The transportation infrastructure plays a significant role in our lives as it enables us to efficiently move from one place to another. Pavements are an essential part of our modern life as they are used as roads, runways, parking lots and driveways. These structures are used as an indicator of the development level of a country. Lay reveals that the total paved road mileage is often used as a measure of a country’s development (1).
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As engineered structures, pavements are required to be strong and durable through the course of their life. They are expected to provide a smooth travelling surface in the face of different weather conditions and physical strain from the loads they are subjected to. The design and construction of pavements is therefore of great significance since it contributes to their proper functioning. Historically, pavements are divided into two major categories, which are flexible pavements and rigid pavements. These two pavement types are constructed in different ways and they have different advantages and disadvantages. This paper will set out to discuss pavements with a focus on rigid and flexible pavements. It will discuss the advantages and disadvantages of the two and proceed to highlight the differences between the two and argue that the rigid pavement is better.
The Role of Pavements
Pavements are constructed to fulfil some important functions in transportation. The first major function is to withstand the load from vehicles on the road or aircrafts in airports, without deforming. Pavements are covered with layered surfaces that ensure that the load is spread out so that the resulting stress at the bottom later is very low and therefore does not cause any damage (Lay 1). Numerous vehicles can therefore use a road network each day without suffering from significant deforming. Without a pavement, road surfaces would be eroded or suffer from depressions after continued use and this would make travelling on the road difficult.
Pavements also ensure that environmental factors such as rain do not interfere with the transportation surface. Without a pavement, road conditions are adversely affected by weather conditions such as rain and snow (Mallick and El-Korchi 45). Pavement design factors in the various effects of weather and ensures that their impact on the road surface is minimal. For example, during design engineers consider the effect that water can have on a road surface and provide adequate draining on the surface. Proper drainage ensures that vehicles can travel with little difficulty even during raining seasons.
The rigid pavement is made of a number of layers that are placed above the subgrade soil. An important characteristic of these pavements is that they include reinforcements. Steel reinforcements increase the strength of the pavement and its ability to support heavy loads. The top layer is the concrete slab, which provides the structural support for the vehicle load that will use the pavement. The surface is designed to withstand the repeated loadings it will be subjected to over its years of service. The rigid pavement’s surface is mostly constructed using Portland cement concrete. This material is preferred due to its wide availability and lower cost (Fwa 92). The next layer is the sub base, which provides the stable platform on which the surface layer is placed. The final layer is the subgrade, which is the compacted soil layer on which the rigid pavement rests.
The rigid pavement is able to produce a relatively uniform distribution of pressure from the traffic load on the surface to the subgrade. The stress developed at the surface is not transmitted to the lower layers, which is why these pavements are suited for areas where the soil sub-grade is weak. Fwa reveals that rigid pavements are mostly found in major highways and airports (92). These types of pavements also serve as floor slabs in heavy-duty industries and surface coverings for harbor yards.
A major advantage of rigid pavements is that they have a long design life. These pavements can be expected to have a service life of between 30 and 40 years. This long lifespan is attributed to the durability of the concrete slab used as the surface of the pavements. Since the road will not have to be replaced for decades, it provides a good return on investment. Fwa admits that the designed life of the pavement is likely to be less than the actual service life (83). One reason for this is that it is difficult to predict the future traffic that the pavement will be expected to carry. However, even when these factors are considered, the rigid pavement still has a long life span.
Another significant merit is the low maintenance cost of the road over its service life. The concrete slab is very durable and resistant to many forms of damages. Routine maintenance mostly involve sealing of joints and cracks as needed (Mallick and El-Korchi 87). The pavement surface generally remains intact over its lifetime. The pavement is resistant to damage from chemicals and oil that might be spilt on it. During the service life of the pavement, it is likely to experience frequent contact with chemical and oil spills from the vehicles that travel on it. The rigid pavement is not damaged by these spills since the concrete surface is not affected.
These pavement types provide a smooth finish and the road has better visibility. This makes it more comfortable and safe to drive on the rigid pavement. The cement slab used at the surface makes it possible to provide a smooth finish that makes driving enjoyable. Ramsamooj notes that a good cement surface is smooth and free from potholes (132). In addition to this, the surface does not reflect light in the way that the flexible pavement does and as such, there is no glare or mirage on the road surface. Drivers are therefore able to enjoy better visibility when using the rigid pavement.
