Ribbed Slab: Cutting, Dimensioning, What is it? and Thickness!

September 17, 2021 by No Comments


The next time you enter a large building, look at the ceiling. If it is “divided” into squares, or has square “holes” along its entire length, the structure is nothing more than a ribbed slab.

The ribbed slab system consists of reinforced concrete and small regularly spaced beams. The reinforced concrete layer is thin, and the distance between one beam and another is supported by steel formwork and beams. As it is lighter, the material ensures that this support is safe.

Another point guaranteed by the use of lighter material is the economy of construction. This is because the amount of steel used is about 30% less than in solid slabs. Are you interested in the option? Follow the text and find out more about the ribbed slab!


What are the types of ribbed slabs?

There are two main categories of the ribbed slabs, each subdivided into others. The basic categories are those of slabs molded-in loco, and prefabricated.

In the case of slabs molded on-site, the structure is assembled on the construction site. The technique guarantees the completion of the work in a medium time and does not require specialized labor. In this category, it is possible to find, for example, the mushroom slab.

Although also available in the solid version (whole, with concrete), the alternative can rely on inert materials, forming the ribbed. Its most common shape is square, but it is also possible to use it in a circular shape.

Pre-molded slabs are those produced in the industry, undergoing strict quality control. Here, there is the slab with reinforced concrete joists and ceramic tiles. It consists of using the unidirectional ribbed slab. That is, its divisions do not form squares, but rather long rectangles that only end at the edge of the property.

In a lattice slab, it is possible to have a system consisting of a ribbed slab also unidirectional. In this case, however, the joists are interspersed with inert elements and a concrete cover. The joist is a thin concrete beam used horizontally to support the slab structure.


Which applications?

The use of the ribbed slab is indicated whenever the property requires a larger space between its spans. After all, the alternative allows for a very large gap between one support and the other.

Thus, theaters, auditoriums and sports gyms are some of the places where this type of slab is used. As in parking lots and basements, where larger space is essential for maneuvering vehicles.

Ribbing can be used for projects of different sizes. Therefore, a small property can take advantage of the option, as well as large industries or shopping malls. The last buildings are the ones that receive the most structure.

In any case, this type of slab is not suitable for an environment where there is a high and concentrated load performance.


How is it done?

For a ribbed slab to be assembled, four basic components are required. First, the precast joists, already mentioned as thin concrete structures. They are usually more used in small works, but can also be indicated for larger buildings.

Second, it is necessary to have prestressed joists. In this case, the concrete structure has bars or wires inside it. The prestressing elements guarantee greater support for the work.

At the same time, it is necessary to rely on forms. The molds are usually made of plastic, and serve exactly to give the ceiling its classic look divided into squares. In addition, of course, to the platform formwork, that is, the iron mold that will be the support for the entire structure of the slab.

Once these materials are obtained, the construction step by step begins with the assembly of the metal shoring and the work beams. Then, the platform is placed on the props, and the plastic molds are inserted side by side.

Soon after, you should attach a strip of wood on the edge of the slab, and place the metal reinforcements. The rebars and stirrups are attached, and then the slab is concreted. The concrete must be leveled and wait for its curing time, that is, drying.

When the concrete dries, the person responsible for the construction removes the structure’s shoring, maintaining it only approximately every 1.5 m². This distance varies according to the project of the property. Then, remove the platform on which the forms were supported, as well as these forms. The result is the visible and classic “holes”. Then, just finish the shoring of the structure.


Design of the ribbed slab

The dimensions of a ribbed slab vary according to the design and size of the desired property. In any case, it is possible to establish basic measures for this type of construction.

According to the standard NBR 6118: 2003 of ABNT (Brazilian Association of Technical Standards), for example, the characteristic resistance of concrete to compression must be, at least, equal to 25 MPa.

The ribs, on the other hand, must be at least 5 cm. The ribs are the spaces between a “hole” and another of this type of side, the concrete parts. When there is compression reinforcement, this measurement must be at least 8 cm.

Ribbed slab thickness

When the buckets (molds) are removed from the structure, an upper layer of concrete remains on the slab. This layer is called a table and must be greater than or equal to 1/15 of the distance between the ribs. This measurement will always be at least 3 cm. This is if there are no pipes embedded in the slab.

If there are pipes, however, this measure grows. The diameter of the piping must be a maximum of 12.5 mm, and the table must be at least 4 cm.

As the shapes have different shapes available on the market, it is difficult to define the depth of each “hole”. The definitions appear in the construction project and must comply with current regulations (NBR’s 6118, 15200 and 15575-3). The thickness of the slab and rib is also determined by the size of the formwork.

Regarding the height of the ribbed slab, it can be estimated based on the span of the structure. The height must correspond to 1/25 to 1/30 of the largest span of the work.


