Construction

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Construction and Building Materials 21 (2007) 1282–1287

and Building

MATERIALS

Self-curing concrete: Water retention, hydration and moisture transport

A.S. El-Dieb *

Department of Structural Engineering, Faculty of Engineering, Ain Shams University, 1 El-Sarayat St., Abbasia 11517, Cairo, Egypt

Received 5 May 2005; received in revised form 7 February 2006; accepted 19 February 2006Available online 1 September 2006

Abstract

Water retention of concrete containing self-curing agents is investigated. Concrete weight loss, and internal relative humidity measure-ments with time were carried out, in order to evaluate the water retention of self-curing concrete. Non-evaporable water at different ageswas measured to evaluate the hydration. Water transport through concrete is evaluated by measuring absorption%, permeable voids%,water sorptivity, and water permeability. The water transport through self-curing concrete is evaluated with age. The effect of the con-crete mix proportions on the performance of self-curing concrete were investigated, such as, cement content and w/c ratio.� 2006 Elsevier Ltd. All rights reserved.

Keywords: Self-curing concrete; Water retention; Relative humidity; Hydration; Absorption; Permeable pores; Sorptivity; Water permeability

1. Introduction

Curing of concrete is maintaining satisfactory moisturecontent in concrete during its early stages in order todevelop the desired properties. However, good curing isnot always practical in many cases. Several investigatorsasked the question whether there will be self-curing con-crete [1,2]. Therefore, the need to develop self-curing agentsattracted several researchers [3]. The concept of self-curingagents is to reduce the water evaporation from concrete,and hence increase the water retention capacity of the con-crete compared to conventional concrete [4,5]. It was foundthat water soluble polymers can be used as self-curingagents in concrete [5].

Concrete incorporating self-curing agents will representa new trend in the concrete construction in the new millen-nium [3]. Curing of concrete plays a major role in develop-ing the concrete microstructure and pore structure, andhence improves its durability and performance. The con-cept of self-curing agents is to reduce the water evaporationfrom concrete, and hence increase the water retention

0950-0618/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.conbuildmat.2006.02.007

* Tel.: +202 6858377; fax: +202 6850617.E-mail address: amr_eldieb@hotmail.com.

capacity of the concrete compared to conventional con-crete [4,5].

The aim of the investigation is to evaluate the use ofwater-soluble polymeric glycol as self-curing agent. Theuse of self-curing admixtures is very important from thepoint of view that water resources are getting valuableevery day (i.e., each 1 m3 of concrete requires about 3 m3

of water for construction most of which is for curing).The benefit of self-curing admixtures is more significantin desert areas where water is not adequately available.

In this study water retention and hydration of concretecontaining self-curing agents is investigated and comparedto conventional concrete. Also, water transport throughthis concrete is evaluated and compared to conventionalconcrete continuously moist-cured and air-cured. Concreteweight loss and internal relative humidity measurementswith time were carried out in order to evaluate the waterretention ability. Non-evaporable water at different ageswas measured to evaluate the hydration of self-curing con-crete. The water transport, as durability index [6–10], isevaluated by measuring water absorption%, permeablevoids%, water sorptivity and water permeability. Briefdescription of tests and specimens is given in Section 2.2.The parameters included in the study were mainly thecement content and the w/c ratio.

Fig. 1. Internal relative humidity set-up.

A.S. El-Dieb / Construction and Building Materials 21 (2007) 1282–1287 1283

2. Experimental work

2.1. Materials and concrete mixes

The main constituent variable parameters in this studywere the cement content and the w/c ratio. Table 1 givesthe details for the mixes used in the study. For each cementcontent and w/c ratio, two concrete mixes were cast; onewhich includes the self-curing agent and the other is con-ventional mix. A total of eight mixes were used in thisinvestigation. The initial slump for all the conventionalconcrete mixes was kept constant (about 90–120 mm) usingvariable dosage of high-range water reducer-retardingadmixture (Type G). The admixture dosage was kept con-stant for concrete mixes when self-curing agent was used.For evaluating water transport, two curing regimes wereused for conventional concrete mixes without self-curingagent; continuously moist-curing under water, and air-curing.

