Concrete Efflorescence 2017-07-04T09:26:50+00:00

Concrete Efflorescence Prevention & Removal

With most concrete, efflorescence is unavoidable. We keep hearing more about issues associated with efflorescence, thanks to stained floors’ popularity. Due to the growing trend of having bare concrete floors, the building industry is battling with the issue of unpleasant efflorescence. This is particularly the moment it gets trapped below a film forming sealer. However, this does not mean that outer decorative concrete is free from concrete efflorescence problems, rather interior floors issues present a bigger challenge and this discussion is actually focused on them.

The objective of this article is to provide a common sense outline of the main efflorescence problems and efflorescence prevention.  Concrete precautionary steps to control the circumstances that tend to encourage concrete efflorescence will be suggested and remedial suggestions also provided.  Valuable explanations of this intricate occurrence will also be offered in real terms without going into full chemical analysis.

Usually, decorative contractors receive the first phone call the moment efflorescence appears on a stained floor. There are several factors that could have led to a certain project that is plagued with efflorescence problems. The least of these could probably be the job executed by the stain contractor, who in the majority of cases is not aware of the previous conditions that initiated the efflorescence. No one is unaffected by efflorescence-related issues including owners, builders, general contractors, concrete finishers, stain contractors, and the design community. Hopefully, this discussion explains some of the problems for better understanding of efflorescence so that proper steps are taken to avert it. Efflorescence prevention costs are minimal as compared with the expenses and inconveniences related to solutions, particularly in an occupied space. While efflorescence on its own does not pose any health risk, it can be an indication of moisture levels that are enough to encourage mold. In fact, efflorescence is not even a structural problem, but an artistic problem that everyone would like to circumvent.


So What Is Efflorescence In Concrete?

Efflorescence can be defined as the white powdery matter appearing on unsealed concrete’s surface as well as the white blush that is seen with floors which are sealed.  It occurs when vapor migrates through the slab, carrying soluble salts to the concrete’s surface.  Also, it usually wears off or is washed off unsealed concrete surfaces. It may be necessary to use a gentle acid rinse or a light sandblasting in dealing with stubborn cases. Efflorescence that gets trapped beneath the sealer is unappealing and becomes more noticeable on darker floors, necessitating efflorescence removal using efflorescence concrete cleaner.

In extreme situations where vapor is unable to escape through the upper coat, for example a urethane and/or an epoxy, there can be an accumulation of hydrostatic pressure to form water blisters, which is a sign of delamination of the upper coat from the concrete. But blistering is something that is hardly seen with acrylics. This type of sealers let vapor escape through, but leave salt deposits, thus forming ugly blush that makes owners to contact the stain contractor for efflorescence removal.


Causal Factors

Efflorescence needs moisture movement. If there is no movement of moisture, no efflorescence would exist on the surface to cause the efflorescence concrete slab problem. It is unfortunate that a great number of (non-ACI Certified) finishers normally tend to add to the mixture huge amounts of unnecessary water called “water of convenience” meant to expedite concrete placement. The main efflorescence occurs when the water in the concrete evaporates from the slabs and leaves soluble salts on the surface of the concrete. The chances of efflorescence appearing following winter concrete placement are increased by the fact that these salts dissolve better in colder temperatures and also there is an increase in bleed water in cold weather. Cold weather pours coupled with using calcium chloride to increase the fixed time further contributes to efflorescence. The main contributors to efflorescence are high slump concrete coupled with the addition of calcium chloride.

Secondary efflorescence is often defined as water originating from either below the slab or introduced from the slab’s surface. Some of the probable causes of secondary efflorescence could be water-logged base material, a poorly drained site or surplus amounts of water that the decorative flooring contractor use while cleaning, for instance during the process of washing off the residue of acid stain.  A certain level of efflorescence will definitely happen the moment more water and more soluble salts from calcium chloride are further added to concrete that has been placed during cool weather and was followed by additional water from the decorative processes.  This is when efflorescence treatment will be required.


