Curing time (Set I)

The following section discusses the effect of curing time on:

 Effective porosity

 Weight changes

 Sorptivity

 Penetration of salt solution and chloride ions

 Apparent diffusion coefficient and surface chloride concentration of concretes conditioned at 20ºC [Table 3.6]

4.3.1 Effective porosity

Figure 4.9 shows the effect of curing time and number of cycles on the effective porosity of OPC concrete conditioned at 20°C. Although the curing time has no effect on the strength and absolute porosity when specimens are conditioned at 20°C [Figures 4.1 & 4.2], it significantly influences the effective porosity.

The effective porosity of specimens cured for a shorter period of time is greater than that of concretes cured for a longer period of time. This is apparently because those cured for a longer period of time are kept under wet hessian and polyethene for a greater number of days and as a result have a shorter drying period (i.e. shorter conditioning time) prior to the exposure to salt solution when the specimens are 28 days old.

The effective porosities of the 1-day and 7-day cured specimens are almost constant during wet/dry cycles, whereas the effective porosities of the 27-day cured specimens increase gradually as the number of cycles increase and eventually achieve a more stable value. This can be explained by the fact that the 27-day cured specimens have a high internal moisture content at first exposure but eventually reach equilibrium with the surrounding environment as the number of wet/dry cycles increase.

Figure 4.9: Effect of curing time (Set I: conditioning at 20°C) on effective porosity at the beginning of each cycle

4.3.2 Weight changes

Figure 4.10 shows the effect of curing time on weight changes of concrete conditioned at 20°C. The weight changes are similar in concretes cured for 1 day and 7 days. The 27-day cured concrete absorbed a smaller amount of liquid but lost a greater amount of liquid compared with the 1-day and 7-day cured specimens, particularly during the first wet/dry cycle.

The weight changes are relatable to the effective porosities of the specimens. The difference in effective porosities of concretes cured for 1 day and 7 days is relatively small and thus they would be expected to experience similar weight changes. The 27-day cured concrete has a smaller effective porosity and therefore it is reasonable that this concrete experiences a smaller weight gain and a greater weight loss than others.

The weight loss of 27-day cured concretes decreases as the number of cycles increase. The reason is that there is a net weight loss at the end of each cycle which indicates that the internal moisture content has reduced. The reduction in moisture content causes a reduction in the amount of evaporation during the next cycle.

There is a very small net weight loss for specimens cured for 1 day or 7 days after six wet/dry cycles. The 7-day cured samples experienced a slightly more weight loss than the 1-day cured samples. The net weight loss is much more significant in cubes cured for 27 1-days.

1 2 3 4 5 6 7 8

1 2 3 4 5 6

Effectiveporosity (%byvolumeofsample)

Number of cycle

curing time: 1 day

curing time: 7 days

curing time: 27 days

The difference in the net weight change for specimens cured for 1, 7 and 27 days is also attributable to the variation in their initial moisture content. The 27-day cured cubes have the greatest initial moisture content of the test specimens. Therefore, they lose weight to reach equilibrium with the environment.

All specimens appeared to achieve a repeatable moisture state after five wet/dry cycles.

Figure 4.10: Effect of curing time (Set I: conditioning at 20°C) on weight changes

4.3.3 Sorptivity

The effect of curing time and number of cycles on the weight and distance sorptivity for concretes conditioned at 20°C are shown in Figures 4.11-a and 4.11-b, respectively. It can be seen that unlike to the results for the effect of conditioning time [Figures 4.5-a and 4.5-b], weight and distance sorptivities show different trends from each other.

At the first cycle the weight sorptivity decreases as the curing time increases. This can be explained by the fact that increasing the duration of curing leads to an increase in the moisture content and a reduction in the effective porosity [Figure 4.9]. However, there is very little difference between the sorptivities of the 1-day and 7-day cured specimens as there is little difference between their effective porosities.

During subsequent cycles, the effect of curing time is less significant. The sorptivities of the 1-day and 7-day cured concretes decrease slightly and equal that of the 27-day cured concrete. The reason is that hydration of the 1-day and 7-day cured samples increases during

-20 -10 0 10 20

0 20 40 60 80

Weightchanges(g)

time(day)

curing time: 1 day

curing time: 7 days

curing time: 27 days

the cyclic regime, thus further refinement of the pore structure occurs. In addition, chloride binding and crystallization may lead to enhancement of the pore structure. Consequently, with a denser pore structure, less absorption occurs.

In the case of the 27-day cured specimens, the effective porosity increases slightly with increasing number of cycles [Figure 4.10] which leads to an increase in the weight sorptivity. Conversely, changes in pore structure which occur due to chloride binding and crystallization decrease the weight sorptivity. The net result is that the weight sorptivity remains almost constant.

As discussed in the literature review (Section 2.5.3.3.6) it is generally accepted that the sorptivity of concrete decreases as the duration of moist curing increases. This is in agreement with the results at the first cycle. Some researchers have found that the effect of the curing decreases as the age of exposure increases such that the different site curing methods were indistinguishable at later ages (120 days) whereas at early ages (28 days) variation in sorptivity existed between the site and wet cured samples. This again agrees with the results here as curing time has no effect on the weight sorptivity during the subsequent cycles.

