The key to the successful application of aggregate is having a well-prepared subgrade, using the right aggregate at the right thickness for the job, and applying it with skilled operators in the right weather conditions.
The subgrade is the road foundation. Make sure this is done well. Use a grader to shape the road. Eliminate depressions and holes, and get the camber right on corners. Metalling is an expensive way to solve poor formation practices. If soft spots are found, dig these out and replace with quality fill. If needed, use geosynthetic fabric as a barrier in weaker subgrades.
Pavement aggregate layers should be placed in a thickness that is not less than 2.5 times the maximum size of the aggregate, but generally not greater than 300 mm thickness in a single layer. For example, a GAP65 aggregate can be placed in layers from 160 mm to 300 mm thickness, whereas aggregate containing quarry rock up to 120 mm particle size should be placed in 300 mm thick layers. Placement of aggregate that has particles exceeding 150 mm in diameter on the pavement should be avoided, except where it is intended that this material will be in-situ crushed with mobile crushing equipment.
The quantity of aggregate required to create each surfacing layer should be calculated before beginning the pavement spreading operation. This is to ensure that the required quantity is available, and to maintain sufficient quality control over the placement so that the required pavement depth is uniformly achieved. If this is not planned and monitored properly, local areas of reduced thickness may be present. These will reveal themselves as soft spots once the road carries heavy traffic in wet conditions.
Pavement aggregate volume requirements are calculated by using the following formula:
Volume = Road length x Pavement width (incl. allowance for shoulders) x Thickness x Compaction factor
A 1.2 km length of 3.5 m wide road with 0.5 m wide shoulders is to have a compacted pavement layer of 200 mm with an aggregate with a known compaction factor of 1.4.
Volume = 1200 x 4.0 (3.5 m plus 0.5 m tapered shoulder both sides) x 0.2 x 1.4 = 1,344 cubic metres (say 1,350 cubic metres allowing for some handling loss)
The compaction factor is the amount that the aggregate will decrease in volume once it is compacted on the road. This will depend on the aggregate being used, but will typically be 1.3 to 1.5. For example, to get a 100 mm depth, between 130 mm and 150 mm will need to be applied. Once the required volume is calculated, the length of road that will be surfaced by each truck load should be calculated. This is then used as a guide for the operation to ensure the correct quantity is applied. For example, if you are putting a m3 of metal on a metre of road, and the truck carts 7m3, then the spread should be 7 m long.
Where a local aggregate is available at low cost, it is less important to monitor quantities because placing additional thickness in some areas has minimal impact on total road cost. Where pavement surfacing aggregate is being supplied from a quarry at some distance from the forest, it is especially important to monitor spreading rates and thickness. This can be achieved by marking the distance each load is to be spread over, and ensuring each truck operator spreads over this distance.
Pavement aggregate should be handled carefully to minimise the effects of segregation. Segregation is the separation of aggregate particles into size categories, and this occurs when aggregate is deposited in a stockpile or is bladed along the road surface. The fine particles tend to fall out of the aggregate mix, while the larger particles roll or are carried along by the blade. As a result, the outer edge of a stockpile will tend to contain a higher portion of large particles, and the bottom of the stockpile will have a higher concentration of fines. Where segregation is observed, the loading operation from a stockpile should include some re-mixing to achieve a uniform distribution of the entire particle sizes in the aggregate mix.
Unscreened metal can be dumped by the trucks then spread with an excavator. Oversized rock may need to be track rolled into the subgrade. The depth of the aggregate depends on the pavement purpose, and the subgrade strength. There are some rules of thumb like ‘no less than 300 mm’ or m3/m of road. There are many opportunities to reduce the volume of aggregate on roads, especially if metalling costs are expensive. A good place to start is determining the subgrade strength. Consider spreading a layer of crushed/screened aggregate to improve the running surface.
Aggregate spreading is best achieved through a spread from a moving aggregate truck. Dumping aggregate, and blading it into position with a small bulldozer, is likely to result in segregation, but this may be the only possible method if a course, uncrushed quarry rock is being applied to form a sub-base layer. A grader should be used to give the final shape. If the subgrade becomes saturated, the placement of pavement aggregate should be delayed until the subgrade material dries out.
On arterial and secondary roads, the placement of the final aggregate pavement layers should be accompanied by a grader and vibrating compaction equipment. The spread aggregate should be shaped by the grader to achieve the required crossfall and to remove ruts, with the minimum of re-working of the aggregate material to avoid segregation. Once the shape is correct, the entire pavement layer should be compacted as soon as possible with a minimum of four passes of a smooth steel drum vibrating roller of at least 6 tonne static weight.
On less critical secondary roads and spur roads, it may be unnecessary to grade and compact the pavement aggregate layers with a grader and vibrating roller. Spreading of aggregate, and reliance on truck wheel compaction, may suffice for these less critical roads. Typically truck wheels, although effective at compacting, cannot compact the crown of the pavement, so there will always be a portion of the pavement where, initially, water can penetrate to the subgrade. This is a problem if the road is to be used immediately, but not so much of a problem if the pavement is left for a winter to stabilise and seal.
Applying aggregate in poor weather conditions can lead to problems. Unsealed and wet aggregate spread on a wet subgrade could lead to major problems. However, in some situations it is better to take the risk on the metalling rather than leave an erosion-prone subgrade.