Durability Characterization And Evaluation Methods Of Ultra-lightweight Foamed Concrete

Ultra-lightweight foamed concrete（ULFC）is a kind of foamed concrete with a dry density of less than 400 kg/m³,which has excellent thermal insulation and crumple energy absorption properties.It attracts more attention in practical engineering applications,such as aircraft arresting systems,thermal insulation walls and roofs.The current standard test methods of foamed concrete are most directly applied to study the ULFC durability.It has been found that the evaluation of the frost and carbonation resistance of ULFC according to the current standards has a few shortcomings,such as easy cracking due to the drying process in pretreatment and too fast failure speeds to show the durability differences of ULFC with different mix proportions and microstructures.A new pretreatment method of specimens is explored based on its special pore structure and application environment.The influence of test parameters on the deterioration process is also studied.According to the results of experiments,a new test procedure and evaluation criteria of frost and carbonation resistance for ULFC are proposed for its future durability design.The main contents are as follows:1.The influence of the pretreatment method,freeze-thaw parameters on the frost resistance and microstructure of ULFC is studied.Draining after soaking in water of the specimen is introduced in the pretreatment method.The uniform distribution of water inside the specimen and the water content adjustment is realized by optimizing the soaking method,reversing the specimen regularly and controlling the draining time.A suitable test method for frost resistance of ULFC is proposed.The volumetric water content of 100 mm cube specimens is adjusted to（8±1）%by controlling the draining time（drained for 6 h and reversed once every 30 min.）after soaking in water（suspended in water soaking for 24 h and reversed once for another 24 h）.After sealing the specimen,it is frozen and thawed in the air at-15℃～15℃,and its frost resistance is characterized by strength loss and mass loss.2.The effects of the pretreatment method,carbonation method and key test parameters on ULFC carbonation performance and microstructure are studied.The pretreatment method of placing specimens in the specified temperature and humidity environment（The temperature and humidity are the same as the subsequent carbonation test）to achieve a constant mass is proposed,which avoids specimen shrinkage and cracking caused effectively.The results show that the carbonation rate of ULFC can be effectively slowed down by reducing CO₂ concentration and increasing relative humidity.Considering that ULFC is actually in service in high humidity,low CO₂ concentration environment,it is proposed that the suitable carbonation test method for ULFC is to conduct three-dimensional carbonation of the specimens at 20℃,90%RH and 2%CO₂ concentration,and the carbonation shrinkage and carbonation change of strength are used to characterize their carbonation resistance.3.The advantages of the new method of frost and carbonation resistance test are obtained through comparison tests with the current test methods using two types of representative ULFC products.The cracking problem of ULFC specimens is effectively solved by the new specimen pretreatment method.As a result,the durability test results are more stable and the deviation is smaller.Furthermore,the durability difference between ULFC with different mix propositions and microstructure can be shown.4.Based on the proposed parameter determination test and comparison test results,the standard for inspection and assessment of the frost and carbonation resistance of ULFC is established.The compressive strength loss shall be less than20%,and the mass loss shall be less than 5%after 15 freeze-thaw cycles.In addition,after complete carbonation of ULFC specimen,the carbonation shrinkage shall be less than 5000×10^-6,and the carbonation change of strength shall be more than-30%.For the EMAS,the frost resistance coefficient shall be 0.80～1.20,and the mass loss rate shall be less than 5%after 15 freeze-thaw cycles.After complete carbonation,the carbonation shrinkage shall be less than 5000×10^-6,and the carbonation change of strength shall be-30%～30%.