Durability And Service Life Prediction Of Concrete Subjected To The Combined Action Of Freezing-Thawing, Sustained External Flexural Stress And Salt Solution

Durability is one of the most important properties to evaluate the quality of concrete. Lots of original achievements and experiences in this field have been reported. A great deal of sound suggestions to improve the durability of concrete has been proposed. Although materials science and technology have being highly developed, there are still many problems remained unresolved by now when concrete structures are exposed in severe environment, for example, the service life prediction and durability evaluation of concrete. Generally, the service life of concrete is estimated based on experiences. Despite having been extensively investigated the service life of concrete still can not been designed accurately. The durability of concrete involves frost resistance, corrosion, permeability, carbonation, stress corrosion, chemical attack and so on. Generally, these properties are studied separately. The properties of concrete have been well understood under the separate action of these durable factors. In fact, the degradation of concrete is the results of combined action of mechanical stress, physical and chemical attack. The degradation of concrete can be accelerated by combined action of several deterioration factors. The conclusions obtained from the separate experimental method are not always correct and its reliability is insufficient. According to this view, a serials of new experimental methods that combining the action of freezing-thawing cycles, sustained external flexural stress, deicing salt and sulfate attack to concrete were designed to evaluate the durability of concrete. An appropriate setup was designed to exert controllable flexural stress to concrete specimens. The properties of concrete under the combined action of two or more damage factors and its synergetic effect were investigated systematically. A model that integrates both the loss of weight and change of relative dynamic modulus of elasticity (Er) of concrete specimen is presented in the paper. The measures of restraining the damage of concrete, such as air entraining, adding steel fiber and combined air entraining and adding steel fiber were discussed. Service life prediction based on the accelerated test results was proposed in this paper as well. The main conclusions are as follows: The experiment results under the action of single deterioration factor of freezing-thawing cycles showed that the degradation of concrete was a pure physical process. During the process the concrete microcracked and became uncompacted gradually. According to the experiment results of concrete C40, C60 and C80, water to cement ratio (w/c) had a significant effect on the frost resistance of concrete. With the decrease of w/c, the frost resistance of concrete improved. When concrete subjected to the action of both the freezing-thawing cycles and external flexural stress, the deterioration of Er of concrete specimens was greatly accelerated compared to that when subjected to the single action of freezing-thawing cycles. The higher the stress level reached, the more quickly the Er deteriorated. When concrete specimens were subjected to the combined action of freezing-thawing cycles and external flexural stress, not only the degradation was accelerated, but also the failure mode exhibited more brittle than that without external stress. The freezing-thawing experimental results of concrete specimens immersed in solution showed that various solutions had different influence on the damage of concrete. The freezing point of pore solution in concrete was depressed by both the sodium chloride solution with concentration of 3.5% and sodium sulfate solution with concentration of 5.0%. The degradation rate was slowed by the lower freezing point. There were two different effects on the frost resistance of concrete for two kinds of solutions. 1) After 300 cycles of freezing-thawing, the decline of Er of concrete in sodium sulfate solution was accelerated compared to that in fresh water due to sulfate attack. However, the decline of Er of concrete in sodium chloride solution was not accelerated throughout the experiment. 2) The weight loss of concrete specimens in sodium chloride solution was accelerated during freezing-thawing cycles compared to that in fresh water, but decelerated in sodium sulfate solution. Freezing-thawing cycles was the promotive deterioration factor for concrete subjected to the simultaneous action of freezing-thawing cycles, sodium chloride solution and sustained external flexural stress. The simultaneous action of three damage factors combined the negative effects on the properties of concrete. The Er of concrete declined promptly. Severe scaling occurred, which lead to the considerable weight loss of concrete. Both the loss of weight and decline of Er were possible to exceed the critical values and resulted in the failure of specimen. The degradation degree and rate of concrete subjected to combing action of two or three deterioration factors were greater than superimposition of damage caused by single deterioration factor, that was to say that synergetic effect of damage existed. Damage restraint measures of air entraining and adding steel fiber were also investigated in this study. The first measure—air entraining could release the stress in concrete caused by freezing-thawing and increase the permeability resistance of concrete, improving the frost resistance of concrete. The weight loss of air-entrained concrete (APC) was only about 30~40% of that of the NPC. The freezing-thawing cycles that APC could undertake were about 30~60% greater than those of NPC. The damage of concrete was restrained significantly by the entrained air. The second measure—adding steel fiber retarded the initiation and propagation of microcracking. Steel fiber bridged the two sides of cracks in concrete, which resulted in remarkable improvement of fatigue resistance of concrete. So, the damage andfailure of concrete caused by freezing-thawing cycles were decelerated, and the freezing-thawing cycles that concrete could undertake increased strikingly. The failure mode of concrete changed from brittle cracking to the gradually decline of Ed. The improvement of Er of concrete reinforced with steel fiber was more significant than that with entrained air. The experimental results also showed that when the air was entrained, the weight loss reduced remarkably while the steel fiber reinforcement had little effect on the weight loss. The most effective measure to mitigate the damage of concrete was the combination of entrained air and steel fiber reinforcement. The steel fiber reinforced concrete with air-entrained (ASFRC) combined the positive effects of entrained air and steel fiber reinforcement and the synergetic effect generated. The combined effect was superior to that of only entrained air or steel fiber reinforcement separately. The durability of concrete was significantly improved by the combined measure and the service life of ASFRC increased remarkably. The damage of concrete subjected to the action of combined several deterioration factors can be considered as a low cycle fatigue process, primarily caused by freezing-thawing cycles. Based on this point, the deterioration models were developed in this thesis. The periodic stress was exerted to concrete specimens during freezing and thawing. The stress distributing in the whole concrete specimen could be simplified as the stress condition of tension in three directions. Undoubtedly, the fatigue stress level was increased by the external flexural stress during freezing and thawing. However, the stress level of concrete immersed in solutions was a little smaller than that in fresh water during freezing and thawing. Based on the systematic experiments and theoretical analysis, the weight loss and the Er change of concrete specimens subjected to the combined action of several damage factors could be described as function of cycles of freezing and thawing as follow. Wl=00GGn ? G=a?lg(b?N+1)(1+c? 1+10 1 (00.(0 01.?0N1 ??Nd?)d)) (5-7) Er=En/E0=Aexp(-(k?N) f ) (5-23) In the accelerate test, degradation degree and life of concrete when subjected to the action of freezing-thawing or to the combination of sustained external flexural load, deicing salt sodium and sulfate attack could be determined using equation (5-7), (5-23) and equation (6-5). Subsequently, considering accelerating factors, the service life of concrete could be predicted. 0.400.600.05ω= 1 ±0.05?Wl ?Er? (6-5) This model assembled the weight loss and Er change which perhaps was the pioneerinvestigation for the service life prediction of concrete subjected to this kind of deterioration. Research works in this thesis can be considered as the groundwork for understanding the durability and service life prediction of concrete comprehensively and systematically in the future.