Moisture Issues in Concrete Floors Part 4


Or Start from the begining

Freshly placed concrete must be protected from rapid moisture loss for the surface to be strong and durable.  Lack of proper curing will result in a surface loss of the desired strength as great as half the desired strength.  For example, 4000 psi concrete with no curing or lack of curing can result in a strength of 2000-2800 psi at the surface.

Tests have shown that when the internal relative humidity of concrete drops below 80%, hydration comes to almost a complete stop.  This is why curing and holding in hydration water as long as possible is so important.

Concrete finishers should always check curing compounds for compatibility with any floor coatings and adhesives that might be applied at later dates.  Pre-job conferences and meetings should address all curing issues.  See NRMCA and ASCC checklist for the Concrete Pre-Construction Conference, Section 20.

Curing compounds should be used according the manufactures specifications and applied according to the proper coverage rates.  Use of compounds containing fugitive/dyes can aid in helping provide a “look” to check for even coverage.  Their use can also be helpful in reflecting heat in hot weather.  Application is best done by spraying.  If manual methods are used, a spray and back-roll method is best when using a wide short nap solvent resistant roller.

Curing compounds have pros and cons.  Holding in the hydration water for an extended period of time will result in a harder, durable and more abrasion resistant floor, however, it extends the drying time.  Some compounds may affect future floor coatings (all coatings and cures should be addressed before construction begins).   The use of sheet plastic or curing papers are being used in place of membrane curing compounds.  These materials offer effective curing, but unless special non-wrinkling paper is used will create a blotchy appearance on the concrete where sheets are in complete contact with the slab.  When admixtures like calcium chloride are used the floors can become much darker.  Issues may also arise with use of sheeting since the material can be a slip hazard.  It is often removed too soon and torn easily.  When curing is done with this method, follow ASTM C 171 specifications.  Water or wet cure methods should be pre-planned and completely addressed before construction begins.  Joints and water confinement can be an issue with this method.  When the use of water for cure is specified, ACI 308 states it should be within 20° F (this is now under consideration for changes).


The role of water in concrete has been previously addressed.  In a normal yard of concrete, there will be around 275 lb of water.  About half of the water used in the hydration process must evaporate in order for the humidity in the concrete to drop below 100%.

Note:  This does not address issues with wet curing which can raise the concrete moisture another 7% in most cases.  This means that several extra pounds of water must leave each square foot of concrete.

Several other factors can affect the dry time and moisture content:

¨        Type of cement.

¨        Type of aggregates.

¨        Water-cement ratio (higher water-cement ratios will affect the capillarity structure of concrete allowing for easier vapor transmissions).

¨        The curing used and the weather the concrete is exposed to.

¨        Thickness of the slab.

¨        Extended rains, wet and cool periods during construction.

¨        Landscaping water systems that are too close to the concrete and along un-waterproofed walls and slabs (should thickened slab be waterproofed? Or at the very least sealed?).

¨        Unclean gutters and poorly placed downspouts.

¨        Excessive cleaning with water both before and after service (excessive use can allow water into joints and cracks creating a reservoir for water to pond in).


Vapor Retarders:

Vapor retarders are materials used under concrete floors to restrict moisture flow while restricting growths of mold and mildew.  These materials are 6-10 mil polyethylene that need to be overlapped at least 6” at the seams.

Care needs to be taken when installing vapor retarders around plumbing and electrical outlets.  Many times, contractors have failed to properly seal and cut around these pipes or conduits.

Puncture holes and tears must be repaired before concrete is placed.

Sub-grades should be even and have uniform bearing capacity (be aware that thickened slabs will dry slower in most cases).  Most thickened slabs also support walls and may, in many cases, have pipes extending out of the slab.  It has been poor construction practice to not take as much care as needed along these thickened slabs.

Understanding Vapor Retarders:

Vapor retarders should be placed.  Floor coatings should be able to comply with the concrete and its curing time.  There are specifications for vapor retarders (see ASTM D 1745 or ASTM D 4397).

The following table outlines a classification of vapor retarders.  Class A has the most resistance while Class C has the least:

Class A

Class B

Class C

Water Vapor Permeance

(ASTM E 96)

0.3 Perms

0.3 Perms

0.3 Perms

Tensile Strength

(ASTM D 828 or D 882)

7.9 kN/m

(45.0 lbf/in)


(30.0 lbf/in)

2.4 kN/m

(13.6 lbf/in)

Puncture Resistance

(ASTM D 1709)

2200 g

(5 lb)

1700 g

(4 lb)

475 g

(1 lb)

Some finishing methods like newer ride on laser screeds may tear some types of polyethylene sheets.  These methods should also be covered in pre-job meetings before the concrete is placed.

Where Should Vapor Retarders be Installed:

ACI 302 still has issues on where and even how vapor retarders should be placed.  One thing that has became clear over the years, however, is that each job must be individually looked at.  Some jobs may require a layer of a granular fill be placed over the vapor barrier (though in recent years this seems to have been phased out in most construction practices).

Careful installation can also avoid issues.  Too long it has been practice to not repair tears or holes, not extend the sheets up along perimeter walls, and have too large of cuts where utilities like plumbing and electrical extend out of the slabs.  Other problems arise from improperly installed wire mesh in floors which can result in tears, cutting of the material along construction joints or failure to properly overlap at seams.  Vapor retarders should also be installed over graded sub-bases and compacted to a uniform bearing capacity. Uneven subgrade can result is both tears and standing water pockets.

Many fast-track construction projects are of the tenant finish type, meaning both plumbing and electrical is added after construction.  These can get installed quickly and often no thought is given to installation or repair of vapor retarders.  Also in today’s market, the floor may be cut up as plans change and areas remain open for a length of time.  When the concrete re-poured, it is many times not with the same care, materials or labor as the rest of the concrete floor.

ACI 360 states that a “vapor barrier should be installed with the same care as a roofing material”.  Leaks from underneath can be just as destructive as those from above.  Making sure everyone on the job understands the importance of vapor barriers should help eliminate many problems.


¨        ACI 301 4.1.5

¨        ACI 301 (302 1R-96)

¨        ACI 302

¨        ACI 360

¨        ASTM E 1643:  Standard Practice for Installation of Water Vapor Retarders used in Contact with Earth or Granular Fill under Concrete Slabs

This guide is to offer assistance in installation of both concrete and decorative concrete toppings and sealers.  It is designed to bring contractors, architects, general contractors and owners to understand issues that might occur before, during and after construction.  Working together during the construction process and understanding what may occur, understanding of materials and having material suppliers at pre-job meeting can be effective means of understanding potential issues.