Understanding concrete batching and applications

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To batch a concrete means to measure out the quantity of materials required for the concrete by volume or weight. We know that concrete is made up of 4 plus (+) constituents. Four (4) basic constituents are cement, sand, aggregate and water while the “plus” could include other additives to improve the concrete properties. The aim of batching either by volume or weight is to ensure same standard and quality.

Volume batching: In volume batching, concrete mixes are often quoted by ratio such as 1:2:4 (cement: fine aggregate/sand: coarse aggregate/gravel). Assuming we want to describe 1:2:4 batch using 50kg cement for instance, 50kg cement has a volume of 0.033m³. Therefore, fine aggregate would be 2 x 0.033 (0.066m³) while coarse aggregate would be 4 x 0.033 (0.132m³). in the measurement for volume batching, same equipment such as gauge box, headpans, wheel barrow etc should be used to measure out the quantities.

Important notes:

(i) While batching, allowance should be made for bulking of damp sand. The percentage bulking could be from 25% to 331/3%.

(ii) It is more ideal to mix materials dry before adding water.

Weight batching: This method is usually more accurate because the risk of variation between different batches obtainable in volume batching is reduced and the bulking effect of sand is also greatly reduced. This method is often more suited to self-loading concrete mixers that have the capacity to measure out and batch each content automatically under the press of buttons.

How to load concrete mixer to avoid clogging of hopper

1. First, put the coarse aggregates: This is necessary because the coarse aggregate tends to push other materials out and leaves the hopper clean.
2. Next, put cement: This is put in second place because if it is put last, the fine cement particles could be blown away if the cement is put last.
3. Lastly, put the sand or fine particles: This is put last to stabilize the fine light weight particles of cement powder.

Typical densities of constituents of concrete

Cement: 1440kg/m³

Sand: 1600kg/m³

Coarse aggregate: 1440kg/m³

Water-cement ratio

Water has two functions in concrete viz:

1. To start the chemical reaction which causes the concrete to set into solid mass.
2. To give the mix workability so that it can be placed, tamped or vibrated into the required position.

Generally, very little water is required to set concrete. This is approximately 0.2 W/C ratio. Other surplus moisture evaporates leaving minute voids. More water is usually added to increase workability but this affects the strength of concrete. W/C ratios of 0.4 to 0.5 are usually adequate. Increasing the W/C ratio to 0.6 create 3 times as much voids in concrete. It is very important to control W/C ratio in concrete.

Concrete grade mix

Concrete grade mixes include: C 7.5, C10, C15, C20, C25, C30, C35, C40, C45, C50, C55 and C60; F3, F4 and F5; IT2, IT2.5, and IT3 where:

C = characteristics compressive strength (N/mm²) at 28 days

F = Flexural strength (N/mm²) at 28days

IT = Indirect Tensile strength (N/mm²) at 28 days

Visit the post CONCRETE MIXES to see more (including the categories of mixes: standard, prescribed, designed and designated)

Use of various grades of concrete compressive strength

Grades C7.5 and C10 are used as unreinforced plain concrete

Grades C15 and C20 are used as plain concrete or if reinforced containing lightweight aggregate

Grades C25 are used as reinforced concrete containing dense aggregate

Grades C30 and C35 are used as post-tensioned reinforced concrete

Grades C40 to C60 are used as pre-tensioned reinforced concrete

Concrete supply

Concrete supply depends on demand or the rate at which mixed concrete can be placed. Concrete can be supplied in small batches of 0.1m³, medium to large batches of 0.1m³ to 10m³ and large batches with mixers capable of producing 6m³ of concrete in one batch.

Important rules for concrete placement

1. Concrete should be placed within 30 minutes of mixing to prevent undue drying.
2. After initial mixing of concrete, no further water should be added.

Reference

Chudley, R. and Greeno, R. Building Construction Handbook. 12^th Edition.

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