Conditioning temperature

The following section discusses the effect of conditioning temperature on:

 Effective porosity

 Weight changes

 Sorptivity

 Penetration of salt solution and chloride ions

 Apparent diffusion coefficient and surface chloride concentration

4.5.1 Effective porosity

The effect of conditioning temperature and number of cycles on effective porosity of OPC specimens [Table 3.8] is shown in Figure 4.21. The effective porosity of specimens increases significantly with increasing conditioning temperature due to the fact that specimens conditioned at higher temperatures lose a greater amount of internal moisture during the conditioning period.

As the number of cycles increase, the effective porosity of the specimens conditioned at 30°C and 40°C decreases. The greatest reduction occurs at the second cycle. The effective porosity decreases because the specimens are quite dry prior to exposure to the NaCl solution. During wetting and drying cycles, the moisture content increases, eventually reaching equilibrium with the environment.

The effect of conditioning temperature on the effective porosity decreases as the number of wet/dry cycles increase.

Figure 4.21: Effect of conditioning temperature on effective porosity at the beginning of each cycle

4.5.2 Weight changes

Figure 4.22 shows the effect of conditioning temperature on weight changes of concrete during wetting and drying cycles. It can be seen that there is a large net weight gain in the case of the specimens cured at 30°C and 40°C. This is because they have very high effective porosities prior to exposure to NaCl solution [Figure 4.21].

During the first wetting phase, the weight gain for specimens conditioned at 30°C or 40°C is much greater than for those conditioned at 20ºC. This is due to the fact that the former have higher effective porosities than the latter. The weight of the salt solution absorbed by the cubes conditioned at 30°C is about three times greater than that absorbed by the cubes conditioned at 20°C. Similarly, the weight of salt solution absorbed by the cubes cured at 40°C is twice as high as those conditioned at 30°C.

The amount of NaCl solution absorbed by the specimens cured at 30ºC and 40°C decreases significantly as the number of wet/dry cycles increase. In addition, the amount of evaporation decreases with increasing number of wet/dry cycles. This is due to an increase in the moisture content of concrete and is most noticeable during the first cycle.

Enhancement of the pore structure due to cement hydration as well as chloride binding and salt crystallization may also contribute to the reduction in absorption.

1 2 3 4 5 6 7 8

1 2 3 4 5 6

Effectiveporosity (%byvolumeofsample)

Number of cycle

conditioning temperature: 20

conditioning temperature: 30

conditioning temperature: 40

The weight gains and losses for the concrete conditioned at 20°C reduce with increasing number of wet/dry cycles despite the fact that the effective porosity is constant. This also suggests that pore refinement occurs during wet/dry cycles.

Figure 4.22: Effect of conditioning temperature on weight changes of specimens

4.5.3 Sorptivity

Figures 4.23-a and b show the effect of conditioning temperature on weight and distance sorptivities of concrete. From Figure 4.23-a, it can be seen that the conditioning temperature has a significant effect on the weight sorptivity at the first cycle. However, this becomes less noticeable during subsequent cycles. Initially, the conditioning temperature also has a significant influence on the distance sorptivity but during subsequent cycles this too becomes less noticeable.

As expected, the specimens conditioned at higher temperatures have higher effective porosities and sorptivities than similar specimens conditioned at lower temperatures at the first cycle. This trend seems to persist during subsequent wet/dry cycles in the case of weight sorptivity but not distance sorptivity.

The effective porosities of specimens conditioned at 30°C and 40°C decrease significantly during the second and subsequent cycles which result in significant reductions in both their weight and distance sorptivities. The reason for the decrease in the effective porosities of specimens conditioned at 30°C and 40°C is that whilst they absorb greater amounts of salt solution during the wetting phase than those conditioned at 20°C, they lose approximately the same amount of salt solution as the specimens conditioned at 20°C [Figure 4.22].

-10 0 10 20 30

0 20 40 60 80

Weightchanges(g)

time(day)

conditioning temperature: 20

conditioning temperature: 30

conditioning temperature: 40

Consequently, both the effective porosity and sorptivity of specimens conditioned at higher temperatures decrease.

