- Appointment of types of foundations
- Formwork construction of the aboveground part
- Calculation and binding of reinforcement
- Pouring concrete, shrinkage
- Protection and insulation
Sooner or later, any builder is faced with the need to organize work to prepare the foundation for the construction of a house. With a lack of practical experience, time should be devoted to studying the features of the processes of formwork construction, reinforcement binding, pouring and protecting concrete foundations.
Appointment of types of foundations
Let’s face it: not all plots have conditions that are ideal for building a house. At first glance, everything looks pretty rosy: good ecology and appearance, minimum noise, proximity to natural nature. But sometimes the results of geological surveys indicate the significant engineering complexity of the foundation for a house. Fortunately, at the moment, quite a few varieties of foundations have been invented, and even for very difficult conditions of geomorphology.
There is no special point in talking about strip foundations. They are designed for stable dry soils of high density and are arranged according to the simplest scheme. However, strip concrete foundations can have varying degrees of burial depending on climatic conditions. You can also distinguish several typical sections of the tape: beam, T-shaped, trapezoidal, etc..
Types of strip foundation: a – recessed; b – shallow; в – T-shaped (T-shaped); g – trapezoidal rubble. 1 – sand pillow; 2 – concrete; 3 – fittings; 4 – rubble stone; 5 – backfilling; 6 – blind area
Pile and pile-grillage foundations are used on soils that do not have sufficient supporting capacity even with normal depth. In this version, several device options are possible, the complexity of which depends on the number of storeys and the architectural features of the building. In the general case, such a foundation is a kind of shallow tape, only the tape does not perform a supporting function. It distributes the load on the piles, which rest on a deeper and denser soil layer.
Pile-grillage foundation: 1 – pile support; 2 – waterproofing; 3 – fittings; 4 – grillage
The third type of foundation for a house is slab structures. They are used when high-quality thermal insulation is needed or when erecting buildings on special categories of soil: highly plastic, heaving, water-saturated and sandy. An ordinary slab is unremarkable, however, in order to save materials, it can have a cross mesh of reinforcing ribs, or contain engineering communications or channels for them.
Slab foundation: 1 – soil; 2 – sand and gravel cushion; 3 – concrete preparation; 4 – foundation slab; 5 – reinforcing cage
Almost all types of foundations require excavation. Firstly, the walls of the trenches are convenient to use as formwork for the underground part. Second, the deeper the reference plane is, the less horizontal cross-sectional area is required for sufficient stability. The depth of excavation is determined by the results of engineering and geological surveys.
It is possible to prepare trenches for the foundation both manually and using an excavator method. In the latter case, the bucket width should be only 75–80% of the belt width at the top. In conclusion, there is always a manual cleaning of the walls and bottom. After that, the bottom of the trench is prepared with an incompressible and non-porous bedding: first sand, then fine gravel. The minimum thickness is 20–25 cm in total, but the backfill can be thicker depending on the size of the foundation and hydrogeological conditions.
Preparation can also be performed with concrete of the M100 or M150 brand. This helps to more accurately withstand the lower protective layer of the reinforcement, as well as more technologically advanced to close horizontal and vertical waterproofing in the presence of a basement. After preparation of the bottom, the walls of the trench are covered with plastic wrap, this is done to reduce the release of cement milk from the mixture.
Formwork construction of the aboveground part
One of the main difficulties in the construction of the foundation is the construction of high-quality and rigid formwork. With an irresponsible attitude at this stage, it is impossible to withstand the geometry of the foundation, which can lead to serious problems in the construction of load-bearing walls and decoration. Also, quite often, due to insufficient strength, the formwork breaks, which ultimately results in damage to a large volume of expensive material..
In total, there are three types of formwork: panel, sheet and non-removable foam. The difference between the first two varieties is not high: in one case, the enclosing surfaces are formed with shields from boards of 25 mm or thicker – on average, 1 mm is added for every 5 cm of the foundation height. In the sheet version, the decks are formed with plywood, OSB or chipboard, reinforced on the outside with stiffening ribs. Mostly waterproof materials with a thickness of 14 to 20 mm are used. The need to use sheets is dictated by high requirements for the smoothness of the foundation surface, which is important both during finishing and when installing hydro and thermal insulation. Additional advantages are the convenience of work, high speed of assembly and dismantling, the possibility of multiple use (movable formwork) and subsequent use, often there is an economic benefit.
When assembling plank decks, the boards are knocked apart and reinforced with vertical inserts from a bar. The thickness of the latter is 2 times greater than the board, the width should be sufficient to overlap the butt ends of the order of 80–100 mm on each side. If the shields have a height of more than a meter with a considerable length, they are additionally reinforced with horizontal ribs of the same section as the vertical inserts.
