ASPHALT AND CONCRETE PAVEMENTS

asphalt pavement

Which way should Uganda go on roads infrastructure: asphalt or concrete pavements? This article gives an overview about the performance and practical distinctions between asphalt and concrete pavements. Hopefully it will further the discussion among engineers, policy-makers and stakeholders on how to make informed and better pavement investment decisions. Eng Javilla Barugahare, PhD, a specialist in asphalt and cement concrete materials, writes:

Uganda’s total road network is estimated to be 126,469km long. Of this, national roads constitute 17%. Only 25% (or 5,350km) of the national roads are tarmacked (MoWT, 2018), and less than 1% of those paved roads are surfaced with concrete. This shows that asphalt pavements are currently preferred, and the principal reasons could be their lower initial construction cost and the unfamiliarity with concrete pavements.  About 1km concrete pavement sections could be found after crossing River Nile on the way to Atiak in Gulu District and at Nalukolongo in Kampala.

Currently, the initial construction cost (bidding price) is the main basis for making investment decisions. However, the future favours life cycle cost analysis (LCCA) as the ideal parameter for pavement selection. LCCA compares the economic cost of different pavement design alternatives (or competing pavements) over their entire service lives (AASHTO, 1993). As an example, pavement projects costing over $0.9m in Canada, are subjected to an LCCA over a 50year analysis period. This analysis includes initial construction costs, the future costs of rehabilitation and maintenance, and the facility operation (Asta, G, 2011).

In this article, I give an overview about the structure and design consideration of asphalt and concrete pavements.

Asphalt pavements are constructed with an asphalt mixture layer(s) built over a granular base, granular sub-base and a compacted subgrade (soil). And, asphalt mixtures consist of an asphalt binder (also called bitumen), rock aggregates plus an additive according to specifications. These pavements are classified among flexible pavements, and their layers are designed to increasingly distribute vehicle loads laterally with increasing depth. The pavement layers’ thickness is determined by limiting stresses on the top of the subgrade not to exceed its bearing capacity, and also to limit pavement deformation.

In contrast, concrete pavements are constructed with cement concrete as their wearing course. They are classified among rigid pavements, and may or may not have a granular base or sub-base layers between the wearing course and the subgrade. Concrete is made of cement, aggregates, water, plus an additive where need be. The concrete wearing course is designed to have a high flexural strength and act as a beam over the underlying layers. It applies less pressure on its underlying courses per metre length as compared to asphalt pavements.

Now in the following section, I am going into further performance and practical distinctions between asphalt and concrete pavements.

The initial construction cost of asphalt pavements is approximately 30-40 percent lower than that of concrete pavements (Asta G, 2011). Fluctuations in crude oil prices cause price instability of the asphalt binder. However, the cost per square metre of asphalt pavement is expected to lie within a range of $27-$43 as compared to $43-$65 of concrete pavements (Internet – USA article, 2015). The initial construction price difference of the two pavements is significant and has historically favoured asphalt pavement choice. About 94% of modern pavements in USA are constructed with asphalt. Also, 25% of Germany’s high-volume roads are constructed with concrete. Germany gives an arbitrary credit of $2.1 per square metre for concrete roads as their maintenance is presumed lower (FHA Report, 2014). Asphalt pavements in Uganda cost $0.7m-$1m per km (2018 estimated price); no cost is available for concrete pavements in Uganda. However, it is expected to be higher than that of asphalt pavements by a factor of 1.35. Ultra-thin concrete pavements (concrete thickness approx. 50mm) designed for light to middle traffic are cheaper to construct as compared to typical highway concrete roads (concrete thickness approx. 200mm). Their LCCA cost could be compared to that of surface-dressed roads commonly designed for a similar traffic class.

