نمونه متن انگلیسی مقاله
The compatibility of the admixture of cement and concrete is an important factor in the creation of highquality concrete. Common problems involving the resistance of concrete to freezing and thawing drew the authors’ attention to gypsum dehydration in cement. The influence of gypsum dehydration on the air-void structure of air-entrained concrete obtained with cement low in C3A was examined. In all studied cases, the negative influence of gypsum dehydration was observed. It is believed that greater solubility and a higher solubility rate of hemihydrate result in higher early-stage Ca2+ and SO4 2 ion concentrations, and thus cause alterations in the chemical admixture’s mechanism of action
Concrete used in road surfaces and bridges in cold-winter regions is exposed to very severe conditions. Over the past several years, it has been observed that many types of concrete used in bridges in many countries have failed to meet industry standards . The problem of the resistance of concrete to freezing and thawing has existed globally for some time . Various methods to test the frost resistance of concrete have been developed [3,4]. The test methods used most commonly rely on the subjection of concrete samples to a cycle of freezing and thawing in water and/or deicing salts. Powers  developed the concept of using the spacing factor and size of air voids as parameters used to ensure the frost resistance of concrete. The mechanism of air entrainment and the formation and stabilisation of air voids in fresh concrete has been described by many researchers, e.g. Folliard and Du . Chemical admixtures known as air-entraining agents are used to produce air-entrained concrete. Air voids create empty spaces within the concrete which act as reservoirs for freezing water moving in the capillary pores . Air-entraining agents are usually surfactants – molecules composed of a hydrophilic head, which may be either charged (ionic) or polar (non-ionic), and a hydrophobic hydrocarbon tail.