It is interesting to note that although there is a difference in effective porosity of specimens at the sixth cycle, they have similar weight sorptivities. From their strength and absolute porosity [Figures 4.1 and 4.2], it can be inferred that they have similar pore structures. Thus, having similar pore structures and exposure to similar service environments seems to result in equal weight sorptivities when equilibrium with the internal moisture content of concrete is achieved.

In fact, the weight sorptivity strongly depends on the effective porosity of the specimens. At the first cycle there are significant differences in the effective porosity of the specimens. As the number of wet/dry cycles increase, the internal moisture content reaches equilibrium and the difference in the effective porosities reduces. When equilibrium is achieved the difference in the effective porosity is relatively small and thus the effect of pore structure (absolute porosity and compressive strength) becomes dominant. Therefore, the sorptivity of

the specimens which perhaps have similar absolute porosity and compressive strength becomes equal.

Distance sorptivity shows a different trend from weight sorptivity. The distance sorptivity increases as the curing time increases at each cycle. This can be explained by the fact that distance sorptivity depends not only on the amount of absorbing solution but also on the available pore space in the concrete. A smaller volume of empty pores leads to deeper penetration of the absorbing solution into the concrete. Specimens with a longer curing time have a smaller effective porosity and less available pores and thus the absorbing solution is capable of penetrating deeper into the concrete.

Figure 4.11-a: Effect of curing time (Set I: conditioning at 20°C) on weight sorptivity

Figure 4.11-b: Effect of curing time (Set I: conditioning at 20°C) on distance sorptivity

4.3.4 Penetration of salt solution and chloride ions

The effect of curing time on depth of salt solution penetration in concrete conditioned at 20°C at the end of each wetting phase is presented in Figure 4.12. The chloride profiles at

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the end of first, third and sixth cycles are shown in Figures 4.13-a, 4.13-b and 4.13-c, respectively.

The depth of salt solution penetration increases as the curing time increases which is consistent with their distance sorptivity [Figure 4.11-b]. This is because depth of salt solution penetration, like distance sorptivity, depends on the amount of salt solution absorbed and the available pore space in the concrete. A smaller volume of empty pores leads to a deeper penetration of salt solution into concrete. Specimens with a longer curing time have a smaller effective porosity and less available pores and thus the absorbing salt solution penetrates deeper into concrete.

The depth of salt solution penetration is about 13 to 17mm from the surface at the first cycle which decreases to between 7.5 to 10mm after six cycles. As discussed previously, this is apparently due to changes in the pore structure and the decrease in absorption. The decrease in absorption also occurs as the internal moisture approaches equilibrium with the surrounding environment.

The curing time shows no significant effect on the penetration of chloride in concretes conditioned at 20°C at the first cycle. At the third and sixth cycle, the 27-day cured specimen tends to have smaller chloride contents at 5mm depth from the exposed surface compared with other specimens.

The smaller chloride content at the surface layer of concretes cured for 27 days may indicate that they have a smaller absolute porosity than other specimens near the surface, but similar absolute porosity at greater depths. The results are in agreement with those reported by Buenfeld and Yang (2000). They investigated the effect of site curing on concrete and found that, at early ages, the curing regime can have a significant effect on the pore structure of concrete very near to the cured surface. Poor curing resulted in higher porosity near to the surface in relation to the bulk concrete (20.5mm depth). The curing affected zone for the OPC specimens was at a depth of 3.5 to 6.5mm from the surface at 7-day age.

The depths of chloride penetration in all the cases are between 7 and 18mm from the surface after the first and sixth cycle.

The depth of salt solution penetration is higher than the depth of chloride penetration at the first cycle. During subsequent cycles, the depth of salt solution penetration decreases whereas the depth of chloride penetration increases and therefore the depth of chloride penetration is greater than the depth of salt solution penetration.

Figure 4.12: Effect of curing time (Set I: conditioning at 20°C) on salt solution penetration at the end of each wetting phase

Figure 4.13-a: Effect of curing time (Set I: conditioning at 20°C) on chloride penetration at the end of first cycle

Figure 4.13-b: Effect of curing time (Set I: conditioning at 20°C) on chloride penetration at the end of third cycle

Figure 4.13-c: Effect of curing time (Set I: conditioning at 20°C) on chloride penetration at the end of sixth cycle

4.3.5 Apparent diffusion coefficient, ܦ

, and surface chloride concentration, ܥ

Figure 4.14 shows the effect of curing time on apparentܦ௖andܥ௦at the first and sixth cycle. As expected from the chloride profiles, the values of apparentܥ௦are similar at the first cycle. However, the apparentܥ_௦of the specimens cured for 27 days is smaller than those cured for 1 and 7 days after six cycles. As previously discussed, this may be related to the smaller porosity of the concrete near the surface due to the longer curing time. The curing time shows little or no effect on apparentܦ௖.

Figure 4.14: Effect of curing time (Set 1) on apparentܦ_௖andܥ_௦at the first and sixth cycle.

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