Milling and recycling HaTelit®-reinforced asphalt

The use of asphalt reinforcement considerably extends the service life of a resurfaced road.

It is now almost 40 years since Huesker developed HaTelit® asphalt reinforcement to prevent the propagation of reflective cracking from an old asphalt layer through a new surface course. Many scientific studies and decades of practical experience have shown that the use of HaTelit® extends the service life of a rehabilitated road by a factor of 3 to 4. The formation of reflective cracking is considerably delayed or even completely prevented.

HaTelit® C 40/17 is a reinforcement grid made from high-modulus polyester which is combined in the factory with an ultra-thin nonwoven installation aid. The grid and the installation aid are given a bituminous coating to ensure the optimum bond between the asphalt layers. Interlayer bonding is an important parameter in the functioning of asphalt reinforcement. A reinforcing effect can only be achieved where the bond is capable of transmitting the forces [1].

However, even the best asphalt reinforcement cannot guarantee an asphalt road will have an infinite life. The ease of removal of surfacing, by milling among other methods, is an increasingly frequent focus for discussion.

Milling trials were carried out by Huesker in conjunction with “Mischwerk Schwelm” (in 2004) and RWTH Aachen (in 2008) to demonstrate that a polyester grid (HaTelit®) can be milled as normal and that the millings can be recycled.

"Investigation of the milling characteristics and recyclability of HaTelit®-reinforced roads" in conjunction with “Mischwerk Schwelm” (2004).

A trial length was laid in May 2004 on land at the asphalt mixing plant at Mischwerk Schwelm to determine the milling characteristics and recyclability of HaTelit®-reinforced roads.

The construction of the trial length was as follows: A 0.6 kg/m² coating of bitumen emulsion (U70K) was sprayed on to an existing asphalt base and a layer of high-modulus polyester reinforcement (HaTelit®) installed. The reinforcement was then overlaid with a 40mm thick asphalt surface course, in accordance with the applicable technical rules.

The trial length was removed after about 6 weeks. The milling depth was 50mm; in other words, it extended about 10mm below the asphalt reinforcement. The milling was carried out using a small milling machine (Wirtgen W 500) with a drum width of 0.50 m.

During the removal process, it was already clear that the reinforcement grid (HaTelit®) had no detrimental effect on the milling operation. Likewise, it was found that only short lengths of fibre residue were present in the millings. The machine operator estimated that his milling machine was working at about 80% of its normal capacity on unreinforced roads.

The millings were collected and then crushed for the investigation into recyclability. The crushed millings were added at a rate of 30% to the mix for a new asphalt base layer, which was then laid (with the recycled millings content) in the test length. Here too, there were no observed detrimental effects on the process.

Testing the mix for the new asphalt base (with recycled millings content) showed negligible differences compared with the reference sample without reinforcement fibres.

Finally it should be mentioned that both milling tests took place using a small milling machine. More powerful machines would be used for the larger areas encountered on full-scale roads. The results of the tests indicate that more powerful machines would not suffer any detrimental effect on their milling speed.

Huesker has been producing asphalt reinforcement for almost 40 years. To date we have not encountered any complaints relating to the removal of HaTelit®-reinforced roads, which also serves to show that the use of HaTelit® results in no detriment to milling performance.

Investigations into the milling characteristics of HaTelit®-reinforced roads and recycling of the millings (RWTH Aachen, 2008 [2])

The influence of high-modulus polyester (HaTelit®) asphalt reinforcement on milling characteristics was investigated under defined conditions by the Institute of Road and Traffic Engineering at the RWTH Aachen University.

On its own 26m long, 1m wide test bed, the Institute laid, and subsequently removed, lengths of different road construction using ordinary methods typically employed on site. Various test lengths of road construction, including some that went beyond the limits of RStO 01, the German road construction design and maintenance manual, were laid on a frost-heave protection layer. The road materials were laid by a rail-mounted paver machine with a high performance compacting screed. The surface was given its final compaction by a tandem vibratory roller.

A small milling machine with a milling drum width of 500mm was used for the milling tests.