The most vulnerable places of the formwork are the corners and the bottom zone. In these places, both high static pressure of concrete and water hammer from the discharge of the mixture act. For this reason, the bottom of the formwork must be tightened and reinforced through the existing stiffening ribs. This is done with the help of studs (you can dress them in sleeves from the MP-pipe to be reused) or wire clamps, sometimes a bunch of reinforcement with a penetration outside is practiced. An important point – metal embeds remain in concrete and should not touch the working and distribution reinforcement. Between the metal rods, a protective layer of about 15–20 mm must be respected. When tightening the ties of the lower zone, temporary spacers of the required length must be inserted inside the formwork.
In order for the formwork to maintain its spatial position, it is burst from the ground with boards at an angle of 45-60% to the horizon. The board, set with an edge, rests against a stake or directly into the ground, is knocked out, and then screwed to one of the vertical crossbars. Sometimes additional strips are tied to the braces to provide support for the lower zone; it is also recommended to tie the stops in the soil together with one or two lines of boards. A well-tightened bottom of the formwork does not need support, but for safety reasons it can be compacted with embankments from the ground.
Calculation and binding of reinforcement
It is allowed to independently calculate reinforcement only for small concrete structures with low responsibility. Correctly designing a reinforcing cage is not an easy task and requires special knowledge. At the same time, the matter is not limited to the calculations themselves, it is also necessary to correctly lay and tie the reinforcement elements together so that they retain their position after pouring concrete.
One of the general principles of the distribution of reinforcement is as follows: the frame follows the shape of the concrete structure with a small uniform inward indentation, due to which outer protective concrete layers of 35–50 mm are formed. The further the reinforcement is spaced from the center of the foundation section, the better it does its job..
It is considered that the total content of steel in the foundation cannot be lower than 0.1% of the cross-section of concrete products. The installation of reinforcement should be carried out taking into account the requirements for ensuring the minimum protective layers of concrete, which are provided for by the project. Plastic rebar clamps will be a good help in this matter: flat plugs for support on the bottom of the formwork and “stars” that distance the frame from the side walls.
Pouring concrete, shrinkage
In general, the foundation device has the following order: preparation of trenches, installation of formwork, lubrication of internal planes, assembly and placement of the frame. Pouring concrete is the finale of the foundation, but even here a great deal of attention and care is required..
The mixture can be fed into the formwork in two ways. The first one – directly from a concrete mixer or a truck mixer, the second – with reloading by means of a concrete pump. With purely gravitational pouring of the mixture into a mold, the physical effect on the formwork is minimal: the reinforcement dampens the fall due to its elasticity, and small portions do not have enough weight to spoil the geometry.
The situation is completely different with pumping. The liquid moves in jerks and is forcefully thrown out of the sleeve in large enough portions. At the same time, the already poured mass transfers the effect on the formwork to the fullest, from which both discrepancies of the joints and swelling are possible with insufficient reinforcement of sheet materials.
In order to avoid such phenomena, the foundation must be poured along the perimeter: first up to half or a third of the height, and then repeat the route once or twice. The optimal pouring height is considered to be 0.5 meters in one pass, however, with a total belt height of more than 1.5 meters, it is recommended to wait some time before setting. In monolithic concreting, a pause of up to 20 hours between layers is allowed, with a longer period of time, the formation of cold seams is possible.
Each poured concrete layer must be compacted. This can be done either with a manual pin or a submersible vibrator. In the latter case, you should try not to be zealous with shrinkage in one place and immerse the tip no deeper than 2/3 of the height. If additional compaction is required, it is better to carry out it according to the same principle as pouring: along the perimeter with breaks for gravitational settling.
Protection and insulation
The formwork is often in a hurry to remove, which causes open edges of the concrete structure and accelerates the evaporation of water. The concrete must retain the moisture content of the surface layers for at least 7 days and in the core – up to 28 days. Therefore, if the formwork breaks down for 3-5 days in clear hot weather, the concrete will have to be wetted several times a day. Early breakdown of the formwork may be dictated by the need to reuse lumber in construction: in this case, they are not lubricated, but torn off before the final setting of the concrete surface.
Waterproofing by injection and penetrating compounds is performed before hydration is completed, approximately 10–12 days after pouring. For the application of roll or coating insulation, concrete is given time to build up the design strength over the full curing period. As mentioned above, the formwork may not be removed at all, performing insulating and waterproofing functions. In this case, decks from the inside are sheathed with special materials: foam plastic, glass insulation and others..