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Mechanical Properties of Ultra-High Performance Concrete before and after Exposure to High Temperatures

机译：高温前后超高性能混凝土的力学性能

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摘要

Compared with ordinary concrete, ultra-high performance concrete (UHPC) has excellent toughness and better impact resistance. Under high temperatures, the microstructure and mechanical properties of UHPC may seriously deteriorate. As such, we first explored the properties of UHPC with a designed 28-day compressive strength of 120 MPa or higher in the fresh mix phase, and measured its hardened mechanical properties at seven days. The test variables included: the type of cementing material and the mixing ratio (silica ash, ultra-fine silicon powder), the type of fiber (steel fiber, polypropylene fiber), and the fiber content (volume percentage). In addition to the UHPC of the experimental group, pure concrete was used as the control group in the experiment; no fiber or supplementary cementitious materials (silica ash, ultra-fine silicon powder) were added to enable comparison and discussion and analysis. Then, the UHPC-1 specimens of the experimental group were selected for further compressive, flexural, and splitting strength tests and SEM observations after exposure to different target temperatures in an electric furnace. The test results show that at room temperature, the 56-day compressive strength of the UHPC-1 mix was 155.8 MPa, which is higher than the >150 MPa general compressive strength requirement for ultra-high-performance concrete. The residual compressive strength, flexural strength, and splitting strength of the UHPC-1 specimen after exposure to 300, 400, and 500 °C did not decrease significantly, and even increased due to the drying effect of heating. However, when the temperature was 600 °C, spalling occurred, so the residual mechanical strength rapidly declined. SEM observations confirmed that polypropylene fibers melted at high temperatures, thereby forming other channels that helped to reduce the internal vapor pressure of the UHPC and maintain a certain residual strength.
机译：与普通混凝土相比，超高性能混凝土（UHPC）具有出色的韧性和更好的抗冲击性。在高温下，UHPC的微观结构和机械性能可能会严重恶化。因此，我们首先研究了在新鲜混合相中设计的28天抗压强度为120 MPa或更高的UHPC的性能，并在7天时测量了其硬化机械性能。测试变量包括：胶凝材料的类型和混合比（硅灰，超细硅粉），纤维的类型（钢纤维，聚丙烯纤维）和纤维含量（体积百分比）。除了实验组的UHPC，实验中还使用了纯混凝土作为对照组。没有添加纤维或辅助胶凝材料（硅灰，超细硅粉）进行比较，讨论和分析。然后，选择实验组的UHPC-1标本，以便在电炉中暴露于不同的目标温度后进行进一步的压缩，弯曲和劈裂强度测试以及SEM观察。测试结果表明，在室温下，UHPC-1混合料的56天抗压强度为155.8 MPa，高于超高性能混凝土对> 150 MPa的一般抗压强度的要求。暴露于300、400和500°C后，UHPC-1标本的残余抗压强度，弯曲强度和抗拉强度没有明显降低，甚至由于加热的干燥效果而增加。但是，当温度为600°C时，会发生剥落，因此残余机械强度迅速下降。 SEM观察证实，聚丙烯纤维在高温下熔融，从而形成其他通道，有助于降低UHPC的内部蒸气压并保持一定的残余强度。

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