Some vital short notes for civil engineering students

Least Cover for several RCC members

Slab = 20 mm
Beam = 25 mm 
Column = 40 mm
Footing = 50 mm

Exact unit weight of steel

The unit weight concerning steel should be 78.5 KN/m3

Proportion of cement, sand and aggregate for different types of concrete mix grades

M10 = 1:3:6
M15 = 1:2:4
M20 = 1:1.5:3
M25 = 1:1:2
M30 = 1:1:1

Exact unit weight of concrete

It is based on the type of aggregates as well as the amount of voids. With adherence to IS : 456-2000, the unit weight of PCC is 24KN/m3 and RCC is 25KN/m3.

Crank Length In Slab

The crank length should be 0.42D. Here, D denotes depth of slab. It is obtained by deducting bottom cover from top cover i.e. Top Cover – Bottom Cover.

Initial & final setting time of standard cement mix

Initial setting time for standard cement mix should remain approx 30 minutes for different types of cements. For masonry cement it should be 90 minutes. Final setting time of standard cement mix should be 10 hours maximum. For masonry cement, it should not go beyond 24 hours.

Compressive Strength Of Brick

For first class brick, the strength should be 105 kg/cm2
For second class brick, the strength should be 70 kg/cm2
For fire brick, the strength should be 125 kg/cm2

The weight of steel bar in reinforcement

The weight of bar should be kg/m = D2/162.2, here dia means diameter of bar in mm.

Steps for design mix concrete according to IS10262

Target denotes strength fm = fck+1.65 σ, σ = standard deviation
Grade of concrete = N/mm2
Standard deviation for M10, M15 = 3.5
Standard deviation for M20, M25 = 4
Standard deviation for M30 to M50 = 5

Strength of concrete

Flexural strength of concrete (modulus of rupture) fcr = .7√fck

Split tensile strength of concrete (fct) fct = 66fcr

Minimum size of column footing

Minimum size of footing for 9” x 9” column should not be under 1.5 x 1.5 feet

Minimum size of footing for 1’ x 1’ RCC column should not be under 2’ x 2’

Standard Height Of Building Floor

The height should be 11 feet but should not be under 10 feet.

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Published By

Rajib Dey

www.constructioncost.co

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Why water cement ratio is vital in concrete mix?

In concrete there are different types of materials like cement, sand, Coarse aggregate and water which are combined in exact ratios. This ratio is provided at either as a nominal mix or by design mix.

All the materials contain definite properties and the strength of concrete is mostly dependent on these properties.

Cement is employed as a binding material in concrete because of the best adhesive and cohesive property whereas water is the cheapest and most vital of concrete. Once these two materials are blended, a chemical reaction alias hydration is produced with a sticky and thick gel that contains a binding property to tie all aggregate in form of hard and strong material. So, cement and water both can be used for binding all the materials of concrete jointly.

When cement and water are applied according to their ratio, it is known as water-cement ratio. Water-Cement Ratio means the ratio of the weight of water to the weight of cement.

Water/cement ratio with adherence to IS 10262 (2009) for the concrete mix differs from 0.4 to 0.6. 0.7 w/c ratio is also applicable for pumped concrete. 0.5 w/c ratio signifies that for each 100 kg of cement, 50 lit. of water is added. When w/c ratio is raised, the amount of water is reduced.

In technical terms, the cement should contain 23% of water for the hydration process and 15% for development of pores and volume increment. So, minimum 38% of the water by weight of cement should be maintained.

There should be adequate amount of water in the concrete mix to maintain full hydration of cement paste and eliminate capillary pores there. The minimum quantity of this water should be similar to 0.38 w/c ratio. Accordingly, if water-cement ratio is under 0.38, complete hydration can’t be performed. If the ratio is surpassed, this concrete mass turns out to be porous and penetrable and if it is less, concrete turns out to be dry and harsh and may not bond and attain full strength.

For work-ability purpose, this water-cement ratio should remain as 0.4, but its selection is based on the requirement of work-ability in various exposure conditions and nature of work. At a water-cement ratio of 0.4, the mix contains adequate water for hydration and simplifies the concreting process for mixing, placing, transporting and compacting since water functions as a lubricant in the concrete mix and decreases the internal friction among particles of concrete.

At this water-cement ratio, the ‘gel’ that is produced remains in good physical structure and solid state. At greater water/cement ratio the ‘gel’ that is produced remains in the poor physical structure. It produces voids throughout subsequent evaporation and develops into more porous. As a result, the strength gets lower in due course of time.

Computation of water quantity for concrete

While measuring the quantity of water for concrete mix, initially determine the cement content for the volume and w/c ratio. Suppose, there is 80 kg of cement for concrete mix and w/c ratio is 0.45.

Required amount of water = Cement volume X w/c ratio
So, essential water quantity is 80 kg X 0.45= 36 lit.

To get further information, go through the following link gharpedia.com




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Published By
Rajib Dey
www.constructioncost.co
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