Money down the drain?

Permeable pavements – one of the main sustainable drainage systems techniques (SUDS) – offer major environmental benefits but the cost advantages of concrete block permeable pavements (CBPP) are less well understood. So Interpave commissioned consultant engineers Scott Wilson to carry out research of various pavement materials and types, including CBPP, used in different applications and ground conditions.

To obtain realistic results, Scott Wilson used projects where they had originally been appointed as designers to calculate the initial pavement and drainage construction costs. These projects were then redesigned using various pavement types and a range of different ground conditions (expressed in terms of California Bearing Ratio values, or CBRs).

More than 250 different cases were considered including supermarket car parks and other car parks, estate roads for housing and industrial developments, parking for warehouses, container yards and airport ‘airside’ pavements.

Owner/operators will be more concerned with whole life costs than initial costs

For impermeable types of paving the same on-site drainage systems were used. Drainage systems were then redesigned to suit the three recognised types of CBPP (see box).

For a range of different application types, initial costs per square metre were plotted in graphs for each pavement type related to a range of CBR values, as shown in the example below.

The second stage involved applying whole life cost (WLC) analysis. WLC analysis was carried out for three of the previously assessed pavement types applied to four applications, all chosen to represent markets particularly suited to block paving and where its value may not yet be fully recognised. In each case, two alternative subgrade conditions were considered – a 3% CBR value representing a fairly poor quality of subgrade and 6% CBR for a reasonable quality – again, as shown in the example.

Most supermarkets will require major maintenance to be undertaken during off-peak hours

The research highlights interesting differences between various applications. For example, with supermarkets and other car parks System A (total infiltration) has the lowest initial construction cost, while conventional, impermeable concrete block paving closely follows asphalt.

Keeping up appearances

In the longer term, national supermarket chains are acutely sensitive to the appearance of stores. As a result, there is a need to maintain the aesthetics of the car park, resulting in a more onerous maintenance schedule than that usually required. Most supermarkets will require any major maintenance to be undertaken during off-peak hours, usually at night, allowing customers to park and shop at peak times.

Concrete block permeable pavements proved the most cost effective paving solution over 40 years

All this means any maintenance perceived as time consuming or requiring a long ‘curing’ duration is likely to be viewed unfavourably – hence the popularity of concrete block paving. Owner/operators will be more concerned with WLC than initial costs. Here the difference between CBPP (System C) and asphalt is minimal, with Systems A or B offering potential cost savings.

Overall, CBPPs proved the most cost effective paving solution over 40 years and where ground conditions allow full infiltration, System A provides the lowest initial costs in all cases. With poor ground conditions, the situation varies from one application to another but Systems B and C are shown to be competitive in all cases. From a WLC perspective, CBPP proved to be the most cost-effective paving solution for all four applications considered. But it should be remembered that the most expensive CBPP option, System C, has been used in this analysis and Systems A or B would allow further significant reductions in WLCs. In addition, the design lives for permeable paving used in the research have intentionally been extremely conservative and form the upper boundary of WLCs which could be expected in practice.