Study On The Properties Of Ultra High Performance Concrete With Common Techniques

As one kind of cement-based materials with high strength, high ductility, lowporosity and good durability, ultra high performance concrete （UHPC） has gainedextensive attention. However, there are numerous challenges for UHPC beforewidespread implementation such as its high cost, complexity of preparationtechniques, severe shrinkage and lack of design code.This paper focuses on UHPC prepared by common techniques, mainly analysesthe influences of common constituents on the fluidity, strength, autogeneousshrinkage and drying shrinkage. The main achievements are stated as follows:Water-to-binder ratio （w/b） is the major factor affecting properties of UHPC.With the increase of w/b, the fluidity of freshly mixed UHPC improves and thestrength of hardened specimens decreases. The autogeneous shrinkage of UHPCdecreases with increasing w/b however drying shrinkage increases. As forcement-based material with very low w/b, autogeneous shrinkage occupies thedominant position in the total shrinkage.Due to the synergistic effect of silica fume （SF） and superplasticizer, the overalltrend of the fluidity of fresh UHPC shows a gradual increase. In the case of standardcuring condition, the influence of SF dosage on strength of UHPC specimens is notobvious. When SF dosage varies from15%to25%, the compressive strengthincreases less than5MPa. Besides, there is a notable effect of SF content on theshrinkage property of UHPC. Both autogeneous shrinkage and drying shrinkagestrains increase with the increases of SF content.The rough and irregular shape of quartz powder particle results in its high waterdemand. Therefore, the fluidity of fresh UHPC decreases with the increase of quartzpowder dosage. Owing to the inertia of quartz powder at room temperature, thestrength of UHPC specimens only has a slight upgrade when quartz powder contentincreases. When the quartz powder content is too large or too small, the autogeneousshrinkage strain is relatively high, however, when the dosage is between theextremums, there is little influence on autogeneous shrinkage with dosage varies.Meanwhile, the incorporation of quartz powder has restrained the drying shrinkage ofUHPC to a certain extent. With the increase of quartz powder content, dryingshrinkage of specimens decreases. The increase of sand-to-cement ratio（s/c） means the rise of sand content, withwhich the paste to wrap aggregate is reduced relatively leading to the decrease offluidity of freshly mixed UHPC. Flexural and compressive strength curves show thatthe when the sand-to-cement ratio is1.0, the specimen owns the highest strength. Therise of s/c inhibits both autogeneous and drying shrinkage of UHPC, but the restrainteffect is more obvious as for drying shrinkage.The superplasticizer（SP） dosage has a certain influence on the fluidity of freshUHPC, with which increases, the mobility of the mixture continuously improves.However, when the dosage reaches to2.5%, the effect of increasing SP dosage toimprove fluidity is no longer prominent.With the aid of automatic mercury intrusion analyzer and synchronous thermalanalyzer, pore structure and Ca（OH）₂content of shrinkage samples are investigated.Results showed that samples with lower w/b owned not only lower total porosity, butalso higher percentage of fine pores, indicating a refined pore structure. Theincorporation of silica fume is conducive to the refinement of the pore structure ofUHPC. And the higher dosage of SF, the lower content of Ca（OH）₂in the shrinkagesamples, demonstrating the pozzolanic reaction area of high-SF-content samples isbroader, which can consume more Ca（OH）₂.