Preparation of concrete for footings of a building with a maximum of two storeys can be carried out in the field without the need for extensive calculations. It involves selecting suitable constituent materials and properly mixing them to produce concrete that can support imposed loads.
Ordinary portland cement, clean and strong coarse aggregate and sand are sufficient to construct a footing strong enough to bear the different loads acting on the building.
A mix of 1:2-3/4: 4 (1 part cement, 2 ¾ parts sand, and 4 parts gravel) or 1:1:1½ is the standard for preparing concrete for the footing of a two-storey building. One can test the concrete mixture to determine whether the right quantity of water is added or not.
Finally, pour concrete within 45 minutes of the mixing process using wheelbarrows or any other suitable equipment to carry out the pouring operation.
Selection of Materials for Concrete Footing
1. Cement
Ordinary portland cement is commonly suitable for preparing concrete for a two-storey building's footing. The normal portland cement should be used within six months from its production and stored in a cold, dry, and shaded place. The portland cement is available from most of the cement manufacturers.
One should discard cement bags that contain hard cement lumps which are difficult to break. For areas with heavy rainfall or tropical regions, it is recommended to use hydrophobic cement as it can be stored in damp conditions for a longer period of time.
2. Sand
The sand used to make concrete footing for a two-storey building needs to be clean. Prevent using more than 3 parts of sand for 1 part of cement; otherwise, low workable fresh concrete and high permeable hardened concrete are produced. A very small amount of sand in the concrete mixture could result in shrinkage cracks.
The sand is responsible for the plasticity of the concrete mix. One can test the cleanliness of sand by adding a 5 cm sample in a container with water.
Shake the jar for one minute and then leave it for an hour. The dirt layer precipitated on the sand should be less than 6 millimeters; otherwise, the sand is considered as dirty.
3. Gravel
The gravel for footings needs to be clean, strong, durable, chemically inactive, and particle sizes should range from 6-18 mm. It is responsible for the strength of the concrete mixture. Concrete mix proportion of 1 cement: 2 ¾ sand: 4 gravel or 1:1:1½ is typical for preparing concrete for footings.
4. Water
The use of potable water is the best and safest option. The water used to make concrete needs to be clean and free from dirt, unwanted chemicals, or rubbish that may affect the quality of concrete. To read more about the quality of water for concrete, please click here.
Mixing Concrete Materials
Mix the footing concrete materials on a flat surface area that does not absorb water. Concrete mix proportions are expressed in terms of unit volume. It is easy to use unit measurement on-site such as containers or wheelbarrows to ensure mixing of the correct amount of materials.
When manual mixing is done, spread the right quantity of sand on a flat surface, add a specified volume of cement to the sand, and then mix them adequately until the mixture gets a uniform color. After that, add the correct amount of gravel to the mix of sand and cement and blend them properly. Make a hole in the middle of the mixture and add the right quantity of water. Finally, turn ingredients over while pushing them towards the center of the hole.
One can test the amount of water in the concrete mix by stepping over it. If the foot settles around 5-6 cm deep into the mix, then it is considered to have the correct quantity of water. However, if the mix settles at the ankle level or higher, excess water has been added.
Excess water in the concrete mix leads to lower strength, shrinkage crack development, and reduced durability. However, too little water creates weak points or air spaces in concrete. One should be aware that manual mixing is one of the least expensive ways to produce concrete.
Pouring of Concrete
- Use wheelbarrows or any other suitable tools to pour concrete. The tool needs to be waterproof to avoid leakage of water out of fresh concrete.
- Pour the fresh concrete within 45 minutes of the mixing process.
- Once concrete placement starts, it should continue till the end of the work. If the concrete sets, then it would be hard to manipulate it.
- Avoid over vibration while delivering the fresh concrete to its final position to prevent the separation of sand and gravel from the mixture.
- Make concrete flow to its final position and avoid the free fall of concrete.
- Work the fresh concrete up and down at its final position to break air pockets.
- If the depth of the footing is less than 30 cm, pour the concrete at once.
- If the depth of the footing is more than 30 cm, then place the concrete in layers with a thickness of 15-20 cm.
- When the concreting process is stopped at the end of the day, it is recommended to use a temporary stop at the end of the poured section. Roughen the edges of the old sections and spray cement-water paste on them, and then resume the placement of concrete.
FAQs
For a building with a maximum of two storeys, normal Portland cement is sufficient. Any sand and gravel that are clean and strong are suitable for use.
One can test the cleanliness of sand by putting 5 cm of sand in a container with water. Shake the jar for one minute and then leave it for an hour. The dirt layer precipitated on the sand should be less than 6 millimeters; otherwise, it is considered as dirty.
Spread the right quantity of sand on a flat surface, add a specified volume of cement to the sand, and then mix them adequately until the mixture gets a uniform color.
After that, add the correct amount of gravel to the mix of sand and cement, and blend them properly. Make a hole in the middle of the mixture and add the right quantity of water.
Finally, turn ingredients over while pushing them toward the center of the hole.
One can test the amount of water in the concrete mix by stepping over it. If the foot settles around 5-6 cm deep into the mix, then it is considered to have the correct quantity of water. However, if the mix settles at the ankle level or higher, too much water has been added.
The addition of excess water to the concrete mix leads to lower strength, shrinkage crack development, and reduced durability.
Excess cement in concrete generates high heat of hydration which induces thermal stress in concrete and crack development. The development of shrinkage cracks is proportional to the amount of cement paste in concrete, provided the appropriate precautions are not followed.
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