Lisa R. Feldman

Bond Strength Variability in Pullout Specimens with Plain Reinforcement

Bond strength results from 252 plain bar pullout specimens are presented. Parameters investigated include: concrete compressive strength, bar size, bar shape, concrete cover, and bar surface roughness. All load-slip curves displayed a characteristic shape: the maximum tensile load occurred at a very small slip (∼0.01 mm) and the load then dropped asymptotically to a residual value as the slip increased to 10 mm. Empirical equations based on least-squares analysis are presented to predict maximum and residual average bond stresses. The load-slip curve can be represented with load as a linear function of the logarithm of slip. The average bond strength was 0.98 MPa for as-received bars, and increased by 124% to 2.2 MPa for bars sandblasted to simulate realistic surface roughness. Coefficients of variation were 8% for maximum average bond stress and 24% for residual average bond stress.

Bond Stresses Along Plain Steel Reinforcing Bars in Pullout Specimens

Although recent research has recognized the existence of relationships between bond stress, slip and bar force, mechanics-based analytical prediction models for bond of plain reinforcement based on pullout specimens have not yet been established. This study establishes mechanics-based relationships between bond stress, bar force, slip at the unloaded end of the bar, and slip along the length of plain steel reinforcing bars in pullout specimens. Two 200 mm (7.9 in.) diameter by 800 mm (31.5 in.) long pullout specimens reinforced with instrumented built-up hollow reinforcing bars were tested. The derived mechanics-based relationships show that bond stress is a function of relative bar slip, slip is a function of bar force, and bar force is a function of bond stress. It is shown both analytically and experimentally that bond stress magnitudes vary along the length of the bar at all applied loads. Maximum pullout resistance was observed just before slip initiated at the unloaded end of the bar, and the bond resistance subsequently reduced as slip increased. The location of the peak bond stress shifts from the loaded end toward the unloaded end of the specimen with increasing applied load. This study also demonstrates that a previously-developed theoretical two-step bond stress-slip model captures the essential features of bond behavior observed experimentally in pullout specimens once the authors accounted for the debonding adjacent to the loaded end of the specimen.

Bond in Flexural Members with Plain Steel Reinforcement

Two concrete T-beams reinforced with instrumented built-up hollow plain bars were tested to investigate the effect of flexural cracking and bond loss on the flexural and shear behavior. The beam with a flexural reinforcement ratio of 0.98% exhibited arch action due to bond failure that initiated when the applied load reached 60% of the failure load. The beam with a reinforcement ratio of 0.33% exhibited predominantly beam action until failure initiated by yielding of the longitudinal reinforcement. When beam action was the primary shear-carrying mechanism, the observed bond demand was greatest within the transition zone between elastic-uncracked and elastic-cracked behavior. The transition from beam action to arch action caused a marked reduction offlexural stiffness that indicated bond loss in beams with plain reinforcement.

Effects of Casting Position and Bar Shape on Bond of Plain Bars

Twenty-five lap splice specimens were reinforced with plain round or square longitudinal bars in the top or bottom position to evaluate the effects of casting position and bar shape on bond. All specimens failed in bond, and the bond of square bars may be evaluated by calculating their equivalent round diameter. Top cast factors of 0.3 and 0.6 for round and square bars, respectively, reasonably capture the reductions in bond resistance. Maximum load predictions based on the CEB-FIP draft Model Code 2010 provisions for bond are overly conservative for all combinations of bar shape and casting position, whereas CEB-FIP Model Code 1990 provisions for bond reasonably and conservatively capture the behavior of specimens with bottom-cast round bars, but do not appear to capture the behavior of specimens with bottom- or top-cast square bars or top-cast round bars.

Behavior of Lap-Spliced Plain Steel Bars

Fifteen lap splice specimens reinforced with plain steel bars were tested under four-point loading to investigate bond resistance as a function of development length and bar diameter. Three of these specimens were instrumented with both steel and concrete strain gages to examine bond loss within the lap splice length. All of the specimens failed in bond. Splice specimens reinforced with plain bars are capable of resisting maximum loads that are approximately 60% of those recorded for two similar specimens that were reinforced with deformed bars. An analysis of 11 of the splice specimens tested shows that CEB-FIP Model Code provisions for average bond stress underestimates the prediction of the maximum load by 16% on average. A flexural analysis conducted for the instrumented specimens showed that strain compatibility did not exist for much of the loading range.

Conventional and high-strength hooked bars-Part 2: Data analysis

Empirical equations are developed to characterize the anchorage strength of hooked bars. The equations are based on tests of 245 simulated beam-column joint specimens with two hooked bars: 146 with confining reinforcement and 99 without. Bar stresses at failure for specimens used in the analysis ranged from 30,800 to 144,100 psi (212 to 994 MPa), and concrete compressive strengths ranged from 2570 to 16,200 psi (17.7 to 112 MPa). For the specimens analyzed, hooked bar anchorage strength was proportional to concrete compressive strength raised to the 0.29 power. For confining reinforcement parallel to and located within eight or 10 bar diameters of the straight portion of the hooked bar, the contribution to anchorage strength was proportional to the area of confining reinforcement; for confining reinforcement perpendicular to the straight portion of the bar, more legs of the confining reinforcement contributed to anchor strength, but each leg made a smaller contribution.

Conventional and High-Strength Hooked Bars—Part 1: Anchorage Tests

This paper presents the results of an experimental study on the anchorage strength of conventional and high-strength steel hooked bars. Three hundred and thirty-seven exterior beam-column joint specimens were tested with compressive strengths ranging from 4300 to 16,500 psi (30 to 114 MPa). Parameters investigated included the number of hooked bars per specimen, bar diameter, side cover, amount of confining reinforcement, hooked bar spacing, hook bend angle, hook placement, and embedment length. Bar stresses at failure ranged from 22,800 to 144,100 psi (157 and 994 MPa). The majority of the hooked bars failed by a combination of front and side failure, with front failure being the dominant failure mode. Test results show that development lengths of hooked bars calculated based on ACI 318-14 are very conservative for No. 5 (No. 16) bars and become progressively less conservative with increasing bar size and concrete compressive strength.

Conventional and high-strength steel hooked bars: Detailing effects

Findings from a study on the effect of hook bend angle, concrete clear cover, and orientation of confining reinforcement on hook anchorage strength are presented. The range of test parameters was much broader than in previous studies. Bar stress at anchorage failure ranged from 33,000 to 137,400 psi (228 to 947 MPa) and concrete compressive strengths ranged from 4300 to 16,500 psi (30 to 114 MPa). Anchorage strength of hooked bars was insensitive to bend angle (90 or 180 degrees) and side cover (between 2.5 and 3.5 in. [65 and 90 mm]). Confining reinforcement was found to increase anchorage strength for 180-degree hooked bars regardless of orientation (parallel or perpendicular to the embedment length). For 90-degree hooked bars, reinforcement oriented parallel to the embedment length had a greater effect on anchorage strength than reinforcement oriented perpendicular to the embedment length.

BUILDING CONSENSUS: Reorganization of the ACI 318 Building Code for Structural Concrete

This paper reviews the steps taken to develop consensus for the structure of the revised ACI 318 Building Code and the progress to date developing the reorganized Code. The principals used to revise the Code are discussed. The block diagram of the revised code structure is introduced and a discussion of upcoming work is presented.

Total expected cost design method for reinforced concrete members based on current Canadian code provisions

Reliability-based provisions for the design and evaluation of reinforced concrete members provided in the National building code of Canada and Canadian standard A23.3 are integrated with a corrosio...