Addition of rice husk ash (RHA) is an effective internal curing method to mitigate self-desiccation and autogenous shrinkage of hydrating cementitious materials. Although a certain number of experimental researches on this topic have been carried out, a comprehensive study about numerical simulation of mitigating effect of RHA on autogenous shrinkage of cementitious materials is still scarce. In this study, a numerical model of autogenous shrinkage of rice husk ash blended cement mortar was proposed. The proposed numerical model was based on Pickett model and improved by taking visco-elastic behaviour of rice husk ash blended cementitious materials into account. Final setting time, chemically bound water, compressive strength, internal relative humidity (RH) and autogenous shrinkage of pure Portland cementitious materials and rice husk ash blended cementitious materials were experimentally studied. Liquid absorption capacity and water vapour desorption isotherm of rice husk ash were also measured. The comparison between the simulated and measured autogenous shrinkage showed that the autogenous shrinkage of rice husk ash blended cement mortar can be predicted accurately with the proposed numerical model.

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Concrete made with Portland cement has good behaviour under compressive stress, but it is weak in tension. Cracking caused by tension is a typical problem in concrete practice. High autogenous shrinkage at early ages is one of the causes of cracking, especially in high/ultra-high performance concrete. In that stage shrinkage is strongly related to the decrease of relative humidity (RH) inside the concrete. To minimize the probability of cracking induced by autogenous shrinkage at early age, internal curing, which can provide moisture inside the concrete, has been proposed. Several internal curing agents, such as Super Absorbing Polymers (SAP) and saturated lightweight aggregate (LWA), are utilized in high performance concrete and have shown good results for mitigating autogenous shrinkage. The use of these agents, however, also have some drawbacks. Searching for an alternative internal curing methods for ultra-high performance concrete remains a challenge....@en

Shrinkage-induced cracking can impair the durability of concrete structures. In the past few decades, this topic has drawn more and more attention. Shrinkage of mortar and concrete is actually the result of the interaction between the shrinking cement paste and the non-shrinking aggregates. In recent years, different models that consider the restraining effect of aggregates, i.e. Series model and Hobbs’ model, have been proposed to predict the autogenous shrinkage of mortar and concrete. However, in these models both aggregate particles and cement paste matrix are considered as elastic materials. In fact, cement paste is not ideally elastic. Creep also plays an important role in autogenous shrinkage of mortar and concrete. In this paper an extended Pickett model, which takes the effect of creep into consideration, is proposed. The autogenous shrinkage of CEM I and CEM III/B cement mortar and concrete is simulated by using this model and compared with the experimental results to evaluate the accuracy of the predictions. The results show that the extended Pickett model can well predict the autogenous shrinkage of mortar and concrete.

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In recent years more and more attention has been given to autogenous shrinkage due to the increasing use of high-performance concrete, which always contains supplementary materials. With the addition of supplementary materials-e.g., fly ash and blast furnace slag-internal relative humidity, chemical shrinkage and mechanical properties of cement paste will be affected. These properties significantly influence the autogenous shrinkage of cement paste. In this study, three supplementary materials-i.e., silica fume, fly ash and blast furnace slag-are investigated. Measurements of final setting time, internal relative humidity, chemical shrinkage, compressive strength and autogenous deformation of the cement pastes with and without supplementary materials are presented. Two water-binder ratios, 0.3 and 0.4, are considered. The effects of different supplementary materials on autogenous shrinkage of cement paste are discussed.

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This chapter reviews the performance of rice husk ash (RHA) as partial cement replace in ordinary Portland cement concrete. After pyroprocessing with controlled combustion, highly pozzolanic RHA can be produced. Due to the high specific surface area and pozzolanic properties, RHA shows very good performance as a supplementary cementitious material in concrete. For early age properties, concrete made of RHA needs more water and high dosages of superplasticizer compared to ordinary Portland cement concrete. The RHA concrete has slightly longer setting times than ordinary Portland cement concrete. When the replacement level of ordinary Portland cement by RHA is about 20% by weight of the total binder material, the compressive strength, tensile strength and flexural strength of concrete made of RHA are enhanced. Due to the special porous structures, RHA shows excellent capacity to keep high relative humidity in the concrete, thus RHA significantly mitigates the autogenous shrinkage of concrete, especially high performance or ultra-high performance concrete made with RHA as addition. From water permeability and chloride diffusivity tests the coefficient of water absorption of RHA concrete is lower and the chloride diffusion coefficient is reduced.

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The term “Time-zero”, i.e., the time for the start of autogenous shrinkage measurement, is usually used for estimating the cracking potential of structural components. Accurate determination of the “time-zero” is therefore critical for autogenous shrinkage measurement, which is the main objective of this study. There is a general agreement about the existence of a relationship between autogenous shrinkage and RH changes in hardening cement paste. An improved hygrometer method was developed for monitoring the change of internal RH in cement pastes 1 h after casting. This provides immediate experimental results of RH change for determining the “time-zero”. It indicates that the internal RH of cement pastes does not decrease immediately after the final setting time. A new “time-zero” is defined as the onset of internal RH drop, which is more reasonable for estimating the cracking potential of cement-based materials, compared to the “time-zero” represented by the final setting time.

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It is well recognized that the high risk of early age micro-crack of HPC/UHPC is attributed to the large magnitude of early age autogenous shrinkage caused by self-desiccation in binder hydration. Over the years, several methods have been proposed to mitigate autogenous shrinkage based on internal curing theory, and a better internal curing agent is always being sought. Rice husk ash (RHA), was recognized having potencial to be an internal curing agent. In this paper, the effect of RHA on mitigating the autogenous shrinkage has been evaluated by different mean particle sizes of RHA, and the internal RH change was measured at the same situation. The results show a high efficiency of RHA for internal curing purpose, to drastically reduce the autogenous shrinkage of cement pastes. Comparing to the cement paste without RHA, the internal RH results show that the internal RH in pastes with RHA is evidently higher at either early or later ages. These were addressed in order to explain the mechanism RHA mitigates the autogenous shrinkage of pastes.@en