Concrete pad: the goals and methods of construction

Why fit concrete cushion under the foundation? In what cases is its use justified? What brands of concrete can I use? Is reinforcement necessary? Let's try to find answers to this list of questions.

About terminology

To begin, let's separate the flies from cutlets.

What is customarily called a pillow is not always according to the current regulatory documents.

• The layer of low-quality concrete, which protects the monolithic foundation from the leakage of cement milk into the soil at the filling stage and from the aggressive effects of the soil and groundwater in the future, is called concrete preparation.

In addition: concrete preparation is often one of the lower layers of the cake of the concrete floor along the ground. In this case, it serves to evenly distribute the load on the insulation: concrete protects it from uneven pressure from below with variable density or uneven shrinkage of the soil.

• Actually, a cushion is needed to distribute pressure on the ground over its greater area and to prevent deformation of the precast foundation.

Goals

If the situation with the objectives of the concrete preparation is quite clear, then the purpose of building the cushion should be somewhat clarified. Actually, they are exhaustively described in clause 2.195 of the manual for the design of foundations, which is an annex to SNiP 2.02.01-83. According to the document, prefabricated foundations of rectangular plates should not be used:

And here is the pillow? And here is where: the precast foundation on a concrete pad of sufficient strength will have all the properties of monolithic.

Especially it is necessary to stipulate the situation when the airbag is needed for pressure distribution on soils with low bearing capacity. In this case, the base does not necessarily have to be monolithic: the base plates of PL are designed specifically to perform this function.

An important point: the vertical seams of the precast foundation (including between the FL plates and the FBS blocks) are performed strictly with bandaging. Placing two or more seams on the same vertical is a direct way to the appearance of cracks in the walls.

Строительство

Training

Instructions for the construction of concrete preparation for the foundation or finishing screed more than simple.

1. The bottom of the pit or trench is leveled and tamped as much as possible.
2. On soils of moderate density and heaving soils, a sand bedding with a thickness of about 10 cm is performed.. Sand is spilled with plenty of water for maximum shrinkage. On dense sandy and sandy soils, bedding is not needed; on rocky ground, for obvious reasons, the pillow itself is not needed.
3. A layer of waterproofing is spread across the bedding (as a rule, a strip of dense polyethylene with a 10-centimeter overlap).
4. On waterproofing the concrete layer is laid out (without reinforcement) 100 mm thick. Минимально допустимая, согласно СП 50-101-2004, марка бетона по прочности - М50. Обычно используется M100, цена куба которого даже несколько ниже за счет использования крупнофракционного заполнителя (с прочностью М50 производятся лишь растворы).

When self-preparing concrete mix, you can use the table of proportions:

1. Further work continues approximately a week after the concrete has been set to its minimum strength.

Pillow

How are reinforced concrete pillows constructed for precast strip foundations?

Payment

It should be based on additional data - the bearing capacity of the soil and the mass per meter of the wall. Suppose that a meter of a wall weighs 7 tons, and the carrying capacity of the soil is 1 kgf / cm2. Then the area per meter of pillow should be equal to 7000/1 = 7000 cm2; width 7000/100 = 70 cm.

Note: non-circular and fractional values ​​are rounded up to a multiple of 10 centimeters. The stock on the carrying capacity will never be over.

The thickness of the cushion is usually 300 - 400 mm.

Reinforcement

It is made of reinforcing rods with a thickness of 12 mm. Mesh cell - 200 mm. The reinforcement cage is made double-layered; the nets should be about 50 mm from the surface of the monolith.

Для крепления стержней используется отожженная вязальная проволока. Reinforcement нигде не должно выходить на поверхность.

Bedding

On heaving soils and in this case, it is strongly recommended to pour the bottom of the trench with sand or gravel. How to ensure the shrinkage of sand - we already know; the simplest tamper for rubble can be built with your own hands from a piece of log with a handle nailed to it.

Laying thickness - 10 - 15 cm.

Fill

And in this case it is better to pre-lay on the bedding layer of polyethylene, which prevent the outflow of cement milk. Then the pillow is poured with two layers with the obligatory splicing of each layer.

During breaks in work, a new section is poured either before setting previously filled in, or after gaining a strength of at least 1.5 MPa (which roughly corresponds to 3-4 days from the moment of laying at a temperature of 20 ° C).

Conclusion

We hope that our recommendations will help the reader in self-construction. As usual, the video in this article will offer him additional subject matter. Successes!