Editor’s note: This article first appeared in the May 2016 issue of Grading & Excavation Contractor.
Concrete has been around since ancient times. Concrete and mortar have been used to hold together bricks and create smooth surfaces by every civilization. Today concrete is the foundation (literally) of modern construction techniques of all types. Our roadway system primarily utilizes concrete pavement for road construction, according to the Portland Cement Association:
Concrete played a major role in the construction of the US Interstate Highway System during the past 60 years . . . The national highway system, which includes the nearly 45,000-mile interstate system, carries 40% of the nation’s total traffic, including 70% of the commercial traffic and 90% of the tourist traffic, according to the Federal Highway Administration [FHWA]. About 60% of the interstate system is concrete, especially in urban areas where FHWA anticipates heavy traffic loads. Concrete was selected, in part, because of its durability.
These roadways are constructed with specially designed heavy equipment, differing in both kind and technique from the equipment used to install asphalt pavement. And while this equipment provides an impressive level of productivity, it has limitations based on theconcrete materials being used.
Concrete Characteristics
What exactly is concrete? First used extensively by the ancient Romans, concrete is a construction material made from two active components (Portland cement and water) and two inert components (fine and coarse aggregate). The active ingredients chemically bond upon mixture while the inert ingredients provide mass and volume. Fine aggregate is usually sand while coarse aggregate can be gravel, crushed stone, or recycled concrete chunks. A typical mixture ratio is one part cement to three parts sand to three parts aggregate. When making concrete, the dry ingredients are mixed prior to the addition of water. The chemical reaction binding the concrete together occurs when water interacts with the Portland cement.
Portland cement is the key ingredient of concrete—it’s what holds the concrete together. It is not a result of the concrete drying out and the water evaporating. Loss of water results in reduction of the hydration process. Instead, the chemical reaction called hydration causes the concrete to harden. Concrete cement must be kept as moist as possible to ensure a complete hydration process.
Concrete is strong in compression but weak in tension. This strength is inversely proportional to the amount of water mixed with the dry ingredients. More water makes concrete more fluid and easier to work with while reducing strength. Less water increases the concrete’s strength but makes its stiffer and harder to work with.
Depending on the actual mixture, concrete compressive strengths can vary from 2,500 pounds per square inch gauge (psi) for residential concrete to 4,000 psi and higher in commercial structures. For special applications, concrete strengths exceeding 10,000 psi are utilized. By comparison, concrete tensile strength is only about a tenth of its compressive strength (300–700 psi). Its flexural or bending strength is similarly weak at 400–700 psi. Concrete’s modulus of elasticity varies from 14,000–41,000 MPa.
Concrete can be formed and poured into a wide variety of applications and shapes whose anticipated loads will result in considerable tensile and flexural forces. These loads would result in cracking and failure of the concrete by itself, so concrete is reinforced with structural steel. The steel is included to carry the anticipated tensile loads. Structural ASTM A36 steel, for example, has a tensile strength against yield of 36,250 psi.
At minimum, the steel can consist of “shrinkage and temperature” mesh that protects the concrete from tensile strain caused by internal stresses generated by variable temperatures due to long-term exposure to climate conditions. Individual rods of steel (referred to as “rebar”) can be installed within and along the length of the proposed concrete structure at sizes, thickness, and directions required by the structure’s engineering analyses. This rebar can be unidirectional, laid across angles, bundled into thicker rod, and extended and anchored into adjacent structures and foundations for stability.
So what does a typical cross-section of concrete pavement look like? At the bottom, the pavement consists of a compacted subgrade with a required optimum density for the soil underling the pavement. Then comes a sub-base typically consisting of a minimum of 4 inches of gravel (though some designs do not include this layer). Above this is the surface run of reinforced concrete pavement, usually a minimum of 4 inches thick with steel rebar or mesh whose size, diameter, length, and strength will be specified by the engineer in accordance with anticipated traffic conditions.
Comparisons With Bituminous Asphalt
Concrete pavement is referred to as “rigid” pavement given its high compressive strength and steel reinforcement. It does not bend under applied vehicle loads. But it is not the only type of pavement. Also used extensively is bituminous asphalt, referred to as “flexible” pavement. This is an over-simplification—concrete can bend somewhat and asphalt can be relatively stiff—but these terms provide a good general explanation of how pavement interacts with applied loads, environments, and underlying subgrades.
The configuration of asphalt pavement differs somewhat from concrete pavement. From top to bottom, bituminous asphalt pavement consists of a relatively thin wearing surface course, a bituminous asphalt base of at least 3–4 inches, a sub-base of gravel or stone measuring 4 or more inches in thickness, and a compacted soil subgrade.
Their primary difference lies in how they distribute applied vehicle loads to the underlying subgrade. Being rigid, concrete pavement distributed loads over a wide area of the subgrade. For the most part, it is the concrete slab itself that provides the bulk of the pavement’s structural strength and load capacity. Bituminous asphalt pavement tends to be less stiff and weaker than concrete on a unit basis (deforming more under the same applied loads). Therefore, it doesn’t spread out loads over as wide an area of subgrade as does concrete pavement. It requires thicker layers and depends more on the strength of the compacted, underlying soil subgrade.
Concrete can also be enhanced and augmented by reinforcement, texture, and color in ways not possible for asphalt. This greatly increases the durability and performance of concrete pavement, allowing it to often outlast bituminous asphalt in the same environment by 10–15 years before needing retaliation. However, this often comes with higher up-front initial construction costs since reinforced concrete can be significantly more expensive to install (however, a sharp increase in the price of oil—a major component in the manufacturing of asphalt—can negate this price difference). It was a desire to reduce these upfront costs that led to the development of the concrete paving machines and equipment that are the subject of this article.
These materials have differing strengths, weaknesses, and applications. As mentioned above, concrete pavements are stronger and more durable, but initial construction costs are higher. Furthermore, asphalt pavement can be laid in less time, yet the required frequency of repair for concrete pavement is much lower and it is less susceptible to damage from extreme weather. Concrete surfaces are also not damaged by oil leaks and fuel spills like asphalt surfaces are.
But when repair and maintenance are required, these activities are easier to perform on asphalt pavement. Maintenance on asphalt pavement can be confined to small areas (such as fixing pot holes) instead of having to replace an entire concrete slab section. Performance wise, asphalt has better skid resistance and traction while snow and ice melt faster on asphalt than concrete due to its color.
Concrete Pavement Types and the Use of Joints
Various types of concrete pavement designs are available to highway engineers. These designs utilize different types of joints and steel reinforcement layouts to control the forces that could damage the pavement—including vehicle and equipment loads, shrinkage of the concrete itself during curing, and environmental changes (precipitation and temperature fluctuations). Over time, these forces can cause cracks to appear in the main slabs of the pavement.
Joints can be considered to be controlled cracks. They provide a gap in the pavement to allow a degree of expansion and movement. By allowing a “breathing space” for the concrete, joints largely prevent the accumulation of stresses within the slab itself, and thus prevent the formation of cracks. The locations, intervals, and extent of joints are determined by the designer based on anticipated loads and forces.
The use of joints allows for a jointed concrete pavement (JCP) design as distinguished from the continuously reinforced concrete pavement (CRCP). These two types represent the main categories of pavement designs. JCP has traverse joints set at regular intervals (typically about 15 feet). It also utilizes longitudinal joints along the pavements’ direction and centerline. These joints are connected with steel tie bars. Design standards govern the actual details of the joints themselves.
CRCP, on the other hand, utilizes both longitudinal and…
