Study Of Effects Of Carbon Fiber On Cement Mortar

Adopting fiber reinforced cement-based materials is a common technique,in which the fiber can play the role in strengthening and toughening the the cement-based materials(CBM)and preventing the CBM from cracking.Due to its high tensile strength,corrosion resistance and electrical conductivity,carbon fiber(CF)can significantly improve the mechanical and electrical properties of cement mortar.In addition,the influence of carbon fiber on the fresh properties of cement mortar can not be ignored.Therefore,it is necessary to systematically carry out a research on the influence of carbon fiber on the properties of cement mortar,mainly focusing on the key factors on its fresh properties.The findings would provide guidance and reference for the mix design of carbon fiber reinforced cement mortar.In this thesis,three parameters were used:water/cement(W/C)ratio,fiber content and fiber length.A total of 64 cement mortar mixes were designed,and a series of tests on fresh properties,mechanical properties,electrical conductivity and packing density were carried out.Based on the test results,film thickness theory and fiber factor theory,the combined effects of water film thickness(WFT)and carbon fiber on the performance attributes of cement mortar was explored.The main work and conclusions are as follows:(1)The mini slump flow test,mini V-funnel test,sieve segregation test and stone rod adhesion test were used to explore the influences of W/C ratio,fiber content and fiber length on the fresh properties of cement mortar.The results show that the increase of W/C ratio has positive effect on workability and fluidity of cement mortar,negative effect on cohesiveness,positive and negative effect on adhesiveness;the increase of fiber content and fiber length has negative effects on workability,fluidity and adhesiveness,but provides positive effect on cohesiveness.In general,W/C ratio,fiber content and length are main factors affecting the fresh properties of cement mortar,and the influence degree is in the order of:W/C ratio>carbon fiber content>carbon fiber length.(2)The prismatic flexural and compressive strength test and volume electrical resistance test were used to investigate the influence of W/C ratio,fiber content and fiber length on the mechanical and electrical properties of cement mortar.The results indicates that the increase of W/C ratio has a negative effect on the mechanical properties;the increase of fiber content and fiber length has both positive and negative effects on the flexural strength and flexural strength/compressive strength(F/C)ratio(the optimum fiber content is 0.3%and the optimum fiber content is 12 mm),but has negative effect on the compressive strength on the whole.The improvement of W/C ratio has a positive effect on electric conductivity;with the increase of fiber content and fiber length,the volume resistivity decreases to a certain extent,and the decrease is obvious when the fiber content is about 0.1%.(3)The influences of carbon fiber on the packing density and WFT were investigated by using the wet packing method and the calculation method of WFT.The research shows that the increase of the fiber content has both positive and negative effects on the packing density and WFT,which would reach a peak value at fiber content of 0.2%;the increase in fiber length has positive effects on packing density and WFT.(4)Nonlinear regression analysis was applied to explore the influence of WFT,fiber content and fiber length on the fresh properties of cement mortar.The results demonstrate that the fresh properties(workability,fluidity,cohesiveness and adhesiveness)are well correlated to the WFT,fiber content and fiber length,and the corresponding prediction formulas are derived and characterized.Therefore,WFT,fiber content and fiber length are important factors influencing the fresh behaviours of carbon fiber cement mortar.Furthermore,the fiber content and fiber length were integrated into a single fiber factor(FF),FF=V_f(L/D),(V_f is the volume percentage of carbon fiber in mortar,L is the fiber length,D is the fiber diameter).The regression analysis method was also used to explore the combined effects of WFT and FF on the fresh properties of cement mortar.The findings show that the fresh properties of mortar have good correlations to the WFT and FF,then their mathematical models are established,which indicate that WFT and FF are the crucial factors affecting the fresh properties of carbon fiber reinforced cement mortar.Therefore,the combined effects of WFT and FF should be considered in the mix design of carbon fiber cement mortar.