Under normal circumstances, a bitumen requires high temperatures (150°-180°C) to make it ‘workable’ on site. However, there are multiple reasons why such high temperatures may not be attainable or even desirable, the most common reason of which is that the mixed material will lose heat as it is transported from the production plant to the site. And so, the bitumen technologists have developed what is known as “Cutback”, a technique that condenses the bitmac hence making it workable at lower temperatures for longer periods. This effect is achieved by ‘doping’ the bitumen binder with a lighter oil (often kerosene, creosote or similar), which acts as a solvent, delaying the setting process unless it is evaporated.
There are different grades of cutback ranging from slow-curing, through medium to fast-curing. Medium-curing cutback is a common choice for hand-laid work, as it gives a good degree of workability for a reasonable amount of time, but still gives a useable surface in a matter of hours.
Slow curing bitmac (aka Deferred Set Bitmac) is mostly used by Local Authorities and the Utility Companies to effect minor or temporary repairs to roads and footpaths, because it can be stored in their depot or on the back of a wagon for several days, and then used as and when required. Once it has been trampled, it will achieve a degree of stability and be firm enough to carry traffic. This type of bitmac is the stuff found in the 25kg packs of DIY/repair macadams being sold in many Builder’s Merchants.
Fast-cure cutback bitmacs would usually be used in machine-laid projects, where a discharge of additional heat from the Paver Machine will solidify the bitmac that bit more workable and easier to level precisely with the screed bar at the rear of the Paver.
Types of Materials
There are dozens of different types of ‘tarmacadam’. Different cumulatives, different cumulative sizes, different binders, different binder colors, the list goes on and on and there is a collection for construction industry devoted to the specification of the various materials and the development of new products.
There are generic names for the swarm of surfacing materials, e.g. a 6mm dense hardstone wearing course, and there are Proprietary Brands, e.g. “ProDrive”, a surfacing course product manufactured exclusively by Bardon – Cumulative Industries. While “ProDrive” may be a 6mm wearing course material, it is to be clearly understood that not all 6mm wearing course materials will be “ProDrive”.
Macadams are all based on the code of a cumulative coated with a binder, usually bitumen, hence “bituminous macadam”. Asphalts are a mixture of asphaltic cement or mortar (often a bitumen with fine cumulative such as sands and grits) and some thicker total, such as gravel or crushed rock.
The major difference between macadams, other than cumulative size and pen grade, is whether they are classed as Open Graded or Close Graded. Open graded macadam is composed of cumulative with very little fines and may be penetrable. It is a popular choice for hand-laid base courses as it remains workable for longer at lower temperatures. Dense macadam contains a significant amount of fines (material of 3mm or less). This means it is often classed as an impermeable material, and the tighter-looking finish make it a popular choice for wearing/surface courses.
The choice of cumulative will affect the presence of the surfacing in the medium-to-long term. All macadam (unless intentionally tinted) tends to look jet black when first laid, which is largely because the bitumen binder coats all of the cumulative, making it appear black.
As the surface is traded, the bitumen on the surface is rubbed and weathered, exposing more and more of the bare cumulative content, enlightening what may be a new color.
Where a darker appearance is required over the longer term, a dark cumulative is required. This is often a basalt but other ‘hardstones’ are used, depending on what is available locally. In some parts of Britain, a granite cumulative, just as hard as basalt, may be used, but its lighter color will result in a much paler, light grey look once the surface has been trafficked. In much or Ireland and parts of northern England, a limestone cumulative is a popular choice, and while not considered a ‘hardstone’ it is perfectly adequate for residential driveways, footpaths, car parks and other areas with slow vehicular traffic, but it wears to a very pale grey, almost white color over time.
The thing to keenly remember is that the color of macadam when first laid may well not remain what it will be in 12 months’ time.