Wednesday, January 19, 2011
Concrete Curing, Sealing, & Care
Curing
Concrete mixes usually have more than enough water initially. However, that water evaporates quickly unless you take proper measures/precautions. The process by which concrete hardens is called hydration. The longer the hydration process continues the stronger and denser the cement paste becomes. If concrete is cured properly it can be as much as 50% stronger and much less porous. That combination makes it more difficult for water and salts to penetrate and damage the surface, thus increasing the concrete's resistance to weather and heavy use.
Curing can be as simple as keeping the concrete moist and cool. Concrete that dries in a relatively arid climate ends up weaker than concrete kept in a cool, moist climate.
Probably the best, and most common method, for curing concrete is moist curing. To moist cure, keep the surface continuously wet/damp for the first week after placement. This can be done by wetted burlap, or water misting and ponding.
Another way to cure new concrete is by using a curing compound. Membrane curing compounds actually seal the new concrete with a thin membrane that helps keep moisture from evaporating too rapidly. Timing is most important when using a curing compound. These products must be applied as soon as final finishing is complete. Otherwise, they could possibly mar the concrete's surface. Not all curing compounds are the same. Be sure to follow the manufacturer's recommendations completely.
Curing can be as simple as keeping the concrete moist and cool. Concrete that dries in a relatively arid climate ends up weaker than concrete kept in a cool, moist climate.
Probably the best, and most common method, for curing concrete is moist curing. To moist cure, keep the surface continuously wet/damp for the first week after placement. This can be done by wetted burlap, or water misting and ponding.
Another way to cure new concrete is by using a curing compound. Membrane curing compounds actually seal the new concrete with a thin membrane that helps keep moisture from evaporating too rapidly. Timing is most important when using a curing compound. These products must be applied as soon as final finishing is complete. Otherwise, they could possibly mar the concrete's surface. Not all curing compounds are the same. Be sure to follow the manufacturer's recommendations completely.
Sealing
Moist curing easily lends itself to sealing because there is no chemical removal involved. However, when using a curing compound, you should use a membrane cure that you may easily remove so that the sealer can penetrate quickly and thoroughly.
By making the sealing choice before your install your concrete, you can inform your contractor of the curing method you prefer. There are two main types of concrete sealers: Those forming a film on the surface of your concrete and those designed to penetrate the concrete.
Membrane/Film Sealers - Film formers are acrylic or rubber-based compounds. They dry leaving a shiny, wet-looking surface. They protect against some stains better and are often cheaper than penetrating sealers. Film formers do have some disadvantages: They tend to darken the finished concrete, are not as durable, and can create a more slippery surface.
Penetrating Sealers - While penetrating sealers are usually more expensive, they last longer than film sealers. Most penetrating sealers are derivatives of silicone called silanes or siloxanes designed to penetrate concrete pores. Once there, they react with the alkaline materials and moisture present to form silicone, making concrete water-repellent. Penetrating sealers do not change the finished concrete's appearance. The only disadvantage is there can be no other membrane cure or sealer on the concrete, and the concrete must be at least 28 days old.
By making the sealing choice before your install your concrete, you can inform your contractor of the curing method you prefer. There are two main types of concrete sealers: Those forming a film on the surface of your concrete and those designed to penetrate the concrete.
Membrane/Film Sealers - Film formers are acrylic or rubber-based compounds. They dry leaving a shiny, wet-looking surface. They protect against some stains better and are often cheaper than penetrating sealers. Film formers do have some disadvantages: They tend to darken the finished concrete, are not as durable, and can create a more slippery surface.
Penetrating Sealers - While penetrating sealers are usually more expensive, they last longer than film sealers. Most penetrating sealers are derivatives of silicone called silanes or siloxanes designed to penetrate concrete pores. Once there, they react with the alkaline materials and moisture present to form silicone, making concrete water-repellent. Penetrating sealers do not change the finished concrete's appearance. The only disadvantage is there can be no other membrane cure or sealer on the concrete, and the concrete must be at least 28 days old.
Concrete Driveway Care/Maintenance Suggestions
1. Owners and tradesmen should not drive on new, cured concrete for at least seven days, the longer, the better.
2. Select and apply a driveway sealer, following manufacturer's recommendations. Application every two years greatly helps protect your driveway from the elements and chemicals. For future treatments, use water repellent / sealer containing a minimum of: 20% silane / siloxane solids (Penetrating Sealer) or a 25% high quality acrylic solids (Membrane Sealer) or equivalent.
