There is a subtle natural phenomenon that affects the life span of pavements and slabs. The life span is affected by thermal movement. There are a growth and shrinkage of pavements with temperature, and if this motion is temporary, there is a cracking, upheaval and a failure of joints.
This article alludes to the mechanics and the details of design which will help the engineers to protect the life span.
Thermal Expansion Basics
All paving material expands and contracts a certain amount with temperature. The amount is defined by the coefficient of thermal expansion (CTE) (see here for more info) and decided by the amount of temperature change and the length of member. The different manner of motion is due to the fact that the asphaltic concrete expands and shrinks more in length due to the fact that it becomes heated and hot in the body of asphalt could become softened and be of low cohesion, while the cementitious structure of concrete is rigid, and does not shrink or expand so readily.
It will be seen that in flexible pavement this great amount is dissipated in the several layers, while in the rigid types of the expansion is more apt to be lateral, thus producing stresses at the joints and edges of the pavement.
Therefore, the fundamental facts are turned into tests by the engineers. The daily and seasonal changes are estimated in temperature, which, multiplied by an assumed C. E. of the mixture gives the amount of expansion. This amount is calculated and compared with the joints that can be made, with the stretch of the sealers used and the curl of the bean, and this helps to determine the passages of joints and the kind of reinforcements which will be used before the project gets into the fields.
Expansion Joints Explained
Expansion and contraction joints are but the intentional weak point, which helps to determine the point of cracking and opening. In the concrete, the sawed junctions for contraction would make the artificial joints, where the pavement would crack nicely, while the expansion joint (less apt to be used now) would provide room for movement next to fixed structures, either buildings or those on long worlds.
Load transfer devices like dowels or aggregate interlock keep slab faces level as the joint is under traffic. Joint sealers then keep out water and dirt so the joint can open and close without raveling. Field practices are as important as drawings. How deep a joint is cut, the timing of the saw cut and the curing conditions are critical elements in how the slab reacts.
For regional insight into workmanship and sequencing, an asphalt contractor in East Anaheim can show how crews stage the saw cutting, sealing and curing of pavement on busy jobs.
Concrete vs. Asphalt Behavior
Concrete and asphalt behave differently when exposed to heat and cold. Concrete behaves like a slab with joints, asphalt like a stiff mixture with layers. Before listing these key differences, a note is appropriate that hot-mix asphalt stiffens as it cools and becomes softer with heating, thus providing for movement and stresses to migrate both during the day and through the depth of the pavement.
- Concrete slabs move less for each degree of temperature change but create internal stresses that can cause curling, opening of joints, or transverse cracks when the joints are spaced too far apart.
- Asphalt layers move more for each degree of temperature change, but flexibility in movement distributes stress. If mixes are too soft at high temperatures, rutting is the primary distress rather than cracking.
- Concrete is smoothed by joints and load transfer, while asphalt relies on uniform compaction and stable gradation to control movement and maintain surface tightness.
- Uneven thermal movement causes curling of concrete (i.e., the top cools faster than the bottom), while asphalt is more affected by diurnal “breathing,” which appears as small cracks at the edges and around utilities.
Seasonal Shifts
Daily cycles are small compared with seasonal ones. Winter contraction can open joints and widen existing cracks. Spring thaw saturates bases and amplifies deflection under truck loads. Summer heat expands materials and lessens stiffness of asphalt material allowing small shear movements caused by braking and turning loads.
Repeated cycles rather than isolated events cause most of the visual impact. The subgrade and base courses tend to dampen those cycles. A well-drained, dense base minimizes moisture induced expansion and contraction thus keeping the slab or mat better supported as the temperature fluctuates. Materials selection is also important; aggregates with low sensitivity to moisture and constant thermal expansion properties help minimize differential expansions of layers.
Controlling Movement Damage
Design, materials, construction and maintenance all have means to manage thermal movement. The goal is not to stop the expansion and contraction, but to provide safe pathways for movement and keep the water and debris from those pathways so they can remain functional over time.
- Refine joint geometry and spacing: For concrete, use spacing in accordance with slab thickness and anticipated thermal range, i.e., ensure that sawed joints will reach the required depth and are cut at the appropriate time.
- Stabilize asphalt mixtures: Use binders and gradations that provide good high-temperature rutting resistance and low-temperature crack resistance, particularly since hot-mix asphalt in those areas experiences large thermal ranges.
- Protect seals and edges: Use functional joint sealants and tight shoulders to minimize infiltration of water and grit. Timely sealing of cracks eliminates small openings from becoming structural defects.
- Control moisture: Maintain positive drainage, stable shoulders, and non-erodible bases, since seasonal contraction will cause pumping of fines followed by weakened support.
- Plan for intercession: Rehabilitation includes such pavement maintenance schemes as thin overlays, mill-and-fill, bonded concrete-on-asphalt, or dowel retrofitting before movement-induced distresses reach the superstructure.
By predicting movement, detailing joints, selecting stable mixes, maintaining seals and drainage periodicities, it is possible for the engineers to turn a reality that is unavoidable in everything designed into a manageable design variable. If done successfully, the need for maintenance will be minimized, service life will be greatly extended, rideability improved and cost be lower for both flexible pavement and rigid systems.
