Heavy-duty asphalt

How does Heavy-duty asphalt work, step by step?

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   Step 1: Prove the formation and the layers beneath

   A flexible pavement carries load by spreading it through successive layers, so the performance of the finished surface is decided in the foundation. Formation, capping and subbase are placed and compacted in controlled layers and tested against the designer's requirements, and the whole area is proof-rolled before any bound material is laid. Weak spots found at this stage are dug out and replaced; the same weak spot found after surfacing shows up as a rutted patch within a season.

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   Step 2: Install drainage and services before paving

   Drainage, ducting, chambers and gullies are installed and surveyed before the bound layers go down. Chamber and gully frames are set accurately to the finished level of the surrounding surface, because a frame standing proud or sitting low in an asphalt yard is hit by every machine that crosses it and becomes a permanent maintenance item. Interception and separation required by the environmental permit are installed and tested at this stage.

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   Step 3: Agree the materials and prove them by trial

   The materials for each layer are proposed by the contractor and the supplier against the specification, and on most projects a trial area is laid to prove the mix, the laying temperature, the rolling pattern and the achievable density and finish. The trial settles arguments before production begins and gives the paving crew a proven method to repeat. The material designations and their acceptance criteria come from the specification and the pavement engineer.

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   Step 4: Lay the bound layers to the paving plan

   Layers are laid by paver in a planned pattern of runs that puts longitudinal joints away from the main wheel paths and away from the edges of the heaviest loaded areas. Continuity of supply is planned so that the paver keeps moving - a paver that stops leaves a transverse joint and a temperature discontinuity that will be visible for the life of the pavement. Laying temperature is monitored continuously, because compaction is only possible within a temperature window and once material is cold the opportunity has gone.

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   Step 5: Compact to the rolling pattern

   Rollers follow the paver in the pattern proved by the trial, working the material while it is within the temperature window to achieve the specified density. Density is the single strongest predictor of how long a bituminous layer will last, and it is measured rather than assumed. Edges and joints get particular attention because they are the least compacted and the first to ravel. Longitudinal joints between adjacent runs are formed hot against hot wherever the paving plan allows.

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   Step 6: Detail the edges, joints and around fixed items

   Joints against kerbs, channels, chambers, existing pavement and concrete areas are cut back to a sound vertical face where necessary, treated with a bond coat, and compacted carefully. The junction between a flexible and a rigid pavement is a permanent point of weakness and is detailed by the designer rather than improvised. Hand-laid areas around fixed items are compacted with vibrating plates or small rollers to the same standard as the machine-laid work.

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   Step 7: Open to traffic and set the loading rules

   The surface can be opened once it has cooled sufficiently, which is the main commercial advantage of the material. The operator is told clearly where containers may and may not be stacked on asphalt, and where equipment may stand for long periods, because sustained point loads on a warm surface are what damage it. On most projects those rules are agreed with the terminal operator in advance and are reflected in the yard markings and the operating procedures.

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   Step 8: Mark out, hand over and plan the maintenance

   Levels, regularity and thickness are surveyed against the specification, then markings, lane delineation and signage are applied. The handover record covers material records, laying and compaction data, density results and the level survey. Because asphalt is a maintained surface rather than a fit-and-forget one, the operator is given an inspection regime, planned resurfacing expectations and the information needed to plane and replace areas as they wear.