What is Cement Concrete?
Concrete can be defined as a composite hardened product that is obtained by the setting of a mixture of cement, aggregates, and water in standard specified proportions. The individual materials when mixed together forms a plastic mixture, which can be mold to any shape. Later, within a specified time period into hardens.
The setting is happening, due to chemical reactions that are carried out, which is called as Hydration Reaction. The hydration products make the concrete into a hard mixture. The conditions for Hydration reaction are:
- Rate of Reaction
- Water Content
- Cement Content
- Admixtures if any
Time Phases in Cement Concrete
Cement Concrete formation have three-time phases. They are classified based on three states of the concrete mixture at three times. They vary based on how extent they have undergone setting.
- Initial Setting Time
- Final Setting
- Progressive Setting Time with Age
Initial Setting Time is the first time phase of concrete initial setting, requires 30 -60 minutes. At this stage, the concrete attains some resistance against its flow. That is, its plasticity is reduced.
Final setting time is the second phase of setting of concrete. This takes about 10 hours. The concrete at this stage will remain solid, without any kind of surface hardness.
The third phase is the strength gain with time and age. This is found to be fast in an initial stage, but the process continues with time. The concrete cast is said to gain its potential hardness and strength, after one month of mixing.
Ingredients of Concrete
The concrete ingredients are:
Cement is the material that binds all the materials in the concrete. It is gray in color. Manufactured in Rotary kiln at high temperature. The raw materials used are calcareous, siliceous and argillaceous materials at definite proportions.
Aggregates are Inert or chemically inactive material. This material is bound together by cement hence forming concrete. Most of the aggregates available are natural aggregates. Now man made aggregates have great scope, as a step towards sustainability.
The aggregates can be classified into:
Coarse Aggregates: Those aggregates that pass through a 75mm mesh sieve and are retained on 4.75mm mesh sieve are classified as coarse aggregates. The maximum aggregate size used commonly for all types of building works are 20 to 25 mm. The resistance towards weather and against deformation under loads are contributed by coarse aggregates. The aggregates vary is size, shape, grade and texture.
Fine Aggregates: All those aggregates that pass through 4.75 ( less than 4.75mm) can be classified as fine aggregates. The voids created by the coarse aggregates are filled up by the fine aggregates. Fine aggregates help in reducing the problem of shrinkage in concrete. Fine aggregates are obtained naturally from river sand, beds, pits. Due to high scarcity, they are made from demolition waste and recycled waste as well as industrial by-products.
Water is a lubricant in the concrete mixture. It performs different responsibilities at different stages. It makes the mixture plastic at the intial stage for proper mixing, then help in hydration reaction and finally for Curing. The water has to satisfy certain qualities before it sis used in construction.
Qualities of Water Used in Construction
- It should have a pH value of 7
- Water should be equivalent to drinking water
- Get Water from a reliable source
- The water should be free from debris, organic matter etc
- Water should not be acidic or alkaline in nature