Richard Goodman, Technical Manager, Dextra
Reinforced concrete design and construction practice has historically focused on the use of bonded straight or bend rebar as a method for rebar anchorage. This relies on bond integrity between the rebar and the concrete so that sufficient anchorage strength is obtained. However, a bonded straight or bent length of rebar is not always the most effective or efficient anchorage method, and there are many situations where the use of headed bars, sometimes called headed reinforcement anchors or ‘T’ bars, are more desirable from a design perspective, and more convenient from a construction perspective, or both.
Headed bars are used as an alternative to the more traditional means of reinforcement anchorage. Load from the rebar is anchored via head bearing or a combination of head bearing and rebar bond.
Figure 1: Headed Bars rely on head bearing or a combination of head bearing and rebar bond as a method of anchorage.
The advantages of using headed bars are particularly apparent in heavily reinforced concrete, where rebar congestion and constructability are often problems. As a result, their use is now becoming more widespread around the world, particularly within the civil infrastructure and nuclear new build sectors. Recent nuclear power plants where headed bars have been used include Taishan 1 China, Hinkley Point C UK, Akkuya Turkey, Flamanville France, Kursk Russia and Ruppar Bangladesh.
In a heavily reinforced concrete element, there is often insufficient space for a straight or bent length anchorage. In addition, a bent anchorage often causes additional rebar congestion problems. The use of headed bars eliminates the requirement for a bent anchorage and allows the reduction of the bond/development length. In certain circumstances (code provisions permitting), they allow the elimination of a required bonded rebar anchorage (development) length altogether. The resultant reduction in congestion facilitates concrete consolidation.
The transportation of straight lengths of rebar incorporating headed bars is easier than transporting rebar with bent anchorages and on-site handling and fixing also becomes easier. The use of headed bars can therefore offer a distinct time and cost advantage as well as a potential improvement in concrete quality.
- ACI 318 90° hook. ldh = Tension development length
- ACI 318 180° hook. ldh = Tension development length
- ACI 318 headed bar. ldt = Tension development length
- Headed bar not requiring a bond/development length. Subject to certain criteria and code provisions
Figure 2: The use of headed bars reduced the required bond/development length when compared to a hooked rebar anchorage.
Headed bas can be used for a variety of applications including beam-column joints, knee joints, pile caps, column-roof slab connections, to the end of cantilever elements, within corbels, for transverse shear reinforcement and for shear wall cross ties. Their use can be broken down as follows:
- The termination of main longitudinal reinforcement, where the development length associated with a straight or bent bar anchorage would be an issue, or where there is rebar congestion. As a result, larger rebar diameters are usually associated with this application.
- The termination of transverse shear reinforcement. The use of headed bars for this application is usually associated with the ease of construction. This is particularly the case for seismically designed structures where code provisions require bends greater than 90° (typically 135°), and where for quality control purposes, uncontrolled, on-site manual bending, or re-bending (to fit the rebar into position), is not allowed. These applications are therefore usually associated with smaller rebar diameters (typically 10- 20mm) and of particular benefit for the shear reinforcement of thick slabs or rafts, where the headed bar can simply be dropped down into position and tied to the top layer of longitudinal reinforcement
- Other applications include their use as an alternative to a standard rebar lap or rebar coupler splice by using overlapping heads, as well as their use as confinement, tortional, and punching reinforcement.
The Indian code of practice IS 456 and IRC 112, as well as the design guide SP 34 allow the use of headed bars. IRC 112 has in part been derived from Eurocode 2. Therefore, the principles of the Arup/Dextra guide to designing with headed bars to Eurocode 2 could be implemented when IRC 112 is the applicable code of practice.
Types of Headed Bar
Most headed bar manufacturers produce products with at least two sizes of head. The smaller typically having a net bearing area of at least four times the cross-sectional area of the bar, (a 4A head), to comply with the requirements of ACI-318, while the larger is typically 9A. 9A heads are usually selected because using this size can reduce the burden of ISO 15698 in concrete testing for static and fatigue loading qualification. The heads themselves are typically round, although some manufacturers produce square, rectangular and other shapes of head.
Heavy rebar congestion can lead to labour intensive site placement and potentially a reduction in concrete quality. Headed bars help to reduce rebar congestion, facilitating concrete consolidation, as well as being easier to handling and fix on site than traditional bent bar anchorages. The use of headed bars can therefore offer a distinct time and cost advantage as well as a potential improvement in concrete quality.
Further details, please contact:
Business Development Manager – Dextra