LESSONS LEARNED FROM LABORATORY AND FIELD EXPERIENCES ON THE USE OF DOUBLE TWIST STEEL WIRE MESH AS A REINFORCEMENT FOR ASPHALT PAVEMENTS

Installation of wire netting in Ontario (1960) INTRODUCTION One of the oldest interface systems used in flexible pavement is steel reinforcement. The idea that appeared in the early 1950s was based on the general concept that if hot-mix asphalt (HMA) is strong in compression and weak in tension, then reinforcement is definitely needed. Steel reinforcement was abandoned in the early 1970s after tremendous difficulties were encountered in the installation. The idea reappeared in the early 1980s with a new class of steel reinforcement products in Europe: many of the early reported problems appear to be solved, and successful experiences with the new class of steel reinforcement were reported. Installation of wire netting in Ontario (1960)

WHAT IS ROAD MESH? The product Road Mesh is manufactured from double twisted steel wire mesh with transverse reinforcing rods evenly spaced throughout at approximately 16 cm centres, as depicted in table 1 and figure 1. The hexagonal mesh size is 80 by 100 mm (nominal) as defined in EN 10223-3. Table 1: Mechanical characteristics of the Road Mesh The wire is protected against corrosion by a zinc coating complying with EN 10244-2 Class A. The thickness of the product varies between the 2.2 mm wire diameter up to 8.3 mm where the transverse rod passes through the double twist. The varying height of the product strands, and distance between them, ensures that the asphalt can encapsulate the wire, without developing a weak shear zone at the product interface. Fig. 1: Geometrical characteristics of the Road Mesh Other reinforcing products, due to their geometry, are able to absorb crack reflection stresses, but because they are not able to integrate themselves into the aggregate matrix, they contribute to reducing the rut resistance as well Road Mesh is manufactured from double twisted steel wire mesh with transverse reinforcing rods evenly spaced 16 cm centres. The hexagonal mesh size is complying with EN 10223-3. The wire has a zinc coating complying with EN 10244-2 Class A.

WHAT IS ROAD MESH? The product Road Mesh is manufactured from double twisted steel wire mesh with transverse reinforcing rods evenly spaced throughout at approximately 16 cm centres, as depicted in table 1 and figure 1. The hexagonal mesh size is 80 by 100 mm (nominal) as defined in EN 10223-3. Table 1: Mechanical characteristics of the Road Mesh The wire is protected against corrosion by a zinc coating complying with EN 10244-2 Class A. The thickness of the product varies between the 2.2 mm wire diameter up to 8.3 mm where the transverse rod passes through the double twist. The varying height of the product strands, and distance between them, ensures that the asphalt can encapsulate the wire, without developing a weak shear zone at the product interface. Fig. 1: Geometrical characteristics of the Road Mesh Other reinforcing products, due to their geometry, are able to absorb crack reflection stresses, but because they are not able to integrate themselves into the aggregate matrix, they contribute to reducing the rut resistance as well

MATERIALS FOR PAVEMENT REINFORCEMENT In a road, the deflections range between 0,3mm for an excellent road to 1,5mm on a poor quality road. For a reinforcement to impart any strength resistance, the reinforcement needs to offer tensile resistance at very low strains as indicated roughly by the green circle. It is quite clear that the red line shows that the steel mesh is able to by far provide the maximum benefit at the lowest strains. It is then also apparent that the bigger the deflections in the pavement, the greater the contribution of the reinforcement. This also makes sense, as we know that with conventional road design, if the road has low deflections in the order of 0,3mm, the pavement will perform quiet satisfactory, and should last through to the end of the next maintenance cycle. If it is a new road, then one could consider allowing the road structure to be thinner, thereby inducing higher design deflections, and incorporate the Road Mesh to take up the increase in tensile strains at the bottom of the asphalt as a result of the increase in deflections.

ROAD MESH APPLICATIONS FATIGUE: Crocodile \Alligator cracks Road Mesh is able to absorb the horizontal tensile stresses resulting from existing cracks and traffic loads, thus increasing the pavement life.

ROAD MESH APPLICATIONS RUTTING: Channelisation Road Mesh is installed at the base of the overlay (> 60 mm) thus intersecting the shear slip circles, eventually reducing the surface rutting

ROAD MESH APPLICATIONS TRANSVERSE CRACKS: Overlays on jointed concrete pavements The active joint is a discontinuity which tends to propagate to the road surface. Road Mesh is installed within the overlay to absorb the tensile forces

LABORATORY AND FIELD EXPERIENCES The key objective of the next slides is to review the existing worldwide experiences on the application of the double twist wire mesh reinforcement in HMA. Road Mesh is new to the industry and therefore requires an explanation of how to install the product. The theory is quite complex, and sometimes the audience will believe that the installation will follow suite. This is not the case as indicated in this presentation.