Vapor Transmission Testing

It is important for the contractor to conduct some independent fact finding to help establish the present vapor transmission rate (VTR). The Plastic Sheet Test ASTM-D-4263) is perhaps the oldest test technique.  It involves fastening down a clear sheet of poly measuring 18″ X 18″ and checking it after 16 hours to see whether there is condensation or the surface of the concrete is darkened. The two are vapor transmission signals. The Calcium Chloride Test is yet another surface moisture test that quantifies vapor transmission rate. It consists of a covered dish which is weighed before and on expiry of 24 hours. The two are affordable measures used to establish whether or not vapor is active.

When using these testing techniques, it should be noted, however, that the two tests are only capable of tracking moisture transmission near the surface only. The test may not show substantial vapor movement when the atmospheric conditions are the same as the slab conditions. This is because transmission takes place when atmospheric conditions vary from the slab conditions. Moisture tends to travel towards cool temperatures, and vapor emissions escape and travel towards heat. You can picture the results of this ugly, but very real situation: Following the completion of the project, and contractor’s money settled, the happy owner occupies the building and turns the HVAC, making the vapor in the slab to travel in the direction of the warmth; or, the conditioned space’s lower humidity coming along with the whitish minerals.  An acrylic sealer will allow the vapor to travel through, leaving efflorescence behind.

Hydrostatic pressure can increase and probably cause delamination in case the urethanes or epoxies are present on the floor. It is possible for efflorescence to happen several months or even years after the contractor has completed the task either because of conditions akin to the ones described above, or from seasonal water seeping below the slab.  Actually, testing the surface might not sufficiently guarantee that the project will be free from efflorescence for many years.  That is why it is crucial to establish the source and the vapor transmission rate before suggesting a remedial measure.

Testing technology for instance the Protimeter Moisture Measurement System which costs $1000 is more accurate. This ASTM Standard F-2170-02 employs a probe in gauging the presence of moisture on the top and bottom of the concrete surface together with comparative humidity, dew point, plus temperature. Aquant and Tramex Concrete Encounter Moisture Meter which cost $300 and $450 respectively are non-invasive hand held gadgets that use the same technology. Aquant is used to measure surface moisture up to a depth of between 10 mm and 20 mm. The other device which is the Tramex Concrete Encounter Moisture Meter, transfers a signal to a depth of half an inch, and projects a 4” moisture measurement. The advantage of these devices is that they measure moisture levels under the surface accurately regardless of whether or not there is any movement at present. Considering the financial risk and the potential harm to a contractor’s reputation, it is prudent for a contactor to examine procedures which show more clearly subsurface conditions so as to prevent the efflorescence issues instigated by moisture.

We stated earlier that efflorescence is deterred by a well-drained site and a vapor retarder or barrier. The case can be constructed for locating the concrete either directly on the vapor barrier, or above granular material atop the vapor barrier. It is argued that the granular material may be turned into a drenched “blotter” prior to the pouring of the slab, which will add even more water that has just one way to escape. Finishers are of the opinion that there will be excessive bleed water leading to dusting; however, a slump of 4” coupled with water reducers and a mix that is well graded will actually display very little bleed water.  Raised slabs are rather common in commercial building and are never put on a blotter material.  Reducing water in all those critical areas including the subgrade, concrete, as well as the procedures executed by the decorative contractor is the very crucial factor here.  Although all these procedures need a certain amount of water, the water should be minimized to help ensure that efflorescence is manageable.

Let us now consider a characteristic residential slab pour so as to get an idea of the amount of water that may be obtainable to bring soluble salt to the slab’s surface when these aspects are not taken into consideration at all:

Let’s take it for granted that there is a blotter course of sand 2’’ thick existing between the concrete slab and a vapor barrier. A cubic foot of sand weighs around 100 pounds. When it is saturated in order to attain compaction, it could easily hold 10% vapor by weight, or 10 pounds of water/cubic foot of sand.

