Hot-Mix Asphalt and Portland Cement Concrete

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Table of Contents

Introduction
Material Properties
Requirements
Production
Construction Techniques
Conclusions
Reference

Introduction

Hot-Mix asphalt (HMA) and Portland cement concrete (PCC) are some of the main materials used in road construction. They are typically used together in constructing a composite pavement system, with a layer of HMA placed over a layer of PCC, which sports considerably better results when compared to traditional paving techniques (NASEM, 2013). The purpose of this paper is to compare and contrast the two materials based on their properties, requirements, production, and the use in road construction.

Material Properties

HMA is a combination of stone, sand, and gravel mixed with asphalt cement, which is made from oil (NASEM, 2013). The material is cheap, while sporting high parameters in stability, durability, flexibility, impermeability and fatigue resistance. However, as asphalt ages, it becomes more brittle and likely to suffer damage from the physical wear-and-tear as well as the elements (NASEM, 2013). PCC, on the other hand, as the name suggests, is made from concrete, mortar, stucco, and grout. It is much less flexible than asphalt and offers much greater durability and resistance at the cost of having very little flexibility (NASEM, 2013).

Requirements

When used in construction of roads, the materials have different performance requirements. The asphalt layer is expected to have sufficient thickness and strength to carry out the traffic load, prevent water penetration, have a smooth surface that is resistant to wear, distortion, and deterioration, and support the wheel load from the pavement surface, equally distributing it along the substratum (NASEM, 2013). PCC shares many of these requirements, with additional care being placed into making sure the cement stratum does not have any empty spots along the layer as well as greater quality demands for the material itself (NASEM, 2013). Mixing different cements or using caked cement is not allowed.

Production

HMA is typically made at a temperature of about 300-330 degrees Fahrenheit, though there are types of HMA that could be mixed at lower temperatures (NASEM, 2013). These are usually referred to as warm-mix or cold-mix asphalts. The materials are extracted from open mines, whereas the oil residue from oil distillation. The components are mixed together and bound by hot liquid asphalt, which is then transported in trucks towards the construction site. Portland cement, on the other hand, is baked in a kiln at about 2600-2700 F, baked to form clinkers, which are then ground up and mixed with gypsum. The materials are also mined in open pits (NASEM, 2013).

Construction Techniques

HMA is laid on site using a specialized machine that mixes up and heats the material components together. After the layer has been placed and is still hot, it needs to be smoothened with a heavy roller and compressed to exclude any pockets of air forming underneath and creating a potential pothole under pressure (NASEM, 2013). In smaller spaces where rollers do not fit, a hand-held pressurizer is used. The asphalt is ready for use after it cools off, meaning that repairs and construction can be done quickly. PCC is brought to the construction site in a cement carrier, which keeps rotating to ensure the cement does not dry up and remains consistent in its viscosity. After it is poured to form the bottom layer of the road, it is compressed using concrete pressurizers. Cement requires at least 30 days to dry to a relative 85-90% humidity (NASEM, 2013). It is the reason why PCC roads are more expensive, as the road takes much longer to become operational.

Conclusions

HMA and PCC are widely used in road construction. They have their distinct properties, with flexibility, cheapness, and quickness being the strong sides of HMA, while PCC sports higher durability and longevity. Due to the nature of roads and their frequent use, HMA remains the favored solution for most roads, while PCC is often used as an underlayer for high-volume traffic areas and highways.

Reference

National Academies of Sciences, Engineering, and Medicine (NASEM). (2013).

Composite pavement systems, volume 1: HMA/PCC composite pavements. The National Academies Press.

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