Reflet de la presse - The demands of asphalt reinforcement - Over 40 years' asphalt reinforcement experience using polyester grids

Introduction

A good bond between asphalt layers is essential for sustaining high traffic loading without incurring damage. For that reason, asphalt reinforcement should not reduce the bond between layers.
The long-term interaction of the reinforcement and the asphalt layers is crucial to the proper functioning of asphalt reinforcement.

Furthermore, the reinforcement must resist as much damage as possible from the stresses and strains applied during installation and overlaying / compaction of the asphalt.

Optimisation of reinforcement

The reconstruction and maintenance of asphalt-surfaced roads using polyester reinforcement grid is often an economically viable alternative to conventional construction solutions.

Experience gained over 40 years using such material has given very good results. Continuous product development and optimisation has produced the HaTelit® C 40/17 reinforcement grid.

The choice of polyester material for this grid is based on its mechanical properties being favourably compatible with the elasticity and stress-strain behaviour of asphalt.

To ensure the minimal effect on the bond between the asphalt layers, HaTelit® reinforcement grid is bituminously coated, thus providing good adhesion to the adjacent asphalt layers. Furthermore, for the past 10 years, HaTelit® C 40/17 has been produced with a very light, nonwoven backing, also impregnated with bitumen. This combination of a grid and a very thin nonwoven is most helpful during installation, ensuring good adhesion to the underlying layer and making reinforcement placement straightforward.

As the nonwoven is fully impregnated with bitumen, only a small amount of additional bituminous spray tack coat (bitumen emulsion) needs to be applied on site, which reduces the danger of bleeding resulting from too much spray being applied, or the danger of a poor bond resulting from too little spray being applied.

Testing the bond between layers

Since 1994, over 100 cores containing HaTelit® C 40/17 reinforcement have been taken from various projects and compared to corresponding unreinforced samples. Tests to determine the bond have been carried out by various testing laboratorles, generally using the Leutner test procedure [9]. The results confirm that the use of HaTelit® C 40/17 has no significant detrimental effect on the bond.

Table 1 shows the test results on cores from the re-surfacing at Jagel Airport, a military airport in Germany (1998), which today is still proving satisfactory in every respect. The milled surface [photo 1] was coated with 0.5 kg/m² bitumen e mulsion U 70 K and overlaid with HaTelit® C 40/17. The final installation depth was 40 - 60mm. The first four cores [photo 2] were taken four weeks after construction. One of the cores was an unreinforced reference sample.


Photo 1: Milled surface (Photo as jpg - 143KB)		Photo 2: Cores with reinforcement (Photo as jpg - 142KB)

The shear strength measured in the tests was very high on all the cores and in this instance even higher in the reinforced samples than in the unreinforced. However, it cannot be deduced from this that HaTelit® C 40/17 improves the bond, but it is clear that the bond is not reduced.

Core	Description	Shear force [kN/m]
No.1	HaTelit® C 40/17	36,42
No.2	Unreinforced	30,17
No.3	HaTelit® C 40/17	37,48
No.4	HaTelit® C 40/17	36,72

Table 1: Results of the bond tests [1]

Reinforced samples without a bitumen coating and composite products (reinforcement with a non-impregnated nonwoven) all showed a considerable reduction in the bond between the layers. In many instances these samples broke up during coring. Similar results showing the reductions in the bond between layers for various reinforcement types are described in [5].

In composite products, the bitumen-impregnated nonwoven is designed to have a SAMI (stress absorbing membrane interlayer) effect and the grid a reinforcing function. If, however, the nonwoven reduces the bond between layers, then the reinforcement cannot mobilise the tensile force. A reinforcing effect can only occur if there is sufficient bond between the layers to transfer the forces. The two effects cannot simply be added together.

Demands during installation and the overlaying / compaction of the asphalt

Even during installation the reinforcement may be subjected to high loading, when trafficked by tracked pavers or 'blacktop' lorries. Very high forces can also be applied to the individual strands of the reinforcement by aggregate movement in the hot blacktop during compaction.


Photo 3: HaTelit® C 40/17 trafficked by a tracked paver (Photo as jpg - 242KB)		Photo 4: HaTelit® C 40/17 trafficked by a 'blacktop' lorry (Photo as jpg - 223KB)

Currently there is still no specific test to determine the amount of installation damage to reinforcement in highway asphalt by the loads mentioned above. However, the standardised test in ENV ISO 10722-1, 'Procedure for the simulation of damage during installation', can be used to compare the resistance of reinforcement materials to mechanical damage.