A significant demerit of rigid pavement is that it requires a high initial investment. The construction process for rigid pavements is elaborate and this combined with the high cost of the materials used makes the initial cost of this type of pavement high. The average cost of constructing a rigid pavement might be as high as $8 per square foot and this figure can increase if reinforced steel and a frost protection layer is included in the pavement (Merrill 103).
The construct of rigid pavements is complicated by the need for expansion joints to prevent cracking. The concrete used to make these pavements lacks the ability to freely contract and expand under thermal stresses. Engineers therefore have to make provision for this by including expansion joints in the pavement (Fwa 86). In addition to this, rigid pavements do not support phased construction since the entire stretch of road has to be constructed at the same time. This is a disadvantage since a phased approach might be more time and cost effective.
The flexible pavement is made up of a surface and a number of layers build over the compacted subgrade, which is the natural soil. The various layers include a surface, base course, and sub base, which are made up of varying materials. The surface layer is made of asphalt concrete, which is hot-mix asphalt. Mancy and Atiq document that the main stages of flexible pavement construction include manufacturing and hauling of hot bituminous mixture, and the spreading and compaction of hot mix bitumen (310).The base course is made up of aggregate base that are unstabilized. Fwa notes that in some cases, the aggregate base could be stabilized using stabilizing agents such as Portland cement or asphalt (82). The sub base is made of unstabilized open-graded crushed aggregate material, which could as well be stabilized using cement.
By looking at the various layers of the flexible pavement, it can be seen that the highest quality material is used at the surface. The quality of material decreases as the depth increases and the lowest quality material is used at the bottom. The reason for this is that when traffic load is applied on the surface, localized deformation occurs in the area on which the load is pressing. As the pressure moves downwards, it is distributed over a larger area of the pavement (Lay 204). Since the surface experiences the greatest pressure, it has to be made of the highest quality. Once the load is removed, the pavement layers rebound.
A major advantage of flexible pavements is that they have a lower initial cost, which makes them easier to implement when cost is a major consideration in the project. The construction process used in making flexible pavements is relatively simple and the materials used are not costly. Safwat reveals that the cost of a square foot of flexible pavement might be as low as $2 (81). This low cost provides an economic motivation for the pavement type to be implemented in many locations.
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Flexible pavements support phased construction, which increases the efficiency and ease of the construction effort. Lay notes that it is not necessary to build an entire stretch of the flexible pavement at a go (44). Contractors can therefore engage in stage construction, which offers many benefits as parts of the pavement can be finished and opened to traffic before moving on to the next part.
Finally, the flexible pavement is ready for use as soon as construction work is completed. As a result, this pavement type causes minimal interference during constructed. In most cases, road construction involves expanding an existing road or upgrading a road that is already in use. The construction effort results in traffic dislocation as vehicles are forced to use other routes while the road is under construction. With Flexible pavement, traffic can be allowed to move on the pavement as soon as it has been rolled out (Lay 182).
A significant disadvantage of this pavement is the high maintenance cost incurred. Flexible pavements are more prone to damage and routine maintenance are needed to keep the surface in good form. The maintenance activities include sealing of the pavement surface, patching, and sealing of cracks as needed (Chiu and Zhongren 179). Due to the frequency with which these activities have to be performed, the cost of maintenance over the service life of the pavement is considered high.
Flexible pavements are unable to support very heavy loads due to their low flexural strength. The pavement’s ability to support the load is dependent on the strength of the subgrade and this limits the maximum load that the pavement can accommodate especially when the sub grade is weak (Chiu and Zhongren 181). This makes the pavement unsuitable for roads that will constantly incur heavy traffic in areas with weak subgrade.
Due to its physical make up, it is prone to damage from chemicals and oil spills. A major component of flexible pavements is bitumen, which is used to make the surface layer. This component breaks down when exposed to certain chemicals and oil (Merrill 102). As vehicles pass over the road, it can be expected that oil spills from the vehicles will fall on the road surface regularly. Other chemicals might also get to the pavement surface. The pavement will be damaged by this, which will decrease its life span and necessitate expensive maintenance.