Hollow ribbed slab

The large gaps in ribbed slab structures are possible thanks to the lightness of the material. This is because the option considerably decreases the use of concrete and steel. The measurements between each span usually rotate between 7 and 9 meters. That is, between one beam or the other, there is at least this space available for the transit of people and vehicles.

In addition, this type of slab promotes rather high ceilings. Thus, in addition to the horizontal space, there is a large vertical space in the internal environment. In addition to the spaciousness, these features guarantee good acoustic and thermal performance to the space.

Calculated ribbed slab. How to Calculate?

When calculating the ribbed slab design correctly, it is essential to have the help of a professional. An engineer or architect is best suited for this step. In addition to the measurement between gaps, quantities of material needed and more, the professional will calculate the number of buckets, molds, necessary for the execution of the plan.

For this, it is necessary to know the height of the formwork, its thickness, the height of the table and formwork combined, and the width of the rib. The number of columns and rows is also considered, and at the end it will indicate the number of shapes required for the construction.


Ribbed slab: shape

In addition to the various physical forms of formwork (square, rectangular, etc.), the ribbed slabs can be classified according to the position of their beams. In bidirectional slabs, four massive beams are used around them, so that they support the entire structure well. The cuvettes are placed in such a way that the joists form 90 ° angles.

What are the advantages?

As mentioned so far, one of the main advantages of the ribbed slab is the savings in materials and cost of the structure. After all, the amount of steel and concrete used is much less than in solid slabs.

Thanks to this smaller amount of material, the project also generates little waste. In other words, it is more sustainable than the others. At the same time, the labor does not need to be specialized, saving again on the final cost.

As there is a large space between the spaces, another benefit of the slab is the space created for transit. It is possible to travel with cars and even large vehicles, without many problems for maneuvers. Therefore, it is ideal for garages, parking lots, and for places like hospitals, clubs and others.

There is also great durability and safety in this type of structure. This is because the parts have no danger of corrosion. The advantages also include its ease of assembly and disassembly and the speed of execution of the work.

What are the disadvantages?

Despite these benefits of the ribbed slab, there are disadvantages to using the option. Disadvantages that must be considered when choosing this or another type of slab.

This is the case, for example, with increasing difficulty in combining the system with others. As hydraulic installations or seals of the structure. The assembly of the structure still requires the fulfillment of a greater number of operations. Although faster, the system can take a little more work.

At the same time, more attention is needed when pouring the surface. After all, there should be, in some ways, empty spaces in the ribs between the cuvettes. Concrete is essential to maintain the safety of the structure, since its quantity will be considerably less than in ordinary slabs. Also consider that the buckets can move during pouring, which is why it is essential to make their positioning precise.


With Styrofoam

So far, we have explained the entire process of making the “classic” ribbed slab, the one that has “holes” at the end of the process. However, there is also the option of a slab that does not have holes, but “division” visible in squares. In this case, part of the construction process is quite different.

This different type of ribbing is created from the use of ready-made blocks. These blocks can be made of three main materials: Styrofoam (also called EPS or expanded polystyrene); ceramic blocks; or hollow concrete blocks.

Any of these blocks, already manufactured, are taken to the construction site and positioned in the same way as they would be with the forms of the other method. They are then fixed, given a steel structure and are concreted. In the end, none of the blocks are removed, but rather make up the internal surface of the property.

Among all, the styrofoam ribbed slab provides the best balance. First, thermal, since EPS does not absorb much heat from the sun. In the case of acoustics, the material ensures that the sounds from outside the property do not enter the space and that those from the interior do not “escape” outside.

Styrofoam pieces have a density of approximately 13 kgf / m³. That is, they are lighter than any other material, and further enhances the lightness already characteristic of the ribbed slab. For the same reason, the parts are easy to transport and assemble, as they are more practical to be loaded.


Should I use ribbed slab?

When choosing the best material for your slab, it is necessary to consider a number of factors. As the desired space between the spans, as there are other ceiling options for similar constructions. It is also important to consider the desired height of ceiling height, and the practicality necessary for the work.

Regardless of your preference, however, you should always count on the help of a professional. He will be responsible for establishing the best project, carrying out the necessary calculations, indicating all the material and labor for the construction and more.

You can, at the same time, clarify the information about advantages and disadvantages to your case and, if necessary, suggest another type of slab. Do not stick to the ribbed slab: if another one is more interesting, it can be “replaced” in your project. The important thing is to obtain the best possible result for your work.

Now you know everything about ribbed slab, look for a reliable engineer or architect and request a project. Indicate your final objective and your initial ideas and, together with the expert, polish the information. This way, you can save time, materials and money.