3. Do not use deicers on driveway, especially during the first year. (Use sand for traction.) Never use products containing ammonium sulfate or ammonium nitrate as deicers.
4. Do not allow snow and ice to accumulate on the slab during the first year.
5. Street snow removal efforts often incorporate the use of deicers. Snow clinging to the underside of cars can melt and result in the salt contents penetrating and damaging the concrete surface. Avoid parking on driveways and garages during such times.
2. Select and apply a driveway sealer, following manufacturer's recommendations. Application every two years greatly helps protect your driveway from the elements and chemicals. For future treatments, use water repellent / sealer containing a minimum of: 20% silane / siloxane solids (Penetrating Sealer) or a 25% high quality acrylic solids (Membrane Sealer) or equivalent.
3. Do not use deicers on driveway, especially during the first year. (Use sand for traction.) Never use products containing ammonium sulfate or ammonium nitrate as deicers.
4. Do not allow snow and ice to accumulate on the slab during the first year.
5. Street snow removal efforts often incorporate the use of deicers. Snow clinging to the underside of cars can melt and result in the salt contents penetrating and damaging the concrete surface. Avoid parking on driveways and garages during such times.
6. Lawn and garden fertilizers can also contain salts which will damage concrete. Always sweep fertilizer from concrete surfaces before it becomes wet.
For more information visit our website at http://www.aaazoellner.com
Cold Weather and Concrete
Cold Weather Concreting
As Fall becomes Winter, the process by which concrete is placed and finished slows in direct relation to temperature drops. In order to expedite cold weather pours, hot water is used is in the mix process and often times accelerators are also added to the concrete mix itself. The most common accelerators are calcium-chloride, and non-chloride accelerators (NCA’s). NCA’s are most commonly utilized in projects using reinforcing structural steel. Unlike calcium chloride, the accelerating additives in NCA’s will not attack or deteriorate the steel, and thus will not damage structure or shorten its lifespan.
Cold Weather Placing
Warm the sub-grade, the forms, and any reinforcing steel.
Do not place concrete on a frozen sub-grade. As the frozen ground thaws it may settle, causing cracking.
Use heated enclosures to block wind, keep out the cold, and conserve heat: If combustion heaters are used make sure the exhaust is vented out and away from the concrete to reduce possible damage.
Place concrete as early in the day to utilize the sun’s thermal heat.
Do not place concrete on a frozen sub-grade. As the frozen ground thaws it may settle, causing cracking.
Use heated enclosures to block wind, keep out the cold, and conserve heat: If combustion heaters are used make sure the exhaust is vented out and away from the concrete to reduce possible damage.
Place concrete as early in the day to utilize the sun’s thermal heat.
Cold Weather Curing
Maintain concrete temperature above 50° F by insulating or heating for three days:
Use insulation blankets to retain internal heat generated by the concrete.
Cover concrete with thermal insulation blankets, or use loose straw (12” deep)
The minimum concrete temperature, as placed and maintained, must exceed 50F. However, caution should be exercised with concrete temperatures above 75F.
Cure to protect against freezing until for at least 2 days.
Remove the heat protection in a manner that ensures the temperature of the concrete changes slowly, cooling the concrete gradually helps reduce the potential for cracking due to thermal stresses.
Appropriate curing and cold weather protection must be incorporated to prevent the concrete from freezing. Always cure the concrete after finishing and protect it from freezing. It is advisable to maintain the in-place temperature at 50°F or greater until required strength has been attained.
Do not seal freshly placed concrete. Sealing retains water in the concrete keeping it saturated during freeze/thaw conditions.
Protect concrete against multiple freeze-thaw cycles until it reaches 3500-psi compressive strength.
Use a good quality curing compound if you are unable to maintain concrete temperatures above 50° degrees Fahrenheit for three to seven days.
The curing period must extend a minimum of 7 days (maintaining the 50F temperature).
Use insulation blankets to retain internal heat generated by the concrete.
Cover concrete with thermal insulation blankets, or use loose straw (12” deep)
The minimum concrete temperature, as placed and maintained, must exceed 50F. However, caution should be exercised with concrete temperatures above 75F.
Cure to protect against freezing until for at least 2 days.
Remove the heat protection in a manner that ensures the temperature of the concrete changes slowly, cooling the concrete gradually helps reduce the potential for cracking due to thermal stresses.
Appropriate curing and cold weather protection must be incorporated to prevent the concrete from freezing. Always cure the concrete after finishing and protect it from freezing. It is advisable to maintain the in-place temperature at 50°F or greater until required strength has been attained.
