH

w/c = 0.30, 70 % RH

w/c = 0.45, 50 % RH

w/c = 0.45, 70 % RH

Moisture Loss, 5 d of drying, kg/m2

Moist Curing Time, days

(b)
34

outlier, as indicated in Figure 17(a). Figure 17(b) shows the average 5 d mass loss per unit area

and the corresponding 95 % confidence intervals. Several observations can be made:

• The average moisture losses for drying at 50 % RH were greater than for drying at 70 %

RH.

• The greater the duration of moist curing, the lower the moisture loss.

• An increase in the duration of moist curing from 1 d to 3 d resulted in a drastic decrease

in the moisture loss for the w/c = 0.45 specimens.

The first two observations are not very revealing, as these are well known phenomena. The

third observation, however, is of particular interest because it shows that, from a durability

viewpoint, mixtures with higher w/c benefit more from extended moist curing than mixtures with

lower w/c. This is not surprising, because more than 40 years ago Powers et al. (1959) noted that

as the w/c increases more hydration is required to disrupt the continuity of capillary pores.

Similar conclusions about curing duration were reached for the durability-related curing criteria

of Hilsdorf and co-workers (Hilsdorf and Burieke 1992; Hilsdorf 1995).

As is common with exploratory studies, this work raised questions that merit further

investigation. Tests are underway to repeat these experiments, but using measurement of degree

of hydration as a function of depth. This will avoid the problem of increasing strength due to

drying that may have confounded the results reported here. Similar studies were conducted by

Kern et al. (1995) for concrete with w/c = 0.60 and ages up to 7 d. The results from these new

tests should provide a clearer understanding of the effects of the duration of curing and

subsequent exposure conditions on the variation of degree of hydration with distance from the

exposed surface. The investigation will be extended to include mixtures containing

supplementary cementitious materials. The long-term objective of the research is to develop a

verified numerical model that will permit investigation of the many factors that affect the

required duration of curing. The results of that investigation should provide the knowledge to

formulate rational curing guidelines for high-performance concrete.
35

5. REFERENCES

ACI 318, 1995, “Building Code Requirements for Structural Concrete (ACI 318-95) and

Commentary (ACI 318R-95),” American Concrete Institute, Farmington Hills, MI.

ACI 308, 1998, “Standard Specification for Curing Concrete (ACI 308.1-98),”American

Concrete Institute, Farmington Hills, MI.

ASTM C 305, “Practice for Mechanical Mixing of Hydraulic Cement Pastes and Mortars of

Plastic Consistency,” 1998 Annual Book of ASTM Standards, Vol. 04.01, ASTM, West

Conshohocken, PA.

ASTM C 778, “Specification for Standard Sand,” 1998 Annual Book of ASTM Standards, Vol.

04.01, ASTM, West Conshohocken, PA.

Bartlett, F. M. and MacGregor, J. G., 1994, “Effect of Moisture on Concrete Core Strengths,”

ACI Materials Journal, Vol. 91, No. 3, May-June, pp. 227-236.

Bartlett, F. M. and MacGregor, J. G., 1999, “Variation of In-Place Concrete Strength in

Structures,” ACI Materials Journal, Vol. 96, No. 2, March-April, pp. 261-270.

Bentz, D.P., 1997, “Three-Dimensional Computer Simulation of Portland Cement Hydration and

Microstructure Development,” Journal of the American Ceramic Society, Vol. 80, No. 1, pp.

3-21.

Carino, N. J., Lew, H. S., and Volz, C. K., 1983, “Early Age Temperature Effects on Concrete

Strength Prediction by the Maturity Method,” Journal of the American Concrete Institute,

Vol. 8-0, No. 2, March-April, pp. 93-101.

Carino, N. J. and Clifton, J. R., 1990, “Outline of a National plan on High-Performance

Concrete: Report on the NIST/ACI Workshop, May 16-18, 1990,” NISTIR 4465, Nat. Inst.

of Stds. and Tech., Gaithersburg, MD.

