Effect of Transverse Reinforcement on Development Length

Transverse reinforcement is used to improve the concrete-steel bond strength. At service loading, the lateral pressure introduced to the concrete due to lateral confinement reduces the tendency of the concrete to crack. Several research findings emphasized the importance of transverse confinement in reducing the splice length required for steel in tension and/or compression. Edina et al. (1999) conducted an experimental program that studied the contribution of the transverse confinement on reducing the lap splice of reinforcing steel bars. In this research, the transverse confinement introduced by spiral stirrups to three different patterns of lap splices, significantly reduced the lap length. Based on the research results, it was recommended to increase the maximum effect of transverse reinforcement, as compared to ACI 318-02 provisions. Tapers (1982) presented one of the first investigations to focus on the prediction of the bond strength for deformed bars. Tapers presented an analytical model, where the bond strength at steel- concrete interface is dependent on the capacity of the concrete surrounding the reinforcing bar to carry the hoop stresses.

There are two prevailing modes of steel-concrete bond failure. These can be explained as follows:

Mode 1 – The steel bars are near to the member face or when minimal transverse reinforcement is used. Concrete splitting is expected and steel-concrete bond failure occurs. This mode of failure is known as splitting-type bond failure.

Mode 2 - The steel bar is surrounded by an adequate concrete section. Or, sufficient confinement is utilized. A bond-shear of the rebar is expected to happen. This mode of failure is known as pullout bond failure.

The failure mechanism, in most cases, could be presented as a combination of the afore- mentioned modes. The steel-concrete bond slip is related to an increased circumferential stress within the transverse reinforcement, and a high level of radial stress within the concrete. There are two distinct types of confinement that affects the steel-concrete bond. These could be explained as follows:

Active Confinement - The active confinement is created through the application of a compression stress field that counteracts radial stress developed around reinforcing steel. Thus, reduce the formation and/or propagation of cracks. The active confinement is best represented by the reaction of the bearing on the end zone of a girder. This reaction creates a compression stress, which can be superimposed to the vertical radial stresses acting around the reinforcing steel. This compressive stresses help confine the girder end zone concrete and reduce cracking. Hence, it positively affects the development of rears.

Passive Confinement – The passive confinement is represented by transverse reinforcement, as stirrups, and spirals. The action of this confinement starts upon crossing internal cracks developed due to radial stresses. Because the action of this confinement system does not start except after the crack pattern is developed, it is so called “passive confinement”. The efficiency of passive reinforcement is highly dependent on the

positioning of rears with respect to the extended crack pattern. The closer the confinement to the cracks, the higher is its efficiency.

Many experimental studies have been conducted to quantify the effect of both active and passive confinement on the bond strength between steel and concrete. In addition, different analytical models are available to describe the behavior of concrete structures under the effect of internal and external confinement. The following represents a background for the research efforts in this regards.

Untrue and Henry (1965)

Untrue and Henry studied the effect of active confinement on the bond strength. They conducted their research program by quantifying the effect of lateral pressure on 6 in. sided concrete cube, with #6 and #9 embedded rears. The lateral pressure imposed on the cube ranged from 0% to 50% of the concrete compressive strength. A slight increase in the bond strength was observed, which was numerically correlated to the square root of the concrete strength.

Oran gun Jars and Breen (1975, 1977)

Oran gun et al. (1975, 1977) tested the bond strength between rears and different types of concrete strength. In their research study, they developed and calibrated an expression correlating the bond strength with the concrete compressive strength. The calibrated equation was as follows:

l

d

d

f

u

C = min of concrete cover or one half of the strand spacing.

Zia et al. (1991)

Research conducted by Zia et al. on the steel-concrete bond proved that higher rates of loading will cause a rapid deterioration on the bond. Hence requires longer development length for reinforcing steel. The same research proved that the bond strength is inversely proportional to the concrete age.

Giuliani et al (1991)

The research conducted by Giuliani et al investigated the effect of transverse (passive) confinement on the steel-concrete bond. In their research, they proved that the effect of confinement could be superimposed to external loading, and residual (tensile) strength of concrete, during its post-cracking non-linear behavior.

Azizinamini et al. (1992, 1993)

Azizinamini conducted an experimental research to investigate the tension splice of #8 and #11 bars within high performance concrete. The concrete specimens varied from 5 ksi to 15 ksi. The research findings showed that the stress distribution at ultimate stage might not be linear in case of high performance concrete. The research findings mentioned that in tension splice of rears in high performance concrete, it is not advisable

to utilize longer splice length. However, a minimum amount of transverse reinforcement may be needed to increase the bond strength.

Malvern (1992)

Malvern conducted an experimental research on the steel-concrete bond using steel bars embedded in concrete cylinders. Malvern reported that the bond strength vanishes when cracks due to radial stresses are formed, incase steel confinement is not available. Higher steel-concrete bond strength was achieved when steel bars were pushed into the concrete compared to the pullout test results. This is attributed to the Poisson’s ratio effect.

The afore-mentioned studies are concerned with reinforcing steel-to-concrete bond. One study was completed on the prestressing strands-to-concrete bond strength. Russell and Burns (1993) investigated the effect of confinement on the prestressing strands-to- concrete bond. Mild steel hoops were used to contain all the strands used within prestressed concrete specimens. The research program concluded that strand confinement were efficient when designed to be near the prospective crack pattern location. The effect of confinement was decreased for specimens including large number of strands.