Preparation,Performance And Quality Control Study Of High Strength Self-compacting Fair-faced Concrete

Self-compacting concrete which has the characteristics of high fluidity,gap traffic-ability,self filling performance and speeding up the construction progress can be applied to most of the projects with vibration difficulty;many projects are expecting a kind of concrete with both the effects of self-compacting and as-cast.However,there are few studies about the effects of mortar properties,release agent and the slurry viscosity on the concrete surface property.Therefore,this paper has a comprehensive study on the self-compacting and as-cast concrete and try to provide the basis for the project application of the self-compacting and as-cast concrete.This paper studies the mortar fluidity,viscosity and outflow time as well as the changing rules within 1 hour of the high fluidity concrete with three low water cement ratios 0.27,0.30,0.33,and different cement aggregate ratios.The results indicate that under the condition of low water cement ratio,water cement ratio has the biggest effect on the mortar fluidity,while cement aggregate ratio has a less effect.When the initial self-compacting mortar fluidity is from 280 to 300mm,the water cement ratios should be controlled within 0.30-0.33,and the cement aggregation ratios should be controlled within 0.63-0.68;while the plastic viscosity should be from 8-18Pa·s,the shear stress has a rough scale of 5-14Pa.Water cement ratio is the main factor of outflow time,the mortar outflow time can up to around 10s with a water cement ratio of 0.33.The suitable sand ratio for self-compacting and as-cast concrete is 48%,the addition of 0.2%%air entraining agent,0.3%%thickening agent,5%silica fume and 10%replacement of fine aggregate with light sand respectively can all improve the concrete working performance and self-compacting property.While group PH4 with a thickening agent proportion of 0.3%%and group PH6 added with silica show high air contents,which are 6.1%and 3.9%respectively,concrete in these two groups have a porous surface and more honeycomb holes,which lead to a bad surface property.Group PH2 and group PH7 have an air content of 1.3%and 2.2%respectively,the surface of which have much less holes and smaller hole sizes,concrete in these two groups have a self-compacting state relatively.Therefore,air entraning agent and light sand can be mixed in order to improve the self-compacting and as-cast properties.The study of mechanical properties shows that the addition of 3%%thickening agent caused a 10%decrease in the mechanical property;group PH4 and group PH6 have an electric flux less than 1000C which is higher than that in other groups;the carbonation depth in 56d are less than 3mm,while there is almost no carbonation depth in 28d;the drying shrinkage of all the concrete are between 241 ×10-6 and 335×10-6.The release agent and mortar plastic viscosity affect the performance of self-compacting concrete surface was studied,the surface properties are analyzed by using the software Image-Pro Plus.The results indicate that the viscosity properties of mortar in the self-compacting and fair-faced concrete are in line with the rheological equation of Bingham fluid with a correlation coefficient R2 greater than 0.96,which shows that the mortar plastic viscosity is a significant factor for the surface properties of self-compacting concrete.With the increase of plastic viscosity,the percentage of surface pore areas gradually increases,the pore size distribution becomes wider,and the number of pore with the diameter more than 2mm increases,furthermore,the diameter of the biggest pore becomes larger.The self-compacting concrete with the plastic viscosity less than 9.37 Pa·s has a better fair-faced surface,in this condition,the percentage of the surface pore areas is less than 0.2%,the diameters of the surface pore distribute between 0 mm to 5mm,and the diameter of the biggest pore is less than 5mm.reasonable contact angle and the adhesion strength between release agent and cement based composite materials are important parameters to improve the surface quality.The contact angle of common resin release agent is approximately 90°,the adhesion strength is 216-252MPa,and concrete is easy to produce the adhesion phenomenon,it will cause a lot of holes.Super hydrophobic materials Si almost no adhesion strength,but its microscopic surface morphology is coarse granular,concrete surface is rough and there are many holes.A new kind of release agent FSi,it can increase the contact angle to 106°,reduce the adhesion strength to 168 kpa,the surface of concrete is smooth and flat,the bughole area of concrete is 0.02%,and the bughole size distribution is between 0 and 1.5mm with the maximum hole diameter less than 2mm.