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Common Flatwork Questions & Answers

DELAMINATION

Delaminations are similar to blisters. Delaminated areas result from water and air getting trapped between a prematurely closed surface. Delaminations are very difficult to detect during finishing and become apparent after the concrete surface has dried and the delaminated area is crushed under traffic. The primary cause of delamination is finishing the surface before the bleeding of water and air has occurred.

SCALING

Scaling is the general loss of surface mortar due to freezing and thawing. The aggregate is usually clearly exposed and often stands out from the concrete. Scaling is primarily a physical action caused by pressure from water freezing within the concrete.

SPALLING

Spalling is a deeper surface defect than scaling, often appearing as circular or oval depressions on surfaces or as elongated cavities along joints. Spalls are caused by pressure or expansion with the concrete, bond failure in two-course construction, impact loads, fire or weathering.

Improperly constructed joints and corroded reinforcing steel are two common causes of spalling.

DISCOLORATION

Factors found to influence discoloration are calcium-chloride admixtures, cement alkalies, Hard-troweled surfaces, inadequate or inappropriate curing, variation of the water cement ratio at the surface, and changes in the concrete mix. Weather can have a large effect on discoloration. Humidity, cloud cover, and temperature can affect the color of the cured concrete. Uneven or equally place subgrade material may affect the color. Shadows from adjacent buildings or trees often affect the color. Often discoloration from these causes appears soon after placing the concrete.

CRACKING

There are several reasons why cracking can occur, listed below are the main types of cracking and their causes:

  • PLASTIC SHRINKAGE CRACKING – Plastic shrinkage cracks are caused by a rapid loss of water from the surface of concrete before it has set. The critical condition exists when the rate of evaporation of surface moisture exceeds the rate at which rising bleed water can replace it.
  • SUB-GRADE SETTLEMENT – Insufficiently compacted subgrades and soils susceptible to frost heave or swells, producing cracks in slabs.
  • CRAZING – Crazing, a network pattern of fine cracks that do not penetrate much below the surface, is caused by minor surface shrinkage. Crazing is not structurally serious and does not ordinarily indicate the start of future deterioration. Low humidity, high air temperature, hot sun or drying wind, either separately or in any combination, can cause rapid surface drying that encourages crazing.

POPOUTS

A popout is a conical fragment that breaks out of the surface of the concrete leaving a hole that may vary in size. Popouts usually occur when a piece of porous rock absorbs moisture or freezes under moist conditions. The rock’s swelling creates internal pressure sufficient enough to rupture the concrete surface. Most popouts appears within the first year after placement. Popouts are considered a cosmetic detraction and generally do not affect the service life of the concrete

CURLING

Curling is the distortion (rising up) of a slab’s corners and edges due to differences in moisture content or temperature between the top and bottom of a slab.

DUSTING

Dusting is the development of a fine powdery material that easily rubs off the surface of hardened concrete. Dusting is the result of a thin, weak layer called laitance, composed of water, cement, and fine particles. Dusting is caused by 1) floating and troweling concrete with bleed water on the surface, 2) water applied during finishing, 3) exposure to rainfall during finishing, 4) spreading dry cement over the surface to accelerate finishing, 5) a low cement content, 6) too wet a mix, 7) lack or proper curing, 8) carbonation during winter concreting caused by unvented heaters, 9) freezing of the surface, and 10) dirty aggregate.

BLISTERS

Blisters are bumps, or varying size, which appear when bubbles of entrapped air or water rise through the unhardened concrete and get trapped under an already sealed, airtight surface. Blistering is commonly caused by an excessive amount of entrapped air held within the concrete, insufficient vibration during compaction, overuse of vibration, or finishing when the concrete is still spongy.