International Concrete Abstracts Portal

The bond property between deformed bars and concrete plays a significant role in the safety of construction. Numerous database-dependent empirical models are proposed to evaluate the bond behavior without considering the effect of additional confinement, whose application range is quite limited as a result of unstable accuracy. In this paper, a new model was established based on the thick-walled cylinder model and fictitious crack theory, which can predict bond strength and bond-slip response with fiber-reinforced polymer (FRP)-steel confinement. The effects of various factors on the bond behavior such as concrete strength, concrete cover, reinforcing bar diameter, bar surface geometry, and FRP/steel confinement were comprehensively discussed. According to radial crack radius, the radial stress and displacement induced on bond interface can be calculated, and thus analytical formulae of bond strength and slip were respectively developed in conjunction with deformed bar surface geometry. Finally, a new analytical model was proposed, which can simulate the bond-slip curves of the specimens with different confinement levels, covering unstrengthened, FRP-strengthened, stirrup-strengthened, and FRP-stirrup dually strengthened specimens. Compared with existing models, the proposed model can provide better agreement with existing test results.

This paper presents the implications of variable bond for the behavior of concrete beams with glass fiber-reinforced polymer (GFRP) bars alongside shear-span-dependent load-bearing mechanisms. Experimental programs are undertaken to examine element- and structural-level responses incorporating fully and partially bonded reinforcing bars, which are intended to represent sequential bond damage. Conforming to published literature, three shear span-depth ratios (av/d) are taken into account: arch action (av/d < 2.0), beam action (3.5 ≤ av/d), and a transition from arch to beam actions (2.0 ≤ av/d < 3.5). When sufficient bond is provided for the element-level testing (over 75% of 5db, where db is the reinforcing bar diameter), the interfacial failure of GFRP is brittle against a concrete substrate. An increase in the av/d from 1.5 to 3.7, aligning with a change from arch action to beam action, decreases the load-carrying capacity of the beams by up to 40.2%, and the slippage of the partially bonded reinforcing bars dominates their flexural stiffness. Compared with the case of the beams under beam action, the mutual dependency of the bond length and shear span is apparent for those under arch action. As far as failure characteristics are concerned, the absence of bond in the arch-action beam prompts crack localization; by contrast, partially bonded ones demonstrate diagonal tension cracking adjacent to the compression strut that transmits applied load to the nearby support. The developmental process of reinforcing bar stress is dependent upon the av/d and, in terms of using the strength of GFRP, beam action is favorable relative to arch action. Analytical modeling suggests design recommendations, including degradation factors for the calculation of reinforcing bar stresses with bond damage when subjected to arch and beam actions.

Due to the excellent deformation coordination ability and permeability, bentonite has been widely introduced to modify concrete in underground geotechnical engineering. However, the underlying mechanism for bentonite modification remains unexplored. A series of experiments was performed to clarify the modification mechanism of bentonite. The results showed that all strengths decreased upon bentonite addition, while high toughness was achieved. The micro-test results revealed that bentonite promotes the dissolution of calcium hydroxide (CH) and the nucleation of calcium silicate hydrate (C-S-H) in the interfacial transition zone (ITZ). The hydration products produced by the reactive ions and ultrafine bentonite particles continuously reduced the porosity and Ca/Si ratio in ITZ, strengthened the interface bonding, and controlled the coalescence of microcracks. Inversely, bentonite particles tend to adsorb large amounts of water and hinder the available water from accessing cement grains, which results in an increased porosity and slower hydration progress of cement grains. The loose microstructure cannot be compensated for by reinforced interfacial bonding and inevitably results in the deterioration of mechanical performance in composites.

This study investigates the bonding behavior of large-diameter steel bars (D43 and D57) embedded in concrete using pull-out tests coupled with acoustic emission (AE) monitoring. These large bars, commonly used in nuclear containment structures from the 1970s, were compared with conventional steel bars (D19 and D32) across three concrete strength levels. All tests were performed under displacement-controlled loading using an MTS testing machine. Results indicate that ACI 408R provisions remain valid for large-diameter reinforcing bars. The test results showed that when specimens reached ultimate bond stress, the D57 bar developed only 12 to 16% of its yield strength, whereas the D19 bar reached at least 70%. AE monitoring effectively captured the debonding process, and cumulative AE hit counts correlated with the strain energy released at each loading stage, offering insight into bond failure mechanisms.

This study investigates the structural performance improvement when bond beams are included in stack bond walls. Nine 4.0 m x 2.4 m x 0.20 m masonry walls were tested under out-of-plane and axial loads. The walls were constructed in three configurations: running bond, stack bond without bond beams, and stack bond with bond beams, following TMS 402/602 standard. Results show similar failure patterns and crack formation between running bond and stack bond walls, but stack bond walls with bond beams exhibited distinct behavior. Stack bond walls with bond beams showed slightly higher out-of-plane flexural capacity compared to running bond walls, with a difference ranging from 4 to 5%. These findings provide valuable insights for evaluating the structural performance of concrete masonry walls with different bonding patterns. This study suggests a potential revision to the Canadian (CSA S304) masonry design standard, potentially lifting restrictions on stack bond masonry wall construction.