Continued from Issue 6...

Role of field staff

Discussion between the field staff and the designers/detailer early can result in simplification of reinforcement details and introduction of joints in bars at points in the structure which best suit the eventual location of the construction joints, determined by casting method. Where the work is of a complex nature, the supervisor should press for adoption of open stirrups and links, which allow ease in adjustments of cages to maintain the required cover.

where, for construction requirements, bars have been diverted, bent into and allowing formed face or for later insertion, failure to ensure that these bars are properly rebent can have serious consequences. where openings are formed in a slab, possibly in accordance with the revised detail, it is important to ensure that additional bars required to trim the openings are installed as detailed – a point which may easily be overlooked in the course of changing or inserting formers into the formwork.

The supervisor should pay special attention to the following:

• where, for construction requirements, bars have been diverted, bent into and allowing formed face or for later insertion, failure to ensure that these bars are properly rebent can have serious consequences.
• where openings are formed in a slab, possibly in accordance with the revised detail, it is important to ensure that additional bars required to trim the openings are installed as detailed – a point which may easily be overlooked in the course of changing or inserting formers into the formwork.

The field staff should carry out the following checks:

• check fixability of the detailed reinforcement and effect on construction sequence.
• check drawings and schedules for errors and inconsistencies.
• check bar bending schedule.
• answer queries from steel fixers and resolve problems, in consultation with the designer, if necessary.

Check fixed reinforcement before starting to place concrete.

Spacers for reinforced concrete

The inadequate use of spacers and chairs has been a major cause of variations in cover to reinforcement and consequently decreased durability of concrete. Unfortunately, there is no comprehensive national standard for the use or the performance of spacers and chairs. IS 456 : 2000 merely states that “all reinforcement shall be placed and maintained in the position shown in the drawings by providing proper cover blocks, spacers, supporting bars etc”. (Clause 12.2). While this is a specification requirement, there are no guidelines in this regard either in the IS codes or in the technical specifications.

The spacers are generally called cover blocks in India. There are no specific guidelines for the cover blocks. In practice, cover is maintained by a variety of crude contraptions ranging from pieces of broken tiles or broken stones or timber to mortar blocks. None of these are really satisfactory. They do not maintain the cover except perhaps the mortar block, which again is not satisfactory because of the porosity. Porous blocks result in premature corrosion to reinforcement. Occasionally, some specifications do call for cover blocks of concrete of the same grade as that of concrete. While this may be satisfactory, its application is limited to upper end of the cover, say more than 50 mm. There is also no guarantee that site-made concrete cover blocks are of the same quality as that of the concrete. After all, hardly any constructor would be using concrete from batching plants for manufacturing of cover blocks.

Considering the above, the specifications of developed countries do not permit any of the above contraptions including site-made concrete cover blocks. The standard practice elsewhere is to provide PVC spacers or cover blocks, Fig 4. Two main types are available – clip-on where the clip action is an integral part of the design of the spacer and wireon where the spacers are attached to the bar by the use of binding wire. These are manufactured in factory and provide cover up to 100 mm and 5 mm increments. Sizes commonly available are 20,25,30,35,45,50,60 and 75 mm.

PVC cover blocks

These blocks are lightweight, non-porous and chemically inert in concrete. The PVC cover blocks should have rounded seating such that holes are not punched in the formwork and do not deform under load nor should they shatter or severely crack. The PVC cover block will not rust thus eliminating blemishes on the surface of the concrete. PVC cover blocks are being manufactured in India, but their use is very limited. Many of the consultants/supervising engineers do not permit the use of PVC cover blocks, perhaps because of the lack of familiarity. This is an excellent recommended material for use as spacers. IS 456 : 2000 states that “spacers, cover blocks should be of concrete of samestrength or PVC”. It is also specified that the spacers or Fig 4 PVC spacers of different types 42 The Indian Concrete Journal * January 2004 chairs should be placed at a maximum spacing of one metre and closer spacing may sometime be necessary. In USA, the most widely used spacers are factory-made wire bar supports, which are made of plain wire or stainless steel wire. The lower portions are provided with special rust protection by a plastic cover or by being made in whole or part stainless steel wires. Precast concrete blocks are used to support bars in footings, slabs on grade and as side formed spacers. Plastic supports are generally used as side formed spacers and on horizontal work.

