PREDICTION OF FROST DAMAGE CONSIDERING PORE STRUCTURE CHANGE BY DRYING CONDITIONS

Abstract

 Frost damage is one of the serious deterioration cases of concrete structures in cold regions. The probability of its occurrence depends on temperature, solar radiation and water content of concrete. In addition, various researches have been conducted to evaluate the regional characteristics of frost damage using weather conditions and to estimate deterioration based on freezing process and material conditions. On the other hand, it has been clarified by Aono et al. that the pore volume of 40-2000 nm in diameter which affects the frost resistance increases due to drying conditions. However, existing methods of frost damage assessment and deterioration prediction still not take into account the decrease in frost resistance caused by drying. Therefore, in this study, we propose a calculation method for predicting deterioration after taking into account the decrease in frost resistance due to drying.

 A formula expression between the amount of pores and the durability factor has been established, but a relational expression between the pore volume and the amount of decrease in relative dynamic modulus per cycle has not been established. Therefore, after obtaining the pore volume range suitable for the objective variable of the relative dynamic modulus decrease per cycle, the relational expression was derived.

 A method using maturity of temperature and humidity has already been proposed as a method for predicting pore volume change of 40-2000 nm in diameter due to the drying conditions. This method cannot take into account the change of pore volume of 40-2000 nm in diameter due to the other factors except the water-cement ratio. Therefore, the ratio of the maturity of temperature and humidity and the initial pore volume of 40-2000 nm in diameter to the pore volume of 40-2000 nm in diameter after drying (the rate of change in the pore volume of 40-2000 nm in diameter) was obtained. As a result, the natural logarithm of the rate of change in the pore volume of 40-2000 nm in diameter and the square root of maturity of temperature and humidity have a linear relationship. The slope and the upper limit can be expressed by the water cement ratio.

 In order to predict the decrease in the relative dynamic modulus, the number of freezing and thawing cycles in winter and the amount of decrease in the relative dynamic modulus per cycle are required. Since the equivalent cycle to ASTM test is the one·year weather condition converted into number of freezing and the.wing cycles, so the number of freezing and thawing cycles in winter can be expressed using this index. Here, a method for calculating the amount of decrease in relative dynamic elastic modulus after drying was established from the relational formula between the amount of decrease in relative dynamic elastic modulus and the amount of pores per cycle and the relational formula between maturity of temperature and humidity and the rate of change in pore volume. By calculating these coefficients, the amount of decrease in the relative dynamic modulus per year can be determined. Therefore, it is possible to predict the decrease in the relative dynamic modulus considering of the decrease in the frost resistance due to drying by integrating the amount of decrease in the relative dynamic modulus which is calculated every year and subtracting it from the initial value.