Carino, N. J., Knab, L. I., and Clifton, J. R., 1992, “Applicability of the Maturity Method to

High-Performance Concrete,” NISTIR 4819, Nat. Inst. of Stds. and Tech., Gaithersburg,

MD.

Gjorv, O. E., 1991, “Norwegian Experience with Condensed Silica Fume in Concrete,”

CANMET/ACI International Workshop on the Use of Silica Fume in Concrete, April 7-9,

1991, Washington, DC, V.M. Malhotra, Ed., pp. 47-64.

Goodspeed, C. H., Vanikar, S., and Cook, R. A., 1996, “High-Performance Concrete Defined for

Highway Structures,” Concrete International, Vol. 18, No. 2, February, pp. 62-67.

Hasni, L., Gallias, J. L., and Salomon, M., 1994, “Influence of the Curing Method on the

Durability of High Performance Concretes,” Durability of Concrete, Third International

Conference Nice, France, SP-145, V.M. Malhotra, Ed., American Concrete Institute,

Farmington Hills, MI, pp. 131-155.

Hilsdorf, H. K., 1995, “Criteria for the Duration of Curing,” Proceedings of the Adam Neville

Symposium on Concrete Technology, Las Vegas, June 12, 1995, V.M. Malhotra, Ed., pp.

129-146.

Hilsdorf, H. K. and Burieke, F., 1992, “A Note on EN 206 - Curing Requirements,” Second

Draft, March, 34 pp. (provided by H. K. Hilsdorf)

Ho, D.W.S. and Lewis, R.K., 1988, “The Specification of Concrete for Reinforcement

Protection—Performance Criteria and Compliance by Strength,” Cement and Concrete

Research, Vol. 18, No. 4, July, pp. 584-594.
36

Kern, R., Cervinka, S., and Weber, R., 1995, “Efficiency of Curing Methods,” Darmstadt

Concrete, Vol. 10, pp. 117-122.

Meeks, K. W., 1997, The Curing of High Performance Concrete,” Ph.D. Dissertation, The

George Washington University, September 1997

Meeks, K. W. and Carino, N. J., 1999, “Curing of High-Performance Concrete: Report of the

State-of-the-Art,” NISTIR 6295, Nat. Inst. of Stds. and Tech., Gaithersburg, MD.

Parrott, L. J., 1992, “Water Absorption in Cover Concrete,” Materials and Structures, Vol. 25,

No. 149, June, pp. 284-292.

Parrott, L. J., 1996, “Some Effects of Cement and Curing Upon Carbonation and Reinforcement

Corrosion in Concrete,” Materials and Structures, Vol. 29, No. 187, April, pp. 164-173.

Powers, T. C., Copeland, L. E., and Mann, H. M., 1959, “Capillary Continuity or Discontinuity

in Cement Pastes,” Journal of the Research and Development Laboratories, PCA, Vol. 1,

No. 2, May, pp. 38-48.

Price, W. H., 1951, “Factors Influencing Concrete Strength,” Journal of the American Concrete

Institute, Vol. 22, No. 6, February, pp. 417-432.

Russell, H.G., 1999, “ACI Defines High-Performance Concrete,” Concrete International, Vol.

21, No. 2, February, pp. 56-57.

Torii, K. and Kawamura, M., 1994, “Mechanical and Durability-Related Properties of HighStrength Concrete Containing Silica Fume,” High-Performance Concrete, Proceedings, ACI

International Conference, SP-149, V.M. Malhotra, Ed., American Concrete Institute,

Farmington Hills, MI, pp. 461-474.

Zia, P., Leming, M. L., and Ahmad, S. H., 1991, “High Performance ConcreteA State- of-theArt Report,” SHRP-C/FR-91-103, Strategic Highway Research Program, National Research

Council, Washington, DC, 246 pp.