If the reinforcement in concrete is not of proper quality and/or is not accurately detailed, the performance of the structure gets affected seriously. In this paper, the authors endorse the need for strict quality assurance and control and describe the certification and validation aspects.

Steel reinforcement is routinely used in reinforced concrete (RC) structures to augment the relatively low inherent tensile strength of concrete. It is also used:

• to carry shear, compressive and torsional forces in excess of concrete capacity
• to control cracking of concrete members under working loads or as a result of early thermal effects.
• to minimise or prevent spalling of concrete in fire conditions, as a result of seismic effects, or in the highly stressed regions around anchorages in prestressed concrete construction.

Reinforcement, therefore, plays a vital role in ensuring the safety, integrity and durability of almost all concrete structures. It can only perform that role satisfactorily if it:

possesses the required physical and metallurgical properties

• is of acceptable quality
• is stored, handled, cut and bent so as to avoid damage and contamination
• is properly and accurately fixed.

In order to realise the above-referred objectives, it is very necessary that the detailing of reinforcement is meticulously carried out and that necessary detailed working drawings and bar bending schedules is made available to the field engineers. It is also necessary for the field engineers to critically scrutinise the drawings and bar bending schedules received by them for constructability. They should have a dialogue with the design engineers if required and bring to their notice any difficulties in realising the reinforcement as per the drawings and schedules. It may also be necessary at times to revise the drawings and/or schedules based on such a dialogue.

• 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.

Design process

The drawings issued to the contractors need to be complete. The contractors should have the option to take the right decisions to rationalise reinforcement in order to achieve early striking of formwork and to optimise productivity.

In any project, reinforcement drawings may be revised several times as a result of technical or human error or a change to redesign initiated by one of the parties involved in the construction contract. Such changes may have implications on the speed and cost of construction. Number of revisions should be reduced since the contractors’ participation minimises the element of change during execution of the project.

During the execution of a flyover project at Ludhiana, drawings for deck girders were approved and 40 girders were cast; after that the consultant revised the drawing (changing the spacing of reinforcement). The owner rejected the 40 beams already cast. This resulted in a serious set back in project completion schedule and cost overrun.

Requirements for bond

The IS code is silent regarding the bond strength of coated reinforcement bars. Any type of coating whether it is epoxy or co-polymer based concrete coating (CPCC) or Karaikudi treatment, reduces the bond between reinforcement and concrete, Fig 1, and this is rarely taken into account by the designers. The loss of bond of coated bars may be as much as 50 percent in some cases As per FIP Bulletin 10 – Bond of Reinforcement Concrete – “The epoxy coating is much smoother than the normal mill scaled surface of non-coated bars and is chemically inert. Chemical addition and friction between bars and concrete are therefore reduced by coating”. It is also reported that anchorage capacity of hooks and bends is reduced by coating. A 20 percent increase in the length of developmental epoxy coated bars is recommended.

Ductility requirements

The requirements are not addressed at all in IS 1786 whereas BS 4449 provides for two ductility categories 460A and 460B. As per the BS Code while the yield strength for both the categories is same, the elongation at fracture is increased to 14 percent and total elongation is increased to 5 percent in the case of 460B. As per the BS Code, the maximum carbon equivalent value is limited to 0.51.

Need for detailing

The satisfactory performance of RC structure is dependent on the accurate placement of carefully detailed reinforcements; otherwise concrete elements are marred by cracking, rust marks and similar problems directly related to workmanship. Certain defects result from poor design work, poor detailing does not permit application of satisfactory workmanship and faulty dimensioning of such critical details as location and cover to reinforcements.

Good detailing and scheduling of reinforcement helps the construction process considerably by minimising costs, delays and disputes, by easing the reinforcement fixers’ task and by providing the contractor with a certain amount of flexibility both in fixing and in construction stage.

It should be appreciated that simple lines indicated in the structural drawings can be misleading. A simple line indicating a link or stirrup bar in a corbel is easy to include on the drawing board. However, a line has little thickness and to include the substantial bar of steel, bent to shape, often proves difficult and sometimes impossible.

At the ends of prestressed beams, considerable reinforcement is included to contain the bursting forces. These steel bars can be simply drawn but when translated into actual bars in three-dimensional form, extreme congestion often results, Fig 2. In consequence, it may prove difficult to place and compact concrete

Many inserts are incorporated into structural concrete for fixings, fastening, bearing plates etc. There are also projecting bars and dowels for connections between precast elements or between insitu and precast concrete. These inserts must be detailed and scheduled. Omissions of any of these fixings can prove expensive.

Bar marking on drawings

The generally accepted parameters for bar identification is as follows:

• number of bars
• type of steel
• diameter of bars
• mark no of bar
• spacing of bars

The detailer should include every information on the drawings using some of the abbreviations given in Table 1.

It is important is to use a standard method of detailing. The advent of computer aids in design and detailing has reduced the number of drawing errors and inconsistency between schedules and drawings. The main problems which now arise are thus where the line on the drawing misleads, radius and thickness are combined in some instances with the result that some bars cannot be fitted within the allocated space in the formwork. At column-beam junctions there is often a considerable amount of reinforcement cramped into a restricted space and the main beams and main column bars may clash.

