International Concrete Abstracts Portal

FRP composite materials have mechanical properties which are beneficial and advantageous for design of bridges. The application of these FRP composite materials are currently being used for demonstration projects for repair/retrofit/ rehabilitation of existing bridges and a few for new installations. The future applications of these materials will depend upon the development of the appropriate material/product standards and performance criteria for special proprietary products. The need for coordinated research projects is outlined for the development of the necessary standards and design requirements. Only with proper documentation will the FRP composites become another material available to the construction marketplace for bridges. An outline for the required research projects is presented for the FRP composites for bridge construction.

Twenty cantilever type specimens and four beam specimens were tested to evaluate the bond behavior in reinforced concrete members confined with carbon or aramid FRP sheets. The main test variables were the vertical cover depth, the diameter and the number of longitudinal bars, and the type and the amount of FRP sheets. The test results showed that the confined specimens had higher bond strength, larger peak load slip displacement and lesser bond degradation after the peak than their unconfined prototype counterparts. Based on the test results, an equation was developed to predict the increase in bond strength due to the FRP sheet confinement. That increase was expressed similarly to that due to transverse steel reinforcement except that the elastic modulus of the FRP sheet was important but the number of longitudinal bars was not. The proposed equation was validated using results of column specimens tested in other research institutes and by cantilever and beam specimens tested by authors. It was proved that as long as the bond strength of an unconfined prototype specimen is evaluated properly, the total bond strength of confined specimens can be predicted accurately using the proposed equation although the limitations of the proposed equation still need to be clarified.

In this paper results of an experimental investigation on bond between a glass fiber reinforced plastic (GFRP) rebar and concrete are presented and discussed. Rebars used in tests are the FRP C-BarTM produced by Marshall Industries Composites, Inc. Bond tests were carried out by using a test machine obtained from a modification of the standard scheme of the beam-test and were conducted on prismatic concrete specimens within which a #4 Grade B E-Glass C-Bar was embedded: the embedment length was ranging from 5 to 30 times the bar diameter, thus obtaining different test arrangements. Bond-slip relationships were obtained and bond mechanisms discussed. Furthermore, values of elastic modulus and tensile strength of rebar were evaluated. Finally, a bond-slip constitutive law obtained by means of a system identification procedure is presented. Numerical simulations of bond tests have been performed by using such bond-slip relationship and the obtained bond-slip curves have been compared with the experimental ones.

One of the major disadvantages of the plate bonding technology is the premature and brittle debonding failure of the bonded plate. It has been assumed, quite logically, that the stress concentration at the plate end is the primary cause of such premature plate debonding failure. However, there is no direct evidence of the validity of these stresses as to whether the predicted stresses agree with the experimental data or not. Also there is concern if they can form the basis and criteria for the design and prevention of debonding failures. This paper presents a critical analysis of the calculated peak shear and normal stress values at the plate end using Roberts’ approximate model, and derived from a wide range of published data involving steel, glass fibre reinforced polymer (GFRP) and carbon fibre reinforced polymer (CFRP) plates. It is shown that these calculated stresses are far too high, and cover unacceptably wide range of values, without any consistent pattern of variation with the plate stiffness. It is clear that the peak stresses are influenced by other parameters which are not taken into account in the approximate model used in the calculations. The wide range of the peak stresses obtained from a large number of tests seems to indicate that these stresses cannot form a reliable basis to explain or design the prevention of premature plate debonding failures.

Although a vast number of papers on plate bonding have been published, certain aspects, especially concerning bond have yet been too insufficiently clarified to be adequately considered in practical design. Bond failure is mostly brittle and often occurs a few millimeters deep in the concrete. A fracture mechanics-based engineering model of bond strength for this failure type, derived from pure bond tests exists. In a beam vertical shear crack mouth displacements can reduce bond strength by reducing the total fracture energy, required to destroy the local bond. To quantify this effect, the shear crack mouth displacements have to be calculated, dependent on the acting forces, the geometry, the reinforcement and the material parameters of the beam. Then a mixed mode fracture mechanics approach is used to quantify the loss of bond strength due to simultaneous action of bending and shear. In most cases, bond strength reduction due to vertical shear crack displacement will range around 5-10%. In CFRP-plates another type of debonding failure, specific to fiber reinforced plastics was observed. Interlaminar failure in the plate, preferably occurring with higher-strength concrete is considered a mixed mode fracture problem and was in-vestigated by simultaneous measurements of the mode I and mode II displace-ments of the bond crack with electronic speckle pattern interferometry (ESPI). A fracture mechanics approach to a criterion for interlaminar plate failure was derived. According to this, a surface tensile strength of the concrete of more than 3,0 MPa cannot be taken advantage of in design, since interlaminar plate failure