Drying temperature

The following section discusses the effect of drying temperature on:

 Effective porosity

 Weight changes

 Sorptivity

 Penetration of salt solution and chloride ions

 Apparent diffusion coefficient and surface chloride concentration

4.6.1 Effective porosity

The effect of drying temperature and number of cycles on the effective porosity of concrete [Table 3.9] is presented in Figure 4.27.

Specimens have similar effective porosity at the first cycle as they are cured and conditioned identically. Specimens dried at 30ºC and 40°C experience a significant increase in their effective porosities after the first drying phase and then their effective porosity remains approximately constant during subsequent cycles. The significant increase in effective porosity at the second cycle is because they are exposed to higher temperatures (30 ºC and 40°C) compared to their conditioning temperature (20°C) for the first time and thus they lose a greater amount of moisture during the drying phase than the conditioning phase.

Figure 4.27: Effect of drying temperature on effective porosity at the beginning of each cycle

1 2 3 4 5 6 7 8

1 2 3 4 5 6

Effectiveporosity (%byvolumeofsample)

Number of cycle

drying temperature: 20

drying temperature: 30

drying temperature: 40

4.6.2 Weight changes

The effect of drying temperature on the weight change of concrete specimens is presented in Figure 4.28. At the first wetting phase, all samples have similar weight gain as they have similar effective porosities. It can be seen that the drying temperature has a significant effect on the weight change of concrete. After the first wetting, the weight changes are significantly greater both during the wetting and drying phases in specimens dried at higher temperatures.

As the number of cycles increase, the weight changes decreases in both wetting and drying phases, particularly in the specimens dried at 30 ºC and 40°C. This is because enhancement in pore structure occurs due to development in hydration, chloride binding and crystallization. The greater reduction in weight changes for specimens dried at higher temperatures can be explained by the fact that the volume of salt solution enters the concrete is higher and therefore chloride binding and crystallization are higher.

There is a net weight loss for all the samples at the end of six cycles. For specimens dried at 30ºC and 40°C, the weight loss is because of the higher drying temperatures than their conditioning temperature. For specimens dried at 20°C, the weight loss is small which may be as a result of further hydration.

All the samples reach a stable moisture state at fifth exposure cycle.

Figure 4.28: Effect of drying temperature on weight changes of specimens

-40 -30 -20 -10 0 10

0 20 40 60 80

Weightchanges(g)

time (day)

drying temperature: 20

drying temperature: 30

drying temperature: 40

4.6.3 Sorptivity

Figures 4.29-a and 4.29-b show the effect of drying temperature on weight and distance sorptivity of specimens subjected to six wet/dry cycles. The distance sorptivity of specimens shows a similar trend to their weight sorptivity.

At the first cycle all specimens have a similar sorptivity as they are cured and conditioned similarly and they have similar effective porosities. The small variation in their first cycle sorptivity is simply due to random fluctuations. During subsequent cycles, the sorptivity increases as the drying temperature increases. However, the sorptivities of samples dried at 20°C are approximately constant and slightly lower than the first cycle sorptivity. The sorptivities of specimens dried at 30ºC and 40°C increase dramatically in the second cycle as a result of exposure to higher temperatures than conditioning temperature and thus significant increase in the effective porosity. Thereafter, sorptivities decrease as the number of cycles increase. It is interesting to note that the reduction in the sorptivity continues until the last cycle. This shows that the stable value of sorptivity is not achieved after six cycles and there might be more reduction in sorptivity during further cycles.

The reduction in the sorptivity is again thought to be as a result of changes in the pore structure (i.e. development of hydration, chloride binding and salt crystallization) as the number of cycles increase.

The reduction in the sorptivity is greater for the specimens with a drying temperature of 40°C than those with a drying temperature of 30°C. This is due to the fact that a greater volume of salt solution enters the concrete dried at 40°C and thus there will be more chloride binding and salt crystallization.

Figure 4.29-a: Effect of drying temperature on weight sorptivity

Figure 4.29-b: Effect of drying temperature on distance sorptivity

4.6.4 Penetration of salt solution and chloride ions

The depth of salt solution penetration in the concrete exposed to different drying temperatures is shown in Figure 4.30. The chloride profiles after the first, third and sixth cycle are presented in Figures 4.31-a, 4.31-b and 4.31-c, respectively.

The depth of salt solution penetration has a similar pattern to the weight and distance sorptivity. At first cycle they are approximately the same. The depth of salt solution penetration in the specimens dried at 20°C decreases during the next cycles and for concretes dried at 30ºC and 40°C it increases at the second cycle and then decreases during subsequent cycles.

The depth of salt solution penetration is higher in the specimens dried at higher temperatures. This is most significant at the second cycle and the variation in depth of penetration decreases as the number of cycles increase.

0

In the case of chloride penetration, all samples have similar chloride profiles at the first cycle as they have similar effective porosities and similar sorptivities. During subsequent cycles, samples dried at higher temperatures have greater chloride contents.

The depth of chloride penetration at the first cycle is about 7.5mm [Figure 4.31-a]. This is half the salt solution penetration depth. At the sixth cycle, chloride penetrates up to 17.5, 25 and 30mm from the exposed surface in concrete dried at 20ºC, 30 ºC and 40°C, respectively [Figure 4.31-c]. This shows the significant effect of temperature on chloride penetration in concrete.

Figure 4.30: Effect of drying temperature on depth of salt solution penetration at the end of each wetting phase

Figure 4.31-a: Effect of drying temperature on chloride penetration at the end of first cycle

0 10 20 30 40

1 2 3 4 5 6

Depthofsaltsolution penetration(mm)

Number of cycle

drying temperature: 20

drying temperature: 30

drying temperature: 40

0 0.2 0.4 0.6 0.8 1 1.2 1.4

0 10 20 30

Cl(%wtconcrete) Firstcycle

depth (mm)

drying temperature: 20 drying temperature: 30 drying temperature: 40 Cl content: 0.05%

Figure 4.31-b: Effect of drying temperature on chloride penetration at the end of third cycle

Figure 4.31-c: Effect of drying temperature on chloride penetration at the end of sixth cycle

4.6.5 Apparent diffusion coefficient, ܦ

, and surface chloride concentration, ܥ

Figure 4.32 shows that effect of drying temperature on apparentܦ_௖andܥ_௦at the first and sixth cycle. The apparentܥ_௦increases as the drying temperature increases after six cycles.

This is probably due to the fact that effective porosity and sorptivity increase as the drying temperature increases. The effect of drying temperature is less significant when the temperature increases from 30°C to 40°C, which is similar to their effective porosity and sorptivity.

The apparentܦ௖ increases significantly as the drying temperature increases. The drying temperature had a significant effect on the effective porosity but had no effect on the compressive strength and absolute porosity. This also suggests that apparentܦ௖is more affected by effective porosity than pore structure of concrete.

0

Figure 4.32: Effect of drying 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

6.70E-12 3.57E-12

6.90E-12 9.80E-12

5.91E-12 3.20E-11

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

1 6

ApparentDc(m²/s)

Number of cycles

drying

temperature: 20 drying

temperature: 30 drying

temperature: 40