M53 Bidston Moss viaduct, UK
Eliminator waterproofing system combined with SA1030 provides long-term protection
The Bidston Moss viaduct provides a vital road link within Merseyside carrying the M53 motorway over the A554. The structure has an unusually thin depth of surfacing which necessitates the omittance of the 20mm red sand asphalt protection layer (APL) that is usually required by the Highways Agency. The limited depth of surfacing is necessary due to the load carrying capacity of the structure and the APL had been omitted in favour of using this weight allowance to provide the most durable road surfacing specification.
Despite taking this option the surfacing had twice needed to be replaced due to large scale break up – in 1993 and 1997. The premature failures have been attributed to the absence of a bond between the asphalt surfacing and the polyurethane waterproofing membrane that had been applied on both occasions to the in-situ concrete deck.
Highways Agency Area 10
Laser Special Projects Ltd
- Bidston Moss
The surfacing had been laid at a minimum depth of between 50 and 65mm and the site records show that it had been laid at sufficient temperatures to activate the tack coat used with the polyurethane system. However on both occasions when the crazed asphalt surfacing was removed, the tack coat was stuck to the base of the asphalt but not to the polyurethane waterproofing membrane.
Some may argue that the presence of water at the surfacing / waterproofing interlace, due to ineffective surface drainage, which is then subjected to the pumping action of traffic may have played a part in causing the detachment of the surfacing. However, evidence obtained from an untrafficked link road that had also been treated showed that practically all of the bond had been lost irrespective of any such effect. Other factors that could have contributed to the failure of the surfacing was the thickness and elastic nature of the polyurethane membrane. A thin layer of stiff surfacing would deflect on such a soft substrate and this would result in flexural cracking.
In 1999 the severity of the surfacing problems had again got to the stage that it had once more become necessary to replace the surfacing -that was less than two years old. The system chosen also had to cope with a varying upper regulating layer to accommodate the large variation in the depth of surfacing across the deck. The new surfacing specification was as in the image on the right.
When looking for a solution to this re-occurring problem, Babtie Group, the consulting engineers responsible for Bidston Moss viaduct, wished to utilise a system that had a proven track record with thin surfacing specifications throughout the world. As such, the system needed to demonstrate a lengthy, successful track record under similar conditions. Within Cheshire itself, the county council’s engineers had waterproofed Weaver Viaduct in 1994 which had a similar thin depth of surfacing.
For that project, Stirling Lloyd’s Eliminator waterproofing and tack coat system was used and no problems had been encountered including with the surfacing - either at the time or since. In addition Stirling Lloyd’s waterproofing membrane in conjunction with the SA1030 tack coat had been tested after six months service on Farndon Bridge, also in Cheshire and the results had shown that the bond achieved between the waterproofing and the surfacing was greater than the bond between the wearing course and the base course. Based on these experiences and extensive consultation with the TRL, on this occasion the engineers specified Stirling Lloyd’s Eliminator System, incorporating the all important SA1030 tack coat that was to provide the bond between the Eliminator waterproofing membrane and the asphalt surfacing.
Before the actual waterproofing works commenced the Engineers requested a full-scale site trial on a concrete slab beneath the viaduct. They wished to conduct this so as to satisfy themselves that the Eliminator waterproofing system could meet the required tensile adhesion strength on a concrete substrate. They also required the tensile adhesion and shear strength between the waterproofing / surfacing to be produced under representative site conditions. The adhesion values required within the contract specification were as follows:
- Waterproofing to substrate
- Tensile Strength 0.5MPa (min)
- Surfacing/waterproofing/ sunstrate composite (cores)
- Tensile Strength @ 23C 0.4MPa (min)
- Shear Strength @ -10C & 23C 0.3MPa (min)
- Shear Strenght @ 40C 0.15MPa (min)
The tests were carried out in accordance with the Highways Agency Technical Memorandum BD 47, by Babtie Group’s in house laboratories in conjunction with Stirling Lloyd. The values achieved in the site trial exceeded the above requirements and the works were allowed to commence on the main viaduct.
The main contract for the works was awarded to RMC Surfacing who appointed Laser Special Projects Ltd, one of Stirling Lloyd’s Authorised Contractors, to carry out the preparation of the deck and the installation of the Eliminator Waterproofing System. The existing failed surfacing was removed with ease since there was no bond between the surfacing and the previously applied polyurethane membrane. The polyurethane membrane was then completely removed and the concrete deck cleaned by grit blasting. completed numerous adhesion tests were carried out to ensure that the level of preparation and cleanliness had been sufficient to meet the prescribed tensile adhesion value of 0.5 MPa.
Once this had been confirmed the deck was primed with Stirling Lloyd’s PAR 1 primer. This sealed the prepared concrete surface and helped enhance the adhesion of the composite that was to follow The PARi Primer took approximately only thirty minutes to cure at which stage the deck was ready to receive Stirling Lloyd’s “Metaset Industry” repair mortar. Metaset Industry is a fast cure methyl methacrylate mortar that is based upon the same reactive resin systems as the primer and membrane and as such is completely compatible. This was only applied to isolated areas of the deck that had been damaged during the previous works in order to provide a reasonably flat surface. Local undulations greater than 3mm were removed, which would allow consistent compaction of the surfacing. The Metaset Industry cured and was ready for overlaying within 30 minutes.
The Eliminator membrane was then spray applied and the thickness continuously monitored using wet film gauge tests to ensure the minimum final film thickness of 2mm was achieved, with a strict maximum of 2.5mm. The Eliminator cured in less than 30 minutes and was then inspected for any potential defects. Once the membrane had been passed Stirling Lloyd’s SA1030 tack coat, which had been heated to achieve the application temperature range of 175 to 20000, was applied.
The SA1030 Tack Coat was poured on to the deck and spread out using a straight bladed rubber squeegee, to a minimum thickness of 1mm. Once the tack coat had been allowed to cool for a minimum of 30 minutes the system was ready to receive the surfacing base course. Maintaining a minimum rolling temperature of 90C the application of the hot base course activated the SA1030 tack coat, which when cooled, provided a strong, intimate bond between the waterproofing and the surfacing.
The change from a polyurethane waterproofing membrane, which had twice failed to form an effective bond with the thin surfacing specification, to an advanced resin system which has successfully waterproofed many structures around the globe, with a variety of thin surfacing specifications, resolved the problems encountered on Bidston Moss Viaduct.
Since completion of the works, many inspections have been made during the ensuing period, no problems have been encountered and the surfacing remains in-tact. The use of the Eliminator system as well as providing protection to the structural deck is expected to show benefits in the improvement of the long term durability of the surfacing.