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In construction, the word "treillis" (mesh) refers to two different things: a layer of steel wires embedded in concrete and a triangulated structure of bars that carries loads. Below, you will find the definition of each, the types of welded wire mesh, their uses in a slab or floor, and the points to check before pouring the concrete.
What is a mesh in construction?
On a construction site or in an engineering office, the term refers to two different objects. The first is reinforcement integrated into concrete. The second is a load-bearing geometry made of triangles. Both share a network-based logic, but they do not handle stress in the same way.
Welded wire mesh embedded in concrete
Welded wire mesh is an assembly of longitudinal and transverse steel wires, crossed and welded at their intersections. It forms a flat layer that serves as reinforcement in concrete. Its mesh is described using two terms:
- Mesh openings: these are the empty spaces left between the wires.
- Nodes: these are the welded intersection points of the wires.
- Steel wires: these are longitudinal in one direction and transverse in the other, which forms the grid.
Do not confuse this reinforcement mesh with fencing mesh: the latter separates or protects an area and does not take up any stress in the concrete. The same word also refers to military clothing, such as the khaki M64 military fatigues, which has no connection to the reinforcement.
The triangulated truss that supports a structure
A triangulated truss is an assembly of bars that form triangles. The chords, which are the long bars at the top and bottom, and the diagonals meet at the nodes. The supports then connect this lattice structure to its environment.
This triangulated system can be planar or spatial. It is notably made of steel, aluminum, or wood: roof trusses, bridges, walkways, or pylons whose vertical and diagonal bars intersect. The stiffening truss is a three-dimensional version, made of chords and diagonals, which can be integrated into a floor beam.
How do these two types of mesh handle stress?
In the ideal model of an articulated truss, the loads reach the nodes. Each bar then works solely in tension or compression. In reality, the joints are not perfect hinges: they can also transmit bending.
Take a truss beam placed on two supports and loaded by its own weight and what it carries. The top chord is generally in compression, while the bottom is in tension. The diagonals undergo tension or compression depending on the loading and geometry.
Reinforced concrete distributes roles differently: the concrete primarily handles compression, and the steel placed in the tension zone handles tension. It is the bond between the two materials that transmits the stresses. Welded wire mesh can limit the opening of cracks, but it does not prevent concrete shrinkage.
What are the types of welded wire mesh?
Welded wire mesh types are classified first by their function, then by their trade designation and manufacturing standard. This classification allows you to read a plan or order without confusing an anti-cracking panel with a structural panel.
Surface mesh and structural mesh
Surface welded wire meshes use small-diameter wires. They are used to control cracking in certain compression slabs. PAF (anti-crack) panels specifically fall into this use: a PAF does not replace an ST structural mesh.
Structural welded wire meshes contribute to the reinforcement of thick slabs and ground-supported slabs. Each type of welded wire mesh is defined by its mesh—square or rectangular—as well as the diameter and spacing of its wires. The range of welded wire mesh mainly exists in panels, sometimes in rolls when the structure allows them to be unrolled. According to AFCAB, factory-made mesh offers a guarantee of weld shear resistance that a pre-assembled reinforcement panel does not offer in the same way.
Some designations to identify panels
Mesh panels bear a designation that summarizes their mesh and wires. Several references appear in the characteristic tables:
- ST25 C: 150 × 150 mm square mesh and 7 mm wires. This panel is cited for single-family home slabs.
- ST25 CS: same mesh and same wires, in a shorter 3 × 2.4 m format.
- ST25: 150 × 300 mm rectangular mesh. Its load-bearing steel cross-section is approximately 2.57 cm²/m.
- ST10, ST15 C, ST35, ST50: 5.5 mm wires for the ST10 and 6 mm for the ST15 C. The ST35 and ST50 are cited for certain industrial slabs and floors.
