Sunday, August 06, 2006
Good construction practices
WHAT ARE good construction practices? A clear understanding of these will help you go about building your dream house the proper way.
Laying the foundation
Mark the boundaries prior to excavation. Use a right angle and line-string to mark perpendicular lines at various distances for foundations & columns, as shown in your design drawings. Always maintain the depth and level of the excavation with a water level and plinth level.
Sometimes, the underground water seeps into the foundation pits. Drain out the water before the foundation concrete is cast.
In case of black cotton soil or soil types with low load bearing capacity, use pile foundations or continuous foundations.
Fill in the trench up to the floor level, when the foundation is cast or plinth is constructed, by soil filling. Filling should be done in layers of maximum 6" thickness, sprinkling each soil layer with water before compaction, using a ram or plate compactor. If good quality clay is not available, use sand for filling with adequate watering.
Steel rods
Plain concrete without steel or other reinforcements is strong in compression, but weak in tension. Steel is one of the best forms of reinforcement to take care of stress and to strengthen concrete to bear loads. It forms the basis of the concrete structure and is generally laid out in a circular cross section. Steel rods should be free from loose rust, oil pints, mud, etc., and cut, bent and fixed properly. The bars should be placed and maintained in position by using proper cover blocks, spacers, supporting bars, laps, etc.
For general house construction, 6 to 20 mm diameter bars are used. Thermo-mechanically treated (TMT) and corrosion-resistant steel (CRS) bars with added features have been introduced recently. Remember to purchase ISI certified steel bars, from reputed manufacturers, regardless of the size of you choose.
Lap length
Lap length is provided at the junction where an old bar terminates and joins the new extension bar. Lap length of about 50 times the diameter of the bar is considered safe. All laps should be staggered and not provided at one place. A maximum of 50 per cent bars should be lapped per section.
If the required lap length is not available at the junction owing to space and other constraints, couplers or the correct method of welding should be used to join the bars.
This is the additional length of steel of one structural member necessary for insertion with another member at the junction. For example, the beam with the pillar, or the pillar with the foundation. The required length is similar to the lap length mentioned above or as per design instructions.
Cover blocks
Cover blocks are placed to prevent the steel rods from rusting due to exposure to air, and to fix the bars in place. Sometimes during the concreting activity, the covers tend to get displaced.
To prevent this, tie the cover block using thin steel wires called binding wires to the steel bars. Covers should be made of cement and sand mortar (1:3, 1 part cement and 3 parts sand by volume). The blocks could be cubical or cylindrical in shape. Normally, cubical cover blocks are used. As a thumb rule, a minimum cover of 2" in footings, 1.5" in columns and half inch to three-fourths inch for slabs and beams is recommended.
Shuttering
The formwork or shuttering should be rigid and closely fitted, with sufficient strength to support wet concrete.
It should be leak proof to prevent the cement-sand-water mix from leaking. The face of the formwork should be treated with form release agents available in the market or a mixture of diesel and grease.
Removal of shutters (deshuttering) results in the concrete being subjected to loads and exposure to the atmosphere. If the ambient temperature is above 15 degrees centigrade and the workmanship is good, the following striking/deshuttering period is considered safe:
For vertical formwork of slabs, beams and columns - 24 hours.
Total deshuttering, including supports to slabs (a) For spans/length up to 4.5 m - 7 days (b) Over 4.5 m - 15 days
Total deshuttering including supports to beams (a) For spans/length up to 6 m - 14 days (b) Over 6 m - 21 days
Concreting and Curing
Use wooden or steel measurement boxes to ensure consistency in the mix.
Since at small sites, batching is always done as per bag of cement, the measurement box dimension is in multiples of 1 bag of cement, or 35 litres (1 bag of cement equals 35 litres in volume). A convenient size of the measurement box is 350 x 250 x 400 mm (35 litres). Sand and stone aggregates should be loosely filled in and levelled without compaction. For example, when preparing a mix of 1:1.5:3, you need 1 bag of cement, 1.5 boxes of sand and 3 boxes of stone aggregates - two boxes of 20 mm aggregates and 1 box of 10 mm aggregate.
Use a five-litre measurement can to measure water correctly.
Concrete mix
When mixing concrete by volume ensure that the right measurements and ratios of the different ingredients are strictly followed. Account for the surface water of aggregates. A mixer should be used for a minimum of two minutes to ensure uniform distribution of the material or until the mass is uniform in colour and consistency. Avoid hand mixing.
Placing
Concrete should be placed in the final position before setting starts. In general, concrete should be poured within 30-60 minutes of preparation. Addition of extra water to make the concrete reworkable is not recommended.
Furthermore, please ensure that the shuttering position is not disturbed. Concrete should be poured in layers without any break of continuity, with the maximum free fall of concrete not exceeding 1.5 m.
Compaction
Compaction, especially around steel bars, and the corners of the formwork should be carried out. Use of vibrators will ensure good compaction. Remember, good compaction will ensure high strength and impermeable concrete.
Curing
Curing is the process of preventing moisture loss from the concrete or mortar while maintaining the required humidity within the structure to ensure high strength and durability.