A Comparison of Rigid Pavements and Flexible Pavements
A difference between the pavements is their reaction to weight loads. Flexible pavements are designed to deform when a load is placed on the surface and transfer the pressure to the lower layers. The Rigid pavement is designed to spread the load stresses to a wide region on the surface (Ramsamooj 99). The pressure is not transferred to the lower layers. As such, while the flexible pavement is designed to take advantage of the load-distributing characteristic of the layers making up the pavement, the rigid pavement is designed to take advantage of the strength of the concrete slab.
The two forms of pavement are suitable for different soil conditions. Flexible pavements are ideal for hard subgrades such as gravelly materials. Ramsamooj explains that the flexible pavements utilize the inherent strength of the lower layers and as such, a naturally strong sub layer is desirable (98). Rigid pavements are preferred for soft but uniform clay subgrades. The rigid pavement spreads the load over a large area therefore reducing the stresses on the subgrade. A soft uniform subgrade will therefore not experience excessive pressure when this pavement type is used.
A significant difference between the two pavements is the scientific method used in their design. The rigid pavement is designed using precise scientific data on the stresses of concrete. Rigorous calculations are made to determine the level of stress that the concrete can withstand and the pavement is constructed based on this data. It is therefore easy to predict the behavior of the surface when subjected to a load. On the other hand, flexible pavement design is mostly empirical in nature.
There is no exact scientific data to determine the manner in which the stress is distributed to the lower layers when a load is place on the surface. Fwa admits that the design of flexible pavements has “evolved throughout the years from pure empirical to mechanist empirical (84). There is no pure mechanistic method of design for this pavement type. The load-carrying capacity of the flexible pavement can only be determined using empirical data obtained from various implemented flexible pavements.
The rigid pavement is more prone to cracking failure compared to the flexible pavement. A number of factors cause this cracking, most important of which are the load-induced tension and thermally induced tension. Heat on the road surface leads to tensile stresses that can adversely affect the integrity of the surface. Fwa reveals that thermal stresses are a major design concern when constructing rigid pavements (93). Flexible pavements are not prone to cracking as they react differently to heat. Safwat reports that while the bituminous layers in the flexible pavement are sensitive to changes in temperature, heat does not cause them to crack (80). When the pavement surface is subjected to high temperatures, the asphalt concrete softens. It is therefore not likely to crack as the rigid pavement does under high temperature conditions.
There is a difference in the impact that the strength of the subgrade has on the rigid and flexible pavement. In the rigid pavement, slight variations in the strength of the subgrade will not have significant effects on the load bearing capacity of the pavement (Mallick and El-Korchi 62). On the other hand, the strength of the subgrade is of great significance to the flexible pavement and even minor variations in the subgrade strength will affect the structural capacity of the flexible pavement. While the rigid pavement is able to maintain a flat top surface even when the subgrade strength has minor variations, the surface of the flexible pavement reflects the subgrade strength variations through surface depressions.
The Better Pavement
From the discussions held in this paper, it is evident that the two types of pavements have different merits and demerits. They are best suited for different areas and the choice for the suitable pavement may be determined by factors such as cost consideration, project schedule, and expected traffic load. However, the rigid pavement is the better type of pavement. To begin with, this pavement offers superior performance and a better value for the initial investment. Merrill observes that many countries are interested in long-life pavements that are able to accommodate increasing traffic loads as the years pass by (101). The rigid pavement is able to offer this long-life and the ability to bear heavy loads without rapid deterioration.
Another reason why the rigid pavement is better is that it is able to deliver strong pavements regardless of the natural property of the soil. When constructing a rigid pavement, the strength of the concrete slab determines the strength of the road since the load will be distributed on this surface. The flexural strength of the pavement surface therefore determines how strong a pavement is (Mallick and El-Korchi 62).
This strength can be adjusted by using various material and engineering practices. The strength of the flexible pavement is largely determined by the sub grade since the load is eventually transferred to this layer. This limits the strength of the pavement since it is dictated by the natural properties of the soil. It would be desirable for the contractors to achieve any strength level they want as opposed to having the subgrade dictate the strength of the pavement.
This paper set out to discuss the rigid and flexible type of pavements in order to show that the rigid pavement is better. It began by documenting the important functions fulfilled by pavements in our society. The two types of pavements were then discussed in details and their merits and demerits elaborated. The paper has engaged in a comparison of the two pavement types in order to highlight their major differences. It has concluded by stated that the rigid pavement is superior since it offers a longer life and its strength is not limited by the natural properties of the soil on which the pavement is laid.
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