The cement used was ordinary Portland cement. Thecoarse aggregate was crushed stone with two sizes; S1 (5–20 mm particle size) and S2 (10–25 mm particle size). Thetwo coarse aggregate sizes were mixed with a 1:1 ratio.The sand used was natural sand with fineness modulus of2.58; the percentage of the sand was 32% of the total aggre-gate weight.

The self-curing agent used in the study was water solublepolymeric glycol (i.e., polyethylene-glycol) [3,5]. The dos-age of the self-curing agent was kept constant for all theself-curing concrete mixes. The dosage was 0.02% byweight of the cement.

2.2. Specimens and testing

Concrete weight loss was carried out by filling polypro-pylene containers of capacity 1.5 l, with internal diameter120 mm and height 130 mm, with concrete. The containerswere kept at constant temperature of about 25 �C and rel-ative humidity environment of about 65%. The weight ofthe containers was measured after casting and at severalintervals to determine the weight loss with time. Measure-ment of the weight was carried-out till 28 days of age. Twospecimens were used for each mix and the average valuesare used in the discussion.

A cube specimen of dimensions 158 · 158 · 158 mm wascast from each mix. The cubes were cured in the moulds for24 h. After de-moulding a hole of diameter 20 mm anddepth 100 mm was drilled in each cube from the top face

Table 1Concrete mixes

Concrete mix type Self-curing concrete

Cement content (kg/m3) 350 450w/c Ratio 0.3 0.4 0.3Mix I.D. Self-curingCuring I.D. Self-curing (no curing)

of the cube. The hole was then cleaned using air jet toremove any loose particles. The hole was then sealed usinga rubber stopper. The cube was then sealed from the sur-rounding environment using wax film. A digital relativehumidity probe was used to measure the relative humidityinside the cube at several time intervals up to 91 days ofage; a one-hole rubber stopper was used to seal the humid-ity sensor into the concrete block. The probe was keptinside the whole for about 2–3 h before taking the measure-ments. Measurement of the relative humidity took about20–30 s to stabilize. The holes were kept sealed using thesolid rubber stopper when not being in use to measurethe internal relative humidity. Fig. 1 shows the set-up formeasuring the internal relative humidity. Duplicate speci-mens were prepared for each mix and the average resultsare used in the discussions.

The non-evaporable water content was carried out onconcrete specimens cast from the mixes. The specimenswere left in air (i.e., under drying condition). The non-evaporable water was determined at several time intervalsup to 28 days of age. At each age a concrete specimen fromeach mix was crushed and a cement paste sample wasobtained for the test by sieving crushed concrete sampleto remove aggregate particles. The samples were kept inporpan-ol2 to stop hydration until testing. The propan-

Conventional concrete

350 4500.4 0.3 0.4 0.3 0.4

Conv.Moist-curing Air-curing

1284 A.S. El-Dieb / Construction and Building Materials 21 (2007) 1282–1287

ol2 was dried off before testing (i.e., the specimens weredried in a 105 �C oven). The non-evaporable water wasdetermined as the weight loss after burning in a muffle fur-nace at 1050 �C. The non-evaporable water was calculatedas the sample weight loss to the sample weight (g/g). Rep-licate samples were used for each mix and at each test ageand the average values are used in discussions.

Water absorption% and permeable pores% tests wereconducted according to ASTM C-642. A concrete disc ofdiameter 100 mm and height 50 mm was cut from a cylin-der and used for testing. Water absorption and permeablepore tests were conducted at 28 day of age. The test wasconducted on replicates and the average values arereported and used for discussion.

