Formation of loose powder resulting from disintegration of surface of hardened concrete is called dusting or chalking. The characteristics of such surfaces are:
a. They powder under any kind of traffic
b. They can be easily scratched with a nail or even by sweeping.
Scaling is local flaking or peeling of a finished surface of hardened concrete as a result of exposure to freezing and thawing. Generally, it starts as localized small patches which later may merge and extend to expose large areas. Light scaling does not expose the coarse aggregate. Moderate scaling exposes the aggregate and may involve loss of up to 1/8 to 3/8 inch [3 to 10 mm] of the surface mortar. In severe scaling more surface has been lost and the aggregate is clearly exposed and stands out.
Crazing is the development of a network of fine random cracks or fissures on the surface of concrete or mortar caused by shrinkage of the surface layer. These cracks are rarely more than 1/8 inch [3 mm] deep and are more noticeable on steel-troweled surfaces. The irregular hexagonal areas enclosed by the cracks are typically no more than 11/2 inch [40 mm] across and maybeassmallas1/2 or3/8 inch[12or20mm]in unusual instances. Generally, craze cracks develop at an early age and are apparent the day after placement or at least by the end of the first week. Often they are not readily visible until the surface has been wetted and it is beginning to dry out.
Crazing cracks are sometimes referred to as shallow map or pattern cracking. They do not affect the structural integrity of concrete and rarely do they affect durability or wear resistance. However, crazed surfaces can be unsightly. They are particularly conspicuous and unsightly when concrete contains calcium chloride, a commonly used accelerating admix-ture.
Plastic shrinkage cracks appear in the surface of fresh concrete soon after it is placed and while it is still plastic. These cracks appear mostly on horizontal surfaces. They are usually parallel to each other on the order of 1 to 3 feet apart, relatively shallow, and generally do not intersect the perimeter of the slab. Plastic shrink- age cracking is highly likely to occur when high evaporation rates cause the concrete surface to dry out before it has set.
Plastic shrinkage cracks are unsightly but rarely impair the strength or durability of concrete floors and pavements. The development of these cracks can be minimized if appropriate measures are taken prior to and during placing and finishing concrete.
Concrete expands and shrinks with changes in mois- ture and temperature. The overall tendency is to shrink and this can cause cracking at an early age. Irregular cracks are unsightly and difficult to maintain but gen- erally do not affect the integrity of concrete. Joints are simply pre-planned cracks. Joints in concrete slabs can be created by forming, tooling, sawing, and placement of joint formers.
Curing is the maintaining of an adequate moisture content and temperature in concrete at early ages so that it can develop properties the mixture was designed to achieve. Curing begins immediately after placement and finishing so that the concrete may develop the desired strength and du- rability.
Without an adequate supply of moisture, the cementitious materials in concrete cannot react to form a quality product. Drying may remove the water needed for this chemical reac- tion called hydration and the concrete will not achieve its potential properties.
Temperature is an important factor in proper curing, since the rate of hydration, and therefore, strength development, is faster at higher temperatures. Generally, concrete tem- perature should be maintained above 50°F (10°C) for an ad- equate rate of strength development. Further, a uniform tem- perature should be maintained through the concrete sec- tion while it is gaining strength to avoid thermal cracking.
For exposed concrete, relative humidity and wind condi- tions are also important; they contribute to the rate of mois- ture loss from the concrete and could result in cracking, poor surface quality and durability. Protective measures to control evaporation of moisture from concrete surfaces be- fore it sets are essential to prevent plastic shrinkage crack- ing (See CIP 5).
Hot weather may be defined as any period of high temperature in which special precautions need to be taken to ensure proper handling, placing, finishing and curing of concrete. Hot weather problems are most frequently encountered in the summer, but the associ- ated climatic factors of high winds, low relative hu- midity and solar radiation can occur at any time, es- pecially in arid or tropical climates. Hot weather con- ditions can produce a rapid rate of evaporation of moisture from the surface of the concrete and accel- erated setting time, among other problems. Generally, high relative humidity tends to reduce the effects of high temperature.
Cold weather is defined as a period when the average daily temperature falls below 40°F [4°C] for more than three successive days. These conditions warrant spe- cial precautions when placing, finishing, curing and protecting concrete against the effects of cold weather. Since weather conditions can change rap- idly in the winter months, good concrete practices and proper planning are critical.