Experimental Study On Anti-freeze Mix Ratio And Low Temperature Performance Of Dry Hard Concrete

Experimental study on anti-freeze mix ratio and low temperature performance of dry hard concrete Dry hard concrete refers to concrete whose slump value is no more than 10 mm and whose working degree ranges from 5 s to 30 s.It has the characteristics of low water-cement ratio,high early strength,good shrinkage and good frost resistance.In this paper,the key problems in the application of dry hard concrete in cold water conservancy projects are studied,in order to determine the mix ratio affecting the quality of dry hard concrete through tests,the key factors of forming process,and put forward the freeze-resistant preparation technology of dry hard concrete.The freezing-thawing cycle tests of hard and dry concrete were carried out for 0~300 times by means of fast freezing method.By measuring the changes of mass and dynamic elastic modulus after freezingthawing cycle,the microstructure of hard and dry concrete was observed by SEM electron microscope to reveal the damage mechanism of freezing-thawing cycle.For determination of harsh concrete at 20 ? and 0 ?,-10 ?,-20 ?,-30 ? under the condition of compressive strength(fcu),splitting tensile strength(fts),bending strength(ff)and shear strength(?i),and compared with the ordinary concrete results,it is concluded that under the condition of low temperature change rule of harsh concrete mechanics.The main conclusions are as follows:(1)Through the harsh concrete proportioning test and performance test,from the aggregate gradation and multivariate composite gelled material two aspects of harsh concrete proportioning design,using three kinds of aggregate combination plan,four stages of proportioning design,the conclusion harsh concrete mechanical properties and the main factors of the change trend of antifreeze performance.Based on the analysis of macroscopic and microscopic properties,the freeze-thaw failure mechanism of hard and dry concrete is revealed,and the reasonable range of influence factors is determined.The dry hard concrete under 60 MPa can be mixed with pebbles and gravel,and the mixing proportion of pebbles is 20 %(the proportion of the total mass of the stones).Silica fume plays an important role in improving the frost resistance of concrete.The strength of concrete with a content of 20 %(accounting for the proportion of the total mass of cementitious materials)is increased to more than 60 MPa,and the frost resistance reaches more than F 300.The cementitious materials mixed with silica fume,fly ash and mineral powder can significantly improve the cohesion of the mixture.Mixing high quality and high efficiency water-reducing agent and airentraining agent can reduce water-cement ratio to less than 0.3 and greatly improve the strength and frost resistance of concrete.(2)After freeze-thaw cycle,the mass loss rate of hard and dry concrete shows negative growth.The freeze-thaw loss process is divided into four stages: rapid growth stage(0~50 times),slow growth stage(50~100 times),slow decline stage(100~250 times),and rapid decline stage(>250 times).The reason is that after the freeze-thaw cycle of positive and negative temperature alternation,the closed internal pore crack,so that water immersion.When the number of freeze-thaw is between 50 and 100,the weight gain tends to be stable.(3)After freeze-thaw cycle,corresponding to the change in the quality of hard and dry concrete,the change in the elastic modulus is generally of "S" shape,but the overall elastic modulus shows a downward trend.In the first 50 times,the dynamic elastic modulus decreased rapidly,and in the 50 ~ 250 times,the elastic modulus changed relatively little.After 250 times,the elastic modulus began to decline significantly,mainly because the internal structure of the freeze-thaw cycle of the hard concrete changed.After 300 freeze-thaw cycles,the relative dynamic elastic modulus of hard concrete decreased to 73.7 %.(4)Within the scope of the-30 ? ~ 20 ?,with lower temperature,dry concrete mechanical properties showed a trend of linear increase,similar to ordinary concrete mechanical properties at low temperature.Dry hard concrete fcu increased by 24.4%,fts increased by 27.5 %,ff increased by 36 %,and?i increased by 121%.Ordinary concrete fcu increased by 23.9%,fts increased by 30.9%,ff increased by 160%,and the number of ?i increased by 219%.(5)Within the scope of the-30 ?~20 ?,fts/fcu as temperatures dropped slightly lower,harsh concrete reduced from 0.171 to 0.164,the 4.1% decline,ordinary concrete from 0.141 to 0.133,the 5.7% decline;ff/fcu increases with the decrease of temperature.The increase rate of dry hard concrete is 11.2% from 0.214 to 0.234,and that of ordinary concrete is 68.8% from 0.130 to 0.216.?i /fcu increases with the temperature decrease,dry hard concrete increases from 0.12 to 0.21,an increase of 75%,and ordinary concrete increases from 0.05 to 0.14,an increase of 180%.