A new Next superstore car park in High Wycombe is the first large-scale use of a fresh approach to permeable surfacing.
According to Environment Agency calculations, in the exceptional summer of 2007 swollen rivers bursting their banks inundated around 19,000 homes. A further 38,000, however, were flooded by rainwater running off impermeable paving or overflowing from overloaded drainage systems.
A similar pattern was observed during the winter of 2013/14. Sustainable drainage systems (SuDS) that slow down the release of rainwater into watercourses and drains are now almost universal on new projects, especially those with large areas of car parking. Permeable blocks or permeable asphalt are the usual surfacing options in such cases, but Lafarge Tarmac offers something quite different.
Topmix Permeable can best be described as a highly developed cement-bound material that bears more than a passing resemblance to ‘no-fines’ concrete (see box). “The problem with no-fines as a drainage material is achieving consistent permeability,” says Lafarge Tarmac product development manager Craig Burgess.
“It’s so dry it’s difficult to mix thoroughly. With Topmix Permeable we’ve concentrated on achieving a homogeneous mix with predictable permeability.”
The secret is “paste control”, he adds. Six years of development have yielded a mix based on 6mm single size crushed granite aggregate and containing both superplasticisers and other additives that Burgess says are “trade secrets”. Void content is up to 35%, yielding a drainage rate of up to 1,000l/m2/min.
At High Wycombe the car park for the new Next superstore is split into two sections. The 1,817m2 upper car park is used exclusively by cars and is entirely surfaced in Topmix Permeable, 150mm thick. Below this is a 350mm deep sub base/attenuation layer of 20mm single sized aggregate, all sitting on a geotextile membrane.
“Here the ground is highly fissured chalk, so drainage is excellent,” Burgess reports. “We had to adopt a slightly different approach for the lower car park, however, as there needs to be access for HGVs and fire trucks.”
Here only the actual parking areas are surfaced with the new mix. Traditional impermeable construction is used for the heavily trafficked routes - but these drain onto the parking bays, another 1,037m2 of Topmix Permeable.
Only readymix concrete depots fitted with pan mixers will be cleared to supply Topmix Permeable, Burgess says, although it will be agitated in the truckmixer en route. “It can be transported as far as a normal mix, and is a lot easier to handle than no-fines,” he adds.
“But it isn’t suitable for pumping. And it isn’t available on a supply-only basis when it’s being used as a wearing course. Instead, Lafarge Tarmac Construction Solutions offer a supply and lay package.
Expertise is needed to ensure the right degree of compaction. After coming down the truckmixer chute the mix is spread by hand then levelled with a roller striker and left 10% high. A “very quick” powerfloat pan finish then follows.
Effective curing is also important, so a polyethylene membrane is laid and left in place for seven days. In normal conditions concrete hydration will be virtually complete by then.
“Our target characteristic strength is 10N/mm2, but we usually achieve somewhere between 17 and 20N/mm2,” says Burgess. “Drying shrinkage is negligible, so no movement joints are needed, which saves a lot of time.”
Finally a specially imported proprietary surface hardener is applied. Coloured versions of Topmix Permeable are also offered, based on the use of liquid pigments so the colour is integral to the mix.
As water flows through the surfacing so readily, sub-zero temperatures pose no threat, Lafarge Tarmac claims. And in the highly unlikely event of the attenuation layer failing to drain the wearing course, the open pores allow ice to form and expand without damaging the concrete matrix.
De-icing salts dripping from parked cars will also drain away quickly, minimising the risks of chloride attack. The lightly textured surface of the wearing course is also said to have skid resistance comparable to other traditional carpark surfacings.
One inevitable question that every proposer of permeable surfacing systems has to answer is how vulnerable to clogging they might be.
“The problem with answering that is that we don’t know how the client will look after the surface after we’ve gone, we can only offer guidance on best maintenance practice,” Burgess says.
“If soil or sand are stored on it, for example, obviously there will be clogging. But the drainage capacity of Topmix Permeable is so high it can still perform as specified even with a high degree of blockage.”
Lafarge Tarmac recommends annual cleaning to maintain drainage performance. With regular maintenance, the combination of Topmix Permeable and the pervious aggregate sub-base removes or degrades pollutants such as silts, hydrocarbons from vehicles and heavy metal residues more effectively than other forms of SuDS, the company claims.
Burgess adds: “For heavily trafficked areas where other permeable surfacing such as blocks are used, Topmix Permeable can be used as a stabilisation layer. This gives a higher and more consistent permeability rate than conventional materials.
“For such applications we use 10mm aggregate - and we are happy to supply the contractor and let him lay it.”
Comeback for no fines housing?
No-fines concrete, a simple mix of Portland cement, a “single size” coarse aggregate and water, has been used for many decades. Its open pore structure made it an obvious choice for drainage applications: less obvious was its large scale use for housing construction.
Post Second World War, the massive demand for new housing led to a shortage of both bricks and skilled bricklayers. In response, contractor George Wimpey came up with a no-fines based structural system that was routinely used up to five stories high. It took advantage of some of the unusual properties of the mix to save on both cost and time.
Freshly mixed no-fines has little fluidity, hence the hydrostatic pressures on formwork are significantly lower than with conventional concretes. There is no risk of segregation - pours up to 8m high were commonplace. By the late 1940s Wimpey had developed a lightweight formwork system that enabled the entire outer shell of a normal two-storey house to be poured in a single operation.
A five-story block of flats would be constructed in one single storey and two, two-storey pours. Nominal coarse aggregate size was 20mm. There was no reinforcement in the 250mm thick walls, and compaction was achieved by hand-rodding, using 16mm diameter steel bars or similar. Target 28 day compressive strength was usually between 5N/mm2 and 15N/mm2.
Finally, the walls would be rendered externally and plastered internally. Insulation values equivalent to a conventional brick cavity wall were claimed. Internal walls would be single skin brick or block, laid in a simple stretcher bond.
Towards the end of the 1960s, however, planners began to favour tower blocks for social housing. Wimpey built no more no-fines houses, but many are still giving good service to this day.
And could Topmix Permeable also be used as a structural material for housebuilding? Lafarge Tarmac product development manager Craig Burgess says: “We’ve deliberately developed a high permeability mix specifically for paving.
“Housebuilding isn’t something we’re currently exploring, but Topmix Permeable could be developed for the right project.”
Where the soil below permeable pavements is far from fully permeable, some if not all of the rainwater passing through the pavement will eventually have to be discharged into watercourses or existing surface water drainage systems.
These days planning permissions for new developments will come with strict limits on the maximum rate at which this run-off can enter downstream systems. One simple yet elegant solution to the problem of controlling discharges is the vortex valve, which can offer precise maximum flow rates with a much reduced risk of blocking than traditional methods.
“These were basically manholes where the outflow pipe was significantly smaller than the inflow,” says Wavin product group manager George Gray. “But they were very vulnerable to blockage and silting up.
“Valves based on the vortex principle are much more reliable. They’re self-actuating, have no moving parts and need no external power.”
Predictable performance means that drainage systems can be designed from the discharge upwards, he adds. Each valve is custom made to match the individual location and is not transferable.
Wavin launched a new range of vortex valves at last week’s Ecobuild exhibition. Three versions are available, differing only in the mechanism for clearing any blockages that might occur. The company is best known for its vast range of plastic drainage products, so it had drawn on the expertise of Danish manufacturer Mosbaek to fabricate the stainless steel valves.
Gray says the company sees the management market as having great potential. “It’s really booming. There’s more attenuation, more storage.
“And in a series of focus groups we held we found an increasing awareness of the need for effective valves to control discharges.”