Water sorptivity test was carried out for measuring rateof absorption of hydraulic cement concretes [11]. The spec-imens used were discs of diameter 100 mm and height50 mm cut from a cylinder. The specimens were oven driedat 110 �C for 24 h, and then the specimens were left to coolin dry condition for the following 24 h. The test was carriedout by allowing one surface of the specimen to be in con-tact with water of 5 mm depth using a circular aluminumsupport as shown in Fig. 2. Using the supporting frameand keeping the outside water level at 1–3 mm above thealuminum support allows continuous contact between the

Fig. 2. Water sorptivity set-up.

0

10

20

30

40

50

60

70

0 8 16 24 32 40Time (days)

Wei

ght L

oss

(gm

)

Self-Curing

Conventional

w/c = 0.4

w/c = 0.3

w/c = 0.3

Cement Content = 350 kg/m3

w/c = 0.4

Fig. 3. Weight loss with time for sel

specimen surface and the water without changing the waterdepth during the test time. The sides of the test specimenswere sealed with electric vinyl tape to create unidirectionalflow through the concrete specimen. The weight of thespecimen was recorded at fixed time intervals with a totaltime of 25 min [11–13]. The sorptivity test was conductedat 28 days and 56 days of age on duplicate specimens foreach mix.

The water permeability test was conducted using con-stant water pressure head during a constant time period.The water inflow was measured and the water coefficientof permeability (m/s) was calculated. The water permeabil-ity test was conducted on saturated concrete specimens ofdiameter 100 mm and height 50 mm cut from concrete cyl-inders. The specimens were saturated using vacuum satura-tion before testing. The water permeability coefficient (m/s)was measured at 7, 14, 28 and 56 days of age for each mixon duplicate specimens fro each mix and for each age.

3. Test results and discussion

3.1. Water retention

The weight loss with time due to the moisture evapora-tion was found to be less for the self-curing mixes than thatfor the conventional mixes. This indicates better waterretention for self-curing mixes. Fig. 3 shows the weight losswith time for all the mixes. The weight loss for the concretemixes with w/c ratio 0.4 was greater than that for concretemixes with w/c ratio 0.3 for both cement contents. Also,the weight loss for the concrete mixes with cement content450 kg/m3 was slightly higher than that for concrete mixeswith cement content 350 kg/m3.

Fig. 4 shows the internal relative humidity for the self-curing and conventional concrete with time. The cementcontent and the w/c ratio have a significant effect on theinternal relative humidity of the concrete whether self-cur-ing or conventional mixes, this confirms with the findingspreviously concluded for conventional concrete mixes[14,15]. For the concrete mixes with cement content350 kg/m3, the internal relative humidity for the self-curing

Time (days)

Wei

ght L

oss

(gm

)

0

10

20

30

40

50

60

70

0 8 16 24 32 40

Self -Curing

Conventional

w/c = 0.4

w/c = 0.3

w/c = 0.4

w/c = 0.3

Cement Content = 450 kg /m3

f-curing and conventional mixes.

C.C. 350 kg /m3

75

80

85

90

95

100

0 20 40 60 80 100 120

Self-CuringConventional

w/c = 0.3w/c = 0.4

w/c = 0.3

w/c = 0.4

C.C. 450 kg/m 3

75

80

85

90

95

100

0 20 40 60 80 100 120

Rel

ativ

e H

umid

ity (

%)

Rel

ativ

e H

umid

ity (

%)

Self-Curing

Conventional

w/c = 0.4

w/c = 0.3w/c = 0.4

w/c = 0.3

Time (days) Time (days)

Fig. 4. Internal relative humidity with time for self-curing and conventional mixes.

A.S. El-Dieb / Construction and Building Materials 21 (2007) 1282–1287 1285

mixes was slightly higher than 85% after 91 days, andbelow 85% for the conventional mixes. For the concretemixes with cement content 450 kg/m3, the internal relativehumidity was below 85% for the self-curing mixes while itwas below 80% for conventional mixes. This shows thatthe self-desiccation is more pronounced for the conven-tional mixes compared to the self-curing mixes which couldhave a direct impact on the hydration of the cement.

