Reinforcement Guide

SIZE AND QUANTITY OF REINFORCEMENT FOR BUILDING WORKS

REINFORCEMENT QUANTITY ESTIMATION

REPAIR OF REINFORCEMENT IN CONCRETE

REINFORCEMENT COVER AND PRE-CONCRETE CHECKS

REINFORCEMENT CORROSION-CAUSES & PREVENTION

SPACING OF REINFORCEMENT IN RCC MEMBERS

CURTAILMENT OF REINFORCEMENT IN FLEXURAL MEMBERS

CONCRETE REINFORCEMENT DETAILING REQUIREMENT

PREPARATION OF BAR BENDING SCHEDULE

 

 

REINFORCEMENT QUANTITY ESTIMATION

REINFORCEMENT QUANTITY ESTIMATION

For estimating the cost of the structure, it is necessary for the quantities of the materials, including those of the reinforcement to be known. Accurate quantities of the concrete and brickwork can be calculated from the layout drawings. If working drawings and schedules for the reinforcement are not available it is necessary to provide an estimate of the anticipated quantities. The quantities are normally described in accordance with the requirements of the Standard method of measurement of building works.
In the case of reinforcement quantities the basic requirements are:
1. Bar reinforcement should be described separately by steel type (e.g. mild or high-yield steel), diameter and weight and divided up according to:
(a) Element of structure, e.g. foundations, slabs, walls, columns, etc., and
(b) Bar ‘shape’, e.g. straight, bent or hooked; curved; links, stirrups and spacers.
2. Fabric (mesh) reinforcement should be described separately by steel type, fabric type and area, divided up according to 1(a) and 1(b) above.

There are different methods for estimating the quantities of reinforcement;, three methods of varying accuracy are:

Method-1 for Reinforcement Estimation

The simplest method is based on the type of structure and the volume of the reinforced concrete elements. Typical values are, for example:
• Warehouses and similarly loaded and proportioned structures: 1 tonne of reinforcement per 105m³
• Offices, shops, hotels: 1 tonne per 13.5m³
• Residential, schools: 1 tonne per 15.05m³
However, while this method is a useful check on the total estimated quantity it is the least accurate, and it requires considerable experience to break the tonnage down to Standard Method of Measurement requirements.

Method-2 for Reinforcement Estimation

Another method is to use factors that convert the steel areas obtained from the initial design calculations to weights, e.g. kg/M² or kg/m as appropriate to the element.
If the weights are divided into practical bar diameters and shapes, this method give a reasonably accurate assessment. The factors, however, do assume a degree of standardization both of structural form and detailing.
This method is likely to be the most flexible and relatively precise in practice, as it is based on reinforcement requirements indicated by the initial design calculations.

Method-3 for Reinforcement Estimation:

For this method sketches are made for the ‘typical’ cases of elements and then weighted.
This method has the advantages that:
(a) The sketches are representative of the actual structure
(b) The sketches include the intended form of detailing and distribution of main and secondary reinforcement
(c) An allowance of additional steel for variations and holes may be made by inspection.
This method can also be used to calibrate or check the factors described in method 2 as it takes account of individual detailing methods.
When preparing the reinforcement estimate, the following items should be considered:
(a) Laps and starter bars
A reasonable allowance for normal laps in both main and distribution bars and for starter bars has shall be considered. It should however be checked if special lapping arrangements are used.
(b) Architectural features
The drawings should be looked at and sufficient allowance made for the reinforcement required for such ‘non-structural’ features.
(c) Contingency
A contingency of between 10% and 15% should be added to cater for some changes and for possible omissions.

REPAIR OF REINFORCEMENT IN CONCRETE

Repair of Reinforcement in Concrete

The reinforcement repair techniques are different for mild steel and prestressing steel.

