Study On Internal Defects Of High Strength Concrete With Polypropylene Fiber At Different Temperatures Based Upon CT Images

High strength concrete (HSC) are widely used in modern structural engineering with high-rise, large span and underground space for it’s high compressive strength, large carrying capacity and less energy consumption; however, its structure is compact, poor permeability, easy to burst when suffering from high temperature. Studies have shown that high strength concrete with polypropylene fiber (PPHSC) can resist the crack efficiently. So it has important theoretical and practical significance when study on the high temperature performance of PPHSC.In this paper, the mechanical property test and X-ray computed tomography (CT) are used to investigate the relationship between compressive strength and internal defects and pore structure after high temperatures, which provides the basis of the test for high temperature performance and anti burst mechanism of PPHSC.This article has mainly carried on the following aspects:1. Study of the high temperature performance of PPHSC.It uses thermocouples to detect the internal temperature and burn-through time as well as the quality loss of PPHSC after high temperatures. Studies have shown that when experienced the temperatures between200℃and500℃, the burn-through time of PPHSC are the same, but the mass loss increased.2. Experimental study on compressive strength of PPHSC after high temperature.Compressive strength of PPHSC’s burning-through specimen is measured referring to the ordinary concrete cube compressive strength test method, which used to analyze the relationship between the strength and the temperatures and the mass loss. Studies have shown that with the increase of temperature, the compressive strength of PPHSC descends. Related model is established when fitting the temperature and the compressive strength. At the same time Compared compressive strength of HSC before and after adding polypropylene fiber, it is concluded that mixing polypropylene fibers has little effect on the compressive strength of HSC.3. Qualitative analysis of PPHSC X-ray CT images after high temperatures.Using X-ray CT technology, combining with the morphology of polypropylene fiber after high temperatures test, to analyse the X-ray CT images after the high temperatures qualitatively, Studies have shown that the pores in PPHSC mainly distribute in the cross section when at the room temperature, and there are obvious native defects in the range of CT observation scale. After high temperatures, the cracks are cracking and extending and the pore structure changes obviously, fiber channel finally connected into a whole.4. Defect rate analysis of PPHSC X-ray CT images after high temperatures.Using Image-Pro Plus to analyse the binary images of PPHSC quantitatively, this can obtain the defect rate of different layers after high temperatures. Studies have shown the affecting factors of defects rate and the biggest impact of the temperature threshold section. Related models are established when fitting the quality loss and compressive strength and internal defects. At the same time, to study the relative defect rate defects of HSC before and after adding the polypropylene fiber, it shows that adding polypropylene fiber does not affect the strength of the change, but can effectively improve the development of internal defects under the same temperature, increase the chances of the internal vapor pressure to escape, inhibit the burst of HSC.5. Pore structure analysis of PPHSC X-ray CT images after high temperatures.Using Image-Pro Plus obtains the mean and maximum average pore size of PPHSC, as well as the Pore structure distribution. Studies have presented that with the increase of temperature, the mean and maximum average pore size and the pore number shows a trend of growth as a whole. The number of pore rises constantly under different temperatures at the same scope, and in addition, the pore number mainly focuses below20microns.