In this test, the lower part of a rigid metal box (300 mm x 300 mm x 75 mm) is filled with a synthetic mineral aggregate and compacted. The mineral aggregate used in the damage test consists of sintered aluminium oxide with an aggregate size of 5 - 10 mm. The reinforcement is placed on top and mineral aggregate tipped loosely into the upper part of the box (300 mm x 300 mm x 30 mm). A loading plate (100 mm x 200 mm) is then cycled 200 times (from 5 kPa to 900 kPa at 1 Hz). The tensile strength is then tested again after removal.

Photo 5: Test set-up
(Photo as - 133KB)

Test results

Table 2 shows the average values from tensile tests, each derived from five specimens. The second column shows the actual tensile force from tests on the reinforcement at manufacture. The manufacturer's data for some products made from glass fibres relates to tests on the basic fibres used in the manufactured product. Single fibres are tested and then the theoretical tensile strength calculated based on the number of fibres per metre width. However, this value does not correspond to the tensile strength of the end product. Losses resulting from subsequent processing are not taken into account. It is also impossible, therefore, to take a sample on site and make a test to compare with the original product.
Column 3 shows the tensile strength after the installation damage test.

Product	Tensile strength Wide width tensile test DIN EN ISO 10319	Tensile strength 'Procedure for the simulation of installation damage' ENV ISO 10722-1
HaTelit® C 40/17	52,4 kN/m	37,1 kN/m
Mech. reinforced PP- nonwoven with additional textile glass fibres as reinforcement, manufacturer's data 50 kN/m	30,5 kN/m	3,9 kN/m

Table 2: Tensile test results before and after installation damage [3, 4]

Diagram 1: Results of the tensile strength tests

The very high resistance to mechanical damage also allows HaTelit® C 40/17 to be placed directly onto milled surfaces.

Manufacturers of glass fibre grids point out that, because of its fragility and brittleness, i.e. the low shear strength of glass fibre and the resulting high risk of damage, glass fibre should not to be placed directly onto milled surfaces.

How glass fibre reinforcement behaves when placed directly over the sharp edges of cracks, especially during compaction, has not been clarified up to now and requires further investigation.

Conclusions and the outlook for the future

The basis for an effectively functioning asphalt reinforcement are not the individual parameters of the reinforcement, such as the short-term modulus of elasticity, but rather the long-term modulus of elasticity and the interaction of the reinforced layer components as a system.

The interaction of the bonding between layers and the axial stiffness of the reinforcement are particularly important. To ensure this occurs in the long-term, the reinforcement has to be able to resist the demands made upon it during installation, the overlaying / compaction of the asphalt and also have a dynamic load bearing capacity.

All the important properties that reinforcement requires are tested and proven in the case of HaTelit® C 40/17. Numerous other laboratory investigations [5,6,7,8] and, above all, over 40 years' experience in practice have shown that asphalt reinforcement using HaTelit® C 40/17 grid is often a cost-saving and economically viable alternative to conventional construction solutions.

Bibliography

[1] Urbanski Ingenieurbüro für Geotechnik und Baustoffprüfung, Prüfbericht AsS 21/98/1578, Untersuchung von Asphaltbohrkernen / Bestimmung des Haftverbundes, 1998.
[2] Produktbeschreibung und Einbauanleitung GlasGrid, Stand 2004.
[3] Institut für textile Bau- und Umwelttechnik GmbH, Prüfbericht Nr. 1.1/17810/493-2003, 2003.
[4] Institut für textile Bau- und Umwelttechnik GmbH, Prüfbericht Nr. 1.1/17810/494-2003, 2003.
[5] De Bondt, A.H., Anti-Reflective Cracking Design of (Reinforced) Asphaltic Overlays, Doktorarbeit, Delft, Niederland, 1999.
[6] Elsing A., Sobolewski J., Asphalt layer polymer reinforcement: Long-Term Experience, New Design Method, Recent Developments, Proceedings Fifth International Conference on the Bearing Capacity of Roads and Airfield's BCRA'98, Trondheim, Norway, 1998.
[7] Kirschner R., Kunst P.A.J.C., Vergleichende Laboruntersuchung an polymeren Asphalteinlagen, 2. Kongress Kunststoffe in der Geotechnik K-GEO 92, Luzern, Schweiz, 1992.
[8] Montestruque, Rodrigues, Nods, Elsing: Stop of reflective crack propagation with the use of pet geogrid as asphalt overlay reinforcement, Proceedings of the Fifth international RILEM Conference, 231 - 238, Limoges, 2004.

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