By Engr. Odinaka Victor Okonkwo Ph.D
There have been several talks on the use of flexible or rigid pavements for our Nigerian roads. The merits and demerits of both have been well noted. While rigid pavements are costlier, they can withstand heavier loading. The flexible pavement has a shorter lifespan and is easier to maintain. Interestingly these are based on statistics from other developed countries. To find out what is best for Nigeria we need to make decisions based on our own experience.
While we can conclude that flexible pavements have a shorter lifespan of 15 to 20 years, is that the reality in Nigeria? The answer is no. If our roads do last for that long the consideration for rigid pavements would not have started.
The Road pavement
To be able to propose an option for the country, there is need to briefly x-ray the various components of the road pavement and their functions.
1) The subgrade: This is the natural surface upon which a road is built.
2) The sub-base: This is a layer of granular material (laterite) that receives the load from the base course and transfers it to the subgrade. It is also used as a fill material to establish the required slopes of the road.
3) The base course: This is a layer consisting of mostly crushed stones that serves the purpose of receiving the concentrated loads (point loads) from vehicle wheels and distributes them (spreads them out) over a wider area enabling the sub-base material to carry them. It is the load bearing layer (stratum) in a road pavement.
4) The asphaltic layer: This is the final layer. It provides a waterproof layer that protects the base course material from water. It also provides a wear-resistance surface that protects the base course from wears due to braking action and screeching of vehicle tires.
The asphaltic layers and the base course are the two most importance components of the road pavement. While the base course material is more rigid and capable of carrying vehicular loads and spreading it over a larger area of the weaker subbase material, it is weak when exposed to water. It also has a low resistance to wear. If a vehicle on high speed suddenly brakes on top of a well laid base course, some of the materials of the base course are removed, raising dust. An asphaltic layer on the other hand is very resistant to wear. The screeching of tires on it rather wears off the tires and leaves the asphalt intact. The asphalt is also very resistant to water. Exposure to water does not cause it to lose strength. Asphalts are however not a high load bearing material like the base course. It is flexible and bends under load and so is not excellent at distributing loads over wider surface areas like the base course.
The relationship between the base course and asphaltic layer is therefore symbiotic. The base course material carries the load while the asphaltic layer protects the base course material against wear and water.
The rigid pavement
A rigid pavement mainly varies from the flexible pavement because of the replacement of the asphalt layer with Portland cement concrete. This is a material that attempts to possess all the good qualities of the crushed stone base course and asphalt. It has very good mechanical properties and so can carry loads. It is resistant to water (does not disintegrate in water), the high grade versions of it is also resistant to wear. Its strength (load carrying capacity) can be increased with the addition of steel reinforcement bars.
It appears to be the one final bus-stop with regards to road pavements. However its two major components (cement and reinforcement rods) are expensive. In standard rigid pavement designs, the Portland cement concrete replaces the asphaltic layer in a flexible pavement. To resist wears due to braking actions of vehicles it must be well prepared concrete of high grade (30 and above).
Our problem
While we propose a sudden shift from flexible pavements to rigid pavements it is important for us to know why most of the flexible pavements do not achieve their envisaged lifespan in Nigeria. I think this hovers around quality control during road construction and increased axle loads from vehicles. The construction of our flexible pavement requires some painstaking quality control measures which are increasingly harder to enforce. The sub-base is compacted to a required density (standard compaction) and tested at every specified distance (chainage) to ensure this quality is achieved.
The same applies to the base course layer. The base course material must also be laid to a specified thickness. The priming of the base course surface is done with a cutback (diluted) bitumen, its quality is hard to enforce. The asphalt is laid at a high temperature and compacted at a specified temperature for it to retain the desired properties. These are hardly noted or enforced in today’s road construction in Nigeria. This is outside what happens in the asphalt plants and the fact that they are limited in number and most contractors have to patronize the ones close to the road construction site irrespective of the quality of their products.
Why Portland cement concrete?
