Concrete Flatwork
Sidewalks | Curb & Gutter | Handicap Ramps | Stairs | Patios
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TYPICAL CONCRETE PAVEMENT
CROSS SECTION
A slab-on-grade system is constructed on a foundation of sand, a base course, and adequately compacted soil. Or, in certain applications, the concrete slab may be constructed directly on soil treated with moisture or chemicals.
(a) The surface is composed of a layer of concrete.
(b) Wire mesh provides structural strength to the concrete. (NOT ALWAYS PRESENT)
(c) Concrete is placed on compacted sand.
(d) Base course provides additional support, allows for drainage, and provides resistance to freezing.
(e) Subgrade is compacted soil and provides the foundation of the pavement structure. It can become harder and more stable over time.
CONCRETE 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 FLATWORK
Concrete flatwork 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 flatwork, design criteria dictate that concrete pavement or flatwork be installed at a minimum slope of 1%, or 1/8-inch per one foot. The recommended cross-slope of sidewalks is 2%, or 1/4-inch per one foot.
Standing water on flatwork is caused by inadequate 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.
CRACKS
Multiple cracks in the same proximity can be indicative of the failure of subgrade material below the concrete.
Possible causes of cracks are:
Inadequate depth of concrete 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)
Cracks are common in flatwork and do not necessarily indicate failure. Cracks should be sealed to prevent water from flowing through the separation when possible. 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.
FLAKING AND SCALING
Flaking is a type of spalling. It can be caused by temperature during placement, chemicals such as de-icer, or water freeze-thaw cycles. Flaking can progress into scaling over time.
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.
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 AND HEAVE
A typical cause of the settlement of concrete is inadequate compaction of the subgrade soil or infiltration of water into the subgrade soil.
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 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 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 flatwork. 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 flatwork is not the proper depth or not adequately compacted by the contractor, cracks, vertical movement, and horizontal separation can occur.