风积沙混凝土的抗冻性与冻融损伤机理分析

为探讨沙漠沙(又名风积沙)替代河砂对低温环境下混凝土的耐久性能影响,按照风积沙替代河砂质量的20%、40%、60%、80%、100%共设计了5种强度等级为C25的风积沙混凝土(aeolian sand concrete,ASC)。采用加速试验方法研究了风积沙混凝土在冻融条件下的损伤失效规律,借助环境电镜扫描(environmental scanning electron microscope,ESEM)、应变监测和核磁共振(nuclear magnetic resonance,NMR)等测试手段得到了风积沙混凝土的损伤机理。研究发现风积沙掺量80%以上的混凝土冻融次数超过200次,冻融损伤残余应变小,内部封闭小孔隙数量多对冻融损伤的抑制阻碍作用增强。结果表明风积沙混凝土的抗冻性能随着风积沙掺量的增加而提高,掺量为100%的风积沙混凝土的抗冻性最好。该研究可为风积沙混凝土大范围应用于寒区渠道衬砌及水利设施建设提供理论依据。

Analysis on frost resistance and damage mechanism of aeolian sand concrete

At present, the insufficient storage of the river sand and the rising cost of the construction have been unable to meet the requirements of the concrete projects in the desert area and the surrounding irrigated area. However, this condition can be greatly alleviated by the use of the rich desert sand in the surrounding irrigated area in Hetao region of Inner Mongolia, which is also known as the aeolian sand. Therefore, it is of great significance to conduct the research on the frost resistance of aeolian sand concrete (ASC), which can be an type of alternative material widely used in the construction of water conservancy projects in the cold northwest region, such as irrigated canal lining and the dam. In this paper, 5 types of C25 aeolian sand concretes were designed according to the percentage of the river sand quality replaced by aeolian sand, i.e. 20%, 40%, 60%, 80%, and 100%, which were compared with the ordinary Portland cement concrete (OPC) with the same strength. The frost resistance of these 6 types of concretes was studied through the acceleration test method in the Structure Laboratory of Inner Mongolia Agricultural University, and finally the damage laws of ordinary concrete and aeolian sand concrete were demonstrated. In addition, the damage mechanism of aeolian sand concrete under freeze-thaw conditions was analyzed by using the testing measures such as environmental scanning electron microscope (ESEM), strain monitoring, X-ray diffraction analysis (XRD), nuclear magnetic resonance (NMR), and so on. At the beginning of freeze-thaw, the increase of ettringite did make the structure of concrete dense, but the internal frost heaving force caused the continuous extension of micro cracks and pores at the end of freeze-thaw. As a result, the ASC containing 60% aeolian sand showed the layered dissection on the surface, and the dynamic modulus of elasticity increased first and then decreased, which proved the internal damage was growing gradually. After the freeze-thaw cycle worked, the residual strain of ASC of 60% was the largest and the residual strain of the 6 groups of concretes was 60%>40%>0%>100%>20%>80%. This result indicated that the internal stress caused by the 6 types of concretes through freeze-thaw cycle of 8 times was sufficient to lead to cracks in the matrix. What was more, the porosity and permeability of ASC of 20% and 40% were much larger. As for 6 types of concretes, the porosity of ASC of 60%was relatively small. However, the phenomenon of porosity increasing appeared with the increasing of the amount of mixed aeolian sand, which indicated the more the closed pore in the concrete, the lower the permeability. All the results indicated that the damage occurred when the freeze-thaw cycle of aeolian sand concrete whose amount was no more than 40% reached 100 times, while the ordinary concrete could withstand 125 times. Both of them showed the expansion of cracking and the rapid decline of quality and dynamic modulus of elasticity. Nevertheless, the freeze-thaw time that the aeolian sand concrete could withstand increased when the mixing amount of aeolian sand reached 60%-100%. At the same time, the frost resistance of concrete was improved obviously and the damage was characterized by a layered dissection on the surface. Besides, the frost resistance was the best when the mixing amount of aeolian sand reached 100%, because the aeolian sand concrete had a mass of closed capillaries and an inhibiting effect on suffering freeze-thaw damage. For this reason, it can certainly withstand the highest freeze-thaw cycle times.