Why use double reinforcement concrete

Reinforcement of concrete - why, why, why?

Concrete is a great building material - unfortunately, it can only withstand large compressive forces. When it comes to tensile loads, he tends to be weak. Fortunately, it is exactly the other way around with steel and so the two materials complement each other perfectly. Reinforced concrete - that is, concrete reinforced with steel - is the solution to many construction problems. The specialist speaks here of reinforcement. And I would like to take a closer look at it here.

Table of Contents

The balance of forces must be maintained

Anyone who wants to erect a staircase, a wall, a ceiling - in short a structure made of reinforced concrete, has to know exactly what makes such a structure tick. So how the forces work there and how strong. With the right combination of concrete and steel, the planner has to ensure that the forces remain in equilibrium - and that, if possible, even in the event of unusual events such as an earthquake. In plain English: the thing must stop under all circumstances.

Now someone may come up with the idea of ​​simply distributing the steel evenly in the concrete, and then it will be all right. Somehow, maybe yes, but steel is expensive and no building owner wants to pay more than is absolutely necessary. In addition, the choice of the steel cross-section has a direct influence on the load-bearing capacity of the reinforced concrete. First of all, I have to tell you something about how it works:

This is how reinforced concrete works

Reinforced concrete is a composite material made of concrete and steel, I have already written that. But it is also a material that only works when microscopic cracks have formed in the concrete. Only then does steel develop its full effect. It is therefore not expedient to place too much steel and then with the wrong diameter in the concrete.

Why is reinforcing steel ribbed?

You may have wondered why the irons that go into the concrete are ribbed. The answer is very simple: So that forces from the concrete are better transferred into the steel. The ribs ensure optimal interlocking of concrete and steel. Smooth steel was also used in the past. Here, however, additional constructive measures were necessary to make it work. For example, the steel was shaped into loops.

Avoid too large cracks

Reinforced concrete needs cracks for it to work, but the cracks in the concrete must not be too big either. Then there is not only a risk of rusted reinforcement from penetrating moisture, but also a lot more. Starting with cracks up to the complete failure of the construction. The collapse of a building, for example.

Now it becomes clearer why the ratio between concrete and steel has to fit in every area of ​​the construction. It is the job of a structural engineer to calculate this precisely and then use plans to determine how the reinforcement is to be carried out. The worker on the construction site must then use the reinforcement plans to correctly distribute the steel in the concrete. There are a few rules to be observed, which I will come back to later.

Which reinforcement steels are there?

Reinforcing steel is available either in the form of bars, mats or stirrups. Bar steel is available in a wide variety of diameters from 6 to 40 millimeters. There are also around 20 different types of welded wire mesh. So there is plenty of choice when it comes to reinforcing concrete.

Reinforcement mats are used for flat components such as walls or ceilings made of reinforced concrete, while brackets and bars are the means of choice for columns, beams, strip foundations or lintels. The latter are often processed into reinforcement cages. Here the stirrups and bars are either welded together or connected with the help of binding wire.

Such a reinforcement cage can be round or rectangular in shape - just like the reinforced concrete column. If such a column is loaded evenly from above, the reinforcement is also distributed evenly in the cross-section. But more on that in the following chapter.

It all depends on the load

Statics is quite a complex matter and one of the main reasons I ended up writing. So I really only want to look at the most basic things when it comes to reinforcing concrete.

A reinforced concrete component can basically be loaded in four different ways:

  • Normal forces (tension and compression)
  • Bending forces
  • Shear forces (opposite lateral forces)
  • Torsional forces (twisting)

However, because all of this would be too easy, the loads tend to occur in combination. A wall does not only get pressure from above, but mostly also from the side. For example by wind or by earth forces, if it is a cellar. Or the load moves - like on a railway bridge by a train that drives over it.

Depending on the level and type of load, the reinforcement must be distributed differently in the concrete. In general, the higher the tensile force, the more iron is used. First of all in the train zone. In the case of a horizontal beam supported on both sides, this is the underside.

Why this is so can be easily explained: Place a long piece of foam on two cups and it will bend downwards. Especially when you put a cup on it in the middle. If you look at the cells, they are compressed on the top (compressive forces), but pulled apart on the underside (tensile forces).

Under certain conditions, the pressure zone may also need to be reinforced. For example, in the case of high bending loads or if the same beam dimensions are required for a wide range of loads. However, the compression reinforcement will never be as high as in the tensile zone.

In addition to the statically necessary reinforcement, so-called “constructive reinforcement” is often provided. They are particularly necessary where voltage peaks that are not mathematically recorded are to be expected. For example, with recesses or openings in the concrete. Structural reinforcement then prevents cracks from forming.