This means that it will take about 167 cubic feet of sand so as to cover 1,000 sq. feet of vapor barrier. When calculated, the result will be: every 1,000 sq. feet of concrete surface with 1,670 pounds, or 200 gallons of water lying under the slab without an alternative place to go except up. Now, if additional 60 or 70 more gallons of water (water of convenience) are added by the finisher, we could talk of 270 gallons of excess water moving upwards towards the surface any time the surface conditions are either warmer or drier than the slab.

There is an additional feature to this ominous picture. The permeable slab described earlier acts just like a hard sponge that has huge pores plus capillaries in the entire environment, capable of wick-moisturing up and rather ready to act like a hard sponge , with the ability to absorb water in hundreds of pound from the cleaning processes used by the decorative flooring contractor.


Efflorescence Reduction

The following are efflorescence reducing actions:

  • Drainage of the site surface
  • A concrete mix that is well-graded with water reducer to reduce paste
  • Concrete of up to but not more than 4” slump
  • Well consolidated concrete (a one-man vibratory are recommended for an excellent job) positioned directly on a vapor barrier and somehow remedied


All of these factors play a role in ensuring that a concrete’s bleed water content is kept at a minimum with a reduced pore and capillary network that will repel instead of enabling moisture absorption and movement.  Put another way, a concrete slab that is thick and impervious is recommended.

Now here are additional facts: a waterproofing admixture is an alternative to the water retarder or barrier in helping prevent efflorescence. It is usually included in the concrete at the plant, but there is a disadvantage to it. Being an ingredient that is quite common, stearic acid might present genuine issues for the stain contractor. This is because it is hydrophobic in a similar manner that the stamped concrete’s powdered release agents are. This means that waterproofed concrete does not accept water base stains or acid stains readily. In this case, it is recommended that the surface be opened with a sanding screen.  Another option is to open it with a gelled acid in order to generate some level of profile, thus facilitating an enhanced mechanical bond for the water born mark.

There are some useful mix design factors such as ordering a mix that is suitably graded from the ready mix manufacturer. The benefit from a mix design that is well graded is the lessening of the concrete’s weakest part, the cement paste (that is the mixture of cement and water), producing a thicker concrete than with the typical mix. Replacing 15–20 percent of the Portland cement with fly ash is another significant mix design consideration. This contributes greatly to minimizing efflorescence.  There are three significant advantages of using fly ash to lessen efflorescence.  It normally decreases the quantity of Portland cement and free lime and also chemically binds up a part of the free lime plus salts that generate efflorescence.   Besides, fly ash needs less water; this also this leads to a denser paste which is helpful in preventing moisture from moving up from down as well as from the surface down. Obviously, water reducers are also useful in reducing the amount of water as well as decreasing of cement.

The nearer a mix design attains to the preferred water cement ratio of  0.45 pounds of water to pounds of cement, the fewer efflorescence problems arise, particularly with a mix that comprises fly ash.  There is this old idea that integral color cannot be used with fly ash; the fact is that it can.  Consistent outcomes can be produced by ensuring that many pours are kept consistent with the mix as well as the subgrade conditions and finishing methods.  Salts dissolve better in colder temperatures and concrete has a tendency of bleeding more in colder weather, too. This further encourages the upward movement of moisture plus soluble salts. That is the reason why cold concrete and cold environmental temperatures encourage efflorescence.  Adhering to ACI 306 Cold-Weather Concreting practices whenever possible is vital. This includes increasing the concrete’s temperature to 60 degrees, then covering it overnight in order to maintain the heat. This encourages a more compact pore plus capillary structure, and helps in closing down every moisture transportation route.

Curing becomes necessary since we are aware that moisture travels much slower through thicker concrete from both directions. When concrete remains humid for an extended period – particularly the first few days, it is filled with additional pores and capillaries either partially or fully to create a more compact and impervious matrix which discourages moisture and soluble salts migration. On the other hand, concrete that is positioned at an elevated slump and not cured acts like a sponge with numerous tiny raceways, facilitating easy migration of moisture from top down and bottom up.

