Experimental Study On Behavior Of Concrete Columns Strengthened By Ferrocement Including Steel Bars Under Axial Compression, Eccentric And Seismic Load

Concrete column is the key member in the construction framework which affects its safety and durability greatly. Enlarging concrete section by using new concrete, enveloping steel and strengthening with fiber-reinforced polymer (FRP) are three widely-used methods to strengthen concrete column, but these methods have their own disadvantages, such as construction complex and volume increase of enlarging section method, high technical needs and poor durability of enveloping steel, and the high cost and poor fire proof of FRP method. In order to improve the seismic performance, ductility and durability of concrete columns significantly while improving the bearing capacity greatly, this thesis put forward a new method for strengthening concrete columns with ferrocement including steel bars (large diameter) and its experimental study was carried out.a. The axial compression performanceA total of 74 circular concrete columns were strengthened by using ferrocement including steel bar (SW), steel bar (S) and FRP respectively. Comparative test was performed under axial compressive load, the effect such as different strengthening methods and the area ratios between steel bar and wire mesh were studied according to the index of bearing capacity and ductility. The results showed that, SW not only improved the bearing capacity dramatically, but also improved the displacement ductility significantly comparing with S and FRP methods. For example, on the premise that the bearing capacity of the SW2 (with two layers of wire mesh) columns was 51% higher than the S columns, the ductility of the SW2 columns was still 2.05 times of the S columns. One of the main reasons is that the superior reinforcement dispersion of the SW columns induces a massive criss-cross of tiny cracks during the loading process, leading to a great energy dissipation. The formula of the ultimate load of the SW columns was proposed, the calculated results show a good agreement with experimental data. In addition, a method for measuring the stain of mortar surface was explored.b. The eccentric loaded performance18 circular concrete columns strengthened by SW and S were tested under eccentric loading, and the effect of the eccentricity (small) was studied. The results show that, eccentricities have a significant effect on the ultimate bearing capacity of each strengthened column. Both S and SW methods can not improve the ductility effectively. The measured ultimate load of the SW column was much higher than that by using the code formula.c. The seismic performanceAn experimental study of 6 square concrete columns strengthened by ferrocement including steel bar (FS) and steel bar (S) was carried out under the cyclic loading. The results show that, the FS can not only improve the hysteretic energy dissipation and ductility of square concrete column significantly, but also enhance the bearing capacity and rigidity greatly. When the reinforcement ratio of the FS column was only 11% higher than that of the S column, the total energy dissipation of the former was as much as 1.61 times of the latter. One of the main reasons is that the wire mesh with superior reinforcement dispersion protects the bond between longitudinal bars and concrete/mortar well, meanwhile it makes the mortar keep cracked but not broken, friction and extrusion of the mortar on both sides of the cracks lead to a large energy dissipation. The plastic hinge length at the column bottom was discussed. In addition, it was pointed out that a reasonable evaluation could only be drawn out based on a comprehensive analysis on both the energy dissipation coefficient E and the total energy dissipation.Further research issues were put forward at the end of the thesis.The whole research shows that, ferrocement (including steel bar) strengthening is a reliable strengthening method, coupled with low cost, construction easily (no need of moulds and large construction tools), which is suitable for seismic retrofit of the primary and secondary school buildings in urban and rural China. With broad application prospects, this method will probably get different concrete results.