In terms of pavement service life, concrete pavements could be designed to last 40-60years. According to EUPAVE, 2009, these pavements could last at least 20 years without any major maintenance. However, poor workmanship can cause them to fail within the first five years. Asphalt pavements could last 10-20 years. Nonetheless, they require resealing every 5-7 years to maintain a good-ride ability (or high present serviceability index) under medium-heavy traffic. A lower resealing time between 3-5 years usually occurs under ultra-heavy traffic or trucks with heavy axle loads (Standard design axle load is 100kN). A high macro-texture and joint widening makes concrete pavements noisy (Otieno, 2012). However, it is reported that after five years of service, concrete pavements have a superior profile ride index, riding comfort index, and friction numbers as compared to asphalt pavements (Smith et al, 2001).  On the other hand, asphalt pavements perform slightly better than concrete pavements under rainy conditions. Vehicles tend to slip on concrete pavements under such conditions.

Concrete pavements provide a better fuel economy as compared to asphalt pavements. Passenger vehicles and loaded tractor trailers use 3-5% less fuel on rigid pavements as compared to flexible pavements (Bienvenu and Jiao, 2013). This is attributed to a better smoothness and high flexural stiffness of concrete. Lower fuel usage minimizes greenhouse gas emissions into the environment, especially NOx and CO2 emissions. According to the United States Environmental Protection Agency, 0.00892 metric tons of CO2 are emitted per gallon consumed while travelling exclusively on rigid pavements. CO2 emission is expected to be higher while driving on asphalt pavements. Under heavily trafficked areas around the country, concrete pavements could significantly minimize CO2 emissions into the environment. Generally, climate change is important to the travelling public as the cost of constructing and maintaining of pavements.

Concrete pavements are also reported to be cost-effective based on a life cycle cost analysis (LCCA) (Anne et al, 2011, Michael et al, 2015), especially when the average annual daily traffic (AADT) exceeds 5,000 vehicles per lane per day (Olafsson, H, 1987). Additional LCCA studies in Iceland reported a limiting AADT greater than 8,000 vehicles per lane per day for a concrete pavement choice (Jensson, P, 1993, Johannesson A, 2009). The influence of pavements on the fuel economy and the environment were excluded in the cited studies. Most LCCA studies consider fixed unit prices and discount rate of materials.

However, this method is not without limitations, changes in the long-term discount rates and construction prices significantly affects cost predictions. As an example, an assumed low rate of return could make an expensive project profitable or provide false support for a poor pavement choice. It is also quite difficult to predict the technological and economic situation of a nation over quite a long period of time like 30-50 years (the design life of most pavements especially concrete pavements).

There are other comparisons (even though not quite significant while making investment decisions). These are: (1) The production process of asphalt pavements produces harmful greenhouse gases, but even to a higher extent, the cement industry is reported to be among the highest environment polluters, (2) Strength development of concrete takes a longer time to reach the peak strength (14-28 days); before then, a pavement cannot be opened to traffic unless a fast setting cement is used. On the other hand, (1) asphalt pavements could be used within hours after completing the paving process, (2) it takes less time to build and repair an asphalt pavement, and (3) concrete pavements offer better visibility at night owing to their lighter colour (they minimize lighting costs).

In conclusion, though not a manual, this article highlights significant performance and practical distinctions between asphalt and concrete pavements, which could help in making informed and better pavement investment decisions. It is recommend that relevant engineers, policy-makers, stakeholders and road agencies consider using a life cycle cost analysis (LCCA) method (inclusive of the long-run environmental impact cost of pavements) in making decisions. The current literature on LCCA favours concrete pavements over asphalt pavements, especially under heavy traffic and in areas with high axle loaded commercial trucks. The resealing and maintenance cost associated with asphalt pavements, is mainly influenced by the degree of the rutting and cracking distresses. However, ongoing scientific research into asphalt mixtures of high modulus and elasticity is likely to solve that problem in the future. That will make asphalt pavements even more competitive on the pavement market. So, which way to go, Uganda: Asphalt pavements, concrete pavements or both?

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