3.2. Hydration

The non-evaporable water (Wn) measured on unsealedspecimens (i.e., under drying condition) at different timesfor self-curing and conventional concrete mixes is shownin Fig. 5. It could be seen that self-curing concrete withits ability to retain water resulted in higher non-evaporablewater which in turn imply higher degree of hydration. Theeffect is affected by the mix proportions as found from theresults of the measurement of the weight loss and the inter-nal relative humidity.

3.3. Water absorption and permeable pores

Fig. 6 shows water absorption% and permeable pores%for the self-curing and conventional concretes with different

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.2

0 5 10 15 20 25 30 35 40

Time (days)

Wn

(gm

/gm

)

Self-Curing

Conventional

C.C. 350 kg/m3w/c = 0.4

w/c = 0.4w/c = 0.3

w/c = 0.3

Fig. 5. Non-evaporable water versus time f

cement contents and w/c ratios. The water absorption%and the permeable pores% were found to be slightly higherfor self-curing concrete than those for continuously moist-cured conventional concrete. On the other hand, the self-curing concrete showed lower water absorption% and per-meable pores% compared to air-cured conventional con-crete. This indicates that self-curing concrete developslower permeable pores% compared to the air-cured con-ventional concrete. This could be attributed to the waterretention of the self-curing concrete and the continuationof hydration compared to the air-cured conventionalconcrete.

3.4. Water sorptivity

The water sorptivity was measured at two ages, 28days and 56 days of age, in order to study the effect ofself-curing on the development of the capillary pores,and the capillary water suction of the concrete. Fig. 7shows the water sorptivity for the self-curing and the con-ventional concrete with its two curing regimes at 28 daysand 56 days of age respectively. For the 450 kg/m3 cementcontent conventional concrete mix, the water sorptivityvalues at both ages were not significantly reduced whenw/c ratio was reduced from 0.4 to 0.3 for the continuously

Time (days)

Wn

(gm

/gm

)

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.2

0 5 10 15 20 25 30 35 40

Self -Curing

Conventional

C.C. 450 kg/m 3

w/c = 0.3

w/c = 0.4w/c = 0.3

w/c = 0.4

or self-curing and conventional mixes.

0

0.05

0.1

0.15

0.2

0.25

0.3

0.4 0.3 0.4 0.3w/c Ratio w/c Ratio

Moist -Curing Self -Curing Air-Curing

C.C. 450 kg /m3C.C. 350 kg /m3

0

0.05

0.1

0.15

0.2

0.25

0.3

0.4 0.3 0.4 0.3

56-

Day

Sor

ptiv

ity(m

m/m

in1/

2 )

56-

Day

Sor

ptiv

ity(m

m/m

in1/

2 )

Moist -Curing Self -Curin g Air-Curing

C.C. 450 kg/m3C.C. 350 kg/m3

Fig. 7. Water sorptivity at 28 days and 56 days of age for self-curing and conventional mixes.

0

1

2

3

4

5

6

7

8

0.4 0.3 0.4 0.3w/c Rati o

0

2

4

6

8

10

12

14

16

18

20

28-

DP

ores

%

28-

DP

ores

%

Moist -Curing Self -Curing Air -Curing

C.C. 350 kg/m3

Absorption % Pores %

0

1

2

3

4

5

6

7

8

0.4 0.3 0.4 0.3w/c Ratio

28-

DA

bsor

.%

0

2

4

6

8

10

12

14

16

18

20

28-

DP

ores

%

Moist -Curing Self -Curing Air-Curing

C.C. 450 kg /m3

Absorption % Pores %

Fig. 6. Water absorption% and permeable pores%.

