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Self-curing concrete: Water retention, hydration and moisture transport
Construction and Building
MATERIALS
Construction and Building Materials 21 (2007) 1282–1287
www.elsevier.com/locate/conbuildmat
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 2006 Available online 1 September 2006
Abstract Water retention of concrete containing self-curing agents is investigated. Concrete weight loss, and internal relative humidity measurements with time were carried out, in order to evaluate the water retention of self-curing concrete. Non-evaporable water at different ages was 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 concrete 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 moisture content in concrete during its early stages in order to develop the desired properties. However, good curing is not always practical in many cases. Several investigators asked the question whether there will be self-curing concrete [1,2]. Therefore, the need to develop self-curing agents attracted several researchers [3]. The concept of self-curing agents is to reduce the water evaporation from concrete, and hence increase the water retention capacity of the concrete compared to conventional concrete [4,5]. It was found that water soluble polymers can be used as self-curing agents in concrete [5]. Concrete incorporating self-curing agents will represent a new trend in the concrete construction in the new millennium [3]. Curing of concrete plays a major role in developing the concrete microstructure and pore structure, and hence improves its durability and performance. The concept of self-curing agents is to reduce the water evaporation from concrete, and hence increase the water retention
*
Tel.: +202 6858377; fax: +202 6850617. E-mail address: [email protected].
0950-0618/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2006.02.007
capacity of the concrete compared to conventional concrete [4,5]. The aim of the investigation is to evaluate the use of water-soluble polymeric glycol as self-curing agent. The use of self-curing admixtures is very important from the point of view that water resources are getting valuable every 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 significant in desert areas where water is not adequately available. In this study water retention and hydration of concrete containing self-curing agents is investigated and compared to conventional concrete. Also, water transport through this concrete is evaluated and compared to conventional concrete continuously moist-cured and air-cured. Concrete weight loss and internal relative humidity measurements with time were carried out in order to evaluate the water retention ability. Non-evaporable water at different ages was measured to evaluate the hydration of self-curing concrete. The water transport, as durability index [6–10], is evaluated by measuring water absorption%, permeable voids%, water sorptivity and water permeability. Brief description of tests and specimens is given in Section 2.2. The parameters included in the study were mainly the cement content and the w/c ratio.
A.S. El-Dieb / Construction and Building Materials 21 (2007) 1282–1287
2. Experimental work 2.1. Materials and concrete mixes The main constituent variable parameters in this study were the cement content and the w/c ratio. Table 1 gives the details for the mixes used in the study. For each cement content and w/c ratio, two concrete mixes were cast; one which includes the self-curing agent and the other is conventional mix. A total of eight mixes were used in this investigation. The initial slump for all the conventional concrete mixes was kept constant (about 90–120 mm) using variable dosage of high-range water reducer-retarding admixture (Type G). The admixture dosage was kept constant for concrete mixes when self-curing agent was used. For evaluating water transport, two curing regimes were used for conventional concrete mixes without self-curing agent; continuously moist-curing under water, and aircuring. The cement used was ordinary Portland cement. The coarse aggregate was crushed stone with two sizes; S1 (5– 20 mm particle size) and S2 (10–25 mm particle size). The two coarse aggregate sizes were mixed with a 1:1 ratio. The sand used was natural sand with fineness modulus of 2.58; the percentage of the sand was 32% of the total aggregate weight. The self-curing agent used in the study was water soluble polymeric glycol (i.e., polyethylene-glycol) [3,5]. The dosage of the self-curing agent was kept constant for all the self-curing concrete mixes. The dosage was 0.02% by weight of the cement.
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of the cube. The hole was then cleaned using air jet to remove any loose particles. The hole was then sealed using a rubber stopper. The cube was then sealed from the surrounding environment using wax film. A digital relative humidity probe was used to measure the relative humidity inside the cube at several time intervals up to 91 days of age; a one-hole rubber stopper was used to seal the humidity sensor into the concrete block. The probe was kept inside the whole for about 2–3 h before taking the measurements. Measurement of the relative humidity took about 20–30 s to stabilize. The holes were kept sealed using the solid rubber stopper when not being in use to measure the internal relative humidity. Fig. 1 shows the set-up for measuring the internal relative humidity. Duplicate specimens were prepared for each mix and the average results are used in the discussions. The non-evaporable water content was carried out on concrete specimens cast from the mixes. The specimens were left in air (i.e., under drying condition). The nonevaporable water was determined at several time intervals up to 28 days of age. At each age a concrete specimen from each mix was crushed and a cement paste sample was obtained for the test by sieving crushed concrete sample to remove aggregate particles. The samples were kept in porpan-ol2 to stop hydration until testing. The propan-
2.2. Specimens and testing Concrete weight loss was carried out by filling polypropylene containers of capacity 1.5 l, with internal diameter 120 mm and height 130 mm, with concrete. The containers were kept at constant temperature of about 25 C and relative humidity environment of about 65%. The weight of the containers was measured after casting and at several intervals to determine the weight loss with time. Measurement of the weight was carried-out till 28 days of age. Two specimens were used for each mix and the average values are used in the discussion. A cube specimen of dimensions 158 · 158 · 158 mm was cast from each mix. The cubes were cured in the moulds for 24 h. After de-moulding a hole of diameter 20 mm and depth 100 mm was drilled in each cube from the top face
Fig. 1. Internal relative humidity set-up.
