As per ASTM standards, the minimum concrete cover for steel reinforcement in a footing should be 1.5 inches (38 millimeters).
Footings are usually the first thing you build when starting a new construction project. Because they need to support a significant amount of weight, footings must be made of very strong concrete. It should be at least 3500 psi (pounds per square inch).
ACI 302.1R recommends a minimum thickness of 4 in. (100 mm) for unbonded overlays but, depending on the use, the overlay may need to be substantially thicker. The shrinkage of the overlay concrete should be minimized as curling/warping can be problematic for slabs placed on existing slabs.
In case if the soil type is not provided for the design, it is essential to mention the footing sizes. Besides, the RCC footing should have minimum 150 mm depth for footing on the soil and 300 metres above the piles for footing on the piles. The minimum clear cover for footings is 50mm.
It is important to note that IS 456 mandates that the minimum grade of concrete has to be M20 for RCC structures and buildings. Though some of the lower grades and its strength can be achieved by volume batching, it has to be designed.
Concrete Cover in Footings
The ACI 318-19, "Building Code Requirements for Structural Concrete," specifies a minimum concrete cover of 3 inches (75 mm) for footings that are exposed to earth or weather. This cover is essential for protecting the reinforcing steel from corrosion caused by moisture and soil chemicals.
To find the amount of concrete you need, use our concrete calculator or this simple volume formula: Length × Width × Thickness. Measure your project and multiply the dimensions to get cubic feet or meters. Divide by 27 to convert cubic feet to cubic yards.
The minimum depth of footings below the surface of undisturbed soil, compacted fill material or controlled low strength material (CLSM) shall be 12 inches (305 mm). Where applicable, the requirements of CBC Section 1809.5 shall also be satisfied.
Footings typically must extend below the frost line to prevent shifting during freeze-thaw cycles. Dig footing holes about 6 inches deeper than required. Fill the bottom of the hole with 6 inches of gravel and compact the gravel with a 2x4 or wood post.
Calculation of concrete cover for bond
For reinforcement bars the minimum cover for bond is calculated in accordance with EN1992-1-1 Table 4.2N as: cmin,b = 1.0⋅Φ, where Φ is the diameter of the reinforcement bar (or equivalent diameter of bundled bars). Therefore minimum cover for bond is cmin,b = 20.0 mm.
Protection from corrosion in the reinforcement is achieved by providing minimum covering of concrete. Cover for the reinforcement must be: 40 mm from unprotected ground. 40 mm from external exposure.
For concrete cast in removable forms that will not be exposed to the earth or weather, and for concrete cast in stay-in-place forms, minimum cover shall be 3/4 inch (19 mm).
Footings should extend to a minimum depth of 12 inches below previously undisturbed soil. Footings must also extend at least 12 inches below the frost line (the depth to which the ground freezes in winter) or be frost-protected.
A concrete mix of 1 part cement : 2 parts sand : 4 parts coarse aggregate (by volume) should be used for footings. Concrete must be placed within half an hour of mixing. Brickwork – Place your concrete into your trench. Work the concrete into awkward places and corners using a shovel.
For every 2 feet of adjustment to the width of the house, add or subtract 2 inches of footing width and 1 inch of footing thickness (but not less than 6 inches thick).
A minimum of 12”inches deep for all footings and the width will be determined by the amount of stories, 12” inches for a single story, 15” inches for a two story and 23” inches for a three story. This is based on a soil bearing capacity of 1500(psf).
The footing is what's actually in contact with the ground, while the foundation is the structure that transfers the load to the earth. A simple way to visualize the difference when comparing it to the human body would be to view the footing as the actual feet of the legs and the foundation being the legs themselves.
Even the best contractors occasionally have to build on a sloppy footing. Ian Geisler, a noted ICF expert who has worked on hundreds of projects across the continent, says that realistically a perfect footing is anything within ¼” of level. “If it's within ½”, then it's workable,” he writes.
Footings generally have a covering of 50 to 75 mm.
The effective depth is determined from the cross-section's geometry as 𝑑 = ℎ − 𝑐 − ⌀𝑠𝑤 − Τ ⌀𝑠 2, where ℎ is the height of the cross-section, 𝑐 is the concrete cover, ⌀𝑠𝑤 is the diameter of the stirrups, ⌀𝑠 is the diameter of the longitudinal reinforcement.
The nominal concrete cover is the distance between the surface of the reinforcement closest to the nearest concrete surface (including links and surface reinforcement where relevant) and the nearest concrete surface.
To determine the size of the footing, a simple formula is used: Structural Load (lbs) / Soil Capacity (psf) = Footing Surface Area Required (sq. ft.)
The material cost for concrete slabs generally ranges between $4 and $8 per square foot, with labor costs factored in. For a typical 30×50 concrete slab that is 6 inches deep, the cost can come to around $9000, but this could differ based on multiple factors such as the location of your building, among others.
For the homeowner the easiest way is for the sq ft of the slab (width x length) divided by 80 equals the cubic yards needed here. For a 12″ x 24″ footing just divide the linear feet by 13 and this is the cubic yards you need here.