Lighting Effects on Paved Surfaces and Why Surface Imperfections Appear Under Low Level Lighting
One of the commonest subjects of customer complaint arises from the unevenness of surfaces, especially on resin bound or asphalted driveways and car parks, caused by undulations and poor finishing with hand or mechanical tools. These are often not immediately noticeable at the time of installation, the work having been carried out during the Spring or Summer, the areas little used during daylight hours.
It is sometimes months before the customer becomes aware of issues regarding the work; only when arriving home after dark with the car headlights shining across the surface do they see lines and undulations in the surface.
Low level lighting, fitted into the surface or slightly above, diffusing light across the surface after dark, may show trowel or roller lines, in rows (asphalt) or curved lines (resin bound) appearing as light/shade with high and low spots creating ridges and hollows that seem higher/deeper than they actually are. This is known as ‘raking’ or ‘grazing’ light.
As expert witnesses, called to inspect and survey properties subject to disputes involving complaints from customers dissatisfied with the quality of an installation, and being limited by our legal brief and instructions, unable to offer advice or comment, due to a commitment to impartiality is frustrating.
The duty of the impartial expert is to establish whether or not the contractor has met the brief as set out in the quotation. This article is not intended to be universal, rather a general statement of likely situations, as sites and their uses will vary.
If the contract simply states the area is to be cleaned and prepared for a surface dressing of resin bound gravel at 15-20mm depth across the site, and the work has been completed using the appropriate tools and equipment, to the correct proportions of mix, with no obvious trowel lines evident, the contractor will have met their brief.
However, if the ‘night-time’ vision resembles an Arabian desert once lit with low-level lighting or headlamps, due to surface undulations, the customer is likely to complain about the workmanship.
Fixed points are generally ironwork – drainage inspection and similar covers may have been installed at different levels – or in an uneven manner, with a high and low side creating a plane that cannot be ignored or easily altered. These rigid features will cause issues when creating a surface that cannot be laid to an even level or plane. (They may be adjusted at extra cost)
Although they may have been there for years, only once a new, clean single-colour surface has been applied, do they become aware of their presence.
Resin Bound Gravel
The art of laying resin bound gravel (RBG) is not simple. Knowing the correct temperatures for application, ensuring accurate batching and site handling are skilled jobs, made easier with the professional use of petrol power tools such as a mechanical float trowel with a smoothing pan should reduce the number of ‘laying lines’ in the surface when used correctly. (Hand tooling may leave curved lines in the surface that are difficult to see when laying). This should be laid to a minimum of 15mm, maximum depth of 30mm for strength and stability.
Asphalt Surfacing
Often referred to as Stone Mastic Aggregate (SMA) is a type of asphalt, laid with a surface of either 6mm (SMA6) or 10mm (SMA10). Both are rut-resistant surfacing materials with an aggregate size of either 6mm or 10mm.
Both are hot-laid to a depth of 20 – 40mm (usually 30mm) over a base, or binder course of 50 – 80mm depth of 20mm asphalt, over a foundation of between 100 and 150mm (depending on traffic weight and use) layer of MOT Type 1, forming a multi-layered highly durable surface.
Correctly installed, with each layer (including the original ground – which should be stable and meet correct California Bearing Ratio (or other means of checking base compaction) suited to the site and intended use/weight of traffic – with all materials laid to correct depths and sufficiently uniformly compacted) – there should be no issues with an inaccurate surface.
However, it is not unusual to find that the wearing course – especially SMA6 – will have pressure lines in the surface, where either the wrong compacting machine has been used (vibrating plate) instead of a tandem power roller, where the whacker plate has been forced into an angle by either site ironwork or an inadequately compacted binder course or foundation.
Once these lines in the hot lay tarmac have occurred, they are difficult to remove.
Effects of Low-level Lighting
During daylight, unevenness and roller/trowel lines may be dismissed as unimportant, but once bathed in low-level light, especially atmospheric mood lighting, undulations and marks can become major irritants to a customer, to a point where they claim compensation for remedial works, to eradicate the problems.
Alternative landscape lighting schemes, with downward lights spreading light across the surface in a non-directional manner may improve the visuals, although some customers will not appreciate the solution.
How to Avoid Complaints
By carefully surveying the site before quoting, looking at the various pieces of ironwork and their relative angles and levelness of surfaces within the precincts to make an assessment of likely problems with lighting (including car headlights at night) it is advisable to warn the client, in writing, before tendering, with a note in your terms and conditions to the effect that you cannot be held liable for any problems with final levels due to the ironwork, explaining your logic and reasons.
Ensure that you are using the correct equipment, for all processes including sub-grade, base and surface applications, meeting all specification regarding temperatures and material requirements. Perhaps dissuade the customer from installing low-level lighting if in any doubt.
