اثر شرایط محیطی کنترل شده بر خواص مکانیکی، ریزساختار و دوام ملات سیمان
Abstract
1- Introduction
2- Experimental program
3- Results
4- Conclusions
References
Abstract
Environmental conditions, such as temperature and relative humidity, impact the rate of evaporation, mechanical properties and durability of concrete. Thus, they have a direct effect on the development of transport properties. The purpose of this study is to investigate the effect of controlled environmental conditions on moisture retention, dielectric constant (DC), compressive strength, shrinkage, water sorptivity index, rapid chloride migration test (RCMT), microscopical analysis (thin sections) and electrical resistivity. The mortar specimens were prepared using Ordinary Portland Cement with two water-to-cement ratios of 0.42 and 0.52. The specimens cured in the controlled environments of 25, 46 and 65 °C for temperature, and 30% and 90% for relative humidity. The results showed that curing under higher temperature cause reduction in compressive strength and increase in the sorptivity index. Moreover, a great correlation between DC measurements and sorptivity, the RCMT and electrical resistivity was found. Higher relative humidity helped the performance of samples, considering the durability and mechanical properties.
Introduction
It is well-known that environmental conditions, such as relative humidity and temperature, can have a great influence on both mechanical and especially durability-related properties of concrete, which control the final quality. One of the key factors that controls the concrete’s quality is moisture evaporation through the concrete that can cause cracking, therefore reducing the durability of concrete in terms of service life. Thus, in order to optimize chemical reactions, providing a suitable temperature and relative humidity in the curing process seems necessary to prevent excessive water loss through the surface of concrete during the hydration process [30,8,62]. During continuous evaporation on the surface of early-age concrete, a negative capillary pressure is built-up and continues to rise with the progress of moisture loss that causes surface tension forces, and eventually cracking [56]. Bakhshi et al. [15] with the use of a fluid-mechanic based approach, provided a model for the prediction of evaporation rate and concluded that an increase in the ambient temperature would lead to an extreme rate of water content evaporation in concrete that will be escalated by a decrease in ambient relative humidity and an increase in wind speed [25]. Determining the dielectric constant (DC) of concrete is a nondestructive test (NDT), which can show the water content of concrete in various stages. Basically, the DC is a material’s physical characteristic that demonstrates its ability to store electrical energy. The DC of free water is 80.1 at 20 C, which is considerably higher than solids, such as aggregate, cement and hydration products, which have a DC around 3–8. Therefore, a shift in the free water content in concrete can cause a change in DC values [23,57,66]. Water in concrete can be divided into three categories, including chemically bound water, physically bound water, and free water. Considering that chemical and physical water has a powerful molecular bond, their DC is not remarkable. Hence, a shift in free water content can affect DC and demonstrate the alteration of free water into chemically or physically bounded water, which is evidence of the continuous hydration process and consumption of free water over time. Shen et al. [55] stated that free water content is alluded to capillary pores at early ages, therefore DC can reflect the capillary pores. They also observed that there is a linear relationship between free water content DC. This method is excellent to determine water content in the fresh concrete mixture.