Influence Of High Temperature On Physical And Mechanical Properties Of C40High Performance Concrete

High performance concrete (HPC), with its comprehensive advantages ofhigh strength, durability, workability and volume stability, has been widely usedand studied in the field of civil engineering. Compared with Normal concrete(NC), HPC has much denser internal structure, lower permeability and higherbrittleness, which more seriously degrade its performance, even spalling orburst when subjected to fire or high temperature. More terribly, steel bars willbe barely exposed, resulting in great safety trouble for building, living andproperty. Research has been focused on the high temperature performance ofHPC in order to reveal the degradation evolution, put forward scientific andreasonable improvement measure in the engineering.In this study, the variation regulation of basic physical and mechanicalproperties for design strength C40HPC with high quality local raw materialswas systematically investigated before and after high temperatures. Combining the macro testing and microscopic analysis, based on the qualitative analysisand quantitative analysis, the performance degradation rules, mechanism and itsinfluencing factors of HPC at high temperatures were further discussed frommicro and meso perspective. The main contents are as follows:(1) Experimental study on basic properties of HPC. The experimentsmainly include the slump of mixes, apparent density, cubic compressivestrength, splitting tensile strength, axial compressive strength, modulus ofelasticity and resistance to chloride ion permeability. The results show that theprepared concrete has good mobility, adhesiveness, uniformity, and durability,and all the performance mentioned above meet the specification requirenments.(2) Experimental research on mechanical properties of HPC subjected tohigh temperatures. In this part, the effect of different cooling method on cubiccompressive strength and splitting tensile strength of HPC was studied,furthermore, the variation of compressive strength, splitting tensile strength,axial compressive strength, modulus of elasticity and fracture performance wasrespectively analyzed with the increase of temperature. The results indicate thatall the mechanical properties of HPC deteriorate to different degrees afterexposed to high temperatures, in which, spray cooling more significantlyinfluences the strength degradation of HPC than natural cooling. Besides, thesize effect characteristic of concrete at room temperature no more applies tothose concretes subjected to high temperatures according to the comparison ofcompressive strength and splitting tensile strength for two sizes concrete cubes (100mm and150mm). The relationship between modulus of elasticity andtemperature, modulus and compressive strength with temperature wasestablished, respectively. Moreover, the crack toughness of HPC slightlydecreases with temperatures, while the stability toughness of HPC dramaticallydeclines with temperature increase. The relationship of splitting tensile strengthand stability toughness of HPC was established, so was the compressivestrength and fracture energy.(3) Experimental study on thermal physical properties of HPC afterexposed to high temperatures. The apparent characteristic of HPC subjected tohigh temperatures was macroscopically observed, meanwhile, the variation ofapparent density of HPC with temperature increase was studied, as well as theeffect of different temperature and constant time on thermal conductivitycoefficient of HPC. What’s more, the difference of specific heat for HPC beforeand after high temperature was comparatively analyzed, and the variation ofthermal expansion properties for HPC and its ingredients were discussed duringheating and cooling process.The results indicate that the obvious cracks appearafter400℃. Both apparent density and coefficient of thermal conductivity ofHPC decrease with temperature increase, except for300℃. The relationshipbetween apparent density and coefficient of thermal conductivity of HPC wasestablished. The specific heat value of HPC under high temperature is higherthan that after high temperature and obviously fluctuates with temperatureincrease. There exist two distinct endothermic peaks (approximately at143℃ and483℃) in DSC curve of HPC, which diapear after high temperature. All thethermal expansion ratios including concrete and its ingredients change littlebefore200℃, and then vary differently with temperature increase, among which,the expansion ratio of hardened binder materials decreases, and the sum of thevolume expansion ratio of mortar and aggregate disagrees to that of concrete.The thermal expansion ratio of concrete, aggregate and the four hardened pastescontinuously decrease during cooling. The cooling speed different, thermalexpansion ratio different, in addition, the water reducer delays the expansionratio of hardended pastes. According to the thermogravimetric (tg) anddifferential thermal curves, the mass of HPC mainly declines at100℃,440℃and710℃.(4) Research on microscopic structure evolution of HPC subjected to hightemperatures. Qualitative and quantitative analysis for HPC before and afterhigh temperatures were carried out by Mercury porosimeter and X-raycomputed tomography (CT) scanning apparatus. The total porosity of HPCgenerally increases with the increase of temperature, besides, the pore sizedistribution in concrete varies after exposed to different high temperatures, inwhich, most of the pores lower than50nm in HPC appear before300℃,nevertheless, the amount of pores higher than50nm arises after300℃. The porein HPC has multiple fractal features, in which, the varied pore diameter is50nm.In a certain pore range, the pore volume fractal dimension decreases with theincrease of porosity, and pores with greater size has significant effect on compressive strength of HPC. Moreover, the relationship and model forcompressive strength and porosity with temperature, compressive strength andpore volume dimension with temperature was established, respectively. In termsof the observation for the representative layers (in x-y plane and x-z plane) inCT image, the characteristic of internal structure in different layers is basicallysimilar to each other. After being heated, the amount of pores in concreteincrease, and the pore size extends, moreover, micro cracks across the interfacebetween coarse aggregate and mortar develop, propagate and connect with thepores in mortar. Based on the threshold image of HPC, the porosity of eachrepresentative layer increases with temperature increase, in accordance with thatobtained from mercury intrusion method.This paper is intended to theoretically and experimentally lay thesystematical research foundation for high temperature performance of HPC, andprovides some macroscopic and microscopic references to the relevant study inthe future.