Bond Strength Of Bars Research Articles

Experimental investigation on influence of embedment length, bar diameter and concrete cover on bond between reinforced bars and steel fiber reinforced concrete (SFRC)

The bond between reinforced bars and concrete has a significant impact on reinforced concrete structures. Incorporating steel fibers in concrete not only enhances the performance of the concrete but also influences the bond strength of embedded reinforcing bars. In this paper, experiments have been carried out to investigate the influences of different bar diameters, concrete cover and embedment lengths on bond-slip responses between reinforcing bars and steel fiber reinforced concrete (SFRC). Furthermore, pull-out tests with normal concrete were also executed as a comparative group. The bond-slip results of specimens were collected. The impact of embedment length, reinforcing bar diameter, and the existence of steel fibers on the bond behavior of reinforcing bars was comprehensively analyzed. Meanwhile, the results indicate that variations in the ratio of concrete cover to reinforcing bar diameter lead to different effects on the bond behavior of SFRC compared with normal concrete. Different failure modes of specimens were exhibited and discussed. Additionally, a mathematical model for bond strength of bars in SFRC was proposed based on the correction of the bond strength model with normal concrete, which shows good agreement with experimental results.

Top cast effect: Influence of bond length on splitting mode failure

AbstractIt is widely considered that bond strength of bars cast near the top of a pour is weaker than that of bars cast near the bottom. The lesser strength of top cast bars is attributed to: (a)The increase in binder/cement ratio toward the top of a pour due to consolidation of fluid concrete prior to setting. (b)The formation of voids beneath bars as solids in fluid and setting concrete consolidate when the bar is restrained from vertical movement. The difference in bond strength bars cast near the top of a pour and those cast near the bottom varies widely between individual studies, with little difference reported in some studies and reductions in excess of 50% reported in others. A markedly greater top bar effect observed in pullout type specimens with short bond lengths compared to that in studies based on conditions more representative of practical construction. Through a simple analytical model of behavior of a lapped joint based on the local bond slip model in fib Model Code 2010, it is shown that the casting position effect diminishes as bond length increases. This influence is supported by analysis of trends in the top cast section of the ACI408 bond test database. Conclusions demonstrate the casting position factor in current design Codes significantly overestimates the top cast effect and justifies a casting position factor of 1.0 for laps designed to develop the full design strength of Grade 500 bars. Increased bond lengths would still be required for top cast laps designed for lower bar stresses.

Bond behavior of high-performance fiber reinforced concrete (HPFRC) under direct tension pullout

Bond is the complex interaction that occurs at the interface of reinforcement and concrete, enabling force transfer between the two materials. The bonded interface is highly sensitive to any passive confinement that occurs normal to the contact area. The internal confinement that occurs in high-performance fiber reinforced concrete enhances the bond strength to such an extent that very short anchorage lengths suffice to develop a standard steel reinforcing bar. In order to evaluate and quantify the bond strength of bars embedded in novel cementitious materials, a combined experimental and analytical study was conducted using 13 specimens fabricated with high-performance concrete materials containing either steel or PVA fibers. Bond specimens were tested in a direct tension pullout (DTP) setup in order to quantify a lower bound for bond strength of bars in high performance fiber reinforced cementitious (FRC) matrices with tension-hardening properties, where the bar is developed in a tension stress field. The test setup was selected so as to minimize unaccountable confinement owing to support conditions and to eliminate out-of-plane effects due to flexural curvature. Study variables included the bar cover thickness and the type of FRC matrix used. The behavior of the test-specimens was analyzed in detail using nonlinear finite element modeling and an extensive database of published bond tests in high-performance strain-hardening cementitious matrices. Strain fields obtained from digital image correlation (DIC) were used to verify the kinematic hypotheses traditionally used in solving the field equations of bond (i.e. relationship of bar and concrete strains with the rate of slip along the anchorage), and to study the effect of enhanced fracture energy supplied by the fibers on bond mechanics. Striking improvements were observed on the measured HPFRC and ECC bond strength and failure mode over bond of conventional (unconfined) concrete. Direct comparison with existing pullout setups in the literature shows that bond strength measurements of the DTP formulation are more conservative and illustrate clearly the effect of bar cover on bond strength.

