Concrete Pavement
Concrete pavement includes roadways and parking lots with vehicle loading.
Sidewalks, patios, curb & gutter, swales, and other free-formed concrete on the ground are considered concrete flatwork.
Contraction and Isolation Joints
Control joints are tooled into the concrete during construction to promote cracking in the control joints instead of on the surface of the concrete.
Control and isolation joints are important elements in the proper placement of concrete pavement. The purpose of isolation joints in slabs on grade is to allow horizontal and vertical movement between the slab and adjoining structures such as walls, columns, footings, porches, and stairs.
Common Conditions of Concrete
Concrete pavement experiences the effects of climate along with wear and tear in the form of cracks and surface deterioration and requires maintenance over its lifetime. Concrete failures present in the form of cracking, spalling, settlement, and heaving.
However, failures in the form of large cracks, spalling, and vertical movement can be attributed to construction defects. Construction defects related to asphalt can be due to deficient engineering design, methods, or materials.
SURFACE SLOPE
Due to the surface characteristics of concrete pavement, design criteria dictate that concrete pavement or flatwork be installed at a minimum slope of 1%, or 1/8-inch per one foot. Concrete pavement sloped less than 1% is hard to construct without the formation of flat areas and drainage low points.
Bird baths (shallow standing water on pavement) are caused by inadequate pavement slope, preventing the surface from properly draining. Low points holding water of only a depth more than the thickness of a coin are hazardous and can lead to slippery or icy conditions. It is often difficult for pedestrians to notice or gauge how deep a shallow depth of standing water is and take the proper precautions when traveling on such a surface.
DRAINAGE
Water is the greatest enemy of concrete pavement, which is supported by layers of gravel and soil.
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Typically, water enters the subgrade through cracks, joints, and edges from adjacent areas. Infiltration of water into supporting materials weakens the bearing capacity of the soil and can result in failure.
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More information about maintaining proper drainage around pavement >>
LINEAR STRESS CRACKS
Narrow-width linear stress cracks are common in concrete and do not necessarily indicate pavement failure. These types of cracks should be sealed to prevent water from flowing through the separation.
In some circumstances, linear cracks appear because the pavement section does not contain enough control joints, which function to help control cracking by forcing the cracks to occur in the joint itself instead of the pavement surface.
FATIGUE CRACKS
An abundance of close cracks in the same proximity can be indicative of the failure of subgrade material below the concrete.
Possible causes of fatigue cracks are:
Inadequate concrete pavement section (concrete and/or supporting material is not thick enough).
Weak supporting subgrade materials (due to soil type, water intrusion, or inadequate compaction).
Excessive loading (such as heavy trucks).
SCALING
Scaling is local flaking or peeling away of the near-surface portion of hardened concrete or mortar'. It is primarily a physical action created by repeated cycles of water freezing within the concrete.
FLAKING
Flaking is a type of spalling. It can be caused by temperature during placement, chemicals such as de-icer, or water freeze-thaw cycles.
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Flaking can progress into scaling over time.
JOINT SPALLING
Joint spalling is the breakdown of concrete edges. This type of failure usually does not extend vertically through the slab but instead intersects the joint at an angle.
CRACK SPALLING
Spalling along cracks can be caused by the freeze-thaw cycle of water that seeps into the cracks.
SETTLEMENT
A typical cause of the settlement of concrete is inadequate compaction of the subgrade soil or infiltration of water into the subgrade soil.
HEAVE
A typical cause of upward vertical movement is the expansion of the soil below the slab or frost heave (freezing of soil) due to water infiltration into the subgrade.
SEEPAGE
Seepage is caused by water infiltrating in or flowing out of the soil and gravel below the concrete layer. It can be due to drainage or irrigation problems at the site.
JOINTS
Concrete is prone to cracking, so contractors attempt to control it by making sure sections of concrete elements are separated by isolation joints and control joints. Isolation joints are physical separation between two sections of concrete. Control joints are scored on the concrete surface during construction to try to cause the cracking to occur in the control joint rather than along the slab surface.
Minor cracking in concrete pavement or flatwork does not necessarily indicate that the concrete element is failing or that it needs to be replaced.
Vertical movement and horizontal separation at joints or cracks are indicative of the failure of the soil below the concrete.
Lack of isolation joints, especially at porches and stairs, can contribute to damage to flatwork.
CHEMICALS
De-icer and salt are damaging to concrete, which manifests in flaking and spalling, to the surfaces of all concrete flatwork, including sidewalks, driveways, porches, and curb and gutter. Unfortunately, on concrete surfaces where the freezing of water is prevalent during the winter, the application of chemicals is necessary for life/safety reasons, and replacement of concrete sections as necessary is expected.
Spalling and flaking are not always due to chemicals and may be due to physical damage or bad placement.
CONSTRUCTION MEANS AND METHODS
Placement of concrete in adverse weather (too hot or too cold) can cause the concrete to be weak and can result in damaged.
Concrete producers that mix and deliver concrete can add chemicals to the mix to make it stronger or hold up to placement in adverse temperatures. However, these chemicals add cost, so the contractor has an incentive to omit them.
If the subgrade of concrete pavement or flatwork is not the proper depth or not adequately compacted by the contractor, cracks, vertical movement, and horizontal separation can occur.
Design Standards for Concrete
Engineers design concrete pavement per applicable criteria of a specific Jurisdiction or Agency. These criteria dictate the thickness of the concrete layer, and the depth of the supporting subgrade depends on weight loading and the amount of traffic. It is important to note that concrete flatwork requires regular maintenance and repair, such as sealing of cracks, sealing of separations, and replacement of spalled and damaged sections to achieve the full design life.
State of California Department of Transportation is a leading source of concrete pavement information and field studies due to its varying climates.
