The multiphasic conductive admixtures like macro steel fiber (SF), nano carbon black (NCB) and carbon fiber (CF) are incorporated into concrete to improve the conductivity and mechanical properties. In this study, the mechanical and conductive properties of the concrete beam are explored by the hybrid use of different conductive admixtures. The deflection hardening (multiple cracking) behavior of concrete beam with different fiber content, monitoring of single and multiple cracks of diphasic (SF+NCB) and triphasic (SF+CF+NCB) conductive concrete beam through resistance measurement is investigated. Moreover, the relationship between fractional change in resistance (FCR) and crack opening displacement is established to study the influence of different types and amounts of electric conductive admixtures on self-sensing behavior of concrete beam. Results reveal that the deflection hardening (multiple cracking) behavior is obtained with 70 kg m ⁻³ SF content. The single and multiple cracks monitoring is illustrated through load-time and FCR-time relationship. In addition, FCR improved linearly with an increment of NCB with triphasic conductive admixtures.

In order to study the axial compression behavior of rectangular concrete short columns that have been strengthened with basalt fiber-reinforced polymer sheets, the axial compression test were conducted on 15 groups specimens. The parameters are the cross aspect ratio and the number of basalt fiber-reinforced polymer layers. As the aspect ratio increases, the confined effect of basalt fiber-reinforced polymer decreases. The strength of one ply of basalt fiber-reinforced polymer decreases as the aspect ratio increases, the range is not very obvious, but the two ply of basalt fiber-reinforced polymer has the obvious decreasing tendency. With the increase in aspect ratio, the square and rectangular section columns of basalt fiber-reinforced polymer confined have a certain size effect. When the aspect ratio is 1.0, the values of f_cc/f_c0 by one ply of basalt fiber-reinforced polymer and two ply of basalt fiber-reinforced polymer are 1.14 and 1.50, respectively. And when the ratio is 2.0, the values of f_cc/f_c0 are 1.06 and 1.16. The confined effect is best when the aspect ratio is 1.0, while there is no significant effect with the ratio of 2.0. Results of the theoretical analysis compared with the experimental results show good agreement.

In order to investigate the bond stress-slip constitutive relation between BFRP bar and basalt fiber recycled-aggregate concrete, 81 central pullout specimens were tested. Basalt fiber volume percent, fiber length and concrete strength grade were considered to be experimental variables to obtain the bond stress-slip curves. The results showed that adding basalt fiber into recycled-aggregate concrete will reduce the bond stress between BFRP bar and recycled-aggregate concrete but enhance the ductility of bond property; the bond stress increases with the increasing of fiber length and concrete strength grade. The bond stress-slip constitutive model was proposed and the results showed that the model is good in effect of fitting. The constitutive model can reflect the bonding mechanism between BFRP bar and basalt fiber concrete well, it also can provide reference for theoretical analysis and engineering applications of the bond and anchor property between BFRP bars strengthened recycled concrete.

The use of recycled aggregate concrete (RAC) in construction is one of the most effective ways to solve the environmental problems of concrete waste. The purpose of the present study is to improve the high-temperature resistance of RAC by incorporating hybrid cellulose and basalt fibers. The mechanical properties of the specimens with different volume fractions of cellulose and basalt fibers after high temperature were studied. Additionally, in order to study the degradation behavior of the specimens, the microstructure of interface between fibers and the cement matrix of RAC was examined by scanning electron microscope. Results show that hybrid fiber can reduce the explosive spalling and mass loss of RAC after being subjected to high temperatures. Compared with the brittle failure of RAC, the failure mode of hybrid fiber recycled aggregate concrete (HFRAC) was transformed into ductile failure under compression and splitting test. After high temperature, the compressive strength and splitting tensile strength of HFRAC were higher than those of RAC, and the cellulose–basalt fibers displayed an excellent synergistic effect on the mechanical properties of RAC. Among all the RAC specimens with different volume fractions of cellulose and basalt fibers, specimen hybrid with 0.15% cellulose fiber and 0.15% basalt fiber showed the best mechanical properties. Microstructure analysis research shows that small channels can be left in the matrix after the melting of cellulose fiber after elevated temperature, which improves the permeability of concrete and reduces the spalling damage of RAC.