Building materials
Cement
Cement is the binder that holds concrete and mortars together. Which is why it plays the most critical role in giving strength and durability to your home. It is used to make concrete for slabs, foundations, beams, columns, lintels, chhajja (sunshades), and mortar for brickwork, plastering, flooring and other such work.
Cements used for domestic building such as your home are basically of three types:
- Portland Slag Cement (PSC) conforming to IS:455. A combination of good quality blast furnace slag (from the iron and steel industry) with clinker (which makes OPC) and gypsum.
- Portland Pozzolana Cement (PPC) conforming to IS:1489. A combination of fly-ash (from thermal power plants) with clinker and gypsum.
- Ordinary Portland Cement (OPC) 33 Grade conforming to IS: 269, 43 Grade conforming to IS: 8112 and 53 Grade conforming to IS:12269. A combination of clinker and gypsum.
Good quality cements have the following features
- Reduced water requirement.
- Improved workability .
- Less permeable to moisture.
- Improved resistance to acids and chlorides.
- Reduced heat of hydration.
- Easier to finish.
- Reduced shrinkage.
- Reduced leaching problems because it is low on free lime.
The colour of cement has no relation to the strength characteristics.
The right cement and its application
Please note that good quality blended cements like PSC and PPC will take more time to set and gain early strength in lower temperatures during winter than OPC, but final strengths at 28 days will be high |
Sand
Proper selection of sand is critical in the durability and performance of your concrete mixture.
It should be:
- clear, angular and hard.
- free from clay, mica and soft, flaky material.
- graded, which means it should be a mix of fine, medium and coarse sand.
- free from contaminants like sea salt.
- consistent in moisture (water) content which should not exceed 7%.
When mixing concrete the moisture content must be taken into consideration.
The price of sand includes three or four components - base cost, transportation, handling and number of intermediaries. Procuring sand in bulk directly from the source will be cheaper. Your neighbourhood dealer in this case is likely to be costlier, except when you need smaller quantities.
Stone Chips
Technically known as coarse aggregates, stone chips are a major ingredient of concrete, giving it strength and solidity. The quality of concrete depends very much on the characteristics of aggregates used.
Stone chips should be angular or round, not flat or flaky
They should not contain marks or layers of any other colour
They should be free from mud and other impurities, which are harmful for concreting.
It is advisable to wash the stone chips before mixing to make it free from dust, dirt and mud.
| Tips for selection Aggregates should be well-graded. Which means these should contain sizes from 5mm to 20mm in proper proportion, so that voids are minimal. This will make a strong and durable concrete. At the same time, this will save on cement (see diagram below). |
Stone chips are generally sold or supplied in multiples of 100 cubic feet (cft). A standard truck generally carries 300 cft of aggregates. This is calculated by multiplying the carrying area of the truck by the average depth of the load, measured by inserting a rod at four to five points to calculate the mean depth. A shrinkage of up to 2% per truck is allowed.
Price is a function of ex-works (quarry) price + transportation cost + handling + involvement of intermediaries. One should get aggregates from the source to get the best price. Always insist on graded aggregates and not on 'pure' ones.
| Graded aggregates-all sizes in proportion give denset concrete
| Single-size 'pure' aggregates-result in voids within the concrete & more cement consumption
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| Caution: You may end up paying more for 'pure' aggregates because aggregates are sold in volumes. So, a given volume of 'pure' aggregates will give you less weight because of larger voids between the chips. |
Reinforcing steel
Reinforcing steel contributes to the tensile strength of concrete. Concrete has low tensile, but high compressive strength. The tensile deficiency is compensated by reinforcing the concrete mass through insertion of plain or twisted mild steel bars. Both branded and unbranded bars are available. It is wise to buy good brands, the names of which are marked on the steel. During construction, make sure that steel reinforcement is provided exactly as the engineering design specifies.
| Precautions: Steel bars/rods should be reasonably clean and free of rust. Bars that cannot be easily bent manually or mechanically should be rejected. Optimum length bars must be chosen to reduce wastage in cutting. To avoid laps, shorter bars must not be accepted. Welded lengths of bars should not be accepted. |
Water
It is very important to use clean, potable water in quality concrete production. Brackish or salty water must never be used. Contaminated water will produce concrete and mortars with lower durability, erratic set characteristics and inconsistent colour.
Bricks
Bricks are distinguished by their base (raw) material and size. Standard burnt clay bricks come in the size 10" x 5" x 3". Modular bricks, rarely used because they are not easily available, come in the size 200 mm x 100 mm x 100 mm (including mortar thickness.) Fly ash bricks, sometimes, also come in modular form.
Conventional bricks have a 'frog' (depressed/raised portion) on one of the larger surfaces bearing the manufacturer's brand. These also provide a good mechanical key for bonding (i.e. lockability) with mortar. The modular bricks do not have the 'frog' on them. Fly ash bricks exhibit almost similar mechanical properties as burnt clay bricks. Exposed brickwork with precise pointing is possible if the shapes are perfect.
How do you recognize good bricks?
They show uniform texture and colour.
When broken, they leave no lumps and grit
| Precautions: Ensure that bricks are not made from saline clay. Look for proper and uniform burning. |
The four broad categories of bricks used in construction:
| 1st Class Bricks
| 2nd Class Bricks
| 3rd Class Bricks
| Jhama/Overburnt Bricks
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Perfect in size/shape
| Not so uniform as 1st class bricks in shape/size/quality of burning
| Much inferior to 2nd class bricks in terms of shape/size and burning
| Absolutely out of size and shape, overburnt, fused with more bricks, with a honeycomb texture
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| Buying Tips Bricks may be purchased directly from the brick fields located close to your area at a lower cost Keep samples for conformity to ordered quality. |