Table 1 Concrete mixes Concrete mix type
Self-curing concrete 3
Cement content (kg/m ) w/c Ratio Mix I.D. Curing I.D.
350 0.3 0.4 Self-curing Self-curing (no curing)
Conventional concrete 450 0.3
0.4
350 0.3 Conv. Moist-curing
0.4
450 0.3 Air-curing
0.4
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A.S. El-Dieb / Construction and Building Materials 21 (2007) 1282–1287
ol2 was dried off before testing (i.e., the specimens were dried in a 105 C oven). The non-evaporable water was determined as the weight loss after burning in a muffle furnace at 1050 C. The non-evaporable water was calculated as the sample weight loss to the sample weight (g/g). Replicate samples were used for each mix and at each test age and the average values are used in discussions. Water absorption% and permeable pores% tests were conducted according to ASTM C-642. A concrete disc of diameter 100 mm and height 50 mm was cut from a cylinder and used for testing. Water absorption and permeable pore tests were conducted at 28 day of age. The test was conducted on replicates and the average values are reported and used for discussion. Water sorptivity test was carried out for measuring rate of absorption of hydraulic cement concretes [11]. The specimens used were discs of diameter 100 mm and height 50 mm cut from a cylinder. The specimens were oven dried at 110 C for 24 h, and then the specimens were left to cool in dry condition for the following 24 h. The test was carried out by allowing one surface of the specimen to be in contact with water of 5 mm depth using a circular aluminum support as shown in Fig. 2. Using the supporting frame and keeping the outside water level at 1–3 mm above the aluminum support allows continuous contact between the
Fig. 2. Water sorptivity set-up.
70
Cement Content = 350 kg/ m
3. Test results and discussion 3.1. Water retention The weight loss with time due to the moisture evaporation was found to be less for the self-curing mixes than that for the conventional mixes. This indicates better water retention for self-curing mixes. Fig. 3 shows the weight loss with time for all the mixes. The weight loss for the concrete mixes with w/c ratio 0.4 was greater than that for concrete mixes with w/c ratio 0.3 for both cement contents. Also, the weight loss for the concrete mixes with cement content 450 kg/m3 was slightly higher than that for concrete mixes with cement content 350 kg/m3. Fig. 4 shows the internal relative humidity for the selfcuring and conventional concrete with time. The cement content and the w/c ratio have a significant effect on the internal relative humidity of the concrete whether self-curing or conventional mixes, this confirms with the findings previously concluded for conventional concrete mixes [14,15]. For the concrete mixes with cement content 350 kg/m3, the internal relative humidity for the self-curing 70
3
w/c = 0.4
60
w/c = 0.4 w/c = 0.4
50
Weight Loss (gm)
Weight Loss (gm)
60
specimen surface and the water without changing the water depth during the test time. The sides of the test specimens were sealed with electric vinyl tape to create unidirectional flow through the concrete specimen. The weight of the specimen was recorded at fixed time intervals with a total time of 25 min [11–13]. The sorptivity test was conducted at 28 days and 56 days of age on duplicate specimens for each mix. The water permeability test was conducted using constant water pressure head during a constant time period. The water inflow was measured and the water coefficient of permeability (m/s) was calculated. The water permeability test was conducted on saturated concrete specimens of diameter 100 mm and height 50 mm cut from concrete cylinders. The specimens were saturated using vacuum saturation before testing. The water permeability coefficient (m/s) was measured at 7, 14, 28 and 56 days of age for each mix on duplicate specimens fro each mix and for each age.
w/c = 0.3
40
w/c = 0.3
30 20
w/c = 0.4 w/c = 0.3
50
w/c = 0.3
40 30 Self -Curing
20
Self-Curing
10
Conventional
10
Conventional
Cement Content
0
= 450 kg /m
3
0
0
8
16 24 Time (days)
32
40
0
8
16 24 Time (days)
Fig. 3. Weight loss with time for self-curing and conventional mixes.
