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As a direct result of a EU directive concerning potable water and the need to renovate urban water distribution networks by eliminating lead piping, a broad campaign of works is under way in the Paris area. For this project, SRBG developed lightweight GFRP plates to replace steel hole covers.
(Published on October 2008 – JEC Magazine #44)
1- HONORÉ GOACOLOU, ENGINEER IN TECHNICAL DIRECTION, EUROVIA
2- DONALD HEARN, EXPERT IN COMPOSITE MATERIALS, MAGNYTUDE
3- PHILIPPE PARISSE, DIRECTOR, SRBG
In view of the constraints of this particular site and the need to minimise public inconvenience, SRBG, a subsidiary of Eurovia, looked at ways to reduce on-site times. One important obstacle is the time required to remove and replace hole covers at the beginning and end of each working day.
Traditionally, holes are protected by thick steel plates (12, 15, 20 or even 25 mm, depending on the size of the hole) that require mechanical assistance to be moved. Steel plates can weigh up to 100 kg. With 30 or more holes in an average suburban street, one can easily imagine the time "lost" in such manoeuvres.
The question was to know if it would be possible to replace steel plates with a standard lightweight plate that could be handled by two people (or occasionally one), yet still offer the same safety guarantees as a steel plate. A point in favour of the standard plate approach was the fact that the holes in the road for this type of work are also standard (800 x 800 mm).
SRBG asked Magnytude to come up with a technically and economically viable answer.
The use of steel for hole covers hardly needs justifying as it offers many intrinsic advantages, and a new solution seemed difficult to find. The material is relatively cheap, exists in almost any size and can be cut subsequently on demand for particular applications. In case of damage, it can be cut up into smaller pieces that can still be useful. Steel is durable and hard-wearing, so it is eminently suited to the harsh conditions of the work-site and, importantly, it is easy to recycle. However, it is a heavy material, and this is where composites can come in. In that context, it was clear that the candidate material would be some form of GFRP and that the field of application would have to be carefully circumscribed.
Following discussions between SRBG, Magnytude and Eurovia’s technical and scientific departments, the following specification emerged for the proposed new plate:
Based on Magnytude’s experience in the field of industrial composites, the design process had to include the choice of a moulding technique from the outset. In fact, the application required high-performance GFRP material, and thus a reliable technique capable of producing a fairly high glass content. The construction sector is not keen to develop new processes, so we opted for the tried-and-tested vacuum bagging method, which could easily be extended to vacuum infusion if needed during the product development stage.
In much the same way, sandwich construction was the only practical solution capable of combining stiffness and low weight. However, in view of the high imposed service loads in terms of plate bending and the associated compression, very thick skins would be required, so that the usual skin/core thickness ratios were not feasible. Moreover, the overall plate thickness had to be minimised to avoid jolts to passing vehicles and undue dynamic loads on the road fixings.
Design work started on the basis of a fairly bulky mat combined with a woven-roving complex that would provide good mechanical properties with the low-technology moulding technique, while the mat would naturally provide a good wear surface.
Finite element analysis iterations were performed in the following order: solid plate of uniform thickness, solid plate with variable thickness, sandwich plate with constant thickness skins and, finally, sandwich plate with variable thickness skins. As expected, the last configuration gave the most promising results and was used for the next step.
A simple mould was built using a plywood base 18 mm thick surrounded by a tubular steel frame. The resulting 1,200 x 1,200 mm working area eliminated the need to machine the plates after moulding. Regularly spaced holes were bored on the inside of the frame to enable uniform vacuum application all around the plate from a single vacuum source. The hollow frame also provided a convenient resin trap. Vacuum bagging consumables were used in the usual way, but an absorption layer was placed on both sides of the plate to ensure that the core material does not impair resin distribution throughout the plate thickness.
Isophthalic polyester resin was chosen for the matrix (no gelcoat) and a typical woven roving/mat complex was used as reinforcement (300 g/m² mat + 800 g/m² WR).
A first plate was successfully produced using a PU foam core. Although a carefully calculated quantity of resin was used and very little resin was pulled out of the composite during fabrication, the final thickness was found to be lower than the theoretical value (about 13 mm instead of 15 mm). The plate was then placed over a shallow 800 x 800 mm hole located at the entrance to a SRBG warehouse where mixed traffic would be passing throughout the day. It was held in place by 8 bolts screw-hammered into the asphalt and passing through coneshaped bushings fitted into holes moulded in the plate. A fully laden lorry with a wheel positioned in the centre of the plate was used to define the initial deflection basis by measuring the height of the upper surface with respect to the surrounding ground; then the plate was left to survive as best it could. After a month, new readings were taken, showing increased deflection under load. On investigation, this was caused by a deterioration of the foam core material and partial subsidence of the sides of the hole. It was reassuring to note that, despite the considerable relative movement between the plate and the road surface, there was virtually no wear on the underside. The upper side was also undamaged and had taken on a slight polish due to the traffic.
The main conclusion at this stage was that the application needed a better-performing core material as the foam was slowly reduced to powder by the repeated load cycles.
It was decided to replace the PU foam core with balsa, which offers high transverse compression strength. Balsa would also better transfer shear stresses between skins. Additionally, two slots were moulded in two opposite sides of the plate to provide handgrips, with no impact on performance.
Two plates were produced using the new core material, one for mechanical testing and the other one to replace the first plate and continue the on-site evaluation process.
Testing was carried out on a 50-tonne load frame equipped with an hydraulic actuator and appropriate load cells. A square base was produced using rectangular tubes, with an 800 x 800 mm passage to simulate a hole in the road. Rubber buffer strips were placed between the road plate and the steel base to avoid local high compression spots that might have initiated a failure crack. Similarly, rubber sheeting was piled under a central loading plate (200 x 300 mm) to simulate the behaviour of tyre action.
The loading schedule consisted in a series of ratcheting load increases with pauses up to rupture. The whole test lasted only 20 min. Throughout the test, the main strains were measured using a series of strain gauges (X). The results are presented in Figure 4, showing the failure mode of the plate.
The plate behaviour was linear and reversible up to a load of 11 tonnes, where some internal damage occurred. The sound emitted at that time suggested delamination between the upper skin and the core due to compressive buckling. However, the part was able to carry load in an orderly manner up to 35 tonnes, when the two skins (forming a 200 mm wide band surrounding the plate) separated and subsequently buckled, with local bending failure.
During a test re-run, it was established that even a damaged plate retained sufficient residual strength to remain serviceable at the design load without real danger to traffic. This encouraging result prompted SRBG to order a small production batch of 15 identical plates, to be allocated to various work teams already on site. It was also decided that Magnytude would test certain selected plates at regular intervals to monitor the load/deflection curves. At the same time, a French patent was filed.
A working version of a GFRP hole cover was rapidly developed in response to a specific urgent need. We were able to meet that need and, although the plate (cover) is slightly overweight compared to the initial specification, it can still be easily handled by one man. Still composite plates weigh much less (30 kg) than steel plates that can weigh up to 100 kg. In this specific application, there was no need to consider such effects as fatigue strength, creep failure or weathering.