Concrete is a mixture of cement, water, coarse and fine aggregates. while the reinforce concrete is the mixture of all this components cast with reinforcement rod.
Concrete is extensively used in modern Building constructions and civil engineering .The cost of concrete is also influenced by some other factors apart from its original cost. These factors includes:
- The method employed in executing the work.
- specification of material, Quality of the products and the nature of work.
Proportioning of concrete
The proportioning of concrete is by volume or weight .In the past materials constituting of concrete is being proportioned on bases of volume only .But in recent days the proportion materials constituting concrete materials is based on weight with carefully controlling of water/cement ratio (WCR) have been observed to be producing a high-quality concrete that is much more consistent as we can control more on its fractional quantities.
I will then discourse the two methods by which you can estimate the material cost of a cubic metre mixture of concrete ether by volume or by weight.
The estimator should be able to notice the specified concrete proportion and build-up the price in accordance.
Mixing by volume
By stating a concrete ratio of mix by volume we refer the proportion as dry material because of the moisture content (mc) of the aggregate material .Allowance must be made against bulking, but by looking at the estimate the bulk-age may be ignore if coarse aggregate such as Shingle, Clean Gravels, Broken Stones are involved, since their moisture content is very minimal .But the washed sand and all-in-ballast etc are appreciably due to its moisture content or if it were delivered in a moist state on site. In this case there should be an allowance for the bulk-age during computation of the amount of materials required per cubic metre of Concrete.
Experience has shown that bulk-age are approximates to 15 and 25 per cent in the case of all-in-ballast and washed sand respectively. So that bulk-age should be allow for
Estimations
The Contractors view it as important for bulk-age to be allowed, both in estimating and carrying out the main job .If not the estimated cost of the concrete per cubic metre is too low and the concrete yield per kg of cement is less than what it ought to be.
Mixing by weight
From our above explanations proportioning by weight or weigh batching the materials do produce concrete that is more consistence than the one mixed by volume .through careful grading of the aggregates and properly controlling the water/cement ratio (WCR) considering the moisture content in the aggregates, then best-quality concrete of ultimate high compressive strength is obtained.
Owing to these facts mixing by weight is constantly used when large volume of concrete is needed .for example in the construction of Airport-runways, Motor ways, Bridges etc.
In carrying out this works the aggregates are properly graded and water/cement ratio is determined on site by test, and allowance is being made for the weight of water in the aggregates with hope to achieving the required compressive strength and workability of
The concrete mixture
By estimation to determine the quantity of the required materials per cubic metre of concrete is best computed by absolute volume method.
The estimator should be aware of the specific gravity of the materials concern and this specific gravity represents the fair average values of the materials used.
The estimator may then compute the required materials per cubic metre in this manner.
Quantities of materias required per cubic metre of concrete.
For a concrete mix of 1:2:4, by volume and weight using. Ordinary Portland cement, washed sand, and shingle.
Example 1 mixing by volume 1:2:4 mix water/cement ratio 0.55
- Cement = 1442 kg per m3
- Dry sand = 1602 kg per m3
- Moist sand = 1282 kg per m3
- Shingle = 1442 kg per m3
Weight of 1m3 of ordinary Portland cement, sand, and Shingle concrete = 2335 kg
To compute the required cubic metre of materials of hardened concrete, reduce the terms kg, weight, thus:
- Cement ¼ x 1442 kg = 361 kg
- Dry sand ½ x1602 kg = 801 kg
- Shingle 1 x 1442 kg = 1442 kg
- Water 1/8 x 1442 kg = 180 kg
- = 2784 kg
The required materials are then.
- Cement 361/2784 x 2355 kg = 298 kg = 0.298 tones
- Sand 801/2784 x 2355 kg = 678 kg = 678/1282m3 =0.529 m3
- Shingle 1442/2784 x 2355 kg =1212 kg = 1212/1442 m3 = 0.841 m3
- Water 180/2784 x2355 kg =154 kg = 154 litres
Example 2 : mixture of 1:2:4 by weight, water/cement ratio = 0.55 the materials required for 50kg of cement thus:
- Cement 1 x 50 kg = 50 kg
- Sand 2 x 50 kg = 100 kg
- Shingle 4 x 50 kg = 200 kg
- Water 0.55 x 50 kg = 27.5 kg
The Absolute volume or Minimum volume of materials with all the voids removed is gotten by the equation:
- Absolute volume, = weight of material in kg / specific gravity x weight of 1m3 of water
- = weight of material in kg / specific gravity x 1000 kg
- Thus : = Cement = 50/ 3.15 x 1000 = 0.016 m3
- Sand = 100 /2.60 x 1000 = 0.039 m3
- Shingle = 200 / 2.25 x 1000 = 0.079 m3
- Water = 27.5 / 1000 = 0.027 m3
- = 0.161 m3
Then the materials required per cubic metre of concrete from calculation are.
- Cement = 1.0 x 50 / 0.161 = 306 kg = 0.306 tones
- Sand = 1.0 x 100 / 0.161 = 612 kg = 0.612 tones
- Shingle = 1.0 x 200 /0.161 = 1243 kg = 1.243 tones
- Water = 1.0 x 27.5 / 0.161 = 172 kg = 172 litres.
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