32
40
A.S. El-Dieb / Construction and Building Materials 21 (2007) 1282–1287 100
100
C.C. 450 kg/m 3
C.C. 350 kg /m 3 Self-Curing Conventional
95
90 w/c = 0.4 85
w/c = 0.3 w/c = 0.4 w/c = 0.3
80
Self-Curing Conventional
95 Relative Humidity (%)
Relative Humidity (%)
1285
90
85 w/c = 0.4 w/c = 0.3 w/c = 0.4 w/c = 0.3
80
75
75
0
20
40
60
80
100
120
0
20
40
60
Time (days)
80
100
120
Time (days)
Fig. 4. Internal relative humidity with time for self-curing and conventional mixes.
mixes was slightly higher than 85% after 91 days, and below 85% for the conventional mixes. For the concrete mixes with cement content 450 kg/m3, the internal relative humidity was below 85% for the self-curing mixes while it was below 80% for conventional mixes. This shows that the self-desiccation is more pronounced for the conventional mixes compared to the self-curing mixes which could have a direct impact on the hydration of the cement. 3.2. Hydration The non-evaporable water (Wn) measured on unsealed specimens (i.e., under drying condition) at different times for self-curing and conventional concrete mixes is shown in Fig. 5. It could be seen that self-curing concrete with its ability to retain water resulted in higher non-evaporable water which in turn imply higher degree of hydration. The effect is affected by the mix proportions as found from the results of the measurement of the weight loss and the internal 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
0.2 C.C. 350 kg/m
cement contents and w/c ratios. The water absorption% and the permeable pores% were found to be slightly higher for self-curing concrete than those for continuously moistcured conventional concrete. On the other hand, the selfcuring concrete showed lower water absorption% and permeable pores% compared to air-cured conventional concrete. This indicates that self-curing concrete develops lower permeable pores% compared to the air-cured conventional concrete. This could be attributed to the water retention of the self-curing concrete and the continuation of hydration compared to the air-cured conventional concrete. 3.4. Water sorptivity The water sorptivity was measured at two ages, 28 days and 56 days of age, in order to study the effect of self-curing on the development of the capillary pores, and the capillary water suction of the concrete. Fig. 7 shows the water sorptivity for the self-curing and the conventional concrete with its two curing regimes at 28 days and 56 days of age respectively. For the 450 kg/m3 cement content conventional concrete mix, the water sorptivity values at both ages were not significantly reduced when w/c ratio was reduced from 0.4 to 0.3 for the continuously
0.2
3
0.18
0.18 w/c = 0.4 w/c = 0.3
w/c = 0.3
0.16 Wn (gm/gm)
0.16 Wn (gm/gm)
w/c = 0.4 w/c = 0.4 w/c = 0.3
C.C. 450 kg/m 3
w/c = 0.4
w/c = 0.3 0.14 0.12
0.14 0.12 0.1
0.1
Self -Curing
Self-Curing 0.08
0.08
Conventional
Conventional
0.06
0.06 0
5
10
15
20 Time (days)
25
30
35
40
0
5
10
15
20
Time (days)
Fig. 5. Non-evaporable water versus time for self-curing and conventional mixes.
25
30
35
40
A.S. El-Dieb / Construction and Building Materials 21 (2007) 1282–1287
Pores %
Absorption %
8
18
Absorption %
16
6
14
5
12
4
10
3
8
20
C.C. 450 kg /m 3
7
18
Pores %
16
6
14
5
12
4
10
3
8
6 2
6 2
4
1
2
1
0
0
0
0.4
0.3 0.4 w/c Rati o
Moist -Curing
28 - D Pores %
28 - D Pores %
20
C.C. 350 kg /m 3
7
28 - DAbsor. %
8
28 - D Pores %
1286
4 2 0
0.3
0.4
0.3
0.4
0.3
Self -Curing
Air- Curing
w/c Ratio
Self -Curing
Air -Curing
Moist -Curing
Fig. 6. Water absorption% and permeable pores%.
0.3
0.3
3
C.C. 450 kg /m
3
3
3
C.C. 350 kg/m 1/2
56 - Day Sorptivity (mm/min )
1/2
56 - Day Sorptivity (mm/min )
C.C. 350 kg /m 0.25 0.2 0.15 0.1 0.05 0
C.C. 450 kg/m
0.25 0.2 0.15 0.1 0.05 0
0.4
0.3
0.4
0.3
0.4
w/c Ratio Moist -Curing
Self -Curing
Air- Curing
0.3 Moist -Curing
w/c Ratio
0.4
Self -Curin g
0.3
Air- Curing
Fig. 7. Water sorptivity at 28 days and 56 days of age for self-curing and conventional mixes.
moist-cured regime. For the continuously moist-cured conventional concrete mix with 350 kg/m3 cement content, reducing the w/c ratio resulted in a water sorptivity value very close to that of similar conventional moistcured mix with cement content 450 kg/m3 and w/c of 0.4. This trend was found at both ages. The water sorptivity values for the self-curing concrete were higher than
C.C. 350 kg/m w/c = 0.4
4E-11
9E-12
3
C.C. 450 kg /m
8E-12
w/c = 0.3
Perm. Coeff. (m/s)
Perm. Coeff. (m/s)
5E-11
those for moist-cured conventional mixes, but lower than those of air-cured conventional mixes. This confirms with the results obtained in the water absorption% and permeable pores%. The sorptivity values were found to decrease with time for both self-curing and the moist-curing concrete mixes; the reduction for the moist-curing mixes was higher than
3E-11 2E-11
w/c = 0.4
7E-12
3
w/c = 0.3
6E-12 5E-12 4E-12 3E-12 2E-12
1E-11
1E-12 0
0 7
14
28
Moist-Curing
56 7 Age (days) Self-Curing
14
28
56
Air -Curing
7
14
28
Moist -Curing
56 7 Age (days)
14
Self -Curing
Fig. 8. Water permeability coefficient with time for self-curing and conventional mixes.
