International Concrete Abstracts Portal

Extensive experimental verification has shown that the use of fiber-reinforced polymer (FRP) anchors in combination with externally bonded FRP composites increases the flexural capacity of existing reinforced concrete (RC) structures. Thus, a rational prediction model is introduced in this study so that fiber splay anchors may be accurately designed for practical strengthening applications. Simplified structural mechanics principles are used to build this model for capacity prediction of a group of fiber splay anchors used for FRP flexural strengthening. Three existing test series using fiber splay anchors to secure FRP-strengthened T-beams, block-scale, and one-way slabs were used to calibrate and verify the accuracy and applicability of the present model. The present model is shown to yield very accurate predictions when compared to the results of the block-scale specimen and eight different one-way slabs. The proposed model is also compared with the predictions of a design equation adapted from the case of channel shear connectors in composite concrete-steel construction. Results show a very promising correlation.

This research aims to investigate cyclic responses of axially restrained diagonally reinforced coupling beams, where the applied axial force was proportional to the beam axial elongation. Six diagonally reinforced concrete coupling beams with an aspect ratio of 2.0 were tested under reversed cyclic displacements. The key test parameters included the magnitude of axial restraint and shear stress demand. The test results showed that the specimen deformations were primarily attributed to the beam end rotation. Specimen peak strength, which increased as the axial restraint was applied, can be reasonably estimated using probable flexural strength at the beam ends where the axial restraint force was considered. All specimens exhibited a minimum of 6.0% chord rotation prior to failure, and the failure mechanism was associated with the damage at beam ends and reinforcement anchorage. The ultimate chord rotation capacity, shear rigidity, and flexural rigidity of the specimens were found to be insensitive to both shear stress demand and the magnitude of axial restraint. Axially restrained specimens showed significantly reduced axial elongation compared to those without axial restraint. The axial elongation of specimens without axial restraint can be adequately estimated using existing models. Analysis indicated an average flexural and shear rigidity of 0.13EcIg and 0.41GcAg, respectively, for all tested specimens.

The ACI 318-19 Building Code does not allow the use of headed bars larger than No. 11 (No. 36) due to insufficient experimental data. Thirty large-scale simulated beam-column joint specimens containing high-strength No. 11 (No. 36), No. 14 (No. 43), or No. 18 (No. 57) headed bars were tested to investigate the effects on anchorage strength of key factors, including bar stress at failure, bar size, bar spacing, embedment length, transverse reinforcement, concrete compressive strength, and loading condition. Specimens exhibited concrete breakout, side splitting, or a combination, with four exhibiting a shear-like failure. Anchorage of larger bars is noticeably influenced by joint shear demand and loading condition. Descriptive equations developed based on 164 tests accurately characterize anchorage strength for headed bars up to No. 18 (No. 57). They indicate that anchorage strength is proportional to concrete compressive strength to a power close to 0.2 and that the contribution of parallel ties for large headed bars is lower than that observed for smaller headed bars.

Two large-scale beam-column connections with beam longitudinal headed bars were tested to evaluate their susceptibility to breakout failures. The specimens were designed following the strength and transverse reinforcement detailing provisions in Chapter 15 of ACI 318-19. The variable investigated was the headed bar embedment length, which was determined based on either Chapter 25 of ACI 318-19 or recent research at the University of Kansas, the latter leading to a 22% shorter embedment length. Both specimens exhibited beam flexural yielding, but the specimen with shorter bar embedment length experienced significantly more connection damage followed by a concrete breakout failure. Based on the limited test results, it is recommended that nominal joint shear strength be calculated based on a joint effective depth equal to the headed bar embedment length and a shear stress of 1.0λ√(fc' ) (MPa) [12λ√(fc' ) (psi)]. A method for calculating headed bar group anchorage strength in exterior beam-column connections is proposed, which led to reasonable and conservative strength estimates in the test specimens.

Steel columns are commonly attached to concrete foundations with groups of cast-in-place headed anchors. Recent physical tests and simulations have shown that the strength of these connections can be limited by concrete breakout failure. Four full-scale physical specimens of axially loaded columns attached to a foundation slab were tested, varying the shear reinforcement configuration in the slab. All specimens were governed by concrete breakout failure. The tests suggest that adequately placed distributed shear reinforcement can increase connection strength and displacement capacity. Steep cone failures were observed to limit the beneficial effect of shear reinforcement. Calibrated finite element models were used to investigate critical parameters such as the extent of the shear-reinforced region and bar spacing. A design approach is proposed to calculate connection strength by adding the strength of the concrete and the distributed shear reinforcement. Design detailing is discussed.