On concrete pavements, road infrastructure deficit, challenges ahead – Part 2

Most of our networks, be they highways or minor arterials; are of the flexible pavement type. These road networks by whichever standards they have been designed are supposed to have the pavement (the sub layers described earlier) and the asphalt surfacing (the black stuff) having a design life ranging from 10 for minor roads to up to 20 to 40 years for highways. From a technical viewpoint, the 10 to 20- or 40-year life does not mean a collapse state but a time within which some major work may be required such as removing the asphalt surfacing and replacing it.

With this in mind, some of the re-constructed sections of the old Enugu – Onitsha Road which one would expect to have been designed for a minimum of 20-year life, failed under three years. Likewise, sections of the Lagos-Ibadan-Sagamu-Benin expressway failed woefully under five years until the recent intervention. Several sections of the Ajaokuta – Abuja Highway, the Auchi – Abuja Highway, the East – West Road, the Odukpani section of the federal highway to Calabar, sections of the Enugu – Onitsha Highway, especially at Ugwuoba are nothing but treacherous.

Over the years, budget preparation and implementation had become more of a ritual without any true meaning attached. To drive this home, a former works minister was questioned by a senator during screening regarding why a road in his constituency was abandoned for two years. The minister replied that when they got to the site, they discovered that there was water at the site, hence work had to stop. The question that comes to mind is, were there studies, concept, and detailed designs for the project? Else, how was the bill of engineering measurement (BEME) prepared if the water was not identified and the associated costs to mitigate its adverse effect on building a road through that terrain provided for? The BEME figures would have been the most realistic numbers to inform budgetary provisions.

Second and a major plank of this piece is on the issue of resilient road assets – flexible (asphalt) or rigid (concrete). It is not intended to make this write up a technical one where asphalt fatigue, stress distribution, stresses and strains, beam theory and pavement rutting will dominate the discourse. Rather, it is intended to present a simplistic view of pavement types and the likely killer diseases that have bedevilled the performance of our road assets.

Having followed the discussions on flexible and concrete pavements in the print and social media since the minister mounted the saddle, it will be worthwhile to define what these pavement types are with a view to clearing some of the misconceptions and abstract discussions that had floated around since this issue became a hot potato or do I say a Nollywood classic. In simple terms, flexible pavements are built from aggregates (usually rock from quarry) placed (compacted) in sub layers (usually two to three sub layers) and finished off with a wearing course (usually asphalt, the black stuff we see on all our roads). All these component layers are supported on the natural ground or foundation soil (usually called the subgrade).

The number and thicknesses of the sub layers are always variable depending on the class of road. The class is determined by the number of vehicles that would use the road while it is in service, hence highways, arterials, minor roads etc. On the other hand, rigid or concrete pavements consist of a concrete slab founded on the natural ground (subgrade) with or without the multiple sub-layers common in flexible pavements. Generally, rigid (concrete) pavements have only one sub-layer. The essence of the sub-layers in both the flexible and rigid pavements, multiple or single is to minimise the stresses (load from vehicles, especially trucks, transmitted through the tyres) onto the natural ground.

Concrete pavement capitalises on the strength of concrete to minimise the stress transmitted to the natural ground, hence the reduction in the number of sub-layers. From this simple definition, it can be understood that whichever way we go, flexible or rigid (concrete), the bottom line is about minimising the stresses on the subgrade. At this point, it may be pertinent to ask – why do we need to guard against the stresses imposed on the subgrade (natural ground). Once the subgrade is overstressed through multiple passes of cars and trucks, especially trucks with high axle loads, going through over time, the subgrade will become weak and lose its capacity to take up the stresses from wheel loads and consequently fail.

These failures usually manifest as rutting which gradually progress to potholes and consequent loss of function of the road asset followed by abandonment where no rigorous maintenance scheme is put in place. I may have to breach my earlier promise not to make this piece technical by bringing in an element of stress-strain of the natural ground (foundation soil on which flexible or rigid pavement are built on) now that I have been boxed into a corner for lack of a simplified social jargon to express stresses and strains. Most natural ground (subgrade) have their peak (maximum) strength at about 4 to 6 per cent strain. As the moisture content of the natural ground increases due largely to fluctuations in the existing ground water levels, increased rainfall which in recent times is heightened by climate change for non-sceptics, the strain at which the peak strength of the subgrade (natural ground) is reached drops due to increased pore pressures developed in the inter spaces between the soil particles. In some soils such as intermediate to high plastic clays, the peak strength drops drastically under increased moisture.

The impact of the drop in strength is that the natural ground (subgrade) which supports either flexible or concrete pavement loses its ability to support wheel load which it can previously support. In the worst case, it may lose up to 100 percent of its carrying capacity under fully saturated condition; a ‘boiling’ condition (effective stress = zero) where the generated pore pressure is equal to the vertical stress.

This explains why most failures occur during the rainy season. In the light of this simplified soil – structure interaction view, the prediction of the likely moisture levels (equilibrium moisture) of the subgrade (natural ground) over the life of a road asset is vital to their long-term performance or survival. Maintaining the equilibrium moisture of the subgrade borders on road drainage and must be properly understood by all road engineers.

They must be properly and logically designed and implemented to ensure successful long-term performance outcomes for either flexible or rigid pavement alternatives.

To be continued tomorrow

Dr. Ezeajughi, Civil Engineer, Geotechnical Engineering Consultant and COREN registered contributed this from his Brisbane, Australia office.