Clarification on the Spalling of Concrete Exposed to Fire

通过测定水蒸气压、温度和开裂时的声发射（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.在剥落的混凝土试件中测得蒸汽压大于2.2Mpa.在散斑出现之前，观察到热裂纹。假设热裂纹对随后的层裂规模有很大的影响。湿态试样的剥落由表面向内扩展，并伴有局部剥落，而风干试样的剥落则是在靠近表面的区域内发生。这被认为与水泥中的水分分布有关。剥落是由开裂、蒸汽压爆炸、剥落、蒸汽压再分布、再爆炸、再剥落等连锁反应引起的。声发射的测量可以用来估计裂纹和剥落的发展.在开始加热后20至30分钟后，在样品表面的整个区域上出现AE。与普通强度混凝土试件相比，高强度混凝土试件测得更多的AE计数。在湿标本和风干标本之间未观察到AE计数有意义的差异。

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.