The standard ST25 panel measures 6.00 × 2.40 m, covering 14.40 m². This format serves as a basis for estimating the number of panels for an area. However, the designation alone does not prove that the panel is suitable for your project: the design determines that.
| Designation | Mesh | Wire diameter | Standard panel |
| ST25 | 150 × 300 mm | 7 mm | 6.00 × 2.40 m |
| ST25 C | 150 × 150 mm | 7 mm | 6 × 2.4 m |
| ST25 CS | 150 × 150 mm | 7 mm | 3 × 2.4 m |
| ST10 | — | 5.5 mm | — |
| ST15 C | — | 6 mm | — |
Standards also distinguish between families
The NF A35-080-2 standard, published in March 2022, regulates factory-made welded reinforcement mesh. It covers nominal diameters from 5 to 16 mm. NF EN 10080 deals more broadly with weldable steels for concrete reinforcement.
Below this 5 mm threshold, NF A35-024 applies to surface mesh. Published in July 2019, it was amended in January 2022. NF A35-028, published in March 2022, concerns stiffening mesh, with nominal diameters of 3 to 16 mm. The same manufacturing vocabulary is found in textiles, as described in the CCE fatiguing pants file, but these standards only apply to steel.
What is the purpose of welded wire mesh in a structure?
The use of welded wire mesh covers many horizontal concrete structures and some vertical ones. The right choice depends on the project, and careful installation is just as important as the selected reference.
Slabs, floors, and other applications
In a beam and block floor, the mesh of the compression slab distributes the forces and limits cracking. Its choice depends on the floor system. It is also used in these structures:
- Ground-supported slab: thicker than a compression slab, it rests on a bed of gravel.
- Industrial slabs and roadways: parking areas or certain roadways, where it limits cracking.
- Foundations: in a raft foundation, it reinforces the slab that distributes the loads to the ground.
- Walls, retaining structures, and swimming pools: poured concrete walls, bottom and sides of a pool.
Depending on the structure, the mesh plays two different roles. It can be the main reinforcement that handles the stress, or a skin reinforcement placed near the surface to control the cracking of a wall.
Choosing based on the structure rather than thickness alone
The choice of a mesh depends on the use, loads, support, concrete thickness, and design rules. For a 15 cm or 20 cm concrete slab, thickness alone is not enough to mandate an ST25 or another reference: a patio and a storage area do not have the same requirements.
For a 5 cm structure, first distinguish between the screed, the decorative element, and the load-bearing element. The necessary concrete cover might make standard structural mesh unsuitable for this low thickness. Conversely, the ST15 C is cited for certain 15 to 20 cm slabs, without replacing the calculations.
Concrete fibers do not systematically replace structural mesh. Some aim primarily at cracking, while structural mesh handles tension. Adding steel does not compensate for poorly placed reinforcement: its position is what makes it effective. The same choice principle based on use applies to textiles, as illustrated by the Félin ripstop pants file.
Positioning and checking reinforcement before pouring
The reinforcement plan specifies the type and direction of the mesh, laps, additional reinforcement, and the concrete cover—that is, the concrete that protects the steel. Chairs keep the layer at the intended height, never placed directly on the ground or the formwork. A clearance height of 3 to 5 cm is sometimes mentioned, but only what is specified in the plan matters.
At joints, panels overlap to maintain the continuity of the reinforcement. The length of this overlap follows the plan or the system's specification, not a general two-mesh rule. In an example of a beam floor published by the FIB/CERIB, it reaches 35 cm for a 4 to 5 cm compression slab.
Before pouring, check the type of mesh, its orientation, its positioning, its laps, and its concrete cover. An error found is corrected, then checked again before concreting. NF DTU 13.3 covers ground-supported slabs, NF DTU 23.5 covers beam floors, and Eurocode 2 covers the design of concrete structures.
Frequently Asked Questions
What is the role of welded wire mesh in concrete?
Depending on its type and the design of the structure, it contributes to resistance, stress distribution, and crack control. In a bending slab, the reinforcement in the tension zone handles the pull, especially after local cracking of the concrete. The welds maintain the geometry of the wire network.
What is the difference between welded wire mesh and shaped bars?
Welded wire mesh offers regular geometry and is installed faster than shaped bars. However, it is less easily adapted to complex shapes, where bars remain more flexible in use.
Can the welded wire mesh be pulled up during concrete pouring?
Pulling up the mesh with a rake or your feet does not guarantee the position provided for by the calculation. Walking on it during the pouring can also move it or push it down. Support it before concreting: a panel placed directly on the under-slab film remains too low and loses part of its effectiveness.