1. Mild reinforcing steel

The damaged bars may either be replaced or supplemented by additional reinforcement based on engineering judgment, the purpose of the reinforcement and the required structural strength of the member.
a) Replacement: In case it is decided to replace the bars, splicing of reinforcement with the remaining steel must be done. The lap length must be according to the provision of ACI 318 and the welding (if used) must satisfy ACI 318 and American Welding Society (AWS) D1.4 (or the codal provisions of the respective country). Butt welding is usually avoided due to the high degree of skill required to perform a full penetration weld because the back side of a bar is not usually accessible. Welding of bars larger than 25 mm may cause problems because the embedded bars may get hot enough to expand and crack the surrounding concrete. Mechanical connectors may also be used according to the code requirements.
b) Supplemental reinforcement: This alternative is selected when the reinforcement has lost cross section, the original reinforcement was inadequate, or the existing member needs to be strengthened. The allowable loss of cross-sectional area of the existing reinforcing steel and the decision to add supplemental reinforcement must be evaluated on a case-by-case basis and is the responsibility of the engineer. The damaged reinforcing bar must be cleaned and extra space is to be created by removing concrete to allow placement of the supplemental bar beside the old bar. The length of the supplemental bar must be equal to the length of the deteriorated segment of the existing bar plus a lap-splice length for smaller diameter bar on each end.
Reinforcing bars, having corrosion of their original deformations, give less bond and this factor must be considered while designing the repair of the reinforcement.
c) Coating of reinforcement: New and existing bars that have been cleaned may be coated with epoxy, polymer cement slurry, or a zinc-rich coating for protection against corrosion. The coating must have a thickness less than 0.3 mm to minimize loss of bond development at the deformations.

2. Prestressing steel

Deterioration or damage to the strands or bars can result from impact, design error, overload, corrosion, or fire. Fire may anneal cold-worked, high-strength prestressing steel. The unbonded high-strength strands may need to be detensioned before repair and retensioned after repair to restore the initial structural integrity of the member.
a) Bonded strands: Because the prestressed strand is bonded, only the exposed and damaged section is restressed following repairs. The repair procedure requires replacing the damaged section with the new section of strand connected to the existing ends of the undamaged strands. The new strand section and the exposed lengths of the existing strand must be post-tensioned to match the stress level of the bonded strand.
b) Unbonded tendons: The strands are protected against corrosion by the sheathing, corrosion-inhibiting material (commonly grease), or both. Corrosion of the end connections and the strand has been the primary cause of failure of unbonded tendons. A deteriorated portion of a strand can be exposed by excavating the concrete and cutting the sheathing. Unbonded tendons can be tested to verify their ability to carry the design load. This can be done by attaching a chuck and coupler to the exposed end of the strand and performing a lift-off test. This usually requires at least 20 mm of free strand beyond the bulkhead. If there is excessive corrosion in the strand, failure occurs and the strand must be replaced or spliced. Shoring of the span being repaired and adjacent spans up to several bays away may be required before removing or retensioning unbonded prestressed strands.
The strand is cut on both sides of the deterioration and the removed portion of the strand is replaced with a new section. The new strand is spliced to the existing strand at the location of the cuts. The repaired strand is then prestressed. Carbon fiber or equivalent systems are available to supplement the reinforcement in prestressed, post-tensioned, and mild steel reinforced structures. This system is normally glued onto the exterior surface. Unless the component being reinforced is unloaded, the strengthening system only provides reinforcement for future loadings. Fiber wrapping is commonly used for reinforcing columns, especially in earthquake zones. There are systems available that recover the dried and damaged protective barrier within the sheathing.

REINFORCEMENT COVER AND PRE-CONCRETE CHECKS

Reinforcement Cover:

It is essential that the steel reinforcement bars are surrounded sufficient impermeable concrete to protect them from corrosion, and to allow the combined strength of the reinforcement and concrete to be effective.

From the diagram above, we can see how the process unfolds. When the binding has set, the steel bars are placed into position. The bars are kept the correct distance from the surface by spacers. When the formwork is erected the concrete can be poured. When the concrete has reached the required strength the formwork can be poured. The cover blocks have ensured that the steel has sufficient protection from the elements.
If the steel had been placed too close to the surface and the concrete had been poured then over time the steel may become exposed to moisture and corrosion will commence.
Spacers can be made from plastic, mortar or steel. Plastic spacers are made to fit particular bar size and give specified depths of cover.
Small mortar blocks are also commonly used which are tied to the reinforcement bars using soft iron binding wire. The ends of these ties should be bent away from the surface of the concrete, otherwise the wire may facilitate the corrosion of the reinforcement.
Steel spacers can be used, but only when the structure is not in a corrosive atmosphere or be exposed to water.
The required amount of cover will always be specified on the design drawings, but no time should the cover be less than the maximum size of aggregate plus 5mm.