Quality enforcement is way easier with Portland cement concrete than with asphaltic concrete. We just specify the mix ratios of the cement, fine and coarse aggregate by volume. The cement come in their finished forms in bags ready for use unlike bitumen that will have to be diluted (cut-back) and heated by the contractor often with little enforceable quality control measures. The actual mixing of the cement, water and aggregates is just a simple mechanical process requiring no heating hence most contractors can easily procure a concrete mixing machine unlike the asphalt plant which is more complex and expensive to operate. This is the primary reason why building construction appears to have succeeded more than road construction in Nigeria. Our buildings relied more on Portland cement concrete and we have seen a better percentage of our buildings achieve their expected lifespan unlike our roads.
What we need to know
To achieve success in our road construction we must borrow a leaf from the successes of our building construction. We must design and build roads that require less hard-to-implement quality controls. When many contractors found it difficult to achieve concretes of grade 30 and above our building construction teams began to rely on concrete grade 20. When getting steel bars of grade 460 was an issue we began relying on grade 410 and 380. These were factored into the designs of our buildings leading to success.
If we rely on high grade concrete for our roads to get a wear resistant surface, that will be the beginning of our failure in the implementation of rigid pavements.
Reinforced concrete no doubt is a better load bearing layer than the compacted crushed rock base course. It requires little or no compaction to achieve this strength. It also requires little reinforcement to provide a tougher layer than the compacted crushed rock base course. We can replace the base course layer with a reinforced concrete layer. This is precisely what we do in most of our paved compounds. In our compounds after using the granular lateritic soil to provide the required slopes/gradients we just put a layer of concrete and BRC to reduce cracks due to creep and drying shrinkage. Because our compounds are not heavily trafficked like the roads it takes time for us to observe that our (often low grade) concrete has a low resistance to wear. To overcome this we often have to introduce a thinner layer of high-grade wear-resistant concrete requiring the quality control we are not good at.
A good asphaltic concrete (produced and laid following strict adherence to quality) will be flexible and impermeable even after many years. It bends without the development of cracks. It offers a surface highly resistant to wear. Hence it completely protects the base course layer from water and wear. When we compromise standards in asphalt production and placement we have an asphaltic layer that develops small cracks under loads. These cracks allow water to percolate and make contact with the base course (compacted crushed stone) material. After some period of contact with water the base course material disintegrates, is eroded leading to the formation of potholes.
Most of our poorly prepared asphalts have a higher resistance to wear than our cement concrete. Hence it can protect any material underneath it from wear.
The solution
I propose a hybrid of flexible and rigid pavement. A low-grade reinforced concrete base course and a weak asphaltic finished layer will produce a strong and durable road pavement.
The weak asphalt may allow some water percolation, but this will not damage the reinforced concrete base course as concrete is resistant to water. The weak concrete wears easily but is protected against contact from tires by the asphaltic layer. The weak reinforced concrete is more rigid than the compacted crushed stone base course layer and is therefore able to distribute loads effectively over a wider area of land. This enables the road to carry more loads. This is necessary at a time when a lot of heavy items are hauled using our roads.
The use of a weaker concrete means that concrete production will not be limited to only those made with granite chippings. Aggregates from other rock formations can be used (just as in our building construction) thereby reducing cost. Embarking on road construction will be possible by most contractors (just as in building construction) because most of them can mix and lay Portland cement concrete. The porosity of the weak or poor asphaltic layer will hardly be known because the layer underneath is concrete hence giving the road a longer service life.
[This article was written in simple English and with less jargon for easier comprehension by a wider audience]
Engr. Odinaka Victor Okonkwo Ph.D is a lecturer at the Department of Civil Engineering, Nnamdi Azikiwe University Awka. He can be reached at vo.okonkwo@unizik.edu.ng
This has been interesting and trending topic since the emergence and confession of new minister of works Dave Umahi.
The hybrid that this article recommended is what is what exactly Borno State ministry of work use to construct benesheik ( Beneshiekh is about 70 km drive from Maiduguri and located along the major Kano-Damaturu-Maiduguri highway) highway road to Maiduguri, which is still in use though it is more than 3 years. The technology of hybrid is been utilized in Borno for some of the busiest road being constructed under Eng. Babagana Umara Zulum.
This article was well written and easy to comprehend, it will be more interesting and easier if pictures were use for illustration .
thank you for this information. We will look at the benesheik you reference.