| Ultra-high performance concrete(UHPC)is a new type of cement-based material with ultra-high mechanical properties and durability,which is based on the most compact packing principle and reinforced by multi-component particles and fibers,it has a broad application prospect in municipal,bridge and military fields.However,the ultra-low water-binder ratio,high superplasticizer content and high powder content of UHPC result in prolonged mixing time and increased viscosity,which greatly affects the mixing,pouring and construction efficiency of UHPC,the application of UHPC on a large scale is severely restricted.Therefore,it is urgent to study the rheological properties of UHPC and the regulation mechanism of mixing and construction efficiency.Based on this,the pulping time and paste rate,two key parameters determining UHPC mixing and construction efficiency,were selected as the research objects in this paper.Four kinds of typical powder particles with different sizes and particle shapes were selected to study their effects on the flow properties and rheological properties of the paste with extremely low water-binder ratio.The influence mechanism of powder particles on pulping time and flow rate was revealed from adsorption of superplasticizer,packing density,estimated viscosity of interstitial solutions and water film thickness.The conclusions are as follows:(1)The pulping time.A criterion of mixing paste state is proposed based on the appearance of spherical particles of 2～3 mm in size wrapped in a thin layer of water,the results show that the influence of particles on the pulping time is as follows: micron-sized spherical particles<micron-sized spherical particles<micron-sized irregular particles <nano-sized spherical particles.(2)The flow rate of paste.The whole process measuring equipment of paste fluidity expansion and image processing technology of paste flow rate at different moments were developed,and the influence law of powder particles on paste rate was defined: with the increase of the content of particles,the flow rate of nano-sized and micro-nano-sized spherical paste first increased and then decreased;However,micron particles can increase the flow rate of paste.(3)The rheological properties of paste.When the content of powder is less than 10%,the apparent viscosity of the irregular particle system is higher than that of the spherical particle system.When the powder content of > is 10%,the apparent viscosity of nanoand micro-nano particle systems is higher than that of micro-particle systems.(4)The mechanism of the pulping time.The larger the specific surface area of the powder particles,the stronger the surface energy,and the more flocculation structure in the paste,which requires a longer mixing time to reach the surplus water that meets the pulp requirements of the paste;However,the ball effect of spherical particles under shear is easier to break the flocculation structure and release free water,so that the micron-sized spherical particle system has a shorter pulping time.(5)The mechanism of the flow rate.Spherical particles are beneficial to improve the packing density,and the specific surface area of micron particles is smaller,which makes the micron spherical particle system have a larger water film thickness.Compared with micro-nano spherical and micro-irregular particles,the micron-sized particles have weaker ability to adsorb superplasticizer and more residual superplasticizer,but the reduction of the total amount of superplasticizer the concentration of residual superplasticizer in interstitial solution,resulting in lower viscosity of the micron-spherical particle system.Therefore,the thickness of the film and the viscosity of interstitial solution determine the flow rate of paste. |
PDF Full Text Request