Clarification on the Spalling of Concrete Exposed to Fire

通过测量混凝土的蒸汽压、温度和开裂产生的声发射（AE），研究了强度和含水量对火灾下混凝土剥落的影响。研究结果如下：1。在剥落混凝土试样中测量了大于2.2Mpa的蒸汽压。在剥落层产生之前，观察到热裂现象。假定热裂纹对后续剥落的规模有很大的影响。湿样剥落由表面向内部扩展，并伴有局部剥落，而风干样剥落则在靠近表面的区域延伸。这被认为与样品中的水分分布有关。假定剥落是由于开裂、蒸汽压爆炸、剥落、蒸汽压再分布、再爆炸、再剥落的连锁反应而发生的。声发射的测量可用于估计开裂和剥落的发展。在开始加热20 ~ 30分钟后，试件表面的整个区域都出现了声发射。高强度混凝土试件的声发射计数高于普通强度混凝土试件。湿法和风干法的AE计数无显著差异。

The influences of strength and water content on the spalling of concrete exposed to fire were examined by the experiments, in which vapor pressure, temperature, and AE(Acoustic Emission)arising from cracking were measured. The results obtained by the study are as follows ;1. Vapor pressures of more than 2.2Mpa were measured in the spalling concrete specimens.2. Before the spallins arose, the thermal cracking were observed. The thermal cracks is assumed to have a large influence on the scale of subsequent spalling.3. The spalling with the wet specimens expands from surface toward inside being accompanied by fairly local apalling, meanwhile with the air-dried specimens arises in a region adjacent to the surface at a stretch. This is believed linked to the moisture distribution in the specimens.4. The spalling is assumed to develop due to chain reaction of cracking, explosion of vapor pressure, spalling, redistribution of vapor pressure, re-explosion, re-spalling.5. Measuring of AE can be applied for estimating of the development of the cracking and the spalling.6. AE arose over the whole area of the surface of specimens after 20 to 30 minutes after the beginning of application of heat. More AE counts were measured with the high strength concrete specimens as compared to the normal strength concrete specimens. No meaningful differences in AE counts were observed between the wet specimens and the air-dried specimens.