Experimental Study And Theoretical Analysis On The Anchoring Mechanism For GFRP Anti-floating Anchor

The glass fiber reinforced plastics(GFRP) anchor is the new kind of anchors which composes of resin and glass fiber. Compared with traditional steel anchors, GFRP anti-floating anchors have the advantages of high specific strength, nonelectric conductivity, nonmagnetic nature, corrosion resistance and so on. It can overcome the groundwater and electrochemical corrosion which occurs in traditional metal anchor through GFRP material used for anti-floating anchors. The kind of anti-floating anchors are particularly useful for the anti-floating of MRT station. A series of laboratory and field tests are conducted through the embedded bare fiber Bragg grating sensing technology to study the distribution of stress and strain and failure mechanism of GFRP anti-floating anchors. The anchoring characteristic between GFRP anti-floating anchors and around soil(internal anchorage), GFRP anti-floating anchors and concrete floor and the creep characteristic with loading for long time are also presented in the thesis. Based on the test results, the concept of “whole deformation” about GFRP anti-floating anchors under loads is proposed and improved the design theory and methods. The test results are as bellow:1. The embedded bare fiber Bragg grating sensing technology was successfully used in the pull-out tests on GFRP anti-floating anchors through pull out tests in field. The bearing characteristics, load transfer mechanism and the interface bond properties of GFRP anti-floating anchors installed in moderately weathered granite were studied compared with steel anti-floating anchors. It is indicated that:(1) There are two failure modes of anti-floating anchors: to short anchors, shear failure model mainly occurred between the mortars and surrounding rock mass(the second interface); to long anchors, the shear failure mainly occurred between the anchor and mortars(the first interface) and the failure essence is the shear failure at the peak value.(2) The axial forces of anchor decrease gradually with depth, and the value will be zero beyond certain length; the load transferring depth of GFRP anti-floating anchors is greater than steel anchors and the transferring rate is lower. The shear stress peak value increased gradually with loading increment and moved to deeper position. However, the shear stress peak value is bigger than steel anchors and the shear stress of steel anchors acts earlier than GFRP anti-floating anchors.(3) As to the GFRP anti-floating anchors with anchorage length 2.0 m, the diameter 28 mm and M32.5 cement mortar, the ultimate uplift capacity is about 225 k N. The average bond strength of the first interface is 1.50~1.54 MPa and 0.32~0.37 MPa for the second interface to GFRP anti-floating anchors with diameter of 28 and 32 mm. The average bond strength of the second interface increases with anchor diameter increased.(4) The ultimate uplift capacity of GFRP anti-floating anchor increased with anchorage depth and diameter increment and the value is higher than steel anchors. The proposed value of reasonable anchorage length of GFRP anti-floating anchors installed in moderately weathered granite is about 3.5~5.0 m.2. The external anchorage mechanism of GFRP anti-floating anchors with different anchorage forms and different anchorage length was studied through the pullout test in door with nut pallet as the anchor. The external anchorage deformation(slippage) and the ultimate capacity also monitored during the test. Test results indicated that:(1) There are two failure modes of the GFRP external anchorage. One is the splitting failure with lack of GFRP material strength and the other is pulling out failure due to larger relative slippage between GFRP anchors and concrete.(2) The ultimate capacity of GFRP anti-floating anchor with direct anchoring is about 356 k N when the external anchorage length is 30 d and the value is about 384 k N for GFRP anchors with nut-pallet anchorage. Compared with GFRP anchors with external anchorage length of 15 d, the values increase by 65.6%, 43.8% respectively. For the GFRP anchors with different anchorage form and same anchorage length, the interfacial bond strength of GFRP anti-floating anchors with nut-pallet anchorage increased by about 7.9~24.4% than that with direct anchoring.(3) The average bond strength between GFRP anti-floating anchor and concrete reduced with the incensement of anchorage length, and increased with slippage increment.(4) With the GFRP anti-floating anchors with 28 mm diameter, external anchorage length 15 d and 30 d, the external anchorage deformation is no more than 2.5 mm in the condition of C25 commodity concrete under the loading of 200 k N or less. The value is satisfied with the engineering demand.3. Based on four pull-out creep tests of GFRP anti-floating anchors with loading for long time, the creep mechanical model was studied and verifies the correctness of the mode through the test results. By introducing the concept of time damage, the long-term pulling out capacity of GFRP anti-floating anchors was proposed combined with the creep mechanical model.(1) The creep behavior of GFRP anti-floating anchors occurred when the load reached 40% of the ultimate capacity and the creep deformation is little. In the practical engineering, the 40% of the ultimate capacity can meet the engineering requirement.(2) The Burgers model described well the creep behavior of the GFRP anti-floating anchors, and the predicted results agreed well with the test results.(3) Based on damage mechanics theory, the change law of damage variable with time was proposed combined with Burgers model, and the change law of ultimate capacity with time was also obtained. The given long-term capacity of GFRP anti-floating anchors is little different from actual force, which indicated that the established creep damage model is suitable to predict the long-term capacity of GFRP anti-floating anchors.4. Based on the internal anchorage test, external anchorage test and creep test, the failure mechanism of the first interface, the second interface and material itself of GFRP anti-floating anchors were discussed respectively. The concept of “whole deformation” was proposed which is helpful to fully understand working mechanism of GFRP anti-floating anchor.