Curing - Curing is one of the most important steps in concrete construction, because proper curing greatly increases concrete strength and durability. Concrete gains strength as a result of hydration: the chemical reaction between cement and water. However, hydration occurs only if water is available and if the concrete's temperature stays within a suitable range. During the curing period-from five to seven days after placement for conventional concrete-the concrete surface needs to maintain moisture to permit proper hydration. It is strongly suggested to coat new concrete, immediately following placement, with commercially available curing compounds which maintain moisture content.
Sealing - After concrete has been initially cured the application of a sealer is recommended to help protect the concrete. Sealing involves applying a liquid coating to the surface of hardened concrete to either prevent or decrease the penetration of a liquid or other media. The sealing compound should be approved by ASTM C-1315. The primary concern with sealing is to prevent chlorides (from cars or flakes for ice removal) from coming into direct contact with the concrete. Although there are no guarantees, sealers help provide a good barrier against deterioration. Sealers should be breathable, penetrating and applied at the manufacturer's prescribed rate. Application in the early fall (September) is recommended. Come by or visit our Building Materials Division to purchase sealers.
Blisters - Blisters are bumps, of varying siz.e, 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:
1. An excessive amount of entrapped air held within the concrete by a high percentage of material
2. Insufficient vibration during compaction or overuse of vibration
3. Finishing when the concrete is still spongy
Cracking - There are several reasons why cracking can occur, listed below are the main types of cracking and their causes:
Shrinkage - All concrete shrinks during strength development. Shrinkage is most pronounced during the earty stages. Shrinkage cannot be prevented entirely but can be controlled by proper joint spacing and joint depth.
• 4ft thick slabs should have joint spacing not exceeding 10 feet at a depth of at least 1ft.
• 5ft thick slabs should have joint spacing not exceeding 12.5 feet at a depth of at least 1 W.
• 6" thick slabs should have joint spacing not exceeding 15 feet at a depth of at least 1 W.
1. Drying plastic shrinkage - 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.
2. Thermal contraction - Concrete placed during hot midday temperatures will contract as it cools during the night.
3. Sub grade settlement - Insufficiently compacted subgrades and soils susceptible to frost heave or swells, produce 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.
Curling - Curling is the distortion (rising up) of a slab's corners and edges due to differences in shrinkage between the top and bottom of a slab.
Delamination - Delaminations are similar to blisters. Delaminated areas result from water and air getting trapped between prematurely closed surfaces. 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.
Discoloration - Factors found to influence discoloration are calcium-chloride admixtures, cement alkalis, hard toweled surfaces, inadequate or inappropriate curing, variation of the water cement ratio at the surface and changes in the concrete mix. Discolorations appear soon after placing the concrete.
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 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 of proper curing, 8) carbonation during winter concreting caused by unvented heaters, 9) freezing of the surface, and 10) dirty aggregate.
Pop outs - A pop out is a fragment that breaks out of the surface of the concrete leaving a hole that may vary in size. Pop outs 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 pop outs appear within the first year after placement. Pop outs are considered a cosmetic detraction and generally do not affect the service life of the concrete.
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 then scaling, often appearing as circular or oval depressions on surfaces or as elongated cavities along joints. Spalls are caused by pressure or expansion within the concrete, bond failure in two-course construction, impact loads, fire or weathering. Improper1y constructed joints and corroded reinforcing steel are two common causes of spalling.