Modeling of Onset of Spalling of Concrete during Fire by Pore Pressure Rise

这项工作是关于；1)火灾中混凝土开始剥落时热应力和孔隙蒸汽压的敏感性分析；2)使用小尺寸试样进行实验分析；3)开发混凝土材料内热、质量（水蒸气）和应力同时传递的计算机代码。1)在灵敏度分析方面，建立了简单的二维传热传质程序模型。对采用标准时温曲线加热的钢瓶进行了模拟。在平面应变假设下，计算得到的热应力。结果表明，热应力随混凝土强度的增加而增大。与热应力和孔隙压力相比，热应力在量级上占主导地位。孔隙压力的影响可能有一定的作用，但孔隙压力本身不能成为剥落的主要原因。根据敏感性分析的结果。2)对40、110、150 MPa三种不同强度的圆柱形试件进行了试验。选取轴向载荷和加热不均匀性作为参数进行研究。结果表明：随着强度的增加，剥落发生的频率和程度都有所增加，不均匀加热对剥落程度的影响较大，轴压和端部约束的存在使剥落深度向浅层转变。综上所述，建议在三维场中分析加热初期的应力和孔隙压力。从而将用于灵敏度分析的简单模型扩展为三维模型。此外，采用隆升系数的概念，明确考虑了孔隙压力的影响。分析结果表明，加热不均匀的影响会在加热初期产生较大的热应力。

This work is concerned with; 1) Sensitivity analysis of thermal stress and pore vapor pressure upon the onset of spalling of concrete during fire, 2) experimental analysis by using small scale specimens and 3) development of computer code for simultaneous transfer of heat, mass (water vapor) and stress within concrete material. 1) As to the sensitivity analysis, a simple two-dimensional model for heat and mass transfer program was developed. Simulations were carried out for cylinders heated by standard time- temperature curve. The resulting thermal stress was calculated under the assumption of plane strain. It was found that the thermal stress is increased as the strength of concrete is increased. Comparing with thermal stress and pore pressure, thermal stress is dominant in magnitude. The effect of pore pressure may have certain effect, however pore pressure alone cannot be a main cause of spalling. Following the findings of sensitivity analysis. 2) experiments were conducted for cylindrical specimens of three different strength (40, 110 and 150 MPa). Axial loading and non-uniformity of heating were selected as parameters to be examined. The results showed at least qualitatively that frequency and degree of spalling is increased as strength is increased, that non-uniform heating cause the degree of spalling considerably and that existence of axial compression and end-restraint changes the depth of spalling to shallow layer. By summarizing above, it was suggested to analyze stress and pore pressure in the early stage of heating in three- dimensional field. Thus the simple model used in sensitivity analysis was extended into three dimensional model. In addition, the effect of pore pressure was explicitly taken into account by using the concept of uplift coefficient. The analytical results showed that the effect of non-uniform heating may cause large thermal stress in the early stage of heating.