Research On Characteristics Of Internal And External Damage Of Concrete In Freeze-Thaw Environment

Freeze-thaw action has an important influence on the durability of concretestructures. Particularly in the cold regions, nearly all build ings would suffer freezingdama ge with different degrees. The macro performa nce and micro structure of concreteinternal and external will vary significa ntly after different freeze-thaw cycles. Therefore,it is very important to investigate the influence of frost action on dama ges in differentpart of concrete.Six types of concrete with different water cement ratio and with or without airentrainment had been used in this project. Cubic specimens were used to determine thecompressive strength at age of 7, 14, 28, 56 days. The other prismatic specimens wereput into freeze-thaw cycles. The relative dynamic elastic modulus , weight loss of alltypes of concrete had been measured after different freeze-thaw cycles. After thatcapillary suction were carried out after cutting specimen s. In addition, MIP test wereconducted for different parts of concrete after frost cycles. The changes of porestructures of different concrete were obtained.The results indica te that the compressive strength of air-entrained concretedecreased. And the sensitivity of the influence of water-cement ratio on strength ofconcrete was reduced with addition of air-entrainment. Furthermore, the strengthdecreased in varying degrees. The air-entrained concrete had high frost resista nce. Evenif the water-cement ratio of concrete was 0.6, specimens looked good after 450 freezethawcycles . With sma ller water-cement ratio, concrete had better frost resista nce. After acertain number of freeze-thaw cycle s, the coefficient of capillary suction and watercapacity of concrete increased. The nearer to the concrete surface, the coefficient ofcapillary suction and water capacity were higher. With addition of air-entrainmentconcrete had more pores with dimater of higher than 220nm from the results of MIP test.The changes of pore size distribution of concrete afer freeze-thaw damges could bedementrated by porosity and critica l pore size. But it is relative difficult to make goodjudgments for each part of the specimen. When ordinary concrete suffered a certain number of freeze-thaw cycles , the cumula tive pore volume of the harmless pores, lessharmful pores and harmful pores increased a little. But the cumula tive pore volume ofmore harmful pores increased significa ntly. The higher water-cement ratio, the greaterthe harmful pores increas ed. While the air-entrained concrete suffered 300 freeze-thawcycles , the cumula tive pore volume of the harmless pores, less harmful pores andharmful pores changed a little; but the cumula tive pore volume of more harmful poresincreased a little, too,within 20%. The air-entrained concrete had higher frost resista nce.