2.0. ESTIMATING CONCRETE

Concrete is one of the main components of erecting structures. Estimating concrete would only require one to check on the four main materials in making concrete - cement, sand, gravel, and water.

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Design engineers would always specify the strength of materials needed for the structure. Grade of concrete can be considered through nominal mix or design mix. Nominal mix uses the standard mixes already available with proportions such as class AA. Nominal mixes are limited to the less critical mixes until Grade M25 (IS designation). Design mixes are often for the higher structures because they require higher strength grades. Design mixes are computed in the design phase of the project.

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2.1. DESIGNATION OF CONCRETE

Concrete strength is designated into different standards, but all are based on the concrete's compressive strength after 28 days. ASTM designates concrete in psi or MPa, the strength of concrete. Indian Standards, on the other hand, uses designations but still based on strength.

• ASTM:   

             American Standard of Testing Materials uses the compressive strength either by psi or MPa.

PSI	MPa
	(N/mm2)
2000	13.79
2500	17.24
3000	20.68
3500	24.13
4000	27.58

• Indian Standard:

                    IS works like a counterpart of ASTM. M20 is taken as 20MPa concrete compressive strength after 28 days where "M" means "Mix".

2.2. CEMENT

Cement is bagged according to the weight of 40kilograms and 50 kilograms. Although 40-kg bags are commercially available these days.

2.3. STANDARD USAGE OF NOMINAL MIXES

STRENGTH	APPLICATIONS
4000 PSI	Retaining walls, underwater concrete
3500 PSI	Footings, columns, beams, slab
3000 PSI	Plastering
2500 PSI	Plant boxes, non-critical structures

2.4. NOMINAL MIXES - VOLUME METHOD

Nominal concrete mix maintains a standard of volumes according to the desired concrete strength.

MIXTURE	PSI	MPa	IS	PROPORTION	40-KG CEMENT	50-KG CEMENT	SAND	GRAVEL
AAA	3625	25	M25	1 : 1 : 2	14.0	11.0	0.5	1.0
AA	2900	20	M20	1 : 1-1/2 : 3	12.0	9.5	0.5	1.0
A	2175	15	M15	1 : 2 : 4	9.0	7.0	0.5	1.0
B	1740	12		1 : 2-1/2 : 5	7.5	6.0	0.5	1.0
C	1450	10	M10	1 : 3 : 6	6.0	5.0	0.5	1.0
D	1087	7.5	M7.5	1 : 4: 8	5.0	4.0	0.5	1.0
E	725	5	M5	1 : 5 : 10	4.0	3.0	0.5	1.0

The quantities for cement, sand and gravel make up for 1 cubic meter of concrete.

Example 2.4.1. Volume method: Estimating concrete for a pavement

Example 2.4.2. Volume method: Estimating concrete for footings

Example 2.4.3. Volume method: Estimating concrete for column-footing structure

Example 2.4.4. Volume method: Estimating concrete for a one-storey structure

Example 2.4.5. Volume method: Estimating concrete for a second floor level

2.5. THE GUAGE BOX

When using the volume method, a guage box is used to measure exact amounts of sand and gravel according to the cement bag available and the mix proportion. A 40-kg cement bag requires to use a  ( ) or in SI the guage box dimensions are 30cm x 30cm x 30 cm (width x length x depth).

A 50-kg cement bag is slightly more having  volume requires a dimension of 30cm x 30cm x 37cm.

                        

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Assuming class A mix is to be produced using 40-kg cement (1:2:4). A bag of cement would require two parts of sand and four times of gravel.

When measuring parts, add 25% for sand bulking.

2.6. NOMINAL MIXES - WEIGHT METHOD

When materials are bought by bulk, the weight method of estimating quantities is preferred. Using the proportion for nominal mixes, one can determine the quantities for cement and aggregates using the following steps:

Consider computing the quantity of materials for  of M25 (1:1:2)concrete. 

1. Convert the wet volume (required) to dry volume.

             The estimated volume from the structure is taken as the wet volume of concrete, that is, the combination of cement and aggregates with water. However, the quantity needed to produce this concrete is measured in its dry state.

The standard multiplying factor for converting wet volume to dry volume is 1.54 (although 1.50 or 1.55 sometimes are used)

                               

2. Compute for materials' quantities using the ratio:

             

So, for  dry volume:

•  

              Then using the density of cement =   :

             

              You can have this weight converted into 40-kg bags or 50-kg bags.

           or

               

• 

           Density of sand = .

           

• 

           Density of gravel = .

           

Example 2.6.1. Estimating concrete pavement using the weight method
Example 2.6.2. Estimating concrete footings using the weight method

2.6. WATER-CEMENT RATIO

W/c ratio measures the weight (in pounds, lbs) of water with respect to the weight of cement used in concrete mixes. 0.40 w/c ratio shows 40 lbs of water with every 100 lbs of cement.

Standard w/c ratios:

• Superstructures (higher concrete strength; up to 5600psi): 0.40 to 0.55
• Pavements (driveways) to at least 2000psi: 0.60 to 0.70

Concrete mix will be very stiff at w/c ratio of 0.30 unless using superplasticizers. Similarly, concrete will be wet and weak with w/c ratio of 0.80.

2.7. ALLOWANCE FOR WASTAGE

A certain percentage is added to the estimated quantities to account for the wastage.

• 5% is added to the quantities for members which are bounded by forms such as columns, beams, suspended slab, and the like.
• 10% is added for members with no forms such as footings, slab on grade, wall footings, and the like.

Related Topics:

2.7. ESTIMATING CONCRETE SLABS BY THE AREA METHOD
2.8. ESTIMATING CONCRETE COLUMNS BY LINEAR METER METHOD
2.9. ESTIMATING CIRCULAR COLUMNS
2.10 ESTIMATING CONCRETE PIPES