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Tarmacadam – Cut-Back & Pen Grades
Tarmacadam – Cut-back
Reference has been made earlier in this website to tarmacadam, or bitmac being delivered to sites in a hot box or insulated truck in order to keep it hot and workable. Temperatures as high as 150º to 175º are quite normal, but there are sometimes reasons why such heat cannot be maintained or when the cooling process needs to be slowed down to keep the product workable for a longer period of time.
Cut-back is a process which, by adding a light oil such as kerosene or creosote to act as a solvent, the bitmac retains its workable properties for longer periods at lower temperatures. The amount of solvent added determines the grade of cut-back from slow curing to medium or fast curing without affecting the ability of the bitmac to take traffic within a reasonable time. Slow curing bitmac is used for minor repairs to pavements, as once produced it can be stored for a number of days, often on the back of a truck before being used. It is also the type of bitmac sold in packs in builders’ merchants and DIY stores to be used for minor domestic repairs.
Although there are other brands of instant tarmacadam repair compound, Quick Mac™ comes ready to use for remedial works on driveways, footpaths and patio areas. It can be used to repair holes between 20 mm and 60 mm deep and the surface can be used immediately, though joints with the existing retained surface should be sealed.
Tarmacadam – Penetration, or Pen Grades
Tarmacadam, which is properly known as bituminous macadam, and is abbreviated to bitmac, consists mainly of bitumen and aggregate. Different types and sizes of aggregate are used to produce a particular grade of bitmac, making it more suitable for one application or another. Unsurprisingly then, there are also different grades of bitumen, the binder used in bitmac. The method used to measure the hardness of bitumen is extremely complicated but in simple terms, refers to how easily it is penetrated – hence the penetration, or pen grade.
A needle is used to penetrate the hardened bitumen and a penetration of 0.5 mm indicates a penetration grade for the bitumen of 50. Pen grades range from 15 pen to 450 pen and the lower figure signifies a lower penetration of the needle’and therefore the harder the bitumen is. Generally, the harder types of bitumen are used in asphalts, for stone mastic asphalt (70 pen or less) and the softer ones used in tarmacadam, or bitmac.