Chairs

Chairs are manufactured from reinforcement bars to provide cover in excess of 75 mm. They are commonly used to support top horizontal reinforcement or to support vertical reinforcement in walls, Fig 5. They may be individual or continuous. Where placed on formwork or mud mat the legs of chairs should be provided with protection gaps. Individual chairs are manufactured in standard heights of between 75 and 300 mm and are used to support reinforcement at one point. Continuous chairs provide a straight line of support at a uniformed height. Chairs above a height of 300 mm are normally scheduled as part of the bar bending schedule.

Guidelines for tying reinforcement

There are six common ways of tying reinforcement bars, Fig 6.

Slabs and walls

Cantilevers

Perimeter bar should be tied at every intersection; for bars up to and including 20 mm, alternate intersection should be tied. For bars of 25 mm or larger diameter, they may be tied at greater centres.

Cantilevers

Top reinforcement in cantilevers should generally be located by the use of chairs.

Beams

Every intersections of a corner of a link with longitudinal main bar should be tied. Other bars within the links should be tied at 50D centres.

Spacers within the beams

Spacers within the beams should be at centres not exceeding one metre along the beam. Spacers should be fixed on three sides of the same link.

Columns

Every intersection between vertical bars and links should be tied.

Binding wire

The binding wire is normally a black annealed wire, of 16 gauge. However, at many projects, thinner binding wire (18– 20 gauge) are used but are not desirable. Care should be taken to ensure that the projecting ends of the binding wire do not encroach into the concrete cover.

Bar bending machines

Partially automated machines are now in use, Fig 7. Once set, bending machines produces reasonably accurate bars, but the first one or two bars of each batch are taken as a sample for checking the set and are always scrapped, increasing the wastage. This should not occur if the new automated machines are used.

Fabricators should automate information transfer in their plant. With new generation of equipment, it is possible to transfer bending schedule data from detailers computers directly to cutting and bar bending machines. This feature needs to be fully exploited.

Quality assurance (QA)

An important feature of the quality management system is to ensure that the purchased or specified material complies with quality requirements. It is customary to accept that material from a source, which is working to a recognised quality assurance scheme, and fully certified satisfies this requirement without the need for further inspection and testing.

Responsibility for quality remains exclusively with the

• steel maker — for production of reinforcement steel
• fabricator — for fabrication of the reinforcement
• contractor — for handling and placing of the reinforcement in the formwork
• with the supplier — for joints and devices

Sampling inspection system (SIS)

In all cases when a producer intends to deliver material not covered by the certification scheme the SIS can be applied. The system should give at least the same quality level as control system. The tests included in SIS should generally be performed before the material is delivered (because rejection of a product already delivered to one or more location is extremely difficult).

Guidelines inspection of handling of reinforcement

It is logical that within the inspection guidelines only those properties of the component of the reinforcement are controlled which have been influenced by the handling procedure.

Special recommendations are necessary for straightening of coils, welding procedures, anchoring systems and joints, rebending of rebars.

The inspection shall be ordered not from the manufacturer but from the purchaser, who may be a distributor (usually a steel merchant) or a building contractor. The purchaser is responsible for the measures in case of non-compliance.

Random sampling shall be carried out either at the manufacturers or at the stores of the purchaser. The specimens from the sampling units shall be submitted to an authorised test house.

The test house shall submit a report to the inspector who assesses the results with reference to the relevant properties according to the rules given here and he shall decide between acceptance and rejection.

The test house shall submit a report to the inspector who assesses the results with reference to the relevant properties according to the rules given here and he shall decide between acceptance and rejection.

The inspector shall draw up and sign a sampling inspection report for all the test results. Cases of serious shortcomings must be reported by the inspector to the certificating body.

Sampling

The sampling shall be made either at the manufacturer’s or at the storage premises of the purchaser. Only in exceptional cases it may be performed at the building site.

Third party certification

The third party certification is designed to meet the consistency of raw materials and cut and bent reinforcement ensuring that at all stages the materials consistently meets with the requirement of the relevant standards or specifications allowing reduction in costly and time consuming site testing. The certification rules also provides for full traceability of material from the hot metal through to the construction site. Certification rules allow the use of electronic systems throughout the approval supply chain starting from material ordering, production and supply.

The certification of a product is a guarantee, granted by a recognised authority, that the product satisfies a certain number of characteristics, specified either by the approving organisation, or by the producer. The guarantee, checked both by the producer and by the organisation, avoids important and costly tests having to be carried out again in each case in which the product is applied and enables a limited check, generally only on identification, to be carried out. The official organisations therefore have an obvious interest in enforcing the use of approved products.