28
56
Air- Curing
A.S. El-Dieb / Construction and Building Materials 21 (2007) 1282–1287
that of the self-curing mixes. This could be attributed to the continuation of hydration in both mixes but the effect in the case of self-curing mixes is not significant in reducing the large pores volume (i.e., capillary pores). In the case of air-cured conventional concrete mixes, the reduction in the water sorptivity value with age is marginal for both cement contents. This could be attributed to the slower rate of hydration. 3.5. Water permeability The water permeability was measured at different ages up to 56 days of age. Fig. 8 shows the water permeability coefficient with time for both self-curing and conventional mixes for cement contents 350 kg/m3 and 450 kg/m3, respectively. For both cement contents it was noticed that self-curing resulted in water permeability higher than that of moist-cured conventional mixes, and that the permeability coefficient values decreased with time. This indicates the continuation of hydration and the development of the pore structure of the concrete; nevertheless, the effect of moistcuring is significant on reducing the water permeability values due to the better reduction in the larger pores volume compared to the self-curing concrete. It was noticed that for the air-cured concrete mixes, the reduction in the water permeability coefficient with time was not significant indicating slower rate of cement hydration and high-permeable pores%. This confirms with the results obtained in the water sorptivity test. 4. Conclusions The following could be concluded from the results obtained in this study in spite of the scattering of test results: – Water retention for the concrete mixes incorporating self-curing agent is higher compared to conventional concrete mixes, as found by the weight loss with time. – Self-curing concrete suffered less self-desiccation under sealed conditions compared to conventional concrete. – Self-curing concrete resulted in better hydration with time under drying condition compared to conventional concrete. – Water transport through self-curing concrete is lower than air-cured conventional concrete. – Water sorptivity and water permeability values for selfcuring concrete decreased with age indicating lower permeable pores% as a result of the continuation of the cement hydration.
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Also, the following conclusions could be considered for further research: – Performance of the self-curing agent is affected by the mix proportions, mainly the cement content and the w/c ratio. – Effect of the self-curing agent on the microstructure and the pore size distribution of the self-curing concrete require additional study. – Durability of self-curing concrete to sulphate salts and chloride induced corrosion is needed to be evaluated. – The effect of using higher w/c ratios, different cement types, and supplementary cementing materials (SCM), such as silica fume fly ash and ground granulated blast slag on water retention, hydration and moisture transport of the self-curing concrete needs further investigation. References [1] Mather B. Self-curing concrete, Why not? Concr Int 2001;23(1):46–7. [2] Bentz DP, Lura P, Roberts JW. Mixture proportioning for internal curing. Concr Int 2005;27(2). [3] Dhir RK, Hewlett PC, Lota JS, Dyre TD. An investigation into the feasibility of formulating ‘self-curing’ concrete. Mater Struct 1994;27:606–15. [4] Wang J, Dhir RK, Levitt M. Membrane curing of concrete. Cement Concr Res 1994;24(8):1463–74. [5] Dhir PK, Hewlett PC, Dyre TD. Mechanism of water retention in cement pastes containing a self-curing agent. Mag Concr Res 1998;50(1):85–90. [6] Basheer L, Kropp L, Cleland DJ. Assessment of the durability of concrete from its permeation properties: A review. Constr Building Mater 2001;15(2-3):93–103. [7] Tu¨rkmen I. Influence of different curing conditions on the physical and mechanical properties of concretes with admixtures of silica fume and blast furnace slag. Mater Lett 2003;57(29):4560–9. [8] De Beer FC, Le Roux JJ, Kearsley EP. Testing the durability of concrete with neutron radiography. Nucl Instrum Meth Phys Res Sec A 2005;542:226–31. [9] Zhu W, Bartos PJM. Permeation properties of self-compacting concrete. Cement Concr Res 2003;33(6):921–6. [10] A.S. El-Dieb, Permeation of fluids through high performance concrete, PhD thesis, Civil Engineering Department, University of Toronto, 1994. [11] ASTM C-1585-04, Measurement of rate of absorption of water by hydraulic cement concretes, ASTM manual, vol. 4.02. [12] Hall C. Water sorptivity of mortars and concretes: A review. Mag Concr Res 1989;41(147):51–61. [13] El-Dieb AS. Effect of sorptivity test time on the variation of test results. Ain Shams Univ Faculty Eng Sci Bull 1999;34(2):41–51. [14] Mjo¨rnell K. Self-Desiccation in concrete. Chalmers University of Technology 1994;2(556):94. [15] McGrath PF. Internal self-desiccation of silica fume concrete. MASc thesis, Civil Engineering Department, University of Toronto, 1989.