A mechanical welded splice shall develop at least 125 percent of the specified yield strength, fy , of the bar to ensure sufficient strength in splices so that yield can be achieved in a member and thus brittle failure avoided; the 25 percent Fig 3 BS 8666 : 2000 standard shapes 40 The Indian Concrete Journal * January 2004 increase above the specified yield strength was selected as both an adequate minimum for safety and practicable maximum for economy. A full welded splice is primarily intended for large bars in main members. Wherever practical, direct butt splices are preferable for larger diameter bars.

Preferred shapes of reinforcements

Though a very large number of shapes and configuration are being practised, it is desirable to limit the numbers of shapes. This is in order to reduce mistakes during fabrication and erection and also to ensure practicability in cutting, bending and installation. Fig 3 gives an example-preferred shape recommended by Construction Industry Research and Information Association.

Grades of reinforcement bars

The BS Code allows for the use of three grades of reinforcement bars, namely, Fe 415, 500 and 550. Facilities exist in the country for the manufacturers of these grades. However, Fe 415 is the most predominantly used grade in India. The consultants limit specification to only Fe 415 grade due to unproven prejudice against higher grades in terms of ductility. Consequently, congestion of reinforcement in many industrial structures is very common leading to deficiencies in the quality of the structure. On many occasions it is just not practical to satisfactorily place and compact the concrete with heavily congested reinforcement. This is evident from large number of cases where problems have been reported during the construction of thermal power stations, atomic power stations, etc. Grade Fe 415 is already obsolete in the developed world. The minimum grade as per BS: 4449 is Fe 460.

Recommended diameters for reinforcement

The IS specification (IS 1786) specifies nominal sizes of 4,5,6,7,8,10,12,16, 18, 20, 22, 25, 28, 32, 36, 49, 45 and 50 mm bars. The reasons for specifying such a large number (18) of nominal sizes are not clear. In practice many of the sizes are neither being rolled nor being used. On the other hand, it gives an opportunity for the consultants/designers to specify odd sizes such as 18, 22 mm etc and create consequent difficulties during execution both due to non-availability of odd diameters in the market and in terms of quality assurance. It is not possible to distinguish between 20 and 22 mm diameters or 18 and 20 mm diameters by visual examination. Mistakes are bound to happen.

The European practice recommends only the following diameters: 8-10-12-16-20-25-32-40-50 mm

These diameters have the great advantage of being distinguishable with naked eyes. The section of each bar corresponds to approximately the sum of the sections of the two preceding lower diameter bars which provides for and theoretically facilitate all combinations. The CEB model code recommends bar diameters of 32 mm or lower. Generally larger diameters are not recommended to be used for obvious quality reasons.

The Indian standard code of IS 1786 was last revised in 1985 and in the context of world wide developments, the code needs immediate revision.

Weldability

IS 1786 – Clause 0.2 (Foreword) states that “there is also need for these steel bars to be welded and fabricated on the site easily. For this, strength and ductility had to be achieved at the lowest possible carbon content”. However, the requirement of welding is not specifically dealt with in IS 1786. The corresponding British Standard BS 4449 : 1997 specifies the requirements for weldable steel bars in terms of carbon equivalent value. In the absence of such specifications in IS 1786, quality problems do frequently arrive when welding is resorted to.

Chemical composition

The IS code permits carbon content 0.3 percent whereas the internationally accepted maximum value is only 0.25 percent. The increased permissible carbon content has been responsible for corrosion of reinforcement steel in the large number of cases.

Tolerances on dimensions and nominal mass

IS 1786 Clause 6.2 (Table 2) provides tolerances and nominal mass of ± 8 percent for bars upto and including 10 mm, ± 6 percent for bars upto 12 to 16 mm diameter and ± 4 percent for bars over 16 mm diameter. These are very liberal in the international context and the manufacturers take full advantage by supplying bars upto the specified tolerances. By the very nature of rolling process, the bars generally have plus tolerances. The bars are supplied by weight but the design and construction is based on the specific theoretical bar diameters and in the bargain the project cost goes up. In major projects the loss runs to several million rupees per annum which of course is reflected in higher project costs.

Decoiled material

The processor should operate a documented procedure which assures that the decoiled material continues to meet the mechanical property requirements of the IS (EN 10080).

This procedure should include the following:

• visual inspection of every coil processed. For ribbed steel, a rib height or indentation depth measurement on at least one sample per day and/or at each size change.
• sampling and testing of the decoiled product at a frequency of one sample per diameter and machine per week.

Bending schedules for reinforcement

Shapes and dimensions of bars cannot in general be deduced from the reinforcement drawings in isolation. The main function of the bending schedule is to define exactly the shape of each bar within a group of bars having the same bar January 2004 * The Indian Concrete Journal 41 mark. A standard layout of the bar bending schedule is given in IS 2502. Schedule normally contains information concerning weights of reinforcement.

It is the responsibility of the design team to prepare the bar bending schedules. IS 456 also stipulates that bar bending schedules shall be prepared for all reinforcement work (Clause 12.1) No schedule should cover more than one drawing. Many organisations follow the very good practice of including the bar bending schedule on each structural drawing.

Will be continued in Issue 7...