1286 A.S. El-Dieb / Construction and Building Materials 21 (2007) 1282–1287

moist-cured regime. For the continuously moist-curedconventional concrete mix with 350 kg/m3 cement con-tent, reducing the w/c ratio resulted in a water sorptivityvalue very close to that of similar conventional moist-cured mix with cement content 450 kg/m3 and w/c of0.4. This trend was found at both ages. The water sorptiv-ity values for the self-curing concrete were higher than

0

1E-11

2E-11

3E-11

4E-11

5E-11

7 14 28 56 7 14 28 56Age (days)

Moist-Curing Self-Curing Air -Curing

C.C. 350 kg/m 3

w/c = 0.3w/c = 0.4

Per

m. C

oeff.

(m

/s)

Fig. 8. Water permeability coefficient with tim

those for moist-cured conventional mixes, but lower thanthose of air-cured conventional mixes. This confirms withthe results obtained in the water absorption% and perme-able pores%.

The sorptivity values were found to decrease with timefor both self-curing and the moist-curing concrete mixes;the reduction for the moist-curing mixes was higher than

Age (days)

0

1E-12

2E-12

3E-12

4E-12

5E-12

6E-12

7E-12

8E-12

9E-12

7 14 28 56 7 14 28 56

Per

m. C

oeff.

(m

/s)

Moist -Curing Self -Curing Air- Curing

C.C. 450 kg /m3

w/c = 0.3w/c = 0.4

e for self-curing and conventional mixes.

A.S. El-Dieb / Construction and Building Materials 21 (2007) 1282–1287 1287

that of the self-curing mixes. This could be attributed to thecontinuation of hydration in both mixes but the effect inthe case of self-curing mixes is not significant in reducingthe large pores volume (i.e., capillary pores). In the caseof air-cured conventional concrete mixes, the reduction inthe water sorptivity value with age is marginal for bothcement contents. This could be attributed to the slower rateof hydration.

3.5. Water permeability

The water permeability was measured at different agesup to 56 days of age. Fig. 8 shows the water permeabilitycoefficient with time for both self-curing and conventionalmixes for cement contents 350 kg/m3 and 450 kg/m3,respectively. For both cement contents it was noticed thatself-curing resulted in water permeability higher than thatof moist-cured conventional mixes, and that the permeabil-ity coefficient values decreased with time. This indicates thecontinuation of hydration and the development of the porestructure of the concrete; nevertheless, the effect of moist-curing is significant on reducing the water permeability val-ues due to the better reduction in the larger pores volumecompared to the self-curing concrete. It was noticed thatfor the air-cured concrete mixes, the reduction in the waterpermeability coefficient with time was not significant indi-cating slower rate of cement hydration and high-permeablepores%. This confirms with the results obtained in thewater sorptivity test.

4. Conclusions

The following could be concluded from the resultsobtained in this study in spite of the scattering of testresults:

– Water retention for the concrete mixes incorporatingself-curing agent is higher compared to conventionalconcrete mixes, as found by the weight loss with time.

– Self-curing concrete suffered less self-desiccation undersealed conditions compared to conventional concrete.

– Self-curing concrete resulted in better hydration withtime under drying condition compared to conventionalconcrete.

– Water transport through self-curing concrete is lowerthan air-cured conventional concrete.

– Water sorptivity and water permeability values for self-curing concrete decreased with age indicating lower per-meable pores% as a result of the continuation of thecement hydration.

Also, the following conclusions could be considered forfurther research:

– Performance of the self-curing agent is affected by themix proportions, mainly the cement content and thew/c ratio.

– Effect of the self-curing agent on the microstructure andthe pore size distribution of the self-curing concreterequire additional study.

– Durability of self-curing concrete to sulphate salts andchloride induced corrosion is needed to be evaluated.

– The effect of using higher w/c ratios, different cementtypes, and supplementary cementing materials (SCM),such as silica fume fly ash and ground granulated blastslag on water retention, hydration and moisture trans-port of the self-curing concrete needs furtherinvestigation.

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