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Provisions for the calculation of interface shear strength have remained unchanged in ACI 318 since the 1980s. The shear friction concept, while simple to apply, does not address many of the most important influencing parameters for interface shear strength. It is silent on cyclic loading, intermediate levels of interface roughness, and the strength of interfaces reinforced with short dowels. To assess the approach included in ACI 318 and to enable the formulation of a new approach, a comprehensive database of test results has been assembled. The results of recent cyclic shear tests performed at the National Technical University of Athens (NTUA) have been combined with the results of investigations conducted worldwide between 1960 and 2020--a total of nearly 1240 tests--to provide a definitive basis for the development of a model for the accurate prediction of interface shear strength under both monotonic and cyclic displacements. Keywords: cold joints; dowel action; interface shear; natural cracks; shear friction; surface roughness.

The study presented a shear strength equation for high-strength high-performance fiber-reinforced concrete (HS-HPFRC), including ultra-high-performance concrete (UHPC). This equation was designed for straightforward implementation, catering to the regular tasks of engineers. It considers various influences on sheartransfer mechanisms, including fiber bridging, fiber distribution, dowel action, cross-sectional shapes, and beam size effects. The equation does not rely on uniaxial tensile tests or inverse analysis of flexural tests; instead, it considers the statistical impact of fibers on shear strength. To generate the coefficients for this semi-empirical closed-form equation, an evaluation database of 118 HS-HPFRC and UHPC beams was constructed. The evaluation results revealed that the proposed equation has a mean of 1.00 and a correlation coefficient of 0.92, indicating low variation and high predictive accuracy. Furthermore, it outperformed existing equations and matched the accuracy of the machine learning (ML)-based models including support vector machines (SVM), random forest (RF), and artificial neural network (ANN), despite its comparatively simpler expression. Keywords: beam shape; closed-form equation; fiber distribution; high-performance fiber-reinforced concrete (HPFRC); hybrid fibers; machine learning (ML); shear-transfer mechanism; size effect; ultra-high-performance concrete (UHPC).

This study aims to investigate the two-way shear strength of concrete slabs with FRP reinforcements. Twenty-one strength models were briefly outlined and compared. In addition, information on a total of 248 concrete slabs with FRP reinforcements were collected from 50 different research studies. Moreover, behavior trends and correlations between their strength and various parameters were identified and discussed. Strength models were compared to each other with respect to the experimentally measured strength, which were conducted by comparing overall performance versus selected basic variables. Areas of future research were identified. Concluding remarks were outlined and discussed, which could help further the development of future design codes. The ACI is the least consistent model because it does not include the effects of size, dowel action, and depth-to-control perimeter ratio. While the EE-b is the most consistent model with respect to the size effect, concrete compressive strength, depth to control perimeter ratio, and the shear span-to-depth ratio. This is because of it using experimentally observed behavior as well as being based on mechanical bases.

The shear behavior of reinforced concrete interfaces between new and existing concrete is known to be a key parameter for the effectiveness of strengthening and repair interventions in reinforced concrete structures subject to earthquakes. In this work, the mechanisms mobilizing the shear resistance of interfaces, both under monotonic and cyclic actions, are described. Constitutive relationships based on previous research are adopted for friction, for dowel action, and for their interaction. A simple algorithm is applied whereby for each value of imposed shear slip, the contributions of the two participating mechanisms are summed, as dictated by the adopted constitutive relationships. The algorithm is applied to experimental results as found in the literature and as obtained by tests conducted by the authors. The agreement of experimental and calculated load-displacement curves under monotonic and cyclic shear loading is quite satisfactory.

Renovation, restoration, remodeling, refurbishment, and the retrofitting of buildings often imply applying forces (i.e., concentrated loads) to beams that before were subjected to distributed loads only. In the case of reinforced concrete structures, the new condition causes a beam to bear a concentrated load with the crack pattern that resulted from the distributed loads which had acted before. If the concentrated load is applied at or near the beam’s midspan, the new shear demand reaches the maximum where cracks are vertical or quasi-vertical, and where inclined bars are not common according to any standards. So, the actual shear capacity can be substantially lower than new shear demand due to the concentrated load. This paper focuses on reinforced concrete beams whose load distribution has to be changed from distributed to concentrated and presents a design method to bring the beam’s shear capacity up to the new demand. The method consists of applying fiber composites (fiber-reinforced polymers or fiber-reinforced cementitious material) with fibers at an angle of 45° bonded to the beam’s web. This kind of external reinforcement arrangement has to comply with some practical measures, which are presented as well. The paper also provides the analytical model that predicts the concentrated load-carrying capacity of a beam in the strengthened state. The model accounts for the crack’s verticality, which nullifies the contributions of steel stirrups, aggregate interlock, and dowel action, and for the effective bond length of each fiber, which depends on the distance between the ends of the fiber and the crack it crosses.

