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The use of Fiber Reinforced Polymer (FRP) composites has become a popular solution for retrofitting and strengthening of existing concrete and masonry structures. However, some drawbacks of this technique, mainly associated with the use of organic resins, have been reported. To overcome such drawbacks, the development of composite materials in which the organic resins are replaced with inorganic matrices has recently caught the attention of the civil engineering industry. Among these newly developed systems, Fiber Reinforced Cementitious Matrix (FRCM) composites, which are comprised of high strength fibers embedded within an inorganic matrix, have shown promising results. However, research on this topic is still limited and important aspects, such as the bond behavior between the composite and the substrate, are not fully understood and require further study. This paper presents the results of an experimental campaign aimed at investigating the influence of the type of matrix and substrate on the bond behavior of FRCM composites. Glass-FRCM composite strips were applied onto concrete and masonry substrates and then tested by means of a classical push-pull single-lap direct-shear test set-up. A cementitious and a lime-based matrix were employed to apply the same type of fiber on concrete and masonry substrates, respectively. FRCM-concrete and FRCM-masonry joints reported the same failure mode. However, higher values of the peak load were obtained for the lime-based glass-FRCM composite applied onto masonry substrates than with the cementitious glass-FRCM composite applied onto concrete substrates.

This paper describes the results of an experimental study carried out to understand the behavior of short plain concrete prisms with a square cross-section confined by steel reinforced polymer (SRP) jackets subjected to a monotonic concentric compressive load. The effectiveness of the confinement is studied in terms of load-bearing capacity and ultimate strain with respect to unconfined prisms. Test parameters considered in this study are the density of steel fibers, concrete corner condition, concrete surface treatment, SRP jacket height, and number of confinement layers. Digital image correlation (DIC) is used to qualitatively and quantitatively study the displacement and strain fields on the composite surface. SRP confinement is shown to improve the compressive strength and the ultimate strain of concrete prisms relative to the unconfined condition. Increases in fiber density and number of confinement layers are not proportional to increases in confined compressive strength.

This paper presents the results of an experimental investigation of the flexural response of reinforced concrete (RC) beams strengthened using externally bonded steel fiber reinforced cementitious matrix (steel-FRCM) composite. Steel-FRCM composite strips were bonded to the tension face of four RC beams, which were tested in four-point bending. Parameters varied were the presence/absence of the external (coating) layer of the matrix, presence/absence of U-wrap anchorages, and loading rate. Results are compared with those from single-lap direct-shear tests conducted on the same composite. The direct-shear tests showed that debonding of steel-FRCM joints is characterized by fiber slippage and fracture of the matrix layer at the internal matrix layer-fiber interface. In the beam tests, the strengthening system increased the yield load by 15–21% relative to the unstrengthened beam. The ratio of the load at which debonding occurred to the load at yielding ranged from 1.11 to 1.19 for each strengthened beam. The load rates employed and the presence of the external matrix layer did not appear to significantly affect the failure mode or the load and midspan displacement at debonding. The presence of the U-wraps helped restrain the peel-off of the composite observed in strengthened beams without the U-wrap, however, they did not restrain the fiber slippage at the ends of the composite, which inhibited composite action. Average values of the maximum fiber strain at composite debonding determined using strain profiles from strain gages, an approximate method, and moment-curvature analysis were 0.54%, 0.73%, and 0.83%, respectively.

The use of fiber reinforced cementitious matrix (FRCM) composites has been studied for flexural and shear strengthening of reinforced concrete (RC) members, but currently there are no studies on its use for torsional strengthening. This paper presents the results of an experimental study in which solid rectangular RC beams were externally strengthened with PBO-FRCM composite material in different wrapping configurations to investigate the torsional behavior in terms of strength, rotational ductility, and failure mode. Increases in the cracking torque, torsional strength, and corresponding values of twist were achieved by beams strengthened with a 4-sided wrapping configuration relative to the control (unstrengthened) beam. On the other hand, the 3-sided wrapping configuration was found to be largely ineffective in improving the torsional performance due to excessive fiber slippage. The contribution of the strengthening system to the torsional strength was reasonably predicted (±20%) by the strain measured in the composite fibers. Provisions used to estimate the torsional strength of RC beams with fully-wrapped, externally-bonded fiber reinforced polymer (FRP) composites were found to be applicable to beams strengthened with PBO-FRCM composite.

Investigation of the bond between fiber reinforced composites and the substrates onto which they are applied is of critical importance to understand their failure mechanisms. The bond behavior can be studied using different experimental test set-ups; the most commonly used are the single-lap and double-lap direct-shear tests. Although single-lap shear tests are simpler to carry out than double-lap shear tests, the presence of an eccentricity between the pulling and restraining forces leads to a mixed mode fracture process at the interface, which may influence the results. This study investigates the eccentricity effect on the bond behavior of fiber reinforced cementitious matrix (FRCM) composite–concrete joints. FRCM composite strips with the same bonded length were applied to concrete blocks of different lengths and tested using the single-lap shear test. The use of digital image correlation (DIC) allowed for studying the strain field on the surface of the bonded composite. Results were compared with those from double-lap shear tests of the same composite. The results obtained confirm that the eccentricity effect is negligible for bonded lengths longer than the effective bond length.

Fiber Reinforced Cementitious Matrix (FRCM) composites can be employed as a valid alternative to Fiber Reinforced Polymers (FRP) composites for strengthening existing reinforced concrete (RC) structures. The weakness of FRCM-concrete joints has been reported to be at the matrix–fiber interface rather than within the concrete substrate, as is usually reported in the case of FRP-concrete joints. The experimental investigations regarding FRCM strengthening available in the literature have been performed by applying the FRCM composite onto a surface that was treated before the application, usually by grinding or sandblasting. However, the role of the substrate characteristics on the failure mechanism of the FRCM composite has not been properly investigated. This paper presents the experimental results obtained from FRCM-concrete joints tested using single-lap direct-shear tests. FRCM composites with different bonded lengths and bonded widths were applied onto concrete specimens with different surface preparations and different concrete strengths. The influence of shrinkage cracks and load distribution along the composite width on the failure modes observed is discussed. In order to avoid the possibility of sudden composite detachment failure, the authors recommend that the substrate surface should be treated, and the matrix shrinkage should be controlled.

This paper presents the results of an experimental study conducted to understand the behavior and stress-transfer mechanism of fiber-reinforced cementitious matrix (FRCM) composites externally bonded to a concrete substrate for strengthening applications. The FRCM composite was comprised of a polyparaphenylene benzobisoxazole (PBO) fiber net embedded within two layers of polymer-modified cement-based mortar. Single-lap shear tests were conducted on specimens with composite strips bonded to concrete prisms. Parameters that varied were bonded length and width of composite. Additionally, the external coating layer of matrix was omitted on a limited number of specimens to examine the interfacial behavior between fibers and matrix and the role of the matrix in the stress transfer. Strain measurements along the composite bonded length were used to investigate the stress-transfer mechanism. Results suggest that the effective bond length of this composite is within the range of 250 to 330 mm (10 to 13 in.). Unlike with fiber-reinforced polymer (FRP) composites, no width effect was observed in terms of the maximum load. Finally, the stress-transfer mechanism at the matrix-fiber interfaces on either side of the fiber net was found to be unequal.