Investigation of surface bond behaviour of FRP sheets on concrete and masonry substrates

The majority of non-ductile reinforced concrete buildings are vulnerable to extreme load effects, including those caused by strong earthquakes and blast shock waves. These buildings consist of reinforced concrete frames with and without infill masonry walls, and non-ductile shear walls. They often benefit from retrofit strategies at the system level because of the presence of a large number of poorly designed non-ductile elements. This indicates that a limited number of elements are strengthened and stiffened for deformation control, while the remaining elements remain within the elastic range of deformations during loading. One of the attractive retrofit techniques used to ensure deformation control is the use of surface bonded fibre-reinforced polymer (FRP) sheets or strips on masonry and non-ductile concrete walls for increased strength. Because FRP material itself does not possess ductility, its use in such applications is not intended for ductility enhancement. The surface bond characteristics of FRP sheets have not been fully understood. The sheets exhibit brittle performance and often delaminate prematurely before they attain their material capacity. The current research is aimed at contributing towards the knowledge gap in this area. It consists of both experimental and analytical tasks. The experimental research consists of seventy one small-scale pull-out tests. The test variables includes, the width, length, and the number of layers of Carbon FRP (CFRP) strips; concrete compressive strength; substrate material consisting of concrete, brick and concrete blocks; and loading conditions as monotonic and cyclic loading. The experimental data indicate that surface-bonded FRP strips on concrete and masonry substrates develop limited strength as governed by bond failure. The bond stress shows a bell-shaped distribution, spreading over an effective bond length with a maximum bond stress that varies between 3.0 MPa and 7.0 MPa for the type of application considered in the current investigation. The FRP has an effective bond length beyond which any further increase in length does not result in a substantial increase in bond strength. This length was determined to vary between 90 mm and 115 mm for the specimens tested in this investigation. The effect of increased concrete strength on surface bond characteristics of FRP is to increase ultimate bond capacity and corresponding slip marginally, without much influence on the effective length. The effect of increased number of layers of FRP is to increase ultimate bond while decreasing slippage, with a small increase in effective bond length. The effect of the increase in FRP strip width is to increase ultimate bond force while decreasing ultimate bond strength and the corresponding slip, without much influence on the effective bond length. Cyclic loading improves surface bond characteristics of FRP, increasing ultimate load resistance by approximately a factor of 1.2 while also resulting in increases in ultimate bond stress and effective bond length. Surface bond characteristics of FRP on concrete and clay brick substrates are similar. Concrete block masonry tends to fail prematurely due to the material failure of block cell walls prior to developing bond failures. The experimental results are used to generate and validate a new bond-slip model. Design expressions, in terms of effective bond length and ultimate load capacity, are suggested. Comparisons between the analytical and experimental results show good correlations. It is shown that the ultimate load capacity continues to increase, all be it marginally, with an increase in bond length between the effective length and twice the effective length. Therefore, it is recommended to use twice the effective length in design