Study On Multilevel Regulation And Control Of Workability In Ready-mixed Concrete

The sustained growing demand for concrete raw materials and the country's strict control over natural resources and the environment makes sources of raw materials become increasingly scarce,which leads to the trend of diversification and fragmentation of material sources.As a result,the raw materials of commercial concrete companies change frequently and the stability of quality decreases.People often find no way to start the regulation and control of concrete workability.For medium and low strength concrete,this problem is more prominent.Therefore,it is of great theoretical and practical significance to establish a simple and convenient prediction and control mechanism for the workability of concrete in the case of frequent fluctuations in the performance of concrete raw materials.This paper is aimed at medium-low strength concrete,which will be considered as a coarse suspension with multi-layer structure formed by multi-scale particles in water.On the level of paste,the influence factors and evaluation methods of the action of water reducer with cement and the influence of rheological properties on the bleeding behavior of paste are studied;On the level of mortar,the model for the effect of properties fluctuation of paste and fine aggregate on the rheological properties of mortar is studied.On the level of concrete,the model for the effect of properties fluctuation of mortar and coarse aggregate on the rheological properties of concrete is studied.The relationship between the rheological parameters of concrete and slump,expansion,bleeding rate and the homogeneity of aggregate distribution are established,and the workability box is constructed.Finally,a workability control system is formed,which is progressive and interrelated layer by layer from paste to concrete,to provide technical support for the production and application of ready-mixed concrete in complex raw material environment.Research shows:?1?From micron scale particles and paste layer:The performance fluctuation of micro-scale materials in concrete will fundamentally change the rheological properties of slurry.Compatibility and sensitivity can quantitatively describe the effect of the fluctuation of the properties of cementitious materials and water reducers on the rheological properties of paste.The former focuses on the comprehensive quality of paste,while the latter focuses on the change rate of the rheological properties of paste with the content of water reducers.The formulas for calculating compatibility and sensitivity are?=?M₁/D₁?*?M₂/D₂?+?D₃-D₂?/D₂?K=?F₂-200?/?C₂-C₁?.From the rheological point of view,when the yield stress of the paste is greater than 8.5 Pa and the plastic viscosity is greater than 0.1 Pa·s,the initial stability and homogeneity of the paste can be ensured,and the bleeding risk can be minimized.?2?From millimeter scale particles and mortar layer:When the content of fine aggregate stone powder increases by 1%,the suitable content of water reducer needs to increase by 0.054%.The increase of the content of stone powder improves the water retention and workability of concrete.The influence of stone chips on the workability of concrete is also related to sand rate and the proportion of replacing sea sand.When sand rate is less than 41%,the influence of stone chips on workability of concrete can not be considered.When sand rate is more than 41%,stone chips will significantly reduce the workability of concrete.The larger the proportion,the more obvious the workability decline.At this time,the proportion of stone chips should be controlled not to exceed 60%.Equivalent mortar experiment is used to adjust the amount of water reducer in concrete production,which can accurately reflect the change of the appropriate amount of water reducer caused by the change of stone chips in concrete.?3?From centimeter scale particles and concrete layer:Workability box reflects the relationship between rheological parameters and workability index of concrete.For slump workable box,the rectangular area is 75 Pa?yield stress?145 Pa,12 Pa·s?plastic viscosity?44 Pa·s;For expansion workable box,the rectangular area is 70 Pa?yield stress?165 Pa,20 Pa·s?plastic viscosity?50 Pa·s;For bleeding workable box,the rectangular area is 115 Pa?yield stress?290 Pa,27 Pa·s?plastic viscosity?66 Pa·s;For the workability box of aggregate homogeneity parameter S₂,the rectangular diagonal line of the region is 100 Pa?yield stress?100 Pa,20 Pa·s?plastic viscosity?65 Pa·s.The rheological parameters can achieve good performance of concrete within the range specified above.?4?The correlation of paste and mortar rheological model:Krieger-Dougherty and Coussot model,based on fine aggregate particle morphology Form2D and Angularity modification,can better describe the effect of fine aggregate and paste property fluctuation on the rheological properties of mortar,and establish the rheological relationship between mortar and paste.The model provides a tool for adjusting rheological properties of mortar under complex conditions such as material fluctuation.For medium-low strength concrete,it is necessary to ensure that the rheological parameters of mortar in concrete distribute in the area of yield stress>20Pa and plastic viscosity>5.5 Pa·s.?5?The correlation of mortar and concrete rheological model:The rheological properties of mortar,the thickness of residual mortar and the volume fraction of coarse aggregate can fully consider the suspension state of coarse aggregate in mortar and its influence on rheological properties.The concrete rheological model established by mortar rheological method is?_c=e2.80??_m/t_s?0.84?V_ca^0.46,?=e.??/t?.?V.,which can well describe the influence of the fluctuation of mortar properties and coarse aggregate properties on the rheological properties of concrete and predict the rheological parameters of concrete.In the case of material fluctuation in concrete production,the variables needed for the model based on mortar rheological method are easy to be measured and acquired,which can reflect the rheological characteristics of concrete more conveniently and can give full play to its timeliness.