This paper presents the experimental methods and findings in obtaining the strain behavior of mortar during freezing and thawing cycles (FTC) at the meso-scale under fully saturated condition and the coefficient of thermal expansion (CTE) and elastic modulus of mortar after FTC tests. A heat-cool cycle test comparing an oven-dried and undried mortar was also performed prior to the FTC test, confirming that oven drying at 105°C does not affect the CTE of samples, thereby ensuring the reliability of the results. During FTC with constant moisture content, a limitation in the increase in tensile strain was observed and this tensile strain decreased until contraction was observed. The contraction is attributed to the removal of gel pore water arising from negative pressures. Due primarily to the absence of available water supply, the displaced pore water cannot be refilled, which results in contraction at the end of the FTC. More importantly, the results show that after FTC, the CTE of frost damaged mortar increases while its elastic modulus decreases, primarily owing to microcracking when frost damage sets in. Microcracks act as broken bridges that can detach the aggregate from the hardened cement paste and in effect reduces the thermal restraints that each part (fine aggregate and cement paste) exerts on the other. The hardened cement paste can then expand/contract more freely under temperature variation, and thus can significantly affect (increase) the CTE of the whole composite (mortar). Further, stress transfer in the material is prevented due to microcracking resulting in elastic modulus reduction.
Recycled aggregate concrete (RAC) becomes a great concern for massive natural resoure comsumption and a huge construction waste. Many researchers have carried out numerous investigations on the durability of RAC. The purpose of this paper is to summarize the achievements and share them with researchers involved in the study on RAC. In this paper, the main durability index and test methods have been reviewed from worldwide literature. Secondly, the durability of RAC including chloride diffusion, freezing and thawing resistance, abrasion resistance, absorption, drying shrinkage and carbonation has been summarized based on literature study. Thirdly, the Grey Relational Analysis (GRA) has been applied to find the most critical factor determining the durability of RAC compared with that of natural aggregate concrete (NAC). At last, new types of recycled concrete have been discussed and compared with NAC.
In this research, an existing multi-scale model for shrinkage behavior is enhanced to improve its applicability and precision. The enhancement work focuses on the initial autogenous shrinkage and intrinsic driving forces of the shrinkage at the microscale. The existing model underestimates the rapid development of the autogenous shrinkage of cement with a low water/cement (w/c) ratio. Hence, combined with self-desiccation, the autogenous shrinkage at an early age is first discussed. As an approximation, a portion of the autogenous shrinkage at an early age is quantitatively calculated using the chemical volume change and distance between cement particles, and that portion is added to compensate for the underestimation in the existing model. Furthermore, in the existing model, capillary tension is assumed to be the principal driving force in all of the pores with various sizes, which would cause an overestimation of the long-term shrinkage in the analysis. Therefore in the enhanced model, the driving forces are discussed and modified. The capillary tension is assumed to only be active in relatively coarse pores, whereas the disjoining pressure dominates in fine pores on the nanoscale. These two driving forces are quantified by the water status in the pores using the proposed formulas. With the above enhancements, the autogenous and drying shrinkage behaviors with various w/c ratios and relative humidity can be simulated reasonably.
The service life deterioration of concrete is mainly a function of the quality of cover concrete. Whereas most past designers considered the cover size and diffusion property as the key quality parameters for durability design, whereas recent research has come to the conclusion that the water penetration front is another key issue regarding cover quality that dramatically affects the service life as well as LCC of RC structures. In this study, Cl- ion profiles, the absorption of water, and the liquid water front position along with its variation for two types of concrete, concrete from a bank protection structure located in Okinawa and slag concrete with 70% w/c ratio in the laboratory were examined. The methods of analysis were divided into three groups. Each type of concrete was analyzed with each of the methods and compared with the actual chloride profile data. It was found that optimum method of analysis depends on the type of concrete to be analyzed. A classification of concrete types along with their respective analysis methods that modifies Fick’s law of diffusion based on the behavior of stagnation of the liquid water front in concrete is proposed.