超高掺量粉煤灰大体积混凝土早龄期热膨胀系数

作为大体积混凝土的超高掺量粉煤灰常态大坝混凝土(UHVFA)是发展前景看好的生态节能大体积混凝土。鉴于采用标准试验方法难以测试其早龄期的热膨胀系数,本文采用绝热模式和温度匹配模式(TMC)两种温度历程养护模式,完成参照混凝土(FA)和超高掺量粉煤灰混凝土(UHVFA)的温度–应力试验(TSTM试验),分离温度变形与自生体积变形。由两种温度历程下的不同龄期时间段的应变差和温度差,确定混凝土早龄期的热膨胀系数,并提出大体积混凝土的时变热膨胀系数模型。研究表明:两种大体积混凝土的热膨胀系数在早龄期具有时变性,在初凝时最大,随后迅速下降至最小值,后又增大并趋于稳定。在升温阶段,UHVFA混凝土的热膨胀系数均值大于FA;而在降温阶段,UHVFA的均值小于FA。但UHVFA在升温阶段的初、终凝时刻特征点的热膨胀系数值都小于FA。从总体看,随粉煤灰掺量的增加,混凝土的热膨胀系数有减小的趋势。

Thermal expansion coefficient of ultra high content fly ash mass concrete at early age

Conventional ultra high volume fly ash (UHVFA) dam concrete is a kind of promising ecological and energy-saving mass concrete, but its thermal expansion coefficient (TEC) at early age is difficult to measure by a standard testing method. In this work, we test the specimens on a temperature-stress testing machine (TSTM) for the benchmark fly ash (FA) concrete and the UHVFA concrete and separate thermal deformation and autogenous deformation under two curing temperature histories: adiabatic mode and temperature matching curing (TMC) mode. The TECs of the two mass concretes at early age are determined by the strain difference and temperature difference at different ages under the two temperature histories, and a time-varying TEC model is constructed. It is shown that at early age, the TECs of the two concretes are time-varying, first growing up to the maximum at the initial setting time, then rapidly decreasing to the minimum and growing again, and finally tending to be stable. In the heating stage the TEC average of UHVFA is greater than that of FA, while it is smaller during cooling. And in the heating stage, the TEC is smaller at both the initial and final setting times. In general, with the increasing volume of fly ash, the TEC shows a decreasing tendency.