General reinforcement rules

The DIN specifies exactly how the reinforcement must be introduced into the concrete. This applies, for example, to the distances between the bars, but also to the concrete cover or how the bars are bent.

Bar spacing

It is not only important how much steel gets into the concrete, but also the distance between the individual bars. If it is too small, the fresh concrete cannot be properly placed and compacted. This also ensures a sufficient bond.

The bar spacing depends on the bar diameter and the maximum grain diameter of the concrete. It should be at least 20 millimeters and in any case at least correspond to the diameter of the rod. For a rod with a diameter of 24 millimeters, the distance must therefore be 24 millimeters or greater. If pebbles with a diameter of more than 16 millimeters are processed in the concrete, the rod spacing must be 5 mm larger. At 16 millimeters so 21 millimeters, at 17 millimeters then 22 millimeters….

Concrete cover

The concrete cover - i.e. the distance from the surface to the steel - is very important for the reinforcement. And for three reasons:

  • Protection of the steel from corrosion
  • Bond between steel and concrete
  • Fire protection

Depending on the steel diameter and the expected environmental influences, the concrete cover is between 20 and 55 millimeters.

The required concrete cover is ensured in various ways. For example with spacers in the form of small feet made of plastic or fiber cement. In the case of walls, these are attached to the outside of the reinforcement. In the case of ceilings, the spacers are placed on the formwork before the reinforcement is introduced.

Bending of reinforcing bars

Sometimes it is necessary to bend the reinforcement. For example for hooks, loops and brackets or for inclined bars and other curved bars. Bending is not the big problem, but certain rules must be followed. Otherwise there is a risk of concrete spalling or destruction of the concrete structure in the area of ​​the bend. Correct bending can also prevent cracks in the rod.

When manufacturing hooks, loops or angle hooks, the minimum roll diameter is either four times (diameter less than 20 mm) or seven times the rod diameter (diameter from 20 mm). A minimum roller diameter of 7, 15 or 20 times the rod diameter must be observed for inclined rods and other curved rods.

In the case of welded reinforcement bars and mats, it looks different again. A distinction must be made here between predominantly static and non-predominantly static effects. The minimum roll diameter required here can go up to 500 times the rod diameter. I do not want to go into further details here.

Conclusion: I hope it has become clear why the reinforcement cannot be distributed “by feel” in the concrete. This might work with a garden wall that is not under stress or with a foundation for a garden shed. Or with a structural engineer with years of professional experience. However, as soon as the structure is subjected to correct loads, the precise dimensioning of the concrete reinforcement is important.

I can recommend the following worksheets to anyone who would like to deal more extensively with concrete reinforcement.

Reinforcement of a strip foundation

If you want to create a strip foundation for a garden wall, you should concretize this with a reinforcement cage. This is usually not necessary for static reasons, but the reinforcement minimizes the crack width. So there are smaller cracks, which makes the foundation more frost-proof. In the case of higher garden walls, load-bearing walls or if a slope is to be supported, it is advisable to consult a structural engineer so that there is no damage later.

A strip foundation should be at least 80 cm deep so that it is frost-free. In addition, a 15 to 20 cm thick cleanliness and drainage layer made of crushed stone or gravel is required. If the floor is not stable, you need a formwork made of wooden boards.

The reinforcement cage must be concreted in such a way that it is covered with at least 3 cm of concrete on all sides. At the bottom, spacers provide the necessary concrete cover.

Reinforcement of a floor slab

Garden houses, garden grills or tool sheds need a base plate as a foundation so that they have a secure hold. Here it is always advisable to concretize a welded wire mesh for reinforcement. Where the tensile forces act. Here again the urgent note that a specialist must absolutely carry out a static calculation for larger construction projects.

The slab should be 10 to 15 cm thick, the concrete cover for the reinforcement mesh should be around 5 cm. The reinforcement is to be placed on spacers so that the cover is guaranteed. It is important to ensure that the concrete cover is also adhered to at the edge.

In the case of larger foundation slabs, frost-free aprons must also be planned. These are strip foundations that protrude at least 80 cm into the ground.

The foundation slab is concreted on a blinding layer. This layer of gravel or crushed stone should be six to eight inches thick. It also serves as drainage so that no water accumulates under the concrete slab. This is especially dangerous in frosty conditions, as frozen water expands.

On the side, board formwork is required, with the upper edge of the formwork boards being the same as the upper edge of the surface foundation. The steel mats, including spacers, are placed on the sub-base before concreting.

Suitable for this:

Mixing concrete - the right mixing ratio
Foundation for garden shed and Co.