Do not seal freshly placed concrete. Sealing retains water in the concrete keeping it saturated during freeze/thaw conditions.
Protect concrete against multiple freeze-thaw cycles until it reaches 3500-psi compressive strength.
Use a good quality curing compound if you are unable to maintain concrete temperatures above 50° degrees Fahrenheit for three to seven days.
The curing period must extend a minimum of 7 days (maintaining the 50F temperature).
For more information visit us online at http://www.aaazoellner.com
Placing & Finishing
Placing
Plan your concrete project very carefully. Immediate weather and time of year can be vital factors affecting the desired outcome of your project.
Make sure to prep the desired area properly. It should have good drainage, be free of all organic debris, at the correct dimensions, depth, and be well-compacted (ideally using 1” or smaller clean gravel). If required, also make sure that you have properly placed any jointing materials, rebar, or wire-mesh.
Moisten the prep site just before the concrete arrives.
Chute, wheelbarrow or shovel concrete directly to its final position. Don't dump it in piles and drag or rake it the rest of the way. On a slope, use stiffer/drier concrete. Spread the concrete using a square-ended shovel, a concrete rake, or a come-along. Tamp the concrete with a spade along the edges of the form to release air voids and consolidate the concrete. All concrete should be placed as soon as possible after the mixer truck is loaded. Shorten this time in hot weather. Delays after the concrete arrives can create problems in finishing and reduce final quality. Also, protect any freshly placed concrete from the hot sun, high winds, or freezing temperatures.
Use a lumber or metal straight-edge (screed) to strike off the concrete and level it. Keep a little concrete in front of the screed to fill in any low spots. Repeat, if necessary.
Float the concrete to embed the large aggregate as soon as it has been struck-off. Floating must be done before visible bleed water (water sheen) rises to the surface.
Finishing
Wait for the concrete to stop bleeding. After bleed water has evaporated broom or float the surface. If hand tooling is necessary, cut control joints and edge while the concrete is still plastic.
For the final finish, use a broom, particularly on exterior slabs, as this is most durable finish. Where a smooth finish is desired, such as on a garage floor, use a wooden float finish. Do NOT machine float or use a hand trowel. Never use a steel trowel on concrete exposed to the weather. Never sprinkle water or cement on concrete while finishing it.
Do not over-finish the surface of any concrete slab, particularly not exterior slabs. Not only is it time consuming, but it also tends to bring too much fine material to the surface which forms a membrane and weakens it, usually causing only aesthetic damage.
For more information visit us Online at http://www.aaazoellner.com
Plan your concrete project very carefully. Immediate weather and time of year can be vital factors affecting the desired outcome of your project.
Make sure to prep the desired area properly. It should have good drainage, be free of all organic debris, at the correct dimensions, depth, and be well-compacted (ideally using 1” or smaller clean gravel). If required, also make sure that you have properly placed any jointing materials, rebar, or wire-mesh.
Moisten the prep site just before the concrete arrives.
Chute, wheelbarrow or shovel concrete directly to its final position. Don't dump it in piles and drag or rake it the rest of the way. On a slope, use stiffer/drier concrete. Spread the concrete using a square-ended shovel, a concrete rake, or a come-along. Tamp the concrete with a spade along the edges of the form to release air voids and consolidate the concrete. All concrete should be placed as soon as possible after the mixer truck is loaded. Shorten this time in hot weather. Delays after the concrete arrives can create problems in finishing and reduce final quality. Also, protect any freshly placed concrete from the hot sun, high winds, or freezing temperatures.
Use a lumber or metal straight-edge (screed) to strike off the concrete and level it. Keep a little concrete in front of the screed to fill in any low spots. Repeat, if necessary.
Float the concrete to embed the large aggregate as soon as it has been struck-off. Floating must be done before visible bleed water (water sheen) rises to the surface.
Finishing
Wait for the concrete to stop bleeding. After bleed water has evaporated broom or float the surface. If hand tooling is necessary, cut control joints and edge while the concrete is still plastic.
For the final finish, use a broom, particularly on exterior slabs, as this is most durable finish. Where a smooth finish is desired, such as on a garage floor, use a wooden float finish. Do NOT machine float or use a hand trowel. Never use a steel trowel on concrete exposed to the weather. Never sprinkle water or cement on concrete while finishing it.
Do not over-finish the surface of any concrete slab, particularly not exterior slabs. Not only is it time consuming, but it also tends to bring too much fine material to the surface which forms a membrane and weakens it, usually causing only aesthetic damage.
For more information visit us Online at http://www.aaazoellner.com
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