For a reinforced concrete structural member, sufficient bond between steel reinforcing bar and concrete guarantees a steel/concrete composite behavior, which is essential for a good overall member performance. Under severe loading, high slippage between rebar and concrete leads to matrix cracking and crushing in the bond region followed by degradation of the rebar/matrix bond. Fiber reinforcement provides a fiber bridging mechanism to resist such cracking behavior by modifying the tensile properties of the matrix. This thesis investigated a deflection hardening hybrid fiber reinforced concrete (HyFRC) with micro/macro fiber hybridization and recommends it as a matrix to enhance the rebar/matrix bond by utilizing its superior crack resisting ability.
In the experimental phase, the rebar bond behaviors in ordinary concrete (OC), HyFRC without/with high volume fly ash and Engineered Cementitious Composites (ECC), another type of fiber reinforced cement-based composite (FRCC) with hardening behavior, were studied. The experimental program consists of monotonic and cyclic rebar pull-out test series and was supported by digital image correlation (DIC) and vibration test techniques. Different specimens were made by varying some conditions such as rebar size (no. 4 and no. 8) and absence/presence of transverse reinforcement (spiral) so the effect of such conditions in regards to bond between rebar and different matrices can also be studied. In addition to experiments, Finite element models were developed to further investigate the mechanical behavior of the HyFRC matrix during rebar pull-out and to examine how a transverse steel reinforcement affects such behavior.
The experimental results showed that HyFRC can improve the rebar pull-out behavior by changing the failure mode from brittle splitting failure to a more ductile frictional pull-out failure compared to the OC material. The HyFRC material without fly ash provided better rebar bond performance compared to OC with spiral reinforcement and other type of FRCC under investigation. Compared to the monotonic rebar pull-out behavior, only minimal amount of additional damage was induced in such HyFRC material by repeated loading and unloading process from cyclic loading protocol.
DIC measurement indicated that the width of the splitting cracks that form in HyFRC matrices due to rebar pull-out could be further reduced when such specimens were confined by spirals. The finer the splitting cracks, the higher the rebar gripping stress and hence the higher the rebar pull-out resistance. The DIC measurement was consistent with the results from finite element analysis because both of them showed that confining the HyFRC matrix with transverse steel reinforcement led to a more uniform distribution of the splitting crack width during rebar pull-out. The vibration test showed that rebar pull-out induced similar degree of damage in the bond regions of HyFRC with and without transverse reinforcement.
If severe macrofiber pull-out occurs within the cracks induced by rebar pull-out, the macrofibers become less effective in resisting such cracks. Therefore, good macrofiber pull-out resistance in HyFRC is essential for an improved rebar pull-out performance. To investigate the macrofiber pull-out behavior and how it is affected by the presence of PVA microfibers in HyFRC, single fiber pull-out tests were conducted for various mortar mixtures. In some of these mortars, the cement was replaced by industrial by-products, such as fly ash and slag. In addition, macrofiber pull-out behavior in an ultra-high strength concrete with and without steel microfiber reinforcement was also studied.
The results of the single fiber pull-out tests revealed that the presence of PVA microfibers in a mortar mixture designed based on the HyFRC mixture enhances the steel macrofiber pull-out resistance. This synergy between micro and macrofibers provided a more effective control of the splitting cracks and was responsible for the superior rebar bond performance. Hence, the research revealed a multi-scale bond enhancement in a HyFRC member reinforced by steel rebar. The macrofiber pull-out behaviors from mortars in which 45% and 15% of cement were replaced by slag and fly ash, respectively, showed that slag densified the steel macrofiber/matrix interface and hence, improved the pull-out resistance for the macrofiber. On the other hand, test results of fiber pull-out specimens in which 55% of cement was replaced by fly ash showed that increasing curing age made the PVA microfiber reinforcement less effective in resisting steel macrofiber pull-out. Such characteristic showed how high-volume fly ash weakened the PVA fiber/matrix bond. This mechanism induced by fly ash made PVA fibers in HyFRC less effective in improving steel macrofiber pull-out resistance and hence, reduced the micro/macro fiber synergy, which is beneficial for the rebar bond behavior. Therefore, using high volume fly ash decreased the rebar pull-out resistance in HyFRC. When steel macrofibers were pulled out from extremely strong matrices, almost no microcracks formed around macrofibers and the steel microfiber reinforcement didn’t affect the macrofiber pull-out behavior because such microfibers function by bridging the microcracks.