Another method is to paint concrete surface with acrylic, urethane, or exposy. Fine particles of crystalline silica are abraded from the surface and released into the air we breathe. The most common silica particles are quartz, while the other two forms are cristobalite and tridymite. Mortar and stucco also release silica dust. Many cements contain other hazardous substances like lime, nickel, cobalt, and chromium compounds.
Alkaline substances are corrosive to human tissues and small amounts of chromium can cause allergic reaction. Concrete dust may irritate the nose, throat, and respiratory tract by mechanical abrasion, causing coughing, sneezing, and shortness of breath. Concrete dust may cause chronic bronchitis or aggravate pre-existing lung disease such as emphysema, bronchitis, and asthma. After prolonged exposure, respirable concrete dust can cause severe damage to human lungs.
Crystalline silica has been classified as a human lung carcinogen. In addition, it can cause silicosis, which is incurable. More commonly, silica dust causes the formation of scar tissue in the lungs, reducing their ability to take in oxygen. Use moderate slump concrete not exceeding mm 2. Do not start finishing operation while the concrete is bleeding. Do not broadcast cement or sprinkle water on concrete prior to or during finishing operations.
Ensure that there is adequate venting of exhaust gases from gas-fired heaters in enclosed spaces. Use adequate curing measures to retain moisture in concrete for the first 3 to 7 days and protect it from the environment, especially freezing.
Use design mixes, followed by good construction practice. Taylor, Peter C. Portland Cement Association, We at engineeringcivil. Your email address will not be published. Our contractor feels a bit overwhelmed, but plans the work: clean up the drywall mud today; stain tonight; clean the residue Tuesday, apply epoxy Wednesday, Thursday is cure day, Friday is open for light traffic.
This seems workable, life is good. No matter that the heat is not yet on in the building and the floor is not fully cured out.
Our contractor friend, by the way, does not have floor scrubbers. Mops and buckets make up the clean up and water removal equipment. This is not an uncommon set of circumstances. What do you suppose the chances are that the stained and sealed floor will escape with no efflorescence? Helpful mix design factors include ordering a well graded a mix from the ready mix producer. The advantage gained from a well graded mix design is the reduction of the weakest part of concrete, the cement paste cement and water , making for a denser concrete than with a standard mix.
Fly ash brings three important benefits to reducing efflorescence. Fly ash reduces the amount of Portland cement and free lime as well as chemically binding up a portion of the free lime and salts that cause efflorescence. In addition, fly ash requires less water again resulting in a denser paste which aids in keeping moisture from traveling up from the bottom and from the top down.
Water reducers of course, are also helpful at minimizing the amount of water and a reduction of cement paste. The closer a mix design gets to the desired water cement ratio of. And forget the old notion that you can't use integral color with fly ash, you can.
Keeping multiple pours consistent with the mix design, sub grade conditions and finishing practices will produce consistent results. Heads up on this one Follow ACI Cold-Weather Concreting procedures whenever possible including raising the placement temperature of the concrete to 60 degrees and then cover overnight to retain the heat. This will encourage a more dense pore and capillary structure and help close down the moisture transportation routes. Curing becomes important as we recognize that moisture moves much more slowly through denser concrete from either direction.
When concrete is kept moist for a longer period, especially the first few days, more capillaries and pores fill partially or completely to form a denser and more impermeable matrix that discourages the migration of moisture and soluble salts.
Conversely concrete that is placed at a high slump and not cured acts as a sponge, full of miniature raceways allowing easy movement of moisture from the bottom up and top down.
Cure and seal membrane type cures poses a problem in that the membrane has to be removed to accept acid stains, so they are not often used.
One manufacturer has a thermal degrading wax that breaks down with hot water. Water borne stains are becoming popular and manufacturers are working to formulate compatible curing membranes that will accept subsequent staining. Wet curing is difficult because it must be continuous and may create discoloration if a plastic sheet has wrinkles.
Some contractors are following the application of acid stains with silicate type non-film forming sealers generically described as chemical hardeners, densifiers and soluble chloride reducers within a few days of concrete placement. A Calcium Chloride Test is a good surface moisture test for determining the current vapor transmission rate of concrete. Photo Courtesy of Vaporprecision, Inc. Even if some answers are provided it behooves the contractor to do some independent fact finding to determine the current vapor transmission rate VTR.
Probably the oldest test method is the Plastic Sheet Test ASTM-D which is taping down a clear 18" X 18" sheet of poly and checking 16 hours later for condensation or for a darkened concrete surface.
Both are indications of vapor transmission. Another surface moisture test is The Calcium Chloride Test which quantifies the rate of vapor transmission.
This is a covered dish that is weighed before and after a twenty four hour period. Both are cost effective measures in determining whether vapor is active. A note of caution when using these testing methods When the atmospheric conditions are similar to the slab conditions the tests might not indicate significant vapor transmission because movement happens when the ambient conditions differ from the slab conditions.