MATERIALS
Construction and Building Materials 21 (2007) 1282–1287
www.elsevier.com/locate/conbuildmat
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 2006 Available online 1 September 2006
Abstract Water retention of concrete containing self-curing agents is investigated. Concrete weight loss, and internal relative humidity measurements with time were carried out, in order to evaluate the water retention of self-curing concrete. Non-evaporable water at different ages was 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 concrete 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 moisture content in concrete during its early stages in order to develop the desired properties. However, good curing is not always practical in many cases. Several investigators asked the question whether there will be self-curing concrete [1,2]. Therefore, the need to develop self-curing agents attracted several researchers [3]. The concept of self-curing agents is to reduce the water evaporation from concrete, and hence increase the water retention capacity of the concrete compared to conventional concrete [4,5]. It was found that water soluble polymers can be used as self-curing agents in concrete [5]. Concrete incorporating self-curing agents will represent a new trend in the concrete construction in the new millennium [3]. Curing of concrete plays a major role in developing the concrete microstructure and pore structure, and hence improves its durability and performance. The concept of self-curing agents is to reduce the water evaporation from concrete, and hence increase the water retention
*
Tel.: +202 6858377; fax: +202 6850617. E-mail address: [email protected].
0950-0618/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2006.02.007
capacity of the concrete compared to conventional concrete [4,5]. The aim of the investigation is to evaluate the use of water-soluble polymeric glycol as self-curing agent. The use of self-curing admixtures is very important from the point of view that water resources are getting valuable every 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 significant in desert areas where water is not adequately available. In this study water retention and hydration of concrete containing self-curing agents is investigated and compared to conventional concrete. Also, water transport through this concrete is evaluated and compared to conventional concrete continuously moist-cured and air-cured. Concrete weight loss and internal relative humidity measurements with time were carried out in order to evaluate the water retention ability. Non-evaporable water at different ages was measured to evaluate the hydration of self-curing concrete. The water transport, as durability index [6–10], is evaluated by measuring water absorption%, permeable voids%, water sorptivity and water permeability. Brief description of tests and specimens is given in Section 2.2. The parameters included in the study were mainly the cement content and the w/c ratio.
A.S. El-Dieb / Construction and Building Materials 21 (2007) 1282–1287
2. Experimental work 2.1. Materials and concrete mixes The main constituent variable parameters in this study were the cement content and the w/c ratio. Table 1 gives the details for the mixes used in the study. For each cement content and w/c ratio, two concrete mixes were cast; one which includes the self-curing agent and the other is conventional mix. A total of eight mixes were used in this investigation. The initial slump for all the conventional concrete mixes was kept constant (about 90–120 mm) using variable dosage of high-range water reducer-retarding admixture (Type G). The admixture dosage was kept constant for concrete mixes when self-curing agent was used. For evaluating water transport, two curing regimes were used for conventional concrete mixes without self-curing agent; continuously moist-curing under water, and aircuring. The cement used was ordinary Portland cement. The coarse aggregate was crushed stone with two sizes; S1 (5– 20 mm particle size) and S2 (10–25 mm particle size). The two coarse aggregate sizes were mixed with a 1:1 ratio. The sand used was natural sand with fineness modulus of 2.58; the percentage of the sand was 32% of the total aggregate weight. The self-curing agent used in the study was water soluble polymeric glycol (i.e., polyethylene-glycol) [3,5]. The dosage of the self-curing agent was kept constant for all the self-curing concrete mixes. The dosage was 0.02% by weight of the cement.
1283
of the cube. The hole was then cleaned using air jet to remove any loose particles. The hole was then sealed using a rubber stopper. The cube was then sealed from the surrounding environment using wax film. A digital relative humidity probe was used to measure the relative humidity inside the cube at several time intervals up to 91 days of age; a one-hole rubber stopper was used to seal the humidity sensor into the concrete block. The probe was kept inside the whole for about 2–3 h before taking the measurements. Measurement of the relative humidity took about 20–30 s to stabilize. The holes were kept sealed using the solid rubber stopper when not being in use to measure the internal relative humidity. Fig. 1 shows the set-up for measuring the internal relative humidity. Duplicate specimens were prepared for each mix and the average results are used in the discussions. The non-evaporable water content was carried out on concrete specimens cast from the mixes. The specimens were left in air (i.e., under drying condition). The nonevaporable water was determined at several time intervals up to 28 days of age. At each age a concrete specimen from each mix was crushed and a cement paste sample was obtained for the test by sieving crushed concrete sample to remove aggregate particles. The samples were kept in porpan-ol2 to stop hydration until testing. The propan-
2.2. Specimens and testing Concrete weight loss was carried out by filling polypropylene containers of capacity 1.5 l, with internal diameter 120 mm and height 130 mm, with concrete. The containers were kept at constant temperature of about 25 C and relative humidity environment of about 65%. The weight of the containers was measured after casting and at several intervals to determine the weight loss with time. Measurement of the weight was carried-out till 28 days of age. Two specimens were used for each mix and the average values are used in the discussion. A cube specimen of dimensions 158 · 158 · 158 mm was cast from each mix. The cubes were cured in the moulds for 24 h. After de-moulding a hole of diameter 20 mm and depth 100 mm was drilled in each cube from the top face
Fig. 1. Internal relative humidity set-up.
