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In addition to temperature variation, another key environmental variable in the curing of concrete is the relative humidity or availability of curing water. Because the solid cement hydration products occupy less space than the starting solid reactants and water, external water will be imbibed into the concrete throughout its curing history. When external water is unavailable, empty porosity will be created within the cement paste, influencing reaction kinetics, microstructure, and physical properties. The NIST microstructure model can be operated under either saturated or sealed curing conditions [8]. Figure 9 provides a comparison of model and experimental results for degree of hydration vs. curing time for an OPC paste prepared with initial w/c=0.30. The sealed curing conditions are seen to substantially hamper the progress of the hydration reactions at longer times (> 7 days). The agreement between the model predictions and experimental measurements is observed to be well within the scatter of the experimental data.
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The microstructure model also allows a direct observation of the resulting microstructure for hydration performed under saturated and sealed conditions. Figure 10 provides a set of two-dimensional images from the final three-dimensional simulated microstructures for a w/c=0.30 OPC paste hydrated under the two different curing conditions, as well as two real SEM images obtained on the experimental counterparts of the model systems [40]. The effects of self-desiccation are clearly evident as the microstructure formed under sealed curing contains many large capillary pores and large grains of unhydrated cement, both of which are practically eliminated under saturated curing. The increased volume of unhydrated cement present under sealed curing is consistent with the degree of hydration results presented in Figure 9. The model microstructural images are seen to compare very favorably with their equivalent SEM counterparts obtained directly on the real hydrated cement pastes.
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While the empty porosity created during sealed curing definitely leads to a lower compressive strength, its effects on transport properties are not well determined. If the pores remain empty, they provide no pathway for transport, effectively acting as inert filler material. However, if these pores later refill with water, without any accompanying increase in the formation of hydration products (a worst case scenario), the effects on transport properties could be dramatic. By directly computing the relative electrical conductivity (equivalent to the relative diffusivity [44]) using a conjugate gradient technique [16], these effects can be quantified. Figure 11 provides a comparison of model results for a w/c=0.4 OPC paste. In both cases, the diffusivity is seen to decrease with increasing degree of hydration in a characteristic S-shaped manner [44]. However, because of the rather large volume fraction of empty capillary porosity present in the microstructures at later ages, resaturation may increase the diffusivity of these materials by up to a factor of three or more. Since, for many degradation processes, service life is directly proportional to a diffusion coefficient [50], this could result in a threefold reduction in usable life for a concrete structure. Thus, understanding both the microstructure and environmental exposure of these materials is critical in the design and rehabilitation of concrete structures.
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