Moisture migrates and moves toward cool temperatures. Vapor emissions migrate and move toward heat. Imagine the consequences of the following unpleasant, but very real scenario If the sealer is acrylic the vapor will pass through leaving the efflorescence. If urethanes or epoxies are on the floor, hydrostatic pressure may build and possibly cause delamination. Efflorescence can occur months or even years after the contractor has left the job due to circumstances similar to those just described or from seasonal ground water seeping under the slab.
Testing the surface may not be enough to guarantee an efflorescence free project for years to come. It is important to determine the source and the rate of vapor transmission before prescribing a remedy. Was a granular material placed over the vapor retarder vapor barrier being the outdated term and was the base saturated?
If a vapor retarder was used, was it carefully taped at penetrations, around the plumbing, for instance? Did the mix design include fly ash? Answers to at least some of these questions can give a decorative contractor a heads up before wading into a project completely uninformed and unprepared. More accurate, but also more expensive, testing technology such as the Protimeter Moisture Measurement System ASTM Standard F uses a probe to measure the presence of moisture on and below the concrete surface along with the relative humidity, dew point and temperature.
The benefit of these tools is their accuracy in measuring the moisture levels well beneath the surface whether there is any current movement or not. Given the financial risk, not to mention damage to a contractor's reputation it makes sense for a contractor to explore procedures that more clearly indicate subsurface conditions in order to avoid the efflorescence problems caused by moisture.
We mentioned that a well drained site and a vapor retarder are deterrents to efflorescence. The case can be made for placing the concrete directly on the vapor retarder or over granular material on top of the vapor retarder. The argument against the granular material is that it may become a saturated "blotter" before the slab is poured adding even more water that has only one way to leave.
Finishers argue that there will be too much bleed water resulting in dusting, but a 4" slump with water reducers and a well graded mix will show very little bleed water. Elevated slabs are quite common in commercial construction and they are not placed on a blotter material. The really important factor here is minimizing water in all the important areas These all require some amount of water, but keeping water to a minimum helps ensure that efflorescence is controllable.
To give an idea how much water may be available to bring soluble salts to the surface when these factors are not given any consideration let us consider a typical residential slab pour Dry sand weighs approximately pounds per cubic foot. It will take approximately cubic feet of sand to cover 1, square feet of vapor retarder. That figures out to each 1, square feet of concrete surface having 1, pounds, or gallons of water sitting on the underside of the slab with no where to go but up!
Add that amount of water to another 60 or 70 extra gallons of "water of convenience" added by the finisher and we may have gallons of extra water moving towards the surface whenever surface conditions are warmer or drier than the slab itself.
There is another aspect of this foreboding picture Technology that might offer double relief from efflorescence related issues involves the use of chemical liquid densifiers and hardeners.
First, manufacturers say chemically hardened floors do not require a membrane sealer. This eliminates the trapped efflorescence problem and also greatly reduces maintenance costs. The chemicals include silicates, silicaonates, polysiliconates, silanes, siloxanes and the new lithiums, to name a few. Some of these are used in the conjunction with polished concrete floors. Some are used on floors already acid stained or dye stained and could receive a membrane type sealer. Some of these chemicals have been used for years and some are relatively new to market.
Claims vary from product to product and from manufacturer about how far the chemicals penetrate, their permanence and the levels of shine and slip resistance. We know for certain that the big box stores are getting away from vinyl and going to bare concrete. Decorative floor contractors would be well served to explore this technology even for smaller residential stained floors. Secondly, some contractors are following the application of acid stains with silicate type non-film forming sealers generically described as chemical hardeners, densifiers and soluble chloride reducers within a few days of concrete placement.
Efflorescence is reduced as the silicate causes the matrix to become denser as we described earlier. The contractor has the option of adding conventional membrane forming sealers if subsequent VTR tests indicate acceptable levels. Doug Bannister is owner of The Stamp Store in Oklahoma City, OK, which provides a complete line of decorative concrete materials including products, equipment and training.
The dust occurs when the surface of the concrete is not strong enough and is friable and porous. When the surface of the concrete is weak, the chance of dusting is increased.
There are several causes that make the concrete surface weak. Ecoratio wants to assist you in stopping concrete dusting problems with the following tips that reduce the chance of dusting concrete. Here you find some tips that can be the solution for concrete dusting:. Tip 1 Use the best available materials to cast your concrete.
The aggregates that you are going to use need to be clean and of uniform shape to create the concrete mix design;. Tip 2 Bleed water on top of the surface will weaken the quality of your concrete. A correctly design concrete mix with the lowest possible water cement ratio to achieve the desired workability will result in less bleed water and less dusting;.
Tip 3 The most obvious solution to prevent dusting concrete is to let your concrete cure properly. When the concrete is properly cured, the structural integrity of the concrete will be at its best. The surface may finish easier and it may look like you have achieved a good quality surface finish , but this normally only leads to severe dusting and concrete surface strength problems and should be avoided at all costs.
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