International Concrete Abstracts Portal

Thirty-nine large-scale reinforced concrete beams were tested under monotonic three-point bending to investigate the use of stirrups with mechanical anchors (heads) or hooks and Grade 80 (550) reinforcing steel. Grade 60 and 80 (420 and 550) No. 3, No. 4, and No. 6 (0.375, 0.5, and 0.75 in. [10, 13, and 19 mm]) bars were used as stirrups, which were spaced at one-quarter to one-half of the member effective depth. Other variables included beam depth (12 to 48 in. [310 to 1220 mm]), beam width (24 and 42 in. [620 and 1070 mm]), longitudinal reinforcement strain corresponding to the nominal beam shear strength (nominally 0.0011, 0.0017, or 0.018), and concrete compressive strength (4000 and 10,000 psi [28 and 69 MPa]). Headed stirrups that: a) engage (are in contact with) the longitudinal bars; or b) have a side cover of at least six headed bar diameters and at least one longitudinal bar within the side cover, produce equivalent shear strengths as hooked stirrups, and both details allow stirrups to yield. The results affirm that beams designed for the same Vn with either Grade 60 or 80 (420 or 550) stirrups exhibit equivalent shear strengths. A nominal shear strength based on a concrete contribution equal to 2 √ fc bwd may be unconservative when ρtfytm < 85 psi (0.59 MPa) in members with a/d = 3, h ≥ 36 in. (910 mm), ρ < 1.5%, and no skin reinforcement.

This experimental study investigates the influence of flexural cracks and punching shear failure inclination on double-headed stud anchorage within the critical perimeter. The research also explored the technical feasibility of using synthetic coarse aggregates from bauxite residue as a sustainable alternative in structural concrete production. The results showed that the overall structural integrity is impaired at 40 to 50% due to flexural cracks at the critical perimeter of 2d (30 degrees); however, the perimeter of 1.2d (45 degrees) enhanced the shear reinforcement activation and shear strength up 15%, providing a balanced failure within the strengthening zone. Thus, a concrete anchoring capacity (CAC) method was proposed to calculate the contribution of doubleheaded studs in serviceability and ultimate limit states. In addition, synthetic aggregates performed similarly to natural aggregates, offering environmental benefits such as reducing the carbon footprint and production stages.

Recent tests showed that anchorage failure could be the primary mechanism that limits the strength and deformation capacity of column-footing connections. An experimental program consisting of the reversed cyclic load testing of 16 approximately full-scale column-footing subassemblages was thus conducted to investigate the effect of various reinforcement details on connection strength, drift capacity, and failure mode. The main parameters evaluated were type of anchorage for the column longitudinal bars (either hooks or heads), extension of column transverse reinforcement into the footing, and longitudinal and transverse reinforcement ratios in the footing. Test results indicate that even when column longitudinal reinforcement extends into the joint with a development length in accordance with ACI 318-19, a cone-shaped concrete breakout failure may occur, limiting connection strength and deformation capacity. The use of transverse reinforcement in the connection over a region extending up to one footing effective depth away from each column face proved effective in preventing a concrete breakout failure. However, for the specimens with column headed bars, extensive concrete crushing adjacent to the bearing side of the heads and spalling beyond the back side of the heads led to significant bar slip and “pinching” in the load versus drift hysteresis loops at drift ratios greater than 3%. The use of U-shaped bars in the joint between the column and the footing or slab, as recommended in ACI 352R-02, led to improved behavior in terms of strength and deformation capacity, although it did not prevent the propagation of a cone-shaped failure surface outside the joint region. Based on the test results, the basic concrete breakout strength, Nb, corresponding to a 50% fractile, in combination with a cracking factor ψc,N = 1.25, is recommended when using Section 17.6.2. of ACI 318-19 for calculation of concrete breakout strength in connections similar to those tested in this investigation.

Inner surface reinforcement is one of the most widely adopted techniques for upgrading or strengthening shield tunnels. An important failure mode in this method is the debonding of the thin plate from the segments, resulting in less reinforcement effect than expected. The shield tunnel lining is a discontinuous curved structure formed by connecting segments with bolts, and its structural form and internal force state are essentially different from reinforced concrete beams. However, there are few reports on the evolution process of debonding failure of similar structures. Therefore, to develop a thorough understanding of the debonding failure, a three-dimensional refined numerical model for the shield tunnel strengthened by a thin plate at the inner surface based on the mixed-mode cohesive method was proposed. The validity and rationality of the model were corroborated by a full-scale experiment. Then, the model was applied to other inner surface reinforcement schemes commonly used in practice to explore the debonding mechanism of the adhesive layer. Finally, anti-debonding measures were proposed, and their effectiveness was elucidated by numerical analysis. The results show that the proposed numerical model can accurately predict the failure process of the adhesive interface of the shield tunnel strengthened by a thin plate. There are obvious interfacial stress concentrations at the joints and the plate ends, which are the essential reasons for the debonding failure initiating from those places. Anchoring the thin plate only at the plate ends and joints can significantly and sufficiently increase the debonding load. Therefore, it is not necessary to add anchoring measures elsewhere.

This study introduces a new anchorage strategy using ultra-high-performance concrete (UHPC) to attach unbonded post-tensioning (PT) strands to existing foundations. This solution complements a seismic retrofit scheme investigated by the authors, which transforms non-ductile cast-in-place reinforced concrete (RC) shear walls into unbonded post-tensioned rocking shear walls, following concepts of selective weakening and self-centering. In the proposed PT anchorage scheme, mild steel reinforcements are inserted through the shear wall thickness and into the foundation. Subsequently, UHPC is cast around the wall base, forming a vertical extension connected to the foundation, which is used to anchor the unbonded PT strands. The feasibility and performance of the anchorage scheme were investigated through a combination of laboratory testing and numerical simulations. Pull-out testing on four scaled-down anchorage specimens was conducted in the laboratory. Hairline cracks were observed in the UHPC during testing. Additionally, 3D finite element (FE) models were created, validated, and used to study the performance of the proposed anchorage scheme under lateral loading. The simulation results support the effectiveness of the proposed anchorage strategy.