Frequently Asked Questions
causes efflorescence and how can it be avoided?
A: Efflorescence is a type of
discoloration. It is a deposit, usually white in color that occasionally
develops on the surface of concrete, often just after a structure
is completed. Although unattractive, efflorescence is usually harmless.
In rare cases excessive efflorescence, within the pores of the material,
can cause expansion that may disrupt the surface.
Efflorescence is caused by a combination of circumstances:
soluble salts in the material, moisture to dissolve the salts, and
vapor transmission or hydrostatic pressure that moves the solution
toward the surface. Water in moist, hardened concrete dissolves
soluble salts. This salt-water solution migrates to the surface
by vapor transmission or hydraulic pressure where the water evaporates,
leaving the salt deposit at the surface. Particularly temperature,
humidity and wind affect efflorescence. In the summer, even after
long periods of rain, moisture evaporates so quickly that comparatively
small amounts of salt are brought to the surface.
Moisture testing to determine the vapor pressure
at the slab surface will tell you how much moisture is moving through
the slab. A common value of vapor pressure acceptable for moisture
sensitive floor coverings is 3 to 5 lb./1000
sq.ft./24 hours. The Calcium Chloride Vapor Pressure Test is commonly
used. Testing of the soils and concrete would identify the source
of the soluble salts. and a look at the
drainage, irrigation systems, accommodation of the building runoff
(downspout drops etc.), and ground waters may give some valuable
clues as to the source of moisture that is driving this process
These types of problems can be very complex to resolve.
One possible strategy would be to install a French drain system
which over time will lower the moisture content of the soil under
the slab. With lower moisture content under the slab, the transmission
of water through the slab will slow or nearly cease. Without the
moisture the salts are no longer transported to the slab surface
and the process should stop. Avoid adding additional water to the
system. In general any wet process cleanup converts the buildup
to a solution which is re-deposited onto the concrete surface to
reappear when the concrete dries. In many cases the use of a dry
method cleanup will help to reduce or prevent a re-occurrence of
For more information see Trowel
Tips: Efflorescence (IS239).
The download is free, and although the article is aimed at concrete
masonry, the process description and cleanup recommendations are
also valid for concrete.
PCA's Library has compiled a bibliography on "Efflorescence
in Concrete", which includes journal articles and conference
papers that address the formation, prevention, and removal of efflorescence.