Table 1 Concrete mixes Concrete mix type
Self-curing concrete 3
Cement content (kg/m ) w/c Ratio Mix I.D. Curing I.D.
350 0.3 0.4 Self-curing Self-curing (no curing)
Conventional concrete 450 0.3
0.4
350 0.3 Conv. Moist-curing
0.4
450 0.3 Air-curing
0.4
1284
A.S. El-Dieb / Construction and Building Materials 21 (2007) 1282–1287
ol2 was dried off before testing (i.e., the specimens were dried in a 105 C oven). The non-evaporable water was determined as the weight loss after burning in a muffle furnace at 1050 C. The non-evaporable water was calculated as the sample weight loss to the sample weight (g/g). Replicate samples were used for each mix and at each test age and the average values are used in discussions. Water absorption% and permeable pores% tests were conducted according to ASTM C-642. A concrete disc of diameter 100 mm and height 50 mm was cut from a cylinder and used for testing. Water absorption and permeable pore tests were conducted at 28 day of age. The test was conducted on replicates and the average values are reported and used for discussion. Water sorptivity test was carried out for measuring rate of absorption of hydraulic cement concretes [11]. The specimens used were discs of diameter 100 mm and height 50 mm cut from a cylinder. The specimens were oven dried at 110 C for 24 h, and then the specimens were left to cool in dry condition for the following 24 h. The test was carried out by allowing one surface of the specimen to be in contact with water of 5 mm depth using a circular aluminum support as shown in Fig. 2. Using the supporting frame and keeping the outside water level at 1–3 mm above the aluminum support allows continuous contact between the
Fig. 2. Water sorptivity set-up.
70
Cement Content = 350 kg/ m
3. Test results and discussion 3.1. Water retention The weight loss with time due to the moisture evaporation was found to be less for the self-curing mixes than that for the conventional mixes. This indicates better water retention for self-curing mixes. Fig. 3 shows the weight loss with time for all the mixes. The weight loss for the concrete mixes with w/c ratio 0.4 was greater than that for concrete mixes with w/c ratio 0.3 for both cement contents. Also, the weight loss for the concrete mixes with cement content 450 kg/m3 was slightly higher than that for concrete mixes with cement content 350 kg/m3. Fig. 4 shows the internal relative humidity for the selfcuring and conventional concrete with time. The cement content and the w/c ratio have a significant effect on the internal relative humidity of the concrete whether self-curing or conventional mixes, this confirms with the findings previously concluded for conventional concrete mixes [14,15]. For the concrete mixes with cement content 350 kg/m3, the internal relative humidity for the self-curing 70
3
w/c = 0.4
60
w/c = 0.4 w/c = 0.4
50
Weight Loss (gm)
Weight Loss (gm)
60
specimen surface and the water without changing the water depth during the test time. The sides of the test specimens were sealed with electric vinyl tape to create unidirectional flow through the concrete specimen. The weight of the specimen was recorded at fixed time intervals with a total time of 25 min [11–13]. The sorptivity test was conducted at 28 days and 56 days of age on duplicate specimens for each mix. The water permeability test was conducted using constant water pressure head during a constant time period. The water inflow was measured and the water coefficient of permeability (m/s) was calculated. The water permeability test was conducted on saturated concrete specimens of diameter 100 mm and height 50 mm cut from concrete cylinders. The specimens were saturated using vacuum saturation before testing. The water permeability coefficient (m/s) was measured at 7, 14, 28 and 56 days of age for each mix on duplicate specimens fro each mix and for each age.
w/c = 0.3
40
w/c = 0.3
30 20
w/c = 0.4 w/c = 0.3
50
w/c = 0.3
40 30 Self -Curing
20
Self-Curing
10
Conventional
10
Conventional
Cement Content
0
= 450 kg /m
3
0
0
8
16 24 Time (days)
32
40
0
8
16 24 Time (days)
Fig. 3. Weight loss with time for self-curing and conventional mixes.