Cure and seal membrane cures types are not often used because they pose a problem as the membrane must be removed in order to accept acid stains. One manufacturer produces a thermal degrading wax that gets broken down by hot water.  Since water borne stains are gaining popularity, manufacturers are trying to come up with compatible curing membranes which will be capable of accepting subsequent staining. Wet curing tends to be problematic as it has to be continuous and may form discoloration in case a plastic sheet has creases.


Alternatives to Stainer

The technology which can provide a twofold relief from problems associated with efflorescence comprises the application of chemical liquid densifiers plus hardeners. To begin with, the makers claim that floors which are chemically hardened do not need a membrane sealer.  This actually eradicates the confined efflorescence issue as well as lessens maintenance costs significantly. Some of the chemicals are silicates, silicaonates, silanes, siloxanes, polysiliconates, and the latest one – lithiums, among others. A number of them are used in combination with polished concrete floors, while others are applied on dye stained or acid stained floors and they could be given a membrane kind of sealer. While a number of these chemicals have been used over several years, some of them are comparatively new to the market. Although several efflorescence products are available, there are variations in products and manufacturers regarding the degree of different penetration of different chemicals from different efflorescence products’ manufacturers, as well as their durability, degrees of shine, and slip resistance. We are certainly aware that that the large box stores are abandoning vinyl in favor of bare concrete. It would be helpful for decorative contractors to examine this technology even for those smaller homes’ stained floors.

Next, there are several contractors who, a couple days after concrete placing, follow acid stains application with silicate type non-film forming sealers which are generally labeled as chemical hardeners, densifiers, and soluble chloride reducers.  Efflorescence is lessened since the silicate makes the matrix to become thicker as described previously.  The option available to the contractor is to add conventional sealers that form membrane in case resultant tests show standard levels.

How To Remediate Efflorescence

What options are available when efflorescence shows up? The quickest solution could be using products such as toluene, xylene or another layer of solvent base acrylic that re-emulsifies the initial seals and also removes the blush. But in case vapor continues to surface through the slab, there is a likelihood of this state recurring. It should be noted that health issues may arise due to solvents such as toluene and xylene.

The most sensible action to take includes peeling off the sealer to allow for carrying a test to find out the rate of vapor movement before developing a remedial plan. Getting it right the second time is crucial and that is why you need to take time to identify the causes as best as you can, using the data that is available.  One of the more costly hand-held digital devices can be valuable since they are capable of obtaining accurate moisture reading under the slab’s surface. After determining the moisture levels, a sealer may be chosen depending on the recommendations of the manufacturer.

You should bear in mind that a number of manufacturers have sealers that inhibit vapor and are also non-film forming as mentioned previously.  These sealers can lessen the vapor transmission rate (VRT) to a level which is suitable for a top coat that is both heavier bodied and more abrasion resistant for application in high foot traffic circumstances.

Contractors have used a finish or polish in situations where slabs have had continuous efflorescence-related issues despite having applied a lithium or silicate densifier. These are water based products which are industrial grade, low build and low solids micronized acrylic products that they have used as the last treatment.  These finishes and polishes may as well be used atop film-forming sealers to augment abrasion resistance.

In the majority of efflorescence cases, the issues which contributed to formation of efflorescence were merely inherited by the decorative flooring contractor. Hence, it can be difficult to identify the causes of efflorescence once the floor has been sealed. That is why it is crucial to establish the amount of moisture present in the slab, its origin, and also if conditions such as seasonal ground water could contribute additional moisture at some point.  It is advisable to resist the quick fix and instead consider vapor testing. Lastly, contractors can choose to evade warranty language when it comes to sealers. Alternatively, they may opt to clearly define specific vapor transmission problems.  When it comes to interior stained floors, the decorative saying: “test, test, and test” truly applies. This means conducting a vapor testing rate (VTR) each time you are suspicious of possible efflorescence issues.  You should remember that it is not a must that you grab every job that comes in your direction.


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