Pre-Concrete Checks for Reinforcement:

The pre-concrete check for reinforcement essentially comes in two parts. The first part is a visual inspection by the clerk of works or equivalent.

Using a steel tape, cover thickness will be checked and any spacers that have fallen off or been broken will need to be replaced.
The clerk or equivalent will be looking to see that the reinforcement bars are free of excessive rust and not covered in mud from foot traffic.
Similarly, the bottom of the concrete pour must be free of debris including the cutoffs from the steel tying bars.
The clerk will also check for under-bent bars that may mail to allow the correct cover and that the bars are at the correct spacing.

In the above fig, left side images shows reinforcement have not been placed correctly. Right side images shows correct way of placing of reinforcement.
The second part is carried out by a surveyor, who will check the steel levels against the required levels from the design drawings. If these levels are satisfactory and the clerk has completed the visual checks then the pour will proceed.

REINFORCEMENT CORROSION-CAUSES & PREVENTION

REINFORCEMENT CORROSION-CAUSES & PREVENTION

The corrosion of steel reinforcement is complex, but basically it is an electro-chemical reaction similar to that of a simple battery. The composition of mild steel varies along its length and potential anodic (more negatively charged) and cathodic (positively charged) sites can be set up at various points.

Concrete is capable of conducting and electric current and acts as the electrolyte with the circuit being completed by the bar through which the electrons can flow. However the highly alkaline environment (pH about 12.8) provided by good quality concrete produces a protective layer around the steel preventing the flow of current. This is known as Passivation.
The corrosion reaction can only occur when the following conditions prevail.
1. There is a breakdown of the passivating layer (de-passivation) brought about by
a) A lowering of the alkalinity of the concrete below a critical pH of about 10.5, caused normally by the ingress of carbon dioxide (carbonation).
b) The ingress of chlorides
2. Oxygen and water are present.
With the above conditions prevailing the ferrous ions (Fe++) released from the anode combine with the hydroxyl ions (OH-) from the cathode, in the presence of water and oxygen to produce rust (ferric hydroxide). This is an expansive reaction leading to eventual spalling of concrete cover and reduction in the area of the steel at the anodic site.

THE MECHANISM OF REINFORCEMENT CORROSION:

Carbonation:
Acidic gases like corbon dioxide react with any free alkali that may be present, which can lead to a drop in the alkalinity of the concrete. This process, which starts at the surface of the concrete, slowly penetrates deeper and deeper. The penetration is nearly proportional to square root of time.

Fig: Carbonation leads to the general corrosion along the full length of the bar.

The above figure shows the first outward signs of general corrosion taking place is surface cracking of the concrete along the line of the steel.

The above fig. shows that as the corrosion proceeds, the concrete will spall away completely to expose the steel.
Chlorides:
Chlorides are generally acidic in nature and can come from a number of different sources, the most common being, de-icing salts, use of unwashed marine aggregates, sea water spray, and certain accelerating admixtures (their use is now prohibited).
In the presence of chlorides localized pitting corrosion occurs which does not always have associated with it the early warning signs of surface cracking.
Chlorides induced corrosion is potentially more dangerous than that resulting from carbonation. Like most of the aspects of concrete durability, deterioration due to corrosion of the reinforcement can take place years (5 to 20) to manifest itselt.

MINIMISING THE RISK OF CORROSION:

The quality and depth of concrete in the cover zone are all important in minimizing the risk of corrosion as shown in fig. below.

Quality:
Quality is controlled largely by minimizing permeability.
Depth:
Recommendations for minimum depths of cover are given in the codes of practice and are based on exposure conditions and minimum cement contents. Higher cement contents infer lower water cement rations leading to permitted reductions in cover.
At no time should the normal cover be less than the maximum size of aggregates+5mm.
Materials:
Blended cements made from combinations of PC/PFA and PC/GGBS can lead to significant reduction in chloride penetration. However, in situations where these materials are not cured properly there is a risk of increased carbonation.
Care must be taken that all aggregates and admixtures contain limited amount of chlorides.

SPACING OF REINFORCEMENT IN RCC MEMBERS

Spacing of Reinforcement in reinforced concrete members:
Minimum Spacing between Bars in Tension
The minimum horizontal spacing between two parallel main bars shall be diameter of larger bar or maximum size of coarse aggregate plus 5 mm. However, where compaction is done by needle vibrator, the spacing may be further reduced to two-third of the nominal maximum size of the coarse aggregate.
The minimum vertical distance between two main bars shall be
(a)  15 mm,
(b)  two-third of the nominal size of coarse aggregate, or
(c)  maximum size of the bar or whichever is greater.
Maximum Spacing between Bars in Tension
Normally these spacing will be as mentioned below :
(a)  For beams, these distances are 300 mm, 180 mm and 150 mm for grades of main reinforcement of Fe 250, Fe 415 and Fe 500, respectively.
(b)  For slabs
(i)  the maximum spacing between two parallel main reinforcing bars shall be 3dor 300 mm or whichever is less, and
(ii)  the maximum spacing between two secondary parallel bars shall be 5dor 450 mm or whichever is less.

Fig: Spacing of reinforcement in beams

Reinforcement Requirement in Members

Beams
(a)  Minimum tensile steel is given by the ratio (For Flanged Beams b= bw)
(b)  Maximum Tensile Reinforcement in Beams shall not exceed 0.04 bD.
(c)  Maximum area of compression reinforcement shall not exceed 0.04 bD.
(d)  Beam having depth exceeding 750 mm, side face reinforcement of 0.1% of webarea shall be provided. This reinforcement shall be equally distributed on two faces at a spacing not exceeding 300 or web thickness or whichever is less.

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CURTAILMENT OF REINFORCEMENT IN FLEXURAL MEMBERS

Curtailment of Tension Reinforcement in Flexural Members 

To economise the design of a flexural member, the tensile bars are curtailed at the section beyond which it is no longer required to resist flexure. Such curtailed bars are extended for a distance equal to 12 or effective depth, whichever is greater, except at simple support or free end of cantilever.
At simple support, positive moment tension reinforcement shall be limited to a diameter such that L_d computed for f_d does not exceed

where M1= Moment of resistance of the section assuming all reinforcement at the section to be stressed to f_d,
f_d = 0.87 f_y,
V= Shear force at the section due to design loads,
L₀= Sum of anchorage beyond the centre of the support and the equivalent anchorage value of hook, etc., and
= Diameter of bar.
The value of in the above expression may be increased by 30% when the ends of reinforcement are confined by a compressive reaction. At least one-third of positive moment reinforcement in simple member shall extend along the same face of the member into the support, to a length equal to .

Fig: Curtailment of Reinforcement in RCC Beams

Flexural reinforcement shall not be terminated in tension zone unless one of the following conditions are fulfilled:
(a)  The shear at the cutoff point does not exceed two-thirds of that permitted.
(b)  Stirrup area in excess of that required for shear and torsion is provided along each terminated bar over a distance from cutoff point equal to three-fourth the effective depth of the member. The excess stirrup area shall be not less than 0.4 bs/f_y, where b is the breadth of the beam and s is the spacing. The resulting spacing shall not exceed is the ratio of the area of bars cutoff to the total area of bars at the section.
(c)  For 36 mm and smaller bars, the continuing bars provide double the area required for flexure at the cutoff point and the shear does not exceed three-fourths of that permitted.

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CONCRETE REINFORCEMENT DETAILING REQUIREMENT

Reinforced concrete is a composite material, hence a proper bond between the two materials – concrete and reinforcement – is the first requirement. A minimum length of reinforcing bar is needed to develop the full bond strength between concrete and steel. This length is expressed in terms of development length.
‘Development length’ on each side of any section is the length over which the force in the reinforcement at that section will be developed without bond failure between these two materials. Sometimes a reinforcing bar is extended and/or bent at its ends to satisfy development length requirement. Such extension and/or bending of a bar at its ends is called anchorage. A bent bar provides a greater safety against bond failure as during a pull-out the whole concrete is to be crushed.

Fig: Development length of reinforcement in concrete

Reinforcing bars have limited lengths for ease of handling and transporting; hence in case of a continuous member or a member of large span it is necessary for continuity to join two bars by overlapping the ends at the joint. The overlapping portion are joined together either by concrete itself by providing proper development length or by welding in case of limitation of length for overlapping. Such jointing of two bars for continuity of reinforcing bar at any section is called splicing.

Fig: Detailing of Reinforcement in continuous beam

Reinforcements are round bars. They are provided as straight or shaped appropriately to suit the requirements. These reinforcements are placed at certain spacing to meet the design requirements. But these spacing must be within a range of the minimum spacing and the maximum spacing for ease of casting, compaction, control of cracking, etc. as laid down in the Standard Code of Practice.
All types of reinforcements must have sufficient concrete cover to protect them from environmental exposure conditions and also against fire. Such cover is called Nominal Cover.
Other requirements of detailing are the minimum and the maximum amount of reinforcement, side face reinforcement, distribution of reinforcement, etc.

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PREPARATION OF BAR BENDING SCHEDULE

Preparation of Bar Bending Schedule
Bar bending schedule (or schedule of bars) is a list of reinforcement bars, vis-à-vis, a given RCC work item, and is presented in a tabular form for easy visual reference. This table summarizes all the needed particulars of bars – diameter, shape of bending, length of each bent and straight portions, angles of bending, total length of each bar, and number of each type of bar. This information is a great help in preparing an estimate of quantities.
Figure 1 depicts the shape and proportions of hooks and bends in the reinforcement bars – these are standard proportions that are adhered to:
(a) Length of one hook = (4d ) + [(4d+ d )] – where, (4d+ d ) refers to the curved portion = 9d.
(b) The additional length (la) that is introduced in the simple, straight end-to-end length of a reinforcement bar due to being bent up at say 30^o to 60^o, but it is generally 45^o) = l₁ – l₂ = l_a
Where,

Fig: Hooks and bends in Reinforcement

Giving different values to respectively), we get different values of la, as tabulated below:

Figure 2 presents the procedure to arrive at the length of hooks and the total length of a given steel reinforcement.

Fig: Typical Bar Bending Schedule

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SIZE AND QUANTITY OF REINFORCEMENT FOR BUILDING WORKS

Reinforcement is required for reinforced concrete members such as footings, beams, columns, slabs, lintels etc. Estimation of reinforcement quantity is required prior to tendering stage to calculate approximate cost of project or construction work.

Following table gives the estimated quantities of reinforcement and its size generally used for various building works:

Sl. No	RCC Member	Quantity in kg/m3	Size of reinforcement required
1	Column footings	75	10mm  or 12mm
2	Grade beams	100	12mm, 16mm – 85% Stirrups – 6mm or 8mm– 15%
3	Plinth beams	125	8mm diameter – 85%, Stirrups 6mm – 15%
4	Columns	225	16mm, 20mm and 25mm – 90% Ties – 6mm or 8mm – 10%
5	Lintel beam	125	12mm, 16mm dia – 85% Stirrups – 6 mm or 8mm – 15%
6	Sunshades	60	8mm dia – 75% Distributer – 6mm – 25%
7	Canopy slab upto 2.0 m span	125	10mm dia – 80% Distributor bars – 6mm or 8mm – 20%
8	Staircase waist slab	150	12 or 16mm dia – 80% Distributor 8mm dia – 15%
9	Roof slab
	(a)  One way slab	80	8mm dia – 70% Distributor – 6mm – 30%
	(b)  Two way slab	100	8mm dia – 100%
	(c)  Square slab – 4m to 6m size	150	10 – 12mm dia – 100%
10	Main beams above 6m	250	20mm, 16mm, 12mm – 80 – 85% Stirrups – 8mm – 15 – 20%

All above mentioned steel are round tapered steel. This data is just for estimation of quantity of steel for various RCC works. This does not provide actual steel required for all the members. Actual quantity of steel required can be calculated from the drawings prepared after structural design.
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