Geosynthetics
Physical and Mechanical Properties:
Such as strength, impermeability, puncture resistance, and most importantly, their extremely high tensile strength relative to their weight, have led to a wide range of applications for these materials in civil engineering projects.
In this article, an attempt has been made to comprehensively introduce the most important members of the geosynthetics group, namely geogrids, geotextiles, and geomembranes, along with their physical and chemical structures. In addition to explaining their applications in various parts of civil engineering and environmental engineering, their limitations and methods of implementation are also discussed.
Geogrids (Geosynthetics)
The Best Option for Soil Reinforcement Geogrids are open-mesh lattices made of fibers and polymers, primarily polyethylene or polypropylene, and sometimes PVC. Their main application is generally to reinforce various layers where there is a high possibility of slippage between them, such as soil layers in foundations or asphalt layers in road and airport construction. They are also used in bituminous surface layers in hydraulic engineering to prevent water infiltration. The most important role of the large pores in geogrid layers is to bond two layers together, with the geogrid (geosynthetics) placed in between.
Geogrids primarily withstand horizontal forces and deformation, thus resisting the development and propagation of reflective (transitional) cracks from the existing structure to the overlaying layer. This type of repair is mainly performed by reinforcing the entire surface.
Applications of Geogrids (Geosynthetics)
Increase in Strength: The Primary Application of Geogrids
These products can be used in road construction and soil reinforcement. The major role of geogrids (geosynthetics) in asphalt paving is as a material to increase the tensile strength of the asphalt. The weakness of asphalt against tensile forces causes cracks that form in the underlying layers of the road structure to transfer to its surface, leading to pavement failure. The two important roles of geogrids (geosynthetics) in asphalt layers are:
Increasing the tensile strength of asphalt layers.
Bearing a significant amount of the horizontal tensile forces in the asphalt and distributing them over a larger area. This action reduces the peak tensile forces resulting from heavy loads.
The main uses of geogrids in road construction are to prevent the occurrence of reflective cracks and to prevent cracks caused by thermal stresses. Asphalt cracking is mostly due to fatigue (wear) caused by heat, although fatigue from traffic also plays a role.
Pavement cracks usually occur due to tensile and shear strains in the asphalt. The ability of asphalt to withstand tensile and shear strains varies and depends on parameters such as the amount and type of bitumen, the loading speed, and the duration of the applied strain.
As mentioned, overall, asphalt's ability to withstand tensile strains is weak and limited, such that tensile cracks often occur at strains of one percent at temperatures below zero degrees Celsius and two to three percent at temperatures above 20 degrees Celsius within the asphalt. The concentration of strain in asphalt due to the presence of old or existing cracks contributes to the increase in the rate and amount of asphalt cracking. Therefore, reinforcing the asphalt, increasing its tensile strength, and ultimately enhancing its final strain levels are necessary. One of the best solutions for this purpose is reinforcing the asphalt using polymer-reinforced geogrid mesh lattices.
Non-woven geogrids can absorb more than one kilogram of bitumen per square meter and, in addition to their tensile properties, form a barrier that prevents water infiltration in bituminous coverings. In reinforced flexible overlays (geogrids), larger deformations and higher load-bearing capacities can be achieved without cracking.
Creating deformation in the overlay generates shear between the asphalt and the reinforcement. Transferring this shear stress to the reinforcement through contact between the asphalt and the reinforcement generates tensile force in the reinforcement. The amount of this transferred shear stress depends on the deformation properties of the asphalt and the reinforcement, as well as the interaction between the asphalt and the reinforcement. The maximum tensile stress in the reinforcement is limited by the amount of transferable shear stress between the asphalt and the reinforcement.
Applications of Geogrids in Road Construction
To strengthen the asphalt of road surfaces with relatively weak substructures. In this case, substructure deformations have less destructive effects on the pavement.
To effectively prevent reflective cracks and cracks caused by temperature changes, especially in extremely cold weather.
In multi-phase asphalt paving of roads, when forced to stop asphalt paving at a specific point along the road, creating a time gap between two paving phases. In such cases, sufficient bonding between the asphalt layers does not form at the junction of the end and start of the paving sections.
Geogrids can effectively resolve this issue. This problem also arises during the asphalt paving of roads with more than one lane.
When forced to widen the road, due to the difference between the substructures of the old and new parts of the road, the settlement of the subbase of the newly constructed edge section, and the lack of sufficient bonding between the old and new asphalt layers, longitudinal cracks form along the road at the junction of the lanes. Using geogrids (geosynthetics) in these areas can largely prevent potential issues.
In airport runways covered with asphalt, due to high traffic loads, cracks, particularly at landing points, may form. Given the limited time available for runway repairs, efforts should be made to extend the intervals between repairs as much as possible. Using geogrids (geosynthetics) is one of the possible solutions.
When roads need to be excavated for various purposes such as installing water, gas, sewage pipes, or any other unforeseen issue during the road's establishment, matching the subbase of the newly created section with the previous road is difficult and sometimes impossible due to the pressure exerted on the newly installed infrastructure. This difference between the two subbases causes depressions and cracks at the site, leading to its collapse. Using geogrids can effectively cover this issue.
In areas where asphalt needs to cover surfaces with different contraction properties from asphalt, as well as over joints between them, such as metal or concrete bridges, this difference in contraction between the two materials causes cracks and deterioration in the asphalt. The same applies when using asphalt to cover gaps between two sections that are spaced apart. Using geogrids in these situations can significantly prevent such damage.