32
40
A.S. El-Dieb / Construction and Building Materials 21 (2007) 1282–1287 100
100
C.C. 450 kg/m 3
C.C. 350 kg /m 3 Self-Curing Conventional
95
90 w/c = 0.4 85
w/c = 0.3 w/c = 0.4 w/c = 0.3
80
Self-Curing Conventional
95 Relative Humidity (%)
Relative Humidity (%)
1285
90
85 w/c = 0.4 w/c = 0.3 w/c = 0.4 w/c = 0.3
80
75
75
0
20
40
60
80
100
120
0
20
40
60
Time (days)
80
100
120
Time (days)
Fig. 4. Internal relative humidity with time for self-curing and conventional mixes.
mixes was slightly higher than 85% after 91 days, and below 85% for the conventional mixes. For the concrete mixes with cement content 450 kg/m3, the internal relative humidity was below 85% for the self-curing mixes while it was below 80% for conventional mixes. This shows that the self-desiccation is more pronounced for the conventional mixes compared to the self-curing mixes which could have a direct impact on the hydration of the cement. 3.2. Hydration The non-evaporable water (Wn) measured on unsealed specimens (i.e., under drying condition) at different times for self-curing and conventional concrete mixes is shown in Fig. 5. It could be seen that self-curing concrete with its ability to retain water resulted in higher non-evaporable water which in turn imply higher degree of hydration. The effect is affected by the mix proportions as found from the results of the measurement of the weight loss and the internal 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
0.2 C.C. 350 kg/m
cement contents and w/c ratios. The water absorption% and the permeable pores% were found to be slightly higher for self-curing concrete than those for continuously moistcured conventional concrete. On the other hand, the selfcuring concrete showed lower water absorption% and permeable pores% compared to air-cured conventional concrete. This indicates that self-curing concrete develops lower permeable pores% compared to the air-cured conventional concrete. This could be attributed to the water retention of the self-curing concrete and the continuation of hydration compared to the air-cured conventional concrete. 3.4. Water sorptivity The water sorptivity was measured at two ages, 28 days and 56 days of age, in order to study the effect of self-curing on the development of the capillary pores, and the capillary water suction of the concrete. Fig. 7 shows the water sorptivity for the self-curing and the conventional concrete with its two curing regimes at 28 days and 56 days of age respectively. For the 450 kg/m3 cement content conventional concrete mix, the water sorptivity values at both ages were not significantly reduced when w/c ratio was reduced from 0.4 to 0.3 for the continuously
0.2
3
0.18
0.18 w/c = 0.4 w/c = 0.3
w/c = 0.3
0.16 Wn (gm/gm)
0.16 Wn (gm/gm)
w/c = 0.4 w/c = 0.4 w/c = 0.3
C.C. 450 kg/m 3
w/c = 0.4
w/c = 0.3 0.14 0.12
0.14 0.12 0.1
0.1
Self -Curing
Self-Curing 0.08
0.08
Conventional
Conventional
0.06
0.06 0
5
10
15
20 Time (days)
25
30
35
40
0
5
10
15
20
Time (days)
Fig. 5. Non-evaporable water versus time for self-curing and conventional mixes.
25
30
35
40
A.S. El-Dieb / Construction and Building Materials 21 (2007) 1282–1287
Pores %
Absorption %
8
18
Absorption %
16
6
14
5
12
4
10
3
8
20
C.C. 450 kg /m 3
7
18
Pores %
16
6
14
5
12
4
10
3
8
6 2
6 2
4
1
2
1
0
0
0
0.4
0.3 0.4 w/c Rati o
Moist -Curing
28 - D Pores %
28 - D Pores %
20
C.C. 350 kg /m 3
7
28 - DAbsor. %
8
28 - D Pores %
1286
4 2 0
0.3
0.4
0.3
0.4
0.3
Self -Curing
Air- Curing
w/c Ratio
Self -Curing
Air -Curing
Moist -Curing
Fig. 6. Water absorption% and permeable pores%.
0.3
0.3
3
C.C. 450 kg /m
3
3
3
C.C. 350 kg/m 1/2
56 - Day Sorptivity (mm/min )
1/2
56 - Day Sorptivity (mm/min )
C.C. 350 kg /m 0.25 0.2 0.15 0.1 0.05 0
C.C. 450 kg/m
0.25 0.2 0.15 0.1 0.05 0
0.4
0.3
0.4
0.3
0.4
w/c Ratio Moist -Curing
Self -Curing
Air- Curing
0.3 Moist -Curing
w/c Ratio
0.4
Self -Curin g
0.3
Air- Curing
Fig. 7. Water sorptivity at 28 days and 56 days of age for self-curing and conventional mixes.
moist-cured regime. For the continuously moist-cured conventional concrete mix with 350 kg/m3 cement content, reducing the w/c ratio resulted in a water sorptivity value very close to that of similar conventional moistcured mix with cement content 450 kg/m3 and w/c of 0.4. This trend was found at both ages. The water sorptivity values for the self-curing concrete were higher than
C.C. 350 kg/m w/c = 0.4
4E-11
9E-12
3
C.C. 450 kg /m
8E-12
w/c = 0.3
Perm. Coeff. (m/s)
Perm. Coeff. (m/s)
5E-11
those for moist-cured conventional mixes, but lower than those of air-cured conventional mixes. This confirms with the results obtained in the water absorption% and permeable pores%. The sorptivity values were found to decrease with time for both self-curing and the moist-curing concrete mixes; the reduction for the moist-curing mixes was higher than
3E-11 2E-11
w/c = 0.4
7E-12
3
w/c = 0.3
6E-12 5E-12 4E-12 3E-12 2E-12
1E-11
1E-12 0
0 7
14
28
Moist-Curing
56 7 Age (days) Self-Curing
14
28
56
Air -Curing
7
14
28
Moist -Curing
56 7 Age (days)
14
Self -Curing
Fig. 8. Water permeability coefficient with time for self-curing and conventional mixes.
