Bar types and identification
The two grades of steel used for bar reinforcement are mild steel, identified by 'R' on the reinforcement drawings and schedules, and high-yield steel identified by T. The letter 'X' is used to denote other steels, for instance stainless steel.
All plain smooth round bars produced in the UK are hot rolled mild steel, which has a characteristic strength of 250 N/mm2.
High yield reinforcement is produced by hot rolling a low-alloy steel. It has a characteristic strength of 460 N/mm2 and is known as 'deformed' steel because of its pattern of raised ribs (Figure 25).
Bar sizes and bending
The preferred nominal diameters of bars are 8, 10, 12, 16, 20, 25, 32 and 40 mm. Should a larger diameter bar be required, bars of 50 mm diameter are normally available by special arrangement with the manufacturer. In the case of bars less than 8 mm diameter, a 6 mm type is sometimes obtainable.
Minimum radii for bends are given in BS 8666 and these should be used unless larger radii are detailed. The minimum inside radius for different types of steel is given below.
n Mild steel - twice the bar diameter
n High-yield steel
- for bars up to and including 20 mm in diameter: three times the bar diameter
- for bars 25 mm and greater in diameter: four times the bar diameter.
All reinforcement should be bent on proper bar-bending machines and should be to the specified dimensions and within the allowable tolerances. It may be impossible to fit steel in the correct position and with the correct cover if the bars have not been bent accurately; reinforcement in the wrong position may reduce the strength of the unit, and a reduction in the specified cover will reduce durability due to an increased risk of the reinforcement corroding.
Reinforcement should not be bent or straightened in a way that will fracture or damage the bars. All bars should preferably be bent at ambient temperature, but when the temperature of the steel is below 5°C special precautions such as a reduction in the speed of bending or increasing the radius of bending may be necessary. Alternatively the steel can be warmed to a temperature not exceeding 100°C. Heated bars should never be cooled by quenching. Reinforcement should not be re-bent or straightened without the approval of the engineer.
Standard shapes of bent bar are readily available from suppliers in a range of bar sizes, and most suppliers will also work from the reinforcement schedule to supply steel ready cut and bent to specified dimensions and tolerances.
Fabric
Factory-made sheets of mesh made from welded bars or wires are known as fabric reinforcement. It is used extensively for ground and suspended slabs and for reinforced concrete roads. Fabric is available in a British Standard range of preferred meshes in stock sheets 4.8 m long by 2.4 m wide, but other mesh arrangements and sizes of sheets are also available to order.
The preferred types of fabric designated in BS 4483 are divided into four categories, each classified by a letter, as shown in Table.
Table : Preferred types of designated steel fabric.
Prefix letter
|
Type of fabric
|
Size of mesh
(mm x mm)
|
Typical applications
|
A
|
Square mesh
|
200 x 200
|
Slabs - suspended and ground
|
B
|
Structural mesh
|
100 x 200
|
Suspended slabs, walls
|
C
|
Long mesh
|
100 x 400
|
Roads, paved areas, ground floor slabs
|
D
|
Wrapping
|
100 x100
|
Sprayed concrete work, concrete
encased steelwork.
|
Each type is available in a limited number of weights, depending on the wire diameter used. References on drawings and schedules use the letter followed by a number denoting the cross-sectional area, in mm2/per metre width, of the main wires. For example, B503 is a structural mesh with a main wire area of 503 mm2 per metre width. An 'A' or square mesh has the same cross-sectional area in each direction. Fabric should be cut and bent to the tolerances and dimensions given in BS 8666.
Prefabricated reinforcement
It is often more convenient to obtain cages and complex reinforcement arrangements already assembled from the supplier's factory. Delivery can be timed to fit in with the construction programmed but the same requirements for storage on site apply a: for conventional reinforcement. Some of the assemblies are heavy and will need suitable lifting equipment.
Handling, storage and cleanness
Whether the reinforcement is being delivered uncut (generally in 12 m lengths) or already cut and bent it is essential to off-load it carefully. Pushing bundles of bars off lorries or throwing them onto stacks inevitably leads to bends or kinks.
The bars should be stacked off the ground and well supported to ensure that they do not become covered with mud and dirt. Bars of different types and diameters for bending on site should be stacked separately and well labeled so that the bar bender can identify them easily. Cut and bent steel should be delivered already bundled and labeled with the bar schedule reference and the bar mark to enable the fixers to find the bars they need.
Before concreting, the reinforcement needs to be free from mud, oil, grease, release agents, paint, retarders, loose flaky rust, loose mill scale, snow, ice or any substance that will affect the concrete or steel chemically or reduce the bond between the two materials.
The effect of loose rust and mill scale on the bond between steel and concrete is often a cause of contention on sites.
Tests carried out on rust-free and rusty bars have shown that, provided the cross-sectional area of the bar has not been reduced, the effect of a little rust is not harmful and normal handling will remove loose rust and mill scale; the same effect can be achieved by dropping bars on the ground or giving them a sharp tap, preferably on the end. Where it is suspected that the cross-sectional area of the bars has been reduced by corrosion, the most accurate way of checking, especially with deformed bars, is to weigh a known length. Mortar or grout droppings on bars projecting from concrete do not need removing provided they are firmly bonded to the bars.
Cover to reinforcement
The strength and durability of a reinforced concrete structure depend on, among other factors, the reinforcement being correctly positioned, within allowable tolerances, in the hardened concrete. The most common cause of corroding reinforcement is insufficient cover. The position of reinforcement should be checked before and during concreting to ensure that the correct cover is maintained; further checks should be carried out using a cover meter after the concrete has hardened, as discussed on page 60 under Electromagnetic cover meter.
The nominal cover should be given on the working drawings. Nominal cover is the depth of concrete cover shown on drawings to all reinforcement including links. The actual cover should nowhere be less than the nominal cover minus a margin which, in British Standards, is currently 5 mm. If the surface of the concrete is to be tooled or the aggregate exposed, the depth of tooling or exposure must be taken into account. The nominal cover will depend on the conditions of exposure of the particular piece of concrete and the cement content and free water/cement ratio of the specified concrete.
Spacers
Reinforcement should be held off the formwork or blinding (for a slab on the ground) by suitable spacers which should be of the same nominal size as the specified cover.
Spacers are generally made of concrete, fibre cement or plastic, in several shapes and various sizes to give the correct cover (Figure 26).
Circular or wheel type spacers are more suitable for reinforcement in vertical members, such as columns and walls, while the block or trestle types are more suitable for reinforcement in horizontal members. They must be durable, and not cause corrosion of the reinforcement or spelling of the concrete. Those made from concrete should be comparable in strength, durability, porosity and appearance if the finish of the surrounding concrete is important. Site-made concrete blocks must not be used.
Fixing reinforcement
The reinforcement bars should be securely tied together with steel wire, tying devices or by welding, and care should be taken to ensure that projecting ends of ties or clips do not intrude into the concrete cover. Welding on site should be avoided if possible, but provided that suitable safeguards and techniques in accordance with the manufacturer's recommendations are adopted, it may be undertaken with the engineer's approval.
Structural connections between two bars can be made by welding or by the use of mechanical couplers if lapping is not feasible. If there is any uncertainty about the arrangement of the reinforcement, or any discrepancy between the bar schedules and the drawings, the engineer should be consulted. The reinforcement must be fixed rigidly in the correct position (Figure 28) and with the correct cover in such a way that it is not displaced during concreting. Top layers of reinforcement in slabs