The science of road rehabilitation

Developing a costeffective maintenance strategy requires an expert understanding, which begins at the design, material selection and construction stages. Saied Solomons, CEO, Sabita, talks to IMIESA about key challenges and preferred interventions.

Do provincial and municipal roads authorities have a clear understanding of road pavement condition assessment/analysis and how this influences current and future maintenance budgets? If not, how can Sabita assist?

SS A Rural Roads Asset Management Systems Grant became available in 2018 to enable effective investment in municipal roads through the development of road asset management systems, the collection of roads data, and the prioritisation of projects.

A budget allocation for this initiative is also made in the current annual budget and covers numerous district municipalities across all provinces. The intention was to train young professionals to assist with the necessary competence in this discipline. Sabita’s membership consists of highly qualified engineers who can assist in the rollout of this programme if required. What would bring clarity on the progress of this initiative is the publication of annual reports on the condition of the respective road networks.

What are your views on alternative products/technologies for road stabilisation (i.e. those that replace conventional materials like cement and lime)?

Bitumen stabilised materials (BSM) incorporating either bitumen emulsion or foamed bitumen are suited to both the construction of new pavements and pavement rehabilitation using in-place recyclers and/or off-site mixing plants. Pavements constructed using a BSM in the base layer are durable, sustainable and can contribute to meeting environmental objectives. BSMs are also suitable for labour-intensive construction.

The use of BSMs normally results in significant project cost and time savings, and can incorporate benefits such as: • reduced energy consumption, therefore lowered emissions • being a flexible material, it is not prone to shrinkage cracking experienced with cementitious stabilisers • improved resistance to moisture damage, as the finer aggregate particles are coated in bitumen • enables lower-quality aggregates, e.g. G5, to be considered for use as a base layer • assists with early trafficking, as material treated with foamed bitumen achieves a significant increase in cohesive strength once compacted. Sabita’s recently updated Technical Guide 2 deals comprehensively with these types of materials.

Could cold in situ recycling help to fast-track the rehabilitation of municipal roads?

Worldwide, the demand for road rehabilitation far exceeds the demand for new roads. In most countries, annual maintenance budgets are insufficient, resulting in an ever-expanding backlog of rehabilitation requirements. This situation has seen the adoption of in-place recycling as one of the preferred procedures for addressing the need for structural rehabilitation by recovering and reusing material in the existing pavement. Bitumen stabilisation enhances the properties of the recycled materials, providing service lives that meet conventional norms, normally at a reduced cost and over a shorter project duration.

In situ recycling offers environmental advantages, with the conservation of natural aggregates and a reduction in material wastage, noise, exhaust and dust emissions.

Pothole repairs are mostly a temporary solution, but is there a more permanent one?

The appearance of a pothole is generally a sign of a road in an advanced stage of distress through neglect of maintenance. While the methods described in Sabita Manual 12 refer to specific actions to repair obviously distressed areas, it is always essential to identify the cause(s) of the distress and to examine adjacent areas for early signs of the same distress types. If present, it is advisable to take preemptive action either to prevent the further development of such distress or to deal with such areas in the same way as the obvious case.

In many instances, damage to road pavements is caused by the inadequate draining of surface or subsurface water. Whenever repairs are made, the client or their agent should investigate this cause and, where appropriate, take action such as deepening or clearing side drains or installing subsurface drainage systems.

The repair of potholes is a consideration where there is no evidence of deep-seated

pavement failure of the base and/or subbase layer(s). Potholes should be repaired as soon as possible before they grow bigger – signifying a serious road traffic hazard – and cause the underlying layers, now exposed to water ingress and strength loss, to disintegrate.

Are road failures mostly due to poor aggregate selection and compaction, or is incorrect asphalt specification a core part of the problem?

It is important in the first instance to define what represents a ‘failure’, as a distinction must be made between the varied conditions that might make the pavement less than perfect.

In certain instances, the problem might not be a failure but rather a repairable defect or a condition that may only be a warning of possible future adverse developments. As an example, if a crack appears in pavement, it could be a sign of lack of maintenance rather than a failure. Failures, however, are either: • attributable solely to the quality of the pavement itself – these may typically appear in the form of stripping, ravelling, disintegration, cracking and rutting of the road surface, which may develop regardless of foundation support • in the form of slippage caused by the lack of bond between the top course of the pavement and the underlying base or intermediate layer • those attributable to deficiencies in the base or the underlying support. In the design of an asphalt pavement, the engineer is guided by documents such as Sabita Manual 35 and Sabita Manual 24 – which comprehensively cover best practice – in conjunction with national pavement design codes. Material selection is critical at this initial stage, which subsequently leads to various laboratory and paving trials of the mix design in order to assess whether the desired functional requirements have been met. The correct placement of the asphalt mix is also of critical importance and this activity is well covered in the recently updated Sabita Manual 5.

South Africa In the design of an asphalt pavement, the engineer is guided by documents such as Sabita Manual 35 and 24 – which comprehensively cover best practice – in conjunction with national pavement design codes

And in closing?

Roads are economic and social imperatives and proper road maintenance of this public asset, which delivers a higher economic return on investment than any other type of infrastructure, is vital.

The real cost of not maintaining roads is difficult to quantify, as poor roads are dangerous and affect road safety, lead to higher transportation costs and bottlenecks on busy routes. If done on time, maintenance can significantly extend the structural and, therefore, service life of a road. However, if road maintenance is deferred, it deteriorates rapidly and, instead of maintenance, road rehabilitation is required, making costs soar.

A well-functioning pavement management system is essential to carry out surveillance programmes to identify areas where maintenance is required, in order to prioritise and action projects. An output of a pavement management system is mapping the condition of the road network in a specific area. This should be published on an annual basis as visibility of the results of the road condition not only informs on progress, but also assists with accountability.

While the challenge of competence and developed skills is real, these can far more easily be dealt with than the impact of declining road conditions. Tackling road maintenance in earnest across our 750 000 km network will help avoid a debt burden on future generations.