As the number of cycles increase, the sorptivity of all the specimens decreases until they reach a more stable value after five cycles. The effective porosity of concrete conditioned at 30°C and 40°C decreases as the number of cycles increase. This causes a reduction in the sorptivity. However, the effective porosity of specimens conditioned at 20°C is constant.

This suggests that the reduction in the sorptivities is partially because of the pore refinement.

The sorptivities of all the samples are similar after six wet/dry cycles. This is due to the fact that they have similar pore structures (i.e. compressive strength and absolute porosity) and the fact that the differences between their effective porosities reduce since they are subjected to the same exposure conditions.

Figure 4.23-a: Effect of conditioning temperature on weight sorptivity

Figure 4.23-b: Effect of conditioning temperature on distance sorptivity

4.5.4 Penetration of salt solution and chloride ions

0

The depth of salt solution penetration at the end of each wetting phase in the concretes conditioned at different temperatures is presented in Figure 4.24. The chloride contents are plotted against the depth of penetration after the first and sixth cycle in Figures 4.25-a and b.

At first cycle, the conditioning temperature has a significant effect on the depth of salt solution penetration. The depth of penetration is double and triple in concretes conditioned at 30°C and 40°C, respectively, as compared to those conditioned at 20°C. However, there is a significant reduction in the depth of salt solution penetration of the specimens conditioned at 30°C and 40°C at the second cycles as their effective porosity and sorptivity decrease and all specimens have similar depths of salt solution penetration. During subsequent cycles, the depth of salt solution penetration decreases for all concretes. This is consistent with their distance sorptivities [Figure 4.23-b].

The conditioning temperature also has a significant effect on the chloride penetration at the first cycle. The chloride contents in the cubes conditioned at higher temperatures are greater than the chloride contents in the cubes conditioned at lower temperatures. The reason is that they lose more moisture during the conditioning period and thus their effective porosity and absorption are greater. This results in higher chloride contents in concrete. This shows the importance of the moisture content of concrete on chloride penetrability.

After six cycles, the difference in the chloride profiles is smaller compared to those obtained after one cycle. This is because the difference in the effective porosity and sorptivity between specimens is smaller [Figures 4.21 and 4.23-a].

The depth of chloride penetration in the concrete conditioned at 20°C is similar to the depth of salt solution penetration (12.5mm) at the first cycle. However, concrete conditioned at 30°C and 40°C shows greater depths of salt solution penetration than chloride penetration.

The depth of salt solution penetration is up to 30mm in the specimens conditioned at 40°C whereas the depth of chloride penetration is about 20mm.

At the end of the sixth cycle, the depth of chloride penetration is 17.5, 21 and 24mm from the exposed surface for the specimens conditioned at 20°C, 30°C and 40°C, respectively.

Figure 4.24: Effect of conditioning temperature on salt solution penetration at the end of each wetting phase

Figure 4.25-a: Effect of conditioning temperature on chloride penetration at the end of first cycle

Figure 4.25-b: Effect of conditioning temperature on chloride penetration at the end of sixth cycle

4.5.5 Apparent diffusion coefficient, ܦ

, and surface chloride concentration, ܥ

The effect of conditioning temperature on apparentܦ௖andܥ௦is shown in Figure 4.26. Like weight sorptivity, apparentܥ௦increases as the conditioning temperature increases at the first cycle. However, conditioning time has no effect on the apparent ܥ_௦as well as weight sorptivity after six cycles [Figure 4.23-a]. As mentioned previously, Bamforth et al (1997)

0

found a relationship between the surface chloride concentration and sorptivity of concrete which agrees with the finding here.

The conditioning temperature has a significant influence on apparentܦ௖, particularly at the first cycle. The apparentܦ௖increases as the conditioning temperature increases. In fact, the apparentܦ௖of concretes follows the same trend as their effective porosity [Figure 4.21].

Figure 4.26: Effect of conditioning temperature on apparentܦ_௖andܥ_௦at the first and sixth cycle

0 0.4 0.8 1.2 1.6

1 6

ApparentCs(%wtconcrete)

Number of cycles

2.10E-11 4.20E-12

3.40E-11 7.70E-12

4.50E-11 1.10E-11

0.00E+00 2.00E-11 4.00E-11 6.00E-11

1 6

ApparentDc(m²/s)

Number of cycles

conditioning temperature: 20 conditioning temperature: 30 conditioning temperature: 40