Recently codified language in ACI CODE-440.11-22 provides an equation for concrete shear capacity and imposes a lower bound on this calculation. An experimental study consisting of 39 flexural members without shear reinforcement and tested to failure in shear was used to evaluate the current code provisions, including, most specifically, the lower bound. Comparison of experimental and analytical shear capacities demonstrates that the current code provisions are conservative. More lightly reinforced specimens have a higher variability in experimental-to-nominal concrete shear strength than more heavily reinforced specimens, and this variability appears to be dominated by the depth between the elastic cracked section neutral axis and the depth of the tensile reinforcement, which is the area where aggregate interlock occurs. Based on a comparative reliability study, the lower bound, kcr = 0.16 (5kcr = 0.8), in the code, causes more lightly reinforced specimens (kcr < 0.16) to have lower factors of safety against shear failure than more heavily reinforced specimens (kcr > 0.16). Rather than imposing a lower bound of 5kcr on the current shear strength equation, it would be more prudent to resolve the overprediction of the equation for all specimens.

GFRP bars are considered an alternative to steel for concrete reinforcement. This project investigated the fatigue behavior of GFRP bars embedded in concrete, studying bond behavior at material and structural scales. GFRP bars (12 mm [0.47 in.] nominal diameter) were embedded in concrete cylinders leaving a 50 mm [2 in.] protrusion at the free end and featuring different bonded lengths. Two types of GFRP bars with different surface treatment (lacquered and unlacquered) were used. Static tests were used to determine the bonded length required for cyclic pull-out tests, Cyclic tests at 1.5 Hz showed GFRP bar failure was possible at just 20% of their reduced tensile strength (0.8ffu) as prescribed in ACI 440.1R-15. Two full-scale slabs internally reinforced with unlacquered GFRP bars were tested using a four-point bending configuration. A quasi-static test was used as a control to determine the fatigue amplitude, considering the fatigue loading provided by the ACI 440.1R-15 document and the pull-out test results with cyclic loading presented in this work. Cyclic load between 10 kN [2.25 kips] and 40 kN [9 kips] at a 1.5 Hz frequency was applied up to 5 million cycles before a subsequent quasi-static test was conducted. The load range was determined using cross-section analysis to cycle the bars between 5% and 20% of their reduced tensile strength (0.8ffu). Both slabs ultimately failed due to shear failure, with cyclic loading having little impact on the slab compliance. Displacements of the load points and supports were measured using linear variable displacement transformers (LVDTs), while digital image correlation (DIC) was utilized to obtain the full-field displacement and strain in the central region of the slab. The strain and displacement fields from DIC were used to determine the opening of flexural cracks and relate it to the stress level in the GFRP bars. A comparison between the static pull-out tests and the four-point bending tests of slabs indicated that the pull-out test could be used to describe the flexural behavior of the slab at low stress level. However, in terms of fatigue behavior, the comparison between the small- and large-scale tests indicated that the fatigue phenomenon in the slab was quite complex and could not be directly described by the results of pull-out tests.

Mehdi Khorasani, Giovanni Muciaccia, and Davood Mostofinejad Synopsis: The externally bonded reinforcement on grooves (EBROG) technique has been recently shown to outperform its rival techniques of surface preparation (such as externally bonded reinforcement, EBR) employed to delay the undesirably premature debonding of fiber reinforced polymer (FRP) from the concrete substrate in retrofitted structure. However, the behavior of EBROG method under fatigue loading has not been assessed yet, and the present study is the first attempt to achieve the above aim. For this purpose, an experimental program is conducted in which 16 CFRP-to-concrete bonded joints on the concrete slab prepared through the EBROG and EBR techniques are subjected to the single lap-shear test and fatigue cyclic loading. Furthermore, the bond behavior of CFRP strips-to-concrete substrate is investigated in this research in terms of the load capacity, slip, debonding mechanism, and fatigue life. The results showed that the grooving method improved the bond properties of CFRP-to-concrete joints under fatigue loading. By using this alternative technique, the number of cycles until failure (fatigue life) increases incredibly under the same fatigue cycle loading and the service life of strengthened members could be improved under fatigue loading. Furthermore, the effects of different loading levels on the behavior of CFRP-concrete joints installed by EBROG method are evaluated. The results showed that fatigue life of strengthened specimens decreases by increasing fatigue upper load limit. Finally, a new predictive equation was developed based on plotting the maximum applied fatigue load versus fatigue life curves for CFRP-to-concrete bonded joints for the EBROG method.

Reinforced concrete slabs can be subjected simultaneously to transverse loads and in-plane tensile forces, as it occurs in top slabs of continuous box girder bridges at intermediate supports, or in flat slabs supported on columns, subjected to horizontal loads. To study the effects of in-plane forces in the slab punching-shear strength, an experimental and theoretical investigation was carried out, which is described in this paper. Five square slabs of 1650 mm (42”) side and 120 mm (4.7”) thickness were tested under a centered transverse point load and different degrees of uniaxial in-plane tensile force. Numerical predictions using non-linear finite element analyses were performed to help in the experiments design. Furthermore, the punching-shear mechanical model, Compression Chord Capacity Model (CCCM), was extended to incorporate the effects of in-plane tensile forces. The experimental results showed that the punching strength linearly decreases with the level of applied tensile force and, if cracking in the slabs is produced by the tensile force, yielding of the reinforcement and further reduction may take place. Excellent agreement was found between theoretical predictions and tests results. Furthermore, the CCCM was verified with available results of punching tests with uniaxial and biaxial tensile forces, obtaining very good results.

Significant research efforts have been devoted to achieving high performance of slab – column connections subjected to lateral loading. Solutions such as using stirrups and headed studs have been shown to work well. With the development of concrete materials with enhanced properties, new possibilities have arisen to employ solutions that are easy to apply and cause less congestion of reinforcement. A total of nine tests on flat slab specimens subjected to combined gravity and lateral loading are discussed, including two new specimens with High Performance Fiber Reinforced Concrete (HPFRC) over a limited region near the column. The main experimental variables were the flexural reinforcement ratio and the punching shear improvement method: none, headed studs, High Strength Concrete (HSC) or HPFRC. It is shown that excellent behavior is achieved with a relatively small amount of HPFRC, extended up to 1.5 times the effective depth of the slab from the face of the column. Punching was completely avoided until the end of the loading protocol (6% drift) for the specimens with HPFRC, whereas reference specimens without punching shear reinforcement failed at 1% drift and specimens with HSC reached 3% drifts. Additionally, the use of HPFRC led to an increased unbalanced moment transfer capacity and lateral stiffness, though this effect was more pronounced for specimens with lower flexural reinforcement ratio.