NOTTINGHAM UNIVERSITY (UK) The research investigated the effectiveness of different interlayer systems (geogrid, steel reinforcement and fiberglass grid) in preventing the reflection of cracks in HMA overlays. A repeated load shear test was first used to evaluate the interface shear strength and stiffness for unreinforced and reinforced samples. Only steel reinforcement provides interface shear stiffness comparable to the unreinforced case; both geogrid and glass fibre caused a significant reduction in the interface shear stiffness.

NOTTINGHAM UNIVERSITY (UK) Pilot-scale pavement tests carried out at the Nottingham Pavement Test Facility enabled the model to be checked under realistic conditions: the pavement consisted of two layers of asphalt with reinforcement between them, overlying individual concrete slabs with air gaps between them. The gaps acted as wide cracks, inducing reflection cracking in the overlying asphalt

NOTTINGHAM UNIVERSITY (UK) The test results have shown that steel reinforcement improves the fatigue life by a factor up to 3, well above the other materials performance

CAGLIARI UNIVERSITY (I) The research investigated the crack propagation process in the presence of steel reinforcement based on a finite element (ANSYS) model. The figure shows the crack development as a function of the load cycles at different frequencies (10 and 20 Hz): by making reference to a crack 1 mm deep, the reinforcement increases the pavement life by a factor variable between 3 and 12 (at 20 Hz and 10 Hz respectively).

SMART ROAD (USA) The Smart Road was conceived by the Virginia DOT in conjunction with Virginia Tech’s Transportation Institute and the Federal Highway Administration and allows testing of various hypotheses on pavement material performance and characteristics. Pavement materials can be tested under different environmental conditions using the All Weather Testing facility. The flexible pavement portion of the Smart Road includes 12 sections 100 m long, closely monitored through a complex array of sensors located beneath the roadway and embedded during construction.

SMART ROAD (USA) All instruments were embedded in the pavement section during construction and included pressure cells, thermocouples and specially made HMA strain gages.

SMART ROAD (USA) In Section I the steel mesh was installed on top of a surface mix base layer (SM-9.5A) and underneath a BM-25.0 base mix layer to investigate its capability as a reinforcing system 38 mm 100 mm 75 mm 150 mm 21B Aggregate layer (E=200 Mpa) Asphalt - Treated drainage layer (E= 1600 Mpa) Wearing Surface (SM 9.5A) (E= 5280 Mpa) Cement Treated Base (21 A) (E= 3700 Mpa) Road Mesh (E= 3500 Mpa) Base Mix (B M 25.0) Base Mix (SM-9.5A)

SMART ROAD (USA) Results of the FE models were compared with actual stress and strain measurements at the Smart Road. To quantify the contribution of steel reinforcement to the pavement’s service life, a classical fatigue law (Arizona DOT) was adopted: N = 9.33 10-7 et-3.84 The improvement is pronounced at intermediate and high temperatures and is approximately 140 % at the average temperature of 25° C both in the transverse and in the longitudinal direction. N = Number of cycles for crack initiation; and et = tensile strain at the bottom of the HMA layers.

SMART ROAD (USA) The Smart Road has provided the design criteria for the calculation of the design life of a Road Mesh reinforced pavement.

CATANIA UNIVERSITY (I) The research focused on the capacity of non-destructive pavement measurement techniques to quantify the increase in terms of resistance to deformation given to flexible reinforced pavements by steel mesh, was carried out elaborating data coming from a survey conducted with Falling Weight Deflectometer (FWD) and Ground Penetrating Radar (GPR) on an experimental section 250 meters long on a national road (SS 121) in West Sicily. N = Number of cycles for crack initiation; et = tensile strain in the HMA at the reinforcement level, E = elastic modulus of HMA.

CATANIA UNIVERSITY (I) Improvement of the pavement’s service life To quantify the contribution of steel reinforcement to the early stages of a pavement’s service life, a classical fatigue law (Asphalt Institute) was adopted N = 0.0796 et-3.291 E-0.854 The presence of the Road Mesh reinforcement provides a service life increase variable between 36 % (reinforcement at 15 cm depth) and 52 % (reinforcement at 8 cm depth) N = Number of cycles for crack initiation; et = tensile strain in the HMA at the reinforcement level, E = elastic modulus of HMA.

CONCLUSIONS - Nottingham Univ.: improvement factor ≈ 3; The performance of Road Mesh in asphalt pavement layers have been thoroughly investigated in the last 10 years through a number of research projects carried out by Universities and Road Authorities around the world, finalised to develop an empirical design methodology for reinforced pavements, validate FEM numerical results with beam tests and field data and evaluate the working life enhancement of a reinforced pavement. The main results of the researches in terms of FATIGUE LIFE IMPROVEMENT due to the Road Mesh are quite similar in terms of reinforcement’s effectiveness: - Nottingham Univ.: improvement factor ≈ 3; - Cagliari Univ.: improvement factor 3 - 12 - Smart Road: improvement factor 1.15 - 3.6 - Catania Univ.: improvement factor 1.36 - 1.52