Crack and joint sealant
Practitioners responsible for pavement maintenance sometimes require a compound to fill and seal cracks in spray sealed, asphalt or concrete pavements. The aim is to seal the pavement to water ingress at the surface. A typical problem is the thermal contraction and expansion of the pavements with seasonal and diurnal temperature variations. This movement can exceed the resilience of conventional bitumen when used as a crack sealant and reopen the pavement to water ingress. Polymer modification of bitumen sealants is often utilised to address this problem and to impart a greater cohesive strength to the sealant at operating temperatures.
The availability of polymer modified crack sealants provides the practitioner with an improved performance option. He must assess whether the enhanced properties are required and provide an economic solution to this problem. In comparison with conventional cationic rapid setting emulsion, the price of polymer modified sealants can range from a modest increment on the basic price to perhaps three times the base price. The price is dependent on the nature of the polymer, its concentration and the manufacturing process involved.
Cracks and joints may be sealed by a variety of techniques including overbanding, blow/rout and fill.
Polymer modified crack sealants are based on road grade bitumen and may be supplied either as a hot bitumen sealant or an emulsified sealant. The polymers typically used are SBS, EVA, scrap rubber, or specialised proprietary polymers. Some products are standard and some are tailored by the producer to meet a client’s needs. The concentrations of polymer vary over a wide range.
Controlled trials have rarely been undertaken thus conclusive results are not available.
The following general rules of thumb apply:
* development of adhesion to the crack walls is probably the most crucial requirement.
* adhesion development will vary in accordance with the pavement type (Eg. granular pavements will vary from asphalt pavements).
* the preparation of cracks and cleaning is important.
* viscosity of the sealant application must be appropriate to the crack width and method of application (e.g. low viscosity for thin cracks).
* for filling of cracks there must be an appropriate width to depth ratio for the sealant “plug” to be sufficiently keyed and the strain geometry optimised. (W/D ~ 1.0 – 2.0).
* performance for three years is very good.
* if cracks are seasonal, treat during the season when they are widest if practicable.
* Do not open to traffic prematurely.
Where preparation and placement conditions and procedures are not ideal, adhesion is more easily attained with the cold applied-emulsion based sealants due to their lower viscosity over the period available for “wetting” the crack walls. Additionally, where crack sealing is considered to be a temporary treatment, conventional cationic rapid set emulsion and sand may sometimes prove an economically effective strategy.
The success of a crack filling and sealant application is largely dependent on the type and extent of preparation. If the preparation is carried out correctly, then the success rate of the sealant can be expected to be satisfactory. The following checklist should be used to ensure good preparation and application practice:
* Use compressed air to clear out fine detritus from cracks.
* If necessary use brushing or other techniques to remove locked in detritus.
* Ensure the crack is as dry as possible.
* Apply a tack prime coat or preheat if appropriate to the sealant and as preparation for a surface stripe.
* Ensure the tack or prime coat is fully dry – if recommended by the manufacturer.
* Apply the sealant with appropriate efficient equipment to meet manufacturer’s instructions.
* Strike off the excess material to provide a striping and blind the sealant surface with an appropriate sand or grit. When sealing a joint or routed crack with hot pour (100% solids) material, the sealant surface is left lower than the pavement surface to allow for seasonal thermal expansion.