- How foundation reinforcement works
- How to strengthen the foundation
- Basic parameters of reinforcement
- Reinforcement configuration for NZLF, tape and slab
- Knitting, installation and control
Hand-made production of reinforced concrete foundations is the most critical of all stages of construction. The required rigidity and strength is provided by embedded reinforcement, so today we will eliminate the gaps in understanding the functions of reinforcement and explain the methodology for calculating reinforcement for the foundation..
How foundation reinforcement works
Concrete has excellent compressive strength. This means that if a concrete block is placed under a press, it will begin to collapse only under very high pressure..
The reality of the operation of reinforced concrete products is such that it is impossible to predict exactly what forces will act at a particular point in the array. This is because the configuration of a concrete product does not mean as much as the physical and mechanical characteristics of the base on which this product is installed. And they are almost always unpredictable..
The load in concrete is unevenly distributed. The maximum stress falls on the fulcrum, while the leverage rule always applies – the force increases in proportion to the leverage. If you suspend a concrete beam at both ends, the impact on the center will directly depend on the length of the beam..
Beam operation scheme for bending: a – concrete beam; b – reinforced concrete beam; 1 – fittings
The nature and direction of deformations at different points are also interesting. When bending, one side will shrink, but this, as we found out, does not bode well. It is much worse that on the reverse side of the product the concrete will stretch, which, with low elasticity values, will result in a crack and break.
The main task of the reinforcement is to prevent the concrete from stretching. This is achieved due to the friction forces that transfer the load from the concrete layer to the embedded elements, which have a modulus of elasticity much higher than that of concrete. And, of course, the reinforcement should be distributed as evenly as possible so that each separate section of the structure does not have weak points with poor dressing. Otherwise, the reinforcement loses all meaning..
How to strengthen the foundation
There are two types of fittings. The working reinforcement performs the direct function of reinforcement – it takes on the load in the applied plane. Structural reinforcement is used to arrange the lines of the working reinforcement in the concrete layer and to obtain additional connections, if necessary..
Hot-rolled bars of periodic or smooth profile according to GOST 5781–82 are traditionally used as working reinforcement. Steel reinforcement can be welded or non-welded, depending on the thermomechanical reinforcement and the area of use.
For the foundation, it is advisable to use a periodic profile as a working reinforcement, which has the highest adhesion to the surrounding mass. Auxiliary reinforcement, on the contrary, is performed with smooth rods, although this is not a definitive rule..
The material is also important, the steel grade determines the class of reinforcement. Classes A400 – A600 are most in demand for a private developer: they are most widespread on construction bases and do not require special means of joining: the entire frame is assembled with a sticky one. Composite reinforcement (GOST 31938) made of plastic reinforced with carbon and fiberglass is increasingly used. Such reinforcement is much lighter than steel and is absolutely not subject to corrosion, but how important it is in a specific project is up to you..
Basic parameters of reinforcement
In each specific calculation, there are a number of key values described in the manual for SNiP 2.03.01:
- Density of reinforcement insertion (reinforcement coefficient). It is determined by the cross-section of the product as the ratio of the sum of the cross-sections of the reinforcing bars to the cross-section of the concrete mass. The minimum established by the norms is 0.05%, although the coefficient can increase as the ratio of segment length to its height grows up to 0.25%.
- The thickness of the rods. With a segment length of over 3 meters, reinforcement with a diameter of at least 12 mm is used, over 6 meters – over 14 mm, and with a length of 10 meters – 16 mm or more.
- Reinforcement distribution. If the foundation has a depth of about a meter, then which edge should be strengthened from stretching: upper or lower? Which is better – a small number of thick bars or many lines of thin reinforcement? In practice, often all the working reinforcement is placed on one face, breaking into as many rods as possible that do not interfere with the pouring of concrete. Then the same belt is duplicated at the opposite face.
- Reliability coefficient (reinforcement) is a concept directly following from the previous paragraph. The strength of the foundation can be deliberately overestimated by 2 or 3 times in case of unforeseen changes in the geomorphology of the region or in the absence of a completed project at the time of construction.
The latter should belong to the category of exceptions, but in practice almost half of the objects of individual housing construction are built this way. The problem is that without exhaustive design data, you are not able to accurately establish the weight of the building, determine from it a sufficient area and depth of occurrence, corresponding to the supporting capacity of the soil, then calculate the linear characteristics of the foundation using standard proportions, and from them deduce the optimal methods of strengthening its structure adequate to the design load.
Reinforcement configuration for NZLF, tape and slab
Strip foundations that lie above the freezing depth are reinforced with a rectangular frame. An unlimited number of reinforcement lines can be located between the outer ribs, between which the standard clearance must be observed. As a rule, such frames consist of separately connected modules, the length of which is convenient for transportation and installation. Structural reinforcement here is represented by U-shaped or closed clamps encircling the working reinforcement bars every 0.6-1.1 meters.
Reinforcement of the straight section of the strip foundation: 1 – working longitudinal reinforcement; 2 – structural fittings (clamps)
Recessed foundations are reinforced, like the tape, with a frame. Reinforcement lines, as mentioned, are duplicated and centered at the top and bottom faces. Additionally, intermediate lines can be laid to compensate for the forces of pressure and heaving of the soil, if required by the project. The reinforcement is connected to each other by vertical rods. This reinforcement looks like a constructive one, but it also performs the function of a worker, to a large extent preventing twisting and lateral pressure deformations..
The slab is reinforced in the simplest way: two reinforcing meshes, each of which can consist of several layers. Meshes are spread to the upper and lower planes in accordance with the standard protective layer. Reinforcement mesh parameters – tabular, rod and cell are calculated depending on the dimensions of the slab. As for the stiffening ribs under the slab, they are formed like the frames of the MZLF, and then fastened to the slab mesh with vertical rods of structural reinforcement..
Knitting, installation and control
With linear sections, everything is simple, but the foundation has turns and intersections. On them, the lines of converging frames are connected by bent embedded elements from reinforcement of the same section. The edges are fitted with an overlap of 40 to almost 100 nominal diameters. The practice of strengthening the corners of the foundation with reinforcing meshes 12x150x150 mm is quite common, especially on soft soils and in earthquake-prone regions..
Reinforcement of abutments and corners of the strip foundation: 1 – working longitudinal reinforcement; 2 – transverse reinforcement; 3 – vertical reinforcement; 4 – L-shaped clamps
We have already described the advantages of tying reinforcement before welding and we strongly recommend using only this method, unless we are talking about special-purpose foundations..
Each subsequent segment of the frame is installed on spacers or rings, which prevent the violation of the protective layers. The rods at the ends are connected with a standard overlap, 2-3 wire clamps at each joint.
As a result, the reinforcing frame should be formed in such a way that people can easily move along it. Before casting, the frame is carefully checked for bond strength. If, when pouring concrete, the bandages of the lines disperse, this is fraught with complete rejection of the entire structure. Therefore, during casting and shrinkage, special attention must be paid to the position and integrity of the reinforcement joints..