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Starter bars are rebars cast into a concrete to give a lapped connection to provide continuity of reinforcement across a cold joint or construction joint. Oriented perpendicular to the existing concrete element in which they are installed, they are also used to extend or rehabilitate existing bridges, roads and building structures.

The easiest and the inexpensive way of designing starter bars is to have them projecting from the concrete surface. However, complications arise when starter bars need to project through the formwork. Timber formwork needs to have pre-drilled holes in the accurate positions which may render the formwork unusable for future projects and cause difficulties when stripping. Not mentioning if it is steel formwork, which creates even greater problems.

Besides, projecting rebars can cause a safety hazard for both trips and falls. The cast in starter bars should have protective caps to minimize these risks. Projecting bars in precast concrete cause additional problems with storage and transport, and as a result are not commonly used.  Rebar will also tend to rust easily when expose to weather.

Organic Resin


Other than having starter bars with cast-in-situ in method, starter bars may be embedded in organic or minerallic mortars from a previously cast surface. Holes are drilled in the concrete at required positions and the bars are fixed with mortar. There are many suitable mortars available, although the higher bond strengths come from organic system, so these tend to be the most popular. The organic resin may be introduced via a gun/dispenser, or in a glass capsule that is broken by the bar, allowing mixing to occur. Care must be taken to treat the drilled hole as recommended to ensure adequate anchorage. The resin method is particularly useful for fixing of items such as posts or barriers where accurate position is needed.
Ankrochim Resin for Anchoring in ConcreteAnkrochim Resin for Anchoring in Concrete2
Injecting Ankrochim resin while withdrawing the nozzle as the hole fills. Insert rebar with a slight twisting motion.

Mechanical Splice System


The easiest way to avoid having to have bars projecting through the formwork is to use a mechanical splice system (Rebar coupler). Simply engage a coupler onto the threaded rebar, which is then cast flush with the concrete (a coupler capping can prevent the concrete slurry from flowing into the coupler).  The second bar is then connected to the coupler. To avoid loss of cross section, the end of the bar is ‘enlarged’ by cold forging the end of the bar before threading.  No drilling is required in the formwork.

Steel coupler for starter bar
Starter bar from diaphragm wall using Moment Jointec Coupler

Reinforcement Continuity


An alternative system (to ensure cost effectiveness for smaller bar diameters) has pre-bent and pre-spaced steel reinforcement supplied in a pre-galvanized steel box that is dimpled and flanged for maximum concrete bond. The system is used to enable efficient and reliable reinforcement continuity for sections of concrete structures that are poured in subsequent phases without the need to drill formwork.

This system not only improves on site productivity due to its installation speed, but also provides significantly higher shear capacities compared to traditional methods due to a higher shear friction. This enables us to reduce the amount of steel bars required to carry the same vertical load; reducing costs and embodied CO2 in the construction.

Installation is simply achieved by nailing the casing to the formwork or wiring it to the existing reinforcement bar cage prior to concreting.  After removal of the formwork the lids of the box is removed and the bended bar being straighten and ready for lapping.

curved rebar boxrebar box as starter bar
Plakabox is the only rebar box capable of making an anchoring which can transfer significant force around the structural joints.

 

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August 10, 2018