The aim of the investigation was to analyse and evaluate the milling characteristics of the reinforced road construction in terms of process engineering and the machinery used. In addition to investigating particle size distribution and the type and size of reinforcement fibres in milled asphalt, the possibility of recycling the removed asphalt containing reinforcement fibres in the form of asphalt granulate in bitumen-bound layers was examined.

Test procedure

A 60mm asphalt binder course (AB 0/16S) and an asphalt base were laid over a frost protection layer on the test bed. After a resting time of about one week, the polyester asphalt reinforcement (HaTelit®) was installed in accordance with the Huesker installation instructions. This was then overlaid with a 40mm asphalt surface course (AB 0/11S). No disturbance of the asphalt reinforcement was detected during the laying of the asphalt surface course. No wave formation or slipping of the asphalt reinforcement was observed.

Removal - milling tests

The test bed was divided into two different test lengths to investigate the influence of the HaTelit® asphalt reinforcement on the removal of the asphalt construction.

On the first few metres (1st test length), the wearing course was removed by milling down to a few millimetres above the asphalt reinforcement, in order not to disturb the asphalt reinforcement itself. The idea behind this was to leave the reinforcement in place during refurbishment by just milling off the surface course material.

Result: Although the asphalt reinforcement had a remaining cover of only a few millimetres, no detachment nor removal of fibres of the asphalt reinforcement by debonding was observed. The millings were solely composed of the asphalt layer material with no evidence of particles of asphalt reinforcement.

In the second trial (the remainder of the test bed length), the asphalt reinforcement was within the milling depth. A milling depth of about 50mm was selected to ensure that the asphalt surface course and the first centimetre of the asphalt binder course (including reinforcement) were removed by the milling machine in a single milling operation. This procedure is recommended by Huesker for the removal of HaTelit®-reinforced roads.

Result: During removal of the material, from the process engineering point of view no detrimental effect on the milling operation was observed. As with the first test length, the millings were finely graded. The fibres of asphalt reinforcement produced from the milling process were evenly distributed in the millings. The fibres had an average length of about 100mm.

After completion of the milling operations, the milling drum was checked for adhering fibres. It was clear that over the whole test bed almost no (only 2) fibres had been trapped in the miling drum. During this test, no detrimental effects on the milling process were observed.

Recyclability: Effect of the reinforcement fibres on Marshall stability

As part of tests on the asphalt, the effect of asphalt reinforcement fibres in the asphalt material on its recyclability was investigated. Marshall test specimens were made from the asphalt binder layer material with and without asphalt reinforcement fibres and their Marshall stability and flow value was determined. The reference sample was equivalent to the asphalt binder course laid on the test bed. The variant with asphalt reinforcement fibres was made up to have the same aggregate grading and binder content as the reference sample. This was achieved by the controlled addition of the appropriate quantity of uncontaminated aggregate and binder to the recovered asphalt. The aggregate and binder used were from the same batches of aggregate and binder as were used for making the asphalt binder mix (reference sample). This ensured that the reference sample and the variant containing the recovered asphalt and asphalt reinforcement fibres had the same aggregate grading and binder content. The asphalt reinforcement fibre content was the major difference between the two variants and the purpose of the tests was to determine the effect of these fibres.

There were only relatively small differences with respect to bulk density and void content between the Marshall test specimens used for the tests. The values for Marshall stability and flow were virtually identical.

The results for the Marshall test parameters are shown in the following table.

	Reference sample	With reinforcement fibres
Marshall stability	8.4 kN	8.5 kN
Marshall flow	3.6 mm	4.3 mm

Conclusion:

No negative indications were observed in the course of the asphalt testing to determine the effect of asphalt reinforcement fibres on recyclability (on the basis of Marshall stability parameters).

[1]  FGSV Arbeitspapier Nr. 69 "Verwendung von Vliesstoffen, Verbundstoffen und Gittern im Asphaltstraßenbau"

[2]  Schlussbericht Nr. 0802791 "Untersuchung des Einflusses der Asphaltbewehrung HaTelit® auf das Fräsverhalten von Asphalt sowie der Wiederverwertung des Ausbauasphaltes"