Highlights on Dalmia Cement's workmanship

• Concrete strength and durability depends upon the quality of its ingredients like cement, sand, aggregate and water. Usually, the ingredients are proportioned volumetrically relative to one cement bag (50kgs by weight or 35 lit by volume). In order to correctly proportion the volume of the sand and aggregates, measuring boxes, having dimensions 35x25x40cms should be used.
• The shuttering should be sufficiently strong to withstand the weight of concrete, reinforcement steel, weight of persons (involved in placing and compacting concrete) and impact loads(due to concrete placement and vibrations of vibrator).
• For footings, columns, beams, slabs etc., the minimum grade of concrete, exposed to mild environment, as per IS 456-2000, is M20 for which the volumetric mix proportion is 1:1.5:3.
• Concrete strength and durability depends mainly on the water-cement ratio used in the concrete mix. It is recommended that the mixing water should be properly measured while preparing the concrete mix. The maximum water that can be added per bag (50 kg) of cement is restricted to 27.5 liters.
• Only that quantity of concrete mix should be prepared that can be consumed within 30-40 minutes. If concrete is not deposited and compacted in forms within the stipulated time, initial setting of the cement takes place and results in reduced workability of concrete.
• Good concrete is homogeneous and is prepared by intimate and thorough mixing of all ingredients. To ensure homogeneity of concrete mix, it should be prepared in mechanical mixers and mixed for at least 2.5 minutes. Under inevitable circumstances, if the concrete is to be made by hand mixing, it should be done on a clean and watertight platform. Cement content of concrete should be increased by 10%, whenever hand mixing is done.
• The concrete mix should be placed in the forms gently and ensuring that it is not dropped from a height of more than 1.5m.
• Freshly prepared concrete contains entrapped air, which should be removed by compaction either by vibration (with the help of needle or plate vibrators) or rodding (with the help of a 16mm bullet nosed rod of 2’ length). Proper compaction of concrete is extremely important because for every 1% voids in concrete, its strength reduces by 5%

• Prior to concreting, it should be ensured that reinforcement steel has been placed properly, as per the drawing issued by the structural engineer. Proper cover to the reinforcing steel is to be maintained by the use of cover blocks (please refer to the section on cover blocks).
• Concreting should not be done in extreme (too hot / too cold) climatic conditions.
• In order to avoid cold joint formation, concreting of slab should be done in one continuous pour (i.e. completed in one stretch without any joints). If joints are necessary, expert advice is to be followed.

• The proportions of cement:sand should be 1:3 to 1:4 (depending on the severity of exposure).
• If the plaster thickness is more than 12.5 mm it should be done in two coats. Prior to the hardening of the first coat, it should be suitably roughened by wire brush or broom to provide mechanical key for the second coat.
• The thickness of the second coat should be between 4-6 mm and should be finished with a wooden float.

In order to ensure that the plaster surface is smooth and uniform both horizontally and vertically, plastering should be done with the help of dots and screeds, laid with help of plumb bob.

• For walls the proportion of cement : sand is 1:4 to 1:5.
• For slab bottom the ratio of cement : sand should be 1:3.
• To ensure proper bond of plaster with concrete surfaces, the concrete surface should be hacked immediately after removal of formwork. Hacking acts as a mechanical key which ensures proper bond of plaster with concrete surfaces.

Slab bottoms and beam soffits should be properly cleaned prior to plastering, so that they are free of any remains of shuttering oil or loose dirt and clay.

Cement is necessary for binding construction materials like aggregates, sand, bricks and reinforcement steel. Dry cement can’t bind these materials. The binding action of cement is due to its reaction with water forming cement gel, which, when cured properly, over a period of time hardens into a rock like material that holds all construction material together.

Gel formation of cement is known as hydration, which is supposed to be completed in 28 days. If water is lost from concrete / mortar before this period then hydration of cement remains incomplete. As a result the concrete/mortar does not gain its potential strength. To prevent this loss of moisture by evaporation, we have to keep the concrete/plaster surface moist by continuous curing.

For members like columns, walls, beams with vertical surfaces, maintaining moisture on the surface is difficult as water does not stay for long on vertical surfaces. In order to ensure proper curing of these, the vertical surfaces should be covered with wet gunny (jute) cloth, wrapped around the member and kept wet continuously for at-least 10 days. The wrapping should be wetted every 2-3 hours during the day.

For proper curing it should be ensured that the concrete surface is kept continuously moist throughout the curing period.

The hardening of slab/floor concrete takes 7 to 8 hours. Once the concrete surface has hardened, water should be flooded on the slab / floor surface and should be kept for 10 days. The flooding of concrete on the surface ensures the concrete does not lose its internal moisture, ensuring proper hydration of cement . As a result, the concrete gains its full strength.

Prompt and effective curing from the first day, increases the strength of concrete from 25-30% and also protects concrete from cracks due to hot weather conditions.