| Recycled Aggregate Concrete(RAC) is a new kind of resource-saving material. Nowadays, human activities have brought an increasing amount of building demolition and construction waste, the RAC technology not only be able to reduce the exploitation of natural gravel resources, but also can solve the deteriorating ecological environment problem of a large number of construction waste, and it has a meaningful significance for the sustainable economy development. The special production process may cause some internal defects within recycled aggregates, consequently there are some differences in the mechanical properties of RAC while it is compared with common concrete, especially when the RAC is affected by some complex factors. In order to explore the mechanical properties of RAC, 19 kinds of mix proportions were measured and certain numbers of 100mm×100mm×100mm cube specimens were produced, then the compressive strength of RAC were tested with many factors influence, such as different temperatures(20 ℃, 200 ℃, 400 ℃, 600 ℃, 800 ℃), different recycled aggregate replacement ratios(0%, 30%, 50%, 70%, 100%), different water-binder ratios(0.34, 0.4, 0.5), different fly-ash dosage(0%, 30%, 50%). The results show that:(1) Generally, the compressive strength of RAC usually is lower than common concrete, there were some loss for the compressive strength and the mass after exposure to high temperatures and the strength loss were increasing with temperatures. The longer standing time extend, the lower compressive strength before 7d, then there was an ascending stage after 7d. In addition, the w/b is still an important factor influencing compressive strength of RAC. The lower w/b led to the worse interface transition zone, but fly-ash can improve the poor surface combination between cement and recycled aggregates, even if it has negative effect on early mechanical properties of RAC. In the heating process, residual fly-ash participate in secondary hydration reaction and formed the hydrates which can help reduce the high temperatures damage. In addition, this phenomenon is more obvious with the increase of fly-ash dosage. Besides, the different w/b or different fly-ash dosage specimen’s tendency of the mass loss rate is similar to compressive strength loss rate.(2) The frost resistance of RAC is far inferior than the common concrete after high temperatures, but both of the compressive strength of RAC and common concrete will further deterioration affected by high temperatures and freeze-thaw effect, higher temperatures also led to more serious degradation. As a consequence of this two factors, the coefficient of high temperatures and freeze-thaw cycles outdistanced the single influence of them. Lower w/b and no fly-ash adding specimens has a good frost resistance in room temperatures, but the lower w/b and appropriate dosage of fly-ash specimens performance better after high temperatures. Salt freeze-thaw cycles caused a terrible mass spalling and higher strength loss than freeze-thaw cycles, which have a greater influence on such specimens of higher porosity. Moreover, the mass loss rate changes of RAC have no direct relationship with its compressive strength loss rate.The article mainly studied the compressive strength of RAC with different factors, concluded its rules after the effect of high temperatures together with freeze-thaw cycles and analyzed the similarities as well as differences in the existing research results. Based on the data of experiment, research also provided some related fitting formulas for the situation of high temperatures, high temperatures with water freeze-thaw cycles and high temperatures with salt freeze-thaw cycles, it has a significant value to the prediction of RAC’s compressive strength and the data support of practical engineering application. |
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