Punching reinforcement systems have significantly developed in recent years as they allow enhancing the punching resistance of slab-column connections as well as their deformation capacity. These systems, with varying geometry and layout, normally consist of vertical or inclined shear reinforcement with both ends anchored on the compression and tension side of the slab. For very high levels of load, when even common punching reinforcement systems cannot safely ensure the transfer of loads, steel shear heads are usually embedded in the slab to enhance the resistance of the connection. Yet, shear heads might be expensive and difficult to place in construction sites. Following the principle of the dowel action of the compression reinforcement, this paper introduces a novel system to efficiently reinforce slabs against punching shear by using large-diameter double-headed studs acting as shear dowels. This system enhances the performance of shear-reinforced slabs with respect to conventional solutions and might be an efficient alternative to shear heads for a large number of practical situations. The system is validated by means of a specific experimental program including 11 axisymmetric punching tests on interior slab-column connections. The results demonstrate not only the increase of the punching strength but also the deformation capacity of the connection. It is also shown that the system can be consistently designed accounting for the doweling forces by making use of the theoretical frame of the Critical Shear Crack Theory (CSCT), allowing to understand the activation of the shear dowels on the basis of the deformation of the member. Keywords: Critical Shear Crack Theory; dowel action; experimental tests; flat slabs; punching; shear reinforcement.

An experimental study was conducted to identify the shear-enhancement and failure mechanisms behind the ultimate shear strength of steel fiber-reinforced concrete (SFRC) slender beams by using the full field-deformation-measuring capability of digital image correlation (DIC) technology. A total of 12 large-scale simply supported SFRC and RC beams with an overall height from 12 to 48 in. (305 to 1220 mm) were tested under monotonic point load up to failure. The greater shear strength in SFRC beams originates from the ability of the fiber bridging effect that delays the propagation of the cracks into the compression zone, whose shear strength is enhanced by the compressive stresses induced by the higher load. The slow progression of the cracks keeps the compression zone depth large, thereby enabling it to contribute to a higher shear resistance. In contrast with the traditional assumption for either plain concrete or SFRC beams, where the shear contribution resulting from dowel action is completely neglected, this research clearly shows that the dowel action has an appreciable effect on the ultimate shear strength. Its contribution varies from 10 to 30% when the beam depth increases from 12 to 48 in. (305 to 1220 mm). On the other hand, the compression zone s contribution decreases from 69 to 36% with the increase in beam depth. In addition, the shear contribution from the fiber bridging effect along the critical shear crack stays approximately unchanged at 20%, irrespective of the beam depth. In this study, the minimum shear strength obtained was in the range of [??] psi (0.42[??] MPa) for the beams with the greatest depth. This indicates thatthe maximum allowed shear stress limit of 1.5[??] psi (0.125[??] MPa) specified inACI 318-14 is on the very conservative side. Keywords: dowel action; hooked-end steel fiber; shear strength; steel fiber-reinforced concrete.

In recent years, there have been several instances of building collapse incidents resulting from shear failure. The complicated stress state and extremely nonlinear behavior seen in RC structures under shear loading are responsible for this phenomenon. Scholars and engineers worldwide have yet to propose a suitable model to address the issue correctly. Most computational approaches currently in use rely on semi‐empirical and semi‐theoretical formulations. The primary modes of shear transmission in cracked reinforced concrete include shear stress inside the uncracked concrete region, aggregate interlock, dowel action, stirrup action, and tensile residual stress. The shear force calculation is contingent upon the choice of the shear transfer model. This study comprehensively overviews the current shear calculation techniques and their respective applications. The pros and cons of these techniques were thoroughly examined. The present study investigated the suitability of these shear calculation formulas for RC structures subjected to intricate natural conditions, mechanical stress, and high‐strength and high‐performance concrete structures.

This paper presents the behavior of glass fiber-reinforced polymer (GFRP)-reinforced ultra-high-performance concrete (UHPC) beams without shear stirrups. An experimental program is conducted to examine the implications of variable geometric, loading, and reinforcement configurations, particularly for the effective depth-to-height ratio (0.75 ≤ d/h ≤ 0.90), shear span-depth ratio (1.18 ≤ a/d ≤ 3.23), and GFRP reinforcement ratio (1.35% ≤ πf ≤ 4.85%) of the beams. Also tested are UHPC cylinders and prisms under axial compression and three-point bending. The strength development of UHPC is remarkable during the first 3 days of casting, after which a gradual and asymptotic growth rate is associated because of saturated pores and the limited attractions of calcium-silicate-hydrate networks. Compared with the factors related to the placement of the GFRP reinforcing bars (d/h and πf), the a/d is more influential in controlling the capacity and the post-peak deformation of the beams by altering load-transfer mechanisms. The stress of a shear-compression zone is redistributed under arch action and results in supplementary cracks along the beam span. The horizontal splitting of UHPC at the reinforcing bar level caused by the dowel action of GFRP is dependent upon the reinforcement ratio. Analytical models are formulated, and design guidelines are recommended.

This study aimed to evaluate the shear friction strength mechanism of monolithically and separately cast concrete members using steel fiber reinforced concrete (SFRC). To achieve this, a total of 30 push-off tests were conducted with variables such as volume fraction of steel fiber, clamping force of shear-friction reinforcement, and concrete compressive strength. The experimental results showed that the inclusion of steel fibers significantly increased the shear friction strength of monolithically cast concrete. Similarly, the strength improvement in the separately cast specimens was notable with the addition of steel fibers. Notably, as the steel fiber content increased, the concrete contribution also improved, which was attributed to the enhancement of dowel action by the shear-friction reinforcement and the increased tensile strength of the concrete. When comparing the experimental results with current design standards, Eurocode2 provided the most accurate predictions, suggesting that the tensile strength of concrete increased by steel fibers can lead to more precise predictions of shear friction strength.