The approval documents summarise and explain the different headings considered below.

1. 1. Object of the guarantee
2. 2. Identification
• name of the producer, description of the steel
• shape of the steel (drawing or photograph)
• mill’s rolling mark
• quality remarks
3. 3. Requirements to be checked by the producer’s quality control
• tolerances on diameter and sections
• geometrical dimensions of the section with minimum values or guaranteed tolerance limits (height, longitudinal spacing and length of ribs)
• guaranteed yield point
• tensile strength
• elongation at fracture
• bending, rebending capacity

The key products covered by a typical certification scheme are:

• steel production and billet casting
• reinforcing bar
• fabric
• cut and bent reinforcement
• fabrication and welding of reinforcement 44 The Indian Concrete Journal * January 2004
• steel fixing on site.

The comprehensive nature of the certification scheme ensures that reinforcing steel consistently satisfies the requirements of the product standard and is fully traceable from start to finish.

All the certified material is labelled and supplied with the necessary electronic or paper documentation to enable the products to be traced. The comprehensive testing and full product traceability required by certification obviates the need for the end-user to undertake any further verification tests.

Failure to specify certification approval places the onus on the purchaser to verify that material complies with the standard. This requires extensive additional product testing with significant costs and potential site delays.

Checklist for initial scrutiny of drawings and schedules

The following queries can be used as a checklist for initial scrutiny of drawings and schedules.

• Can the reinforcement as detailed be fixed?
• Does the detailing permit sensible location of construction joints?
• Is the reinforcement congested?
• Would alternative detailing provide greater flexibility or ease of fixing?
• Is there scope for prefabrication?
• What is the best sequence?
• Is the reinforcement detailed to provide sufficient rigidity and stability of cages during fixing?
• Are set-up bars, bracing bars, chairs, spacers etc required?
• Do schedule agree with drawings?
• Does reinforcement in one member clash with that in an adjacent member?
• Are scheduled bending dimensions compatible with member dimensions and specified concrete cover?
• Are starter bars detailed?

Delivery checklist

For the delivery checklist, one has to:

• ensure adequate offloading space
• check weights given on delivery ticket (by calculation and bar count)
• ensure correct handling during unloading
• check reinforcement is of correct type
• check bundles are correctly labelled
• check reinforcement is of correct grade
• check bar size (for example, by gauge or tape)
• check extend of scale and pitting.

Storage checklist

For the storage checklist, one has to:

• ensure storage area is spacious and well organised
• ensure reinforcement is stored clear of the ground
• keep reinforcement free of mud, oil, grease
• provide a loose protective cover over the reinforcement
• ensure good air circulation around the steel
• store materials according to construction program
• avoid long term storage of reinforcement on site.

Bending checklist

For the bending checklist, one has to:

• use a steel tape when marking bars for bending
• a site bending yard must be properly planned and set up
• reinforcement should be bent cold on a proper powered bending machine
• do not permit high yield steel to be heated on site before bending
• check the bent shape for dimensional accuracy (for example, against a full size template), bend radii and for signs of fracture.

Pre-concreting checklist

For the pre-concreting checklist, one has to:

• ensure correct number of bars have been fixed
• check that all laps are of the correct length
• ensure correct use of cover blocks, spacers etc
• check cover to reinforcement is correct at all locations
• ensure that all twisted ends of ties are bent inwards away from concrete faces
• ensure adequate access for concrete compaction.

Site actions influence costs

Certain activities control the cost incurred. Such activities include:

• ordering in good time in economic loads
• minimising damage by careful handling and storage
• minimising wastage by intelligent cutting and accurate bending
• minimising loss by providing secure storage
• accuracy in cutting and bending
• slinging of reinforcement must be done carefully, Fig 8
• prefabrication, either on or off site
• storing reinforcement in reverse sequence to use to avoid double handling
• avoidance of site welding reduces costs.

Acknowledgement

The content is sourced with due acknowledgement from the following reference for knowledge dissemination.

• The Indian Concrete Journal (January 2004)

References

• ______Indian standard code of practice for plain and reinforced concrete, IS 456 : 2000, Bureau of Indian Standards, New Delhi.
• CIRIA Special Publication 118.
• Concrete Society, U.K. Report CS – 101 (Spacers).
• ACI Manual of Concrete Practices Part III.
• ______ACI building code requirements for structural concrete, ACI 318-02.