28
56
Air- Curing
A.S. El-Dieb / Construction and Building Materials 21 (2007) 1282–1287
that of the self-curing mixes. This could be attributed to the continuation of hydration in both mixes but the effect in the case of self-curing mixes is not significant in reducing the large pores volume (i.e., capillary pores). In the case of air-cured conventional concrete mixes, the reduction in the water sorptivity value with age is marginal for both cement contents. This could be attributed to the slower rate of hydration. 3.5. Water permeability The water permeability was measured at different ages up to 56 days of age. Fig. 8 shows the water permeability coefficient with time for both self-curing and conventional mixes for cement contents 350 kg/m3 and 450 kg/m3, respectively. For both cement contents it was noticed that self-curing resulted in water permeability higher than that of moist-cured conventional mixes, and that the permeability coefficient values decreased with time. This indicates the continuation of hydration and the development of the pore structure of the concrete; nevertheless, the effect of moistcuring is significant on reducing the water permeability values due to the better reduction in the larger pores volume compared to the self-curing concrete. It was noticed that for the air-cured concrete mixes, the reduction in the water permeability coefficient with time was not significant indicating slower rate of cement hydration and high-permeable pores%. This confirms with the results obtained in the water sorptivity test. 4. Conclusions The following could be concluded from the results obtained in this study in spite of the scattering of test results: – Water retention for the concrete mixes incorporating self-curing agent is higher compared to conventional concrete mixes, as found by the weight loss with time. – Self-curing concrete suffered less self-desiccation under sealed conditions compared to conventional concrete. – Self-curing concrete resulted in better hydration with time under drying condition compared to conventional concrete. – Water transport through self-curing concrete is lower than air-cured conventional concrete. – Water sorptivity and water permeability values for selfcuring concrete decreased with age indicating lower permeable pores% as a result of the continuation of the cement hydration.
1287
Also, the following conclusions could be considered for further research: – Performance of the self-curing agent is affected by the mix proportions, mainly the cement content and the w/c ratio. – Effect of the self-curing agent on the microstructure and the pore size distribution of the self-curing concrete require additional study. – Durability of self-curing concrete to sulphate salts and chloride induced corrosion is needed to be evaluated. – The effect of using higher w/c ratios, different cement types, and supplementary cementing materials (SCM), such as silica fume fly ash and ground granulated blast slag on water retention, hydration and moisture transport of the self-curing concrete needs further investigation. References [1] Mather B. Self-curing concrete, Why not? Concr Int 2001;23(1):46–7. [2] Bentz DP, Lura P, Roberts JW. Mixture proportioning for internal curing. Concr Int 2005;27(2). [3] Dhir RK, Hewlett PC, Lota JS, Dyre TD. An investigation into the feasibility of formulating ‘self-curing’ concrete. Mater Struct 1994;27:606–15. [4] Wang J, Dhir RK, Levitt M. Membrane curing of concrete. Cement Concr Res 1994;24(8):1463–74. [5] Dhir PK, Hewlett PC, Dyre TD. Mechanism of water retention in cement pastes containing a self-curing agent. Mag Concr Res 1998;50(1):85–90. [6] Basheer L, Kropp L, Cleland DJ. Assessment of the durability of concrete from its permeation properties: A review. Constr Building Mater 2001;15(2-3):93–103. [7] Tu¨rkmen I. Influence of different curing conditions on the physical and mechanical properties of concretes with admixtures of silica fume and blast furnace slag. Mater Lett 2003;57(29):4560–9. [8] De Beer FC, Le Roux JJ, Kearsley EP. Testing the durability of concrete with neutron radiography. Nucl Instrum Meth Phys Res Sec A 2005;542:226–31. [9] Zhu W, Bartos PJM. Permeation properties of self-compacting concrete. Cement Concr Res 2003;33(6):921–6. [10] A.S. El-Dieb, Permeation of fluids through high performance concrete, PhD thesis, Civil Engineering Department, University of Toronto, 1994. [11] ASTM C-1585-04, Measurement of rate of absorption of water by hydraulic cement concretes, ASTM manual, vol. 4.02. [12] Hall C. Water sorptivity of mortars and concretes: A review. Mag Concr Res 1989;41(147):51–61. [13] El-Dieb AS. Effect of sorptivity test time on the variation of test results. Ain Shams Univ Faculty Eng Sci Bull 1999;34(2):41–51. [14] Mjo¨rnell K. Self-Desiccation in concrete. Chalmers University of Technology 1994;2(556):94. [15] McGrath PF. Internal self-desiccation of silica fume concrete. MASc thesis, Civil Engineering Department, University of Toronto, 1989.