Bond Performance of Sand-Coated BFRP Bars to Concrete

This paper presents an experimental study on the bond behaviour of sand-coated basalt fibre reinforced polymer (BFRP) bars and conventional steel bars of 10mm- diameter. The bond strength of these bars were determined according to ASTM D7913/D7913M-14 standards. The pullout specimens consisted of BFRP bars embedded in concrete cubes (200mm on each side) with the compressive strength of 40MPa were constructed. The pullout test results contain the bond failure mode, the average bond strength , the slip at the free and loaded end, and the bond stress-slip relationship curves.The test results showed that the bond strength of sand-coated BFRP bars was about 70% that of the steel bars.

Residual bond strength of corroded plain round bars

This paper reports results of an experimental programme to measure changes in bond characteristics of plain round reinforcement as a result of corrosion. Principal parameters included in the investigation are the amount of corrosion, the presence of confining reinforcement in the form of links, cover thickness and the initial condition of the reinforcement. In the absence of confining links, bond strength of bars cast near the bottom of a pour is found to reduce with increasing corrosion. However, although top-cast bars were initially weaker, small amounts of corrosion were sufficient to raise bond strength to that of bottom-cast bars. With increasing corrosion, strength reduced in a similar manner to bottom-cast bars. Confining reinforcement had little influence on bond strength prior to the onset of corrosion, but plays an important role in maintaining strength once corrosion becomes established. Results from these tests suggest that surface crack width may provide a better parameter than section loss or corrosion penetration through which to assess residual bond strength. The reasons for this are explained, and empirical relationships for residual strength are derived.

Tension development length and lap splice design for reinforced concrete members

AbstractThe key factors that govern development (anchorage) and splice performance of reinforcing bars in tension are described. The bond design provisions of the AASHTO (American Association of State Highway and Transportation Officials) LRFD (Load and Resistance Factor Design) Bridge Design Specifications, the American Concrete Institute Building Code Requirements for Structural Concrete (ACI 318‐05), ACI Committee 408, CEB‐FIP Model Code 1990, and Eurocode 2 are summarized and compared with test results. In terms of the key factors, the bond strength of bars not confined by transverse reinforcement is proportional to the concrete compressive strength to the ¼ power. The contribution of transverse reinforcement to bond strength increases with the area of the transverse reinforcement per bar and the relative rib area and diameter of the bar being developed or spliced and has been found to be proportional to the concrete compressive strength to a power between ¾ and 1.0. For deep members or for high‐slump (conventional and superplasticized) concrete, the effect of bar placement may be greater than accounted for in current design provisions. Of the five design procedures compared in this paper, those developed by ACI Committee 408 provide the best match with test results for both developed and spliced bars. For developed bars, the other four design procedures result in a high percentage of tests for which the test to calculated strength ratios (T/C) are less than 1.0, with the greatest percentage of low T/C values resulting from the AASHTO requirements. The relative safety of the latter four design procedures improves for lap splices. A provision in ACI 318‐05 that allows a 20% reduction in development and lap splice length for bars smaller than 19 mm appears to be unsafe and should be removed or modified.

Effect of Deformation Height and Spacing on Bond Strength of Reinforcing Bars

The details are described of a study of the effect of deformation pattern on bond strength usign 1 inch diameter machined bars with deformation heights of 0.05, 0.075, and 0.10 inch and deformation spacings ranging from 0.26 to 2.2 inch. The study found that the bond force-slip response of reinforcing bars is a function of the relative rib area of the bars, independent of the specific combination of rib height and rib spacing. Under all conditions of bar confinement, the inititial stiffness of load-slip curves increases with an increase in the relative rib area. Under conditions of relatively low confinement, in which bond strength is governed by splitting of the concrete, bond strngth is independent of deformation pattern. Under conditions in which additional bar confinement is provided by transverse reinforcement or higher cover, bond strength increases, compared to the bond strength of bars with less confinement. The magnitude of the increase in bond strength increases with an increase in the relative rib area.