The Major Ingredients in Cement Concrete
Ancient people used lime, sand, water and stones to construct homes. Over time, we have improved upon the materials and now, it has morphed into the modern concrete. These days, it is the widely used material for construction purposes. At Reitano Concrete Ltd. you can find a variety of concrete options, finishes and products. Many people use ‘cement’ as a synonym for ‘concrete’ but it is not so.
It is a mixture of cement-water paste and aggregates. A chemical reaction called hydration plays a key role in binding cement with all the ingredients. After drying the paste turns into a rock-like mass. The quality of the finished concrete structure depends on the:
- Composition and fineness of the cement
- The mix proportions
- The moisture and temperature conditions
The remarkable traits of cement make it so popular. When dry, it is in a powder form, malleable when it is a paste and strong when hardened.
A cement structure resists weathering, erosion and other natural disasters while requiring very less maintenance. The durability also comes from the fact that steel - rods, bars or mesh are embedded in such a manner in concrete, the structure can withstand compression, bending and tensile stresses. Such concrete is also called as reinforced concrete. This is the reason why skyscrapers, bridges and houses can be all constructed with it.
Selection of Ingredients
Water which has no pronounced taste or odour can be used for mixing water and concrete. Impurities can cause the following problems:
- Longer setting time
- Corrosion of reinforcement
- A weak and fragile structure
The type and size of aggregates also affect the finished structure. Aggregate is a generic term used for particulate material. It can be gravel, crushed stone, sand, slag and geo-synthetic aggregates. They should be free of foreign particles. Thin building-sections require coarse aggregate and large dams can take gravel up to six inches in diameter. Coarse aggregates are particulates in the size range of 9.5mm to 37.5mm in diameter. Finer aggregates are less than 9.55mm in diameter. Similarly, the fineness of sand can vary. Strength of concrete will depend on the voids in it. Fewer voids give stronger structure and vice-versa. You can have graded aggregate to make sure the spaces between bigger particles are filled by particles of smaller sizes.
Ingredients Should Be in Proportion
Cement and other ingredients should be mixed in careful proportions to achieve a durable result. The strength of the cement paste depends on the ratio of water to cement. It can be calculated by dividing the weight of the water to be mixed, by the weight of the cement. Lower ratios are preferred, without sacrificing the workability of fresh concrete. Mostly, the mix is about 10 to 15 per cent cement, 60 to 75 per cent aggregate and 15 to 20 per cent water.
Less than required cement or excess of it can contribute towards a weaker concrete structure:
- When the voids in between the gravels are not filled with a sufficient amount of cement paste, it forms rough surfaces and weak porous concrete.
- When there is an excess of cement paste, the resulting surface is smoother but more prone to cracking.
After mixing all the ingredients thoroughly, it is placed into casts, where it hardens and takes shape. Due to hydration, when mixed with water, all cement particles interlink with each other to form the final hardened structure.
As it stiffens, it should be compacted to eliminate potential flaws like air pockets. To create slabs, the mixture is left to stand until the surface moisture film disappears. Later, a wood or hand float is used to smooth off the surface. Even, but a slightly rough surface is preferred to have a good slip resistance. Steel troweling can result in a smooth dense surface if required.
Curing is Important
The concrete structure should harden enough to resist marring after which, the process of curing can begin. It involves sprinkling water on concrete structures. Burlap or cotton mats are put around the concrete structures to retain the moisture. In colder environments, concrete should be never allowed to freeze during the first 24 hours. Later, live steam can be exhausted around the structure to maintain both heat and moisture. The longer the curing continues, the more durable the structure becomes. It is extremely important in the first month of the concrete’s life cycle.
When construction happens in colder temperatures, enclosures are used to prevent temperatures below 40 degrees Fahrenheit at the job site. Due to the heat capacities of cement, it cannot be heated. But water and gravel can be heated to maintain the concrete temperature as it is delivered. Generally, the ideal time for repairing concrete damage is spring.