FEM analysis of the influence of reinforcing mesh on concrete pavement performance

. Reinforcement of concrete pavement slabs with steel reinforcing mesh (KARI mesh) has been regularly used in the Czech Republic. Reinforcing mesh is used for atypical shapes and non-standard (larger) dimensions of pavement slabs (intersections, runways, transhipment areas, etc.). The placement of mesh across the thickness of the slab varies in designs (top, bottom or both surfaces, eventually in centre). The mesh has a positive effect on cracking occurrence during concrete hardening. It is assumed that the reinforcing meshes also contribute to the reduction of tensile stresses caused by cyclic loading from traffic and temperature. The amount of the reduction in concrete depends on the mesh positioning, but also on the specific loading conditions. In this paper, the FEM analysis of concrete pavements with steel reinforcing mesh is carried out. The reduction of maximal value of tensile stress induced by overweight vehicles, temperature and their combination is inspected while using different mesh positioning. The slab dimensions used in the simulation are larger than the usual highway slabs, the dimensions are based on the real transhipment area pavement. The contribution of the reinforcement to limitation of the maximum tensile stress in the pavement due to different reinforcing mesh positioning was determined.


Introduction
For roads with extreme loads such as airports, motorways or container transhipment points, concrete pavements are proposed instead of asphalt alternatives.These pavements are usually designed as unreinforced jointed plain concrete pavements (JPCP), although in some cases, the joints are reinforced using welded mesh or other reinforcement to ensure the durability of the construction or resistance against cracking.The proposed numerical model uses linear elastic theory, the material properties are retrieved from TP170 technical norm [1], the analysis was performed using Abaqus software.
The objective of this research was to model the reinforcing welded wire mesh in the concrete pavement slab so that a time-saving stress analysis could be performed in the concrete pavement, taking into account the specific geometry of the slab, material properties of the subsoil system, the specific loading, the specific location of the reinforcement within the cross section, and also the geometry of these reinforcement elements.A well-known benefit of reinforcement elements is the reduction of cracking in the phase of concrete hardening [2].However, this research was focused on the contribution of reinforcement mesh to reduction of the maximum tensile stresses generated during pavement serviceability for selected loading conditions and specific reinforcement locations along the concrete slab thickness.Thus, a series of calculations were performed for wheel loading, temperature loading (positive temperature gradient) and a combination of these scenarios.For the analysis, a concrete slab with a slightly atypical shape designed for a specific transhipment area was selected, for which reinforcement with welded mesh (WWF) was considered.The specific conditions for the pavement subsoil system as well as the loading are taken from the design of the site.For the purpose of the research, the position of the reinforcement nets along the thickness of the slab was chosen in several variations.These include locations at the top surface, bottom surface, in the middle and at both surfaces of the concrete slab.Three different reinforcement elements were modelled with the same mesh size (100 x 100 mm) and different steel bar diameters (8, 10 and 12 mm).In this analysis, the differences in stresses comparing different reinforcement to a plain unreinforced concrete JPCP pavement and the percentage contribution to reducing the maximum tensile stresses for specific load conditions are calculated.The fatigue benefits of the reinforcing elements to the concrete slab during service life lead to a more complex analysis [3] and will be the subject of follow-up research.

3D FEM model of concrete pavement
As already mentioned in the introduction, the 3D model of the concrete pavement (see Figure 1) was created based on a specific transshipment area and its concrete pavement area.
-The dimensions of the concrete slab (according to the design) are : 8.06  -Temperature loading : a linear positive temperature gradient was considered along the thickness of the plate.The temperature difference between the top and bottom surface of the CB plate was set to 10°C (Figure 2).

Modelling of reinforcement elements in ABAQUS
In Abaqus [4], several approaches have been presented to model reinforced regions in concrete.From the point of view of modelling and lower computational time requirements, the most suitable approach was chosen to "embed structural elements in solid elements" [5].
In this technique, reinforcement elements are inserted into host elements, with the proviso that the element networks do not have to coincide.The program searches for elements close to the embedded elements.Then the nodes of the embedded elements are constrained by the response of the host elements.In this work, by first using the element "wire" and then assigning a specific profile, a reinforcement steel network was created (see Figure 3).This mesh was then placed in the concrete plate model lying on the subsoil system (see Figure 4).When placing the net to the plate surfaces, the prescribed coverage was always maintained.

Calculated stress in the concrete pavement slab
The tensile stresses for the selected loading conditions, i.e. from the reach stacker wheels, from the temperature and from the combination of these loads were determined by means of a computational analysis in ABAQUS (see Figures 5, 6 and 7).Table 1 summarizes the values of the maximum tensile stresses in the concrete slab for different positions of the reinforcement welded mesh or without any reinforcement.Table 1 also shows the effect of the diameter of the reinforcing mesh member on the resulting stress magnitude.The differences in stresses (in MPa) compared to the unreinforced slab are shown in Table 2 and the percentage comparison is shown in Table 3

Δσ (%) Ø 12mm/100x100
The above results fulfill the logical assumption that a larger wire diameter of the reinforcing welded mesh means a smaller tensile stress in the concrete pavement and therefore a smaller fatigue stress.For more severe loading conditions with higher values of tensile stresses (with simultaneous loading with wheel and temperature), the percentage savings in tensile stresses for the different reinforcement diameters and their different positions within the cross section are as follows: -Welded wire mesh Ø 8 mm -1.47 -3.46 % -Welded wire mesh Ø 10 mm -2.31 -5.35 % -Welded wire mesh Ø 12 mm -3.29 -7.63 % The most beneficial in terms of minimizing tensile stresses is the placement of reinforcement networks at both surfaces of the concrete slab.Conversely, the position of the net in the centre of the slab is the least beneficial in terms of minimising tensile stresses.From the point of view of stress reduction in the pavement, the use of larger wire diameters of the reinforcement mesh is suggested.However, due to the significant price differences between different wire diameter nets at the same mesh size (these are easily two to three times the price), it may be more efficient to use smaller profiles.

Conclusion
Therefore, in addition to the benefit of limiting the induction of cracks during hardening of the concrete, reinforcing welded mesh can also be beneficial in terms of reducing tensile stresses and improving fatigue behaviour against wheel and temperature loading, if properly installed and all technological principles are followed.However, it is important to note that the use of the reinforcing element in the concrete pavement slab and therefore the magnitude of the tensile stress reduction in the concrete is dependent on the specific loading condition and the position of the element within the cross section.This analysis shows that the stress reduction can be in the range of a few percent, but even close to twenty percent for some loading scenarios, which could have a significant positive effect in terms of fatigue behaviour.Thus, the service life of the pavement could be significantly extended.However, only the stress values at the more demanding loading conditions, which are closer to the tensile strength of the concrete (i.e.wheel + temperature combination) should be assessed, as these have a greater influence on the cumulative fatigue damage of the pavement.In the case of the reinforcement for both surfaces, the computational analysis showed a saving of tensile stresses between about 1.5 to 7.6%.A more detailed fatigue analysis and possible cost comparison of mesh reinforced and unreinforced concrete pavements will be the subject of further research.

Fig. 3 .
Fig. 3. Reinforcing welded mesh.Adjusting the radius of the steel bar of the reinforcing mesh.

Table 2 .
Tensile stress difference between reinforced and not reinforced slab (MPa).