Plain and Reinforced Cement Concrete

While cast-in-place plain concrete is laid either with or without formwork, most reinforced cement concrete construction involve some form of centering and shuttering, which in short is called the formwork. Moulds are used largely in prefabricated construction. 

Quantification of any reinforced concrete work can be split in to three components. They are as follows : 

1. Formwork, 
2. Reinforcing Grill Work, and 
3. Concrete. 

Formwork 

The purpose of formwork is to mould the plastic concrete into specific shapes in specific positions as required by the structural design. Centering or falsework supports the formwork that provides a level area or an area of the required shape to receive concrete. Centering consists of a framework of verticals, horizontals and inclined supports, either of timber or mild steel sections or pipes. Concrete receiving formwork is made of wooden planks, mild steel plates or plywood sheets. The surface is oiled and the gaps are properly filled up. Reinforcement grills are placed on the surface over suitably placed cover blocks and concrete is poured and compacted. After the concrete has gained sufficient strength, the formwork is struck off. Falsework and formwork are then inspected for damage, if any, repaired, if  necessary and stored carefully for future reuse. 

Formwork is quantified in area basis. It is the inner surface area of the box (formwork:) that receives the concrete. 

Slab 

Formwork area = Bottom area + Area of sides all around 

Bottom area is generally the clear size of rooms or between beams and area retaining the sides is arrived at as Perimeter x Depth of slab. 

Beam 

Formwork area = Clear span of the beam x [width of beam + (2 x rib depth of beam)] + (area of end faces given by 2 x width x rib depth). 

Lintel with Sunshude 

Lintels over openings in the external walls are usually cast integrally (monolithically) with sunshades, as shown below 

Formwork area = Bottom surface (It should be seen whether  the sunshade has bearing over the entire length of lintel or out of opening only) + sides and ends of lintel + 2 x sides of sunshade. 

(The front surface of sunshade compensates for the side of lintel at the junction). 

Column 

Formwork area = Perimeter x height of column 

Normally, moulds of the standard size of columns of a particular height will be used. 

Reinforcing Grill Work 

Reinforcing grills are fabricated out of mostly mild steel rods of suitable (design) grade and estimated as Quintilian (100 kg) or tomes. There are following two methods to estimate the quantity of reinforcing  grill : 

• Approximate Method 
• Exact Method 

Approximate Method 

In the approximate method, the quantity of concrete is first arrived at. Based on tables available in handbooks, giving the reinforcement quantity per cubic meter of concrete, for different structural members, the quantity of reinforcing grills is arrived at. In the absence of any handbook, the estimator can use his/her own previous experience. 

The estimator is expected to use his judgement to select and use a particular value .

  Quantity of Reinforcing Grill 

Exact Method 

The exact method involves actual quantification of the reinforcement grills. This presupposes the availability of detailed design of the structural concrete members and detailed drawings. 

The designer provides the structural drawings along with detailing of reinforcement as per design requirements and according to the provisions of the codes oi-' practice. Normally, this is made available in the form of a Bar Bending Schedule (BBS) using which the estimator prepares a Bar Requirement Schedule (BRS). The total quantity of reinforcement  is arrived at by multiplying the length of bars with its corresponding linear weight. Table 6.4 gives the linear weight of different diameters of commonly available reinforcing steel. 

 Linear Weight of Different Diameters

Note : The linear weight can also be calculated using unit weight of steel as 7850 kg/m3. To calculate the linear weight per metre length (kg/m) of any diameter rbd, multiply the cross-sectional area expressed in sq m by 7850.