Effects Of Fiber And Admixture On Shrinkage Performance Of Foamed Concrete

Foamed concrete has been accepted by engineering circles for its advantages of light weight,heat preservation and heat insulation.However,due to the characteristics of its porous structure,its early shrinkage is large,and the structure is easy to crack,hollow and leakage,which makes the shrinkage problem become one of the main problems that restrict the application of foam ed concrete.The fiber can resist the stress produced during the contraction of the cement paste,thus reducing the shrinkage;the expansive agent is added to the foamed concrete to produce the expansive crystalline hydrate,which can produce a moderate expansion and counteract the tension stress in the hardening process of the foam ed concrete,and thus reduce the shrinkage;the shrinkage reducing admixtures reduce tensile stress by reducing the surface tension of free water in the capillary,thereby reducing shrinkage.By studying the effect of water cement ratio,sand binder ratio and dosage of water reducer content on the autogenous shrinkage,drying shrinkage,fluidity and compressive strength of the foamed concrete with dry density of 800kg/m~3,the optimum mix ratio of foamed concrete is obtained.Then the effect and mechanism of water cement ratio of 0.4,0.5,0.6 and 0.7 on drying shrinkage of foam ed concrete with dry density of 800kg/m~3 was studied.The effects and mechanism of polypropylene fiber,glass fiber,plant fiber,expansive agent and shrinkage reducing admixtures on the autogenous shrinkage,drying shrinkage,fluidity and compressive strength of the foamed concrete with dry density of 800kg/m~3 were studied.The results of the study are as follows:(1)The effects of water cement ratio,sand binder ratio and water reducer content on the autogenous shrinkage,drying shrinkage,fluidity and compressive strength of the foamed concrete with dry density of 800kg/m~3 are studied.The optimum mix proportion is 0.5 of water cement ratio,1 of sand binder ratio,and 0.5%of the dosage of water reducer content.(2)When the water cement ratio increased from 0.4 to 0.7,the drying shrinkage of foamed concrete gradually increased.One reason is that the porosity and average pore size of the macroporous structure increase,and the pore size distribution of0~200?m decreases sharply,and the pore size distribution of more than 400?m increases sharply,causing more capillary hole water loss.On the other hand,for micro pore structure,when the ratio of water to cement increases from 0.4 to 0.5,the volume of pore volume in the 2.5~8.7nm aperture range is larger than that in the8.7~50nm aperture range,and the small capillary pores within the 2.5~8.7nm aperture range cause greater capillary tension,which leads to the increase of drying shrinkage.The ratio of water to cement increased from 0.5 to 0.7,the increase of porosity of the capillary pores caused more loss of capillary pore water,and the increase of the pore volume of 2.5~50nm caused the increase of capillary pore tension,which resulted in the increase of drying shrinkage.(3)Fiber,expansive agent,shrinkage reducing admixtures can effectively reduce the autogenous shrinkage and drying shrinkage of foam ed concrete,and its effect of resistance to shrinkage is:shrinkage reducing admixtures>expansive agent>fiber.The best addition is a shrinkage reducing admixtures of 1%mass,which can reduce the 37%3d autogenous shrinkage and 57.5%56d drying shrinkage of foamed concrete under the premise of satisfying the fluidity and compressive strength.(4)The mixing of the fibers causes the fragmentation of some foam and causes the larger drying shrinkage of the matrix,but its physical constraint on the foam ed concrete reduces the final drying shrinkage.The reduction mechanism of expansive agent and shrinkage reducing admixtures is due to its own chemical action.