Floor Preparation

Slab Curing and Finishing

We were able to save a customer over 25% on a job by changing the slab finishing and curing methods to a moisture cure and light trowelled finish, thereby eliminating a great deal of expensive floor prep.

If you are specifying a floor on a new project, factor in your slab finishing and curing methods. It is important to consider what you plan on putting over the slab; boilerplate slab specifications do not take into account finishes.

Most floorcovering adhesive and resinous system manufacturers recommend avoiding curing compounds and over-trowelling the slab. When left up to the concrete contractor, substantial floor prep expense may be the result.

Mix Design

Some lightweight aggregates are very absorbent and can retain significant moisture during curing. This increase the amount of time needed for the floor to adequately dry and contribute to Alkali-Silica reaction (which can cause spalling and structural damage) so lightweight aggregates should be selected carefully. Some lightweight concrete and gypsum based concretes can also be incompatible with certain systems so these should be selected carefully and with consideration of planned finishes.

Water / Cement Ratios

Cement requires a .25 water/cement ratio for the hydration reaction. A .42 to .5 will typically provide adequate workability for placement. Ratios above .5 can drastically increase drying times, weaken the concrete and increase potential moisture vapor issues.

After you have taken the .25 water mix required for hydration into consideration, a .6 water/cement ratio a 1000 sqft slab would need to shed a percentage of the remaining 246 gallons of water to “dry” to the point where it meets the RH requirements for floor installation.

Floor Flatness

Historically we have been happy with specifying 3/16” in 10’, but what does that really tell you? Standard methods of determining floor flatness ignore the frequency of the troughs and peaks. This inconsistency can have an impact on the appearance of the floor and may increase your costs by requiring extensive patching to achieve a smooth appearance. If polishing concrete, it may make your slab unsuitable for polishing without an overlay. The American Concrete Institute recommends testing floor flatness in accordance with ASTM E1155 and specifying the level of flatness required for your specified finish. There is some ambiguity as to when the flatness should be tested (some suggest testing as soon as you can walk on the floor,) but testing will provide the most accurate readings after the floor has cured. Waiting until cure allows you to account for slab curl during curing, which will affect finishes.

Existing Conditions

What is on the floor (sealers, glues, mortars, epoxies, etc.) can have a monumental financial impact on the project if proper procedures are not utilized to abate the current conditions. While some of these can be easily removed, some may require shaving the slab and removal of significant material. Whether you need to remove ¼” of epoxy or simply a cure and seal there is still a financial impact on the project that needs to be considered and specified. It is important as well to know what may lie under current finishes that are to be removed. We worked with a general contractor on a small project that required a polished concrete finish, but as they removed the carpet they discovered that the floor was well beyond any conditions that could be polished and would require an overlay.

Moisture

There are two types of moisture issues that can affect your floor; fluid and moisture vapor. Fluid is less common and typically found only when the slab lies below the surrounding water table or during flooding. Moisture vapor emission (MVE) is the most common and has become more of an issue with fast track construction schedules and changes in adhesives over the past decade. Excessive MVE can be caused by lack of cure time under controlled conditions and excessive “water of convenience” used during the placement of the slab (this is why it is important to specify and monitor the proper water/cement ratio for your needs - ACI 211.1,) as well as improper, damaged, or non-existent vapor retarders (ASTM E1745-11.) There are options to remediate MVE (link to moisture remediation) as well as finishes that are not sensitive to moisture vapor emissions (some resinous systems, ceramic tile and polished concrete.)

Control Joints

Control joints are important to slab design, but must be addressed when installing floor coverings or resinous systems. Some people choose to leave them untreated when installing polished concrete or in warehouse areas, but treating them with a semi-rigid resinous filler can help appearances, eliminate dirt and build up as well as help the slab hold up better under heavy wheeled traffic. Any cracking that occurs in the floor can be telegraphed through most hard surface and resinous finishes if not properly addressed. Joints in Concrete Construction – ACI 224.3R-22

Expansion Joints

Similar to control joints, expansion joints need to be addressed properly, refer to Joints in Concrete Construction – ACI 224.3R-22. Expansion joints should never be covered. Damaged expansion joint edges can be repaired and joints should be filled using flexible fill material, proper backer rod and elastomeric joint sealer filled to a depth of 2/3 the width of the joint.

Surface Profile

When installing overlays, some adhesives, moisture mitigation systems, and resinous systems it is essential to have a proper surface profile. There are typically four proper methods of creating a profile; milling, scarifying, