A Research On Effects Of Pumping On Rheological Properties Of Concrete

Pumping is the most commonly used method for fresh concrete transportation and pouring,owing to its advantages of high efficiency,economy and safety.In practice,the work of concrete pumping is highly dependent on field experience.As for long-distance pumping,pumping circuit tests are required to determine the pumping process and concrete proportion.The fresh concrete is subjected to strong physical effects including shearing,mixing and axial pressure during pumping.These effects influenced the properties of fresh concrete,and changed the flow behavior in the pipes and concrete workability after pumping.Aiming at the changes in properties of fresh concrete during and after pumping,in this work,the influences of pumping and various physical coupling effects of pressure,shear,temperature were investigated through a combination of large-scale pumping circuits tests and experimental work in the laboratory.Hydration process and superplasticizer adsorption in fresh concrete were evaluated to reveal the mechanism on changes in fresh properties induced by pumping.Lastly,the effects of the rheological changes in fresh properties on the pumping pressure loss were clarified.Firstly,basing on monitoring data of the full-scale pumping circuits,the flow behavior of fresh concrete in pipes were revealed,and pressure,shearing,and temperature change effects imposed on fresh concrete were quantified.The fresh concrete behaves as laminar flow in the pipes,and can be divided as two regions:shearing zone and plug zone.Test results show that the scale of shearing zone increased logarithmically with the discharge rate,and the maximum shearing rate in the shearing zone increased linearly with the discharge rate.At certain discharge rate,the scale of shearing zone decreased with the growing ratio of the yield stress to the plastic viscosity;and the maximum shearing rate in shearing zone was influenced by the ratio of the yield stress to the plastic viscosity and the ratio of viscosity constant of lubrication layer to the plastic viscosity.The changes in concrete temperature were mainly influenced by the combined effects of environmental temperature and changes in pipe wall temperature.Then,the changes in rheological properties and dynamic stability of fresh concrete after pumping were investigated.The pressure in the pipes and discharge rate during pumping were monitored.Rheological parameters,workability,and material temperature of fresh concrete were measured before and after pumping.The rheological properties at different sites of the pipelines were evaluated by sampling in the dismantled pipes.The yield stress increased,initial tangential viscosity decreased,shear-thickening phenomenon were eliminated,and dynamic stability of fresh concrete were enhanced after pumping.And,the increase in yield stress mainly occurred in the first half of the pipeline,while the viscosity decreased linearly along with the pipeline.Besides,the accumulated shearing actions during pumping were quantified by considering the shearing rate,shearing duration,and apparent viscosity in fresh concrete.Relationships between shearing energy and viscosity reduction reduced by pumping were established.Moreover,the early age hydration process in fresh concrete before and after pumping were evaluated by non-contact electrical resistivity meters.And changes in hydration process induced by pumping were related to the changes in yield stress/slump flow of fresh concrete after pumping.The constant shearing effect during pumping can accelerated the dissolving of cement particles in fresh concrete,the end of dormant periods was shortened by 10.3-27.3%after pumping,and the end of setting periods were shortened by 15.6-28.4%after pumping,besides,the acceleration in hydration process after pumping can be well correlated with the pumping durations.The slump flow loss of concrete after pumping can be well related to the percentage reduction in dormant period in concrete after pumping.Due to the increase in air content and accelerated hydration process,the resistivity value of concrete increased after pumping.Furthermore,the influences of pumping conditions,including pumping distance,discharge rate,and initial flowability of concrete,on the properties of fresh concrete after pumping were investigated.Considering the different concrete pumping scenarios in engineer projects in practice,pumping pressure or discharge rate were controlled as constant in different pumping distance jobs(342,545,1044 m);and concrete were pumped at different discharge rates(5.2-11.4 L/s)in same pipelines.Besides,two groups of fresh concrete with different slump flow level(660-670 mm and 700-730mm)were used in the experimental work.Rheological tests were performed on fresh concrete before and after pumping.Total organic carbon in the pore solution of fresh concrete were measured to evaluate the changes in superplasticizer adsorption after pumping.The increase in yield stress after pumping increased with the increasing discharge rate,and the changes in yield stress were larger for concrete with smaller initial slump flow.The changes in viscosity increased with the increasing discharge rate regardless of the initial slump flow.When the pumping pressure was controlled,the changes in yield stress decreased with the increasing pumping distance;and that increased with the pumping distance when the discharge rate was controlled.The changes in viscosity in different pipelines increased logarithmically with the shearing energy during pumping.Lastly,the influences of rheological changes during pumping on the pumping pressure loss were evaluated.The pressure in the pipes along the pipeline were monitors by several pressure sensors.Relationships between rheological changes and variations of pumping pressure loss along the pipeline were established.The pressure loss decreased along the pipeline,and the changes in pressure loss is related to the ratios between Kaplan’s model predication values based on properties after pumping(?_a)to that based on properties before pumping(?_b),and the pipeline length.The smaller?_a/?_b and longer pipeline length induce a smaller change in pressure loss along the pipeline.In this dissertation,the study on the influence pattern and mechanism of pumping on the rheological properties of concrete is helpful to control the fresh properties of concrete after pumping;By considering the change of rheological properties in the pumping process,the prediction accuracy of pump pressure loss is improved.