International Concrete Abstracts Portal

Seven one-half-scale reinforced concrete coupling beams, designed using ACI 318-19, were tested with constant stiffness axial restraint. The test variables were the span-depth ratio, reinforcement configuration (conventional or diagonal), primary reinforcement ratio and bar diameter, and level of axial restraint. Six beams consisted of three nominally identical pairs, with the two beams in each pair tested at a different level of axial restraint. The two conventionally reinforced beams reached peak strength at 2.0 and 3.0% chord rotation and experienced rapid post-peak strength degradation with the opening of diagonal cracks and the formation of splitting cracks along the longitudinal reinforcement. Strength degradation in diagonally reinforced beams initiated with buckling of diagonal reinforcement, and variation in axial restraint on identical pairs of beams did not lead to a significant difference in deformation capacity. Deformation capacity was larger for beams with a larger diagonal bar diameter, which corresponded to a larger reinforcement ratio and a larger ratio of transverse reinforcement spacing to diagonal bar diameter (s/db). For the diagonally reinforced test beams, the maximum measured shear strength reached as high as 2.4 times the nominal shear strength computed using ACI 318-19 and exceeded the 0.83 √____fc ′ A cw MPa (10 √ ____fc ′ A cw psi) limit on nominal shear strength by more than a factor of 2.0 in the test with the smallest span-depth ratio. Based on strut-and-tie behavior, modifications to the ACI 318-19 equation to include axial load were examined. When the location of the compressive strut and tension tie at the beam ends was consistent with nominal moment calculations, the resulting ratio of the average maximum measured shear strength in the positive and negative loading directions to shear strength calculated using the modified equation ranged from 1.16 to 1.33. For the diagonally reinforced beams, a larger spandepth ratio, bar size, and reinforcement ratio were associated with larger rotation at yielding and larger effective flexural rigidity.

This paper presents the behavior of unreinforced cylindrical concrete elements confined with a hybrid system, consisting of an ultra-high-performance concrete (UHPC) jacket and basalt fiber-reinforced polymer (BFRP) grids. For exploring the feasibility of the proposed strengthening scheme, a series of tests are conducted to evaluate material properties and to obtain results related to interfacial bond, load-bearing capacity, axial responses, and failure modes. To understand the function of the individual components, a total of 57 cylinders are loaded under augmented confining conditions, including plain cores with ordinary concrete (CONT), plain cores with UHPC jackets (Type A), and plain cores with UHPC jackets plus BFRP grids (Type B). By preloading the cores at up to 60% of the control capacity (60%fc′) before applying the confinement system, the repercussions of inherent damage that can take place in vertical members on site are simulated. The compressive strength of UHPC rapidly develops within 7 days, whereas its splitting strength noticeably ascends after 14 days. The adhesion between the ordinary concrete and UHPC increases over time. While the Type B specimens outperform their Type A counterparts in terms of axial capacity by more than 18%, reliance on the BFRP grids is reduced with the growth of UHPC’s strength and adhesion because of the interaction between the hardened UHPC and the core concrete. The adverse effects of the preloading are noteworthy for both types, especially when exceeding a level of 30%fc′. The BFRP grid-wrapping alleviates the occurrence of a catastrophic collapse in the jacketed cylinders, resulting from a combination of the axial distress and lateral expansion of the core. Analytical models explain the load-carrying mechanism of the strengthened concrete, including confinement pressure and BFRP stress. Through parametric investigations, the significance of the constituents is clarified, and design recommendations are suggested.

It is well known that the estimates of most shear capacity prediction models for reinforced concrete (RC) components are of high dispersion, due to its elaborate failure mechanism and elusive. A probability prediction model is more appropriate for estimating the shear capacity of RC members than a deterministic prediction model. Therefore, this study proposed a probabilistic model to evaluate the shear capacity of RC T-beams and employed a Bayesian-Markov Chain Monte Carlo (MCMC) approach to determine the posterior parameter in the shear strength prediction model by Bayesian updating. The analysis results indicate that the probabilistic model achieves minimal variance, offering the most accurate predictions that closely match test data compared with other prediction models. The shear capacity of the T-beam increases with changes in flange width and flange height ratio but remains constant once beyond a certain level. The shear capacity varies rapidly when the shear-span ratio (λ) is less than 2.5 or larger than 4.0, due to a notable shift in the failure mechanism. Besides, the shear capacity raises linearly by increasing the characteristic value of stirrups (ρ_vf_yv).

This paper presents the torsional behavior of hollow reinforced concrete beams strengthened with carbon fiber-reinforced polymer (CFRP) U-wraps. Test parameters involve a variable wall thickness in the section and the width and spacing of the externally bonded CFRP sheets. An experimental program is conducted with 27 beams (3 unstrengthened and 24 strengthened) to examine their capacities, shear flows, and force distributions when incorporating a ratio of 0.27 to 0.46 between the areas of the hollow and gross cross-sections. The stiffness and capacity of the test beams are dominated by the wall thickness and the effectiveness of CFRP-strengthening becomes pronounced as the void of the beams decreases. The presence of CFRP redistributes internal shear forces in the cross section, which is facilitated by narrowing the spacing of the U-wraps. The effective zone of CFRP retrofit is positioned near the outer boundary of the strengthened section. Regarding crack control, multiple discrete U-wraps with narrow spacings outperform wide U-wraps with enlarged spacings. While the location of a shear-flow path is dependent upon the wall thickness, the width of the U-wraps controls the effective shear-flow area of the beams. The size of the void is related to the stress levels of internal reinforcing components, including yield characteristics. Transverse stirrups are the principal load-bearing element for the unstrengthened beams; however, the reliance on the stirrups is reduced for the strengthened beams because the U-wraps take over portions of the torsional resistance. Through a machine learning approach combined with stochastic simulations, design recommendations are proposed.

This laboratory study employed synthesized urea-formaldehyde (UF) microcapsules and polyvinyl alcohol (PVA) microfibers as a self-healing system to improve the durability of concrete in cold climates. The resistance of concrete specimens to rapid freezingand- thawing (F/T) cycles was evaluated by measuring the change of relative dynamic modulus of elasticity (RDM) with respect to the number of F/T cycles. The control specimens (either with or without PVA microfibers) approached the failure state with a reduction of 38% in RDM after being subjected to 54 F/T cycles, whereas the self-healing specimens (either with or without PVA microfibers) remained in a good state with a reduction of approximately 10 to 15% in RDM after 732 F/T cycles. A polynomial regression model was developed to establish the relationship between the RDM and number of F/T cycles, and a three-parameter Weibull distribution model was employed to conduct the probabilistic damage analysis and characterize the relationship between the number of F/T cycles (N) and the damage level (D) with various reliabilities. The results revealed that the benefits of UF microcapsules and PVA microfibers to the frost durability of concrete diminish once the damage level exceeds a certain high level. Based on the Weibull distribution model, the relationships were established and validated between N and D by comparing the experimental data, the predicted data based on the nonlinear polynomial regression model, and the predicted data based on N-D relationships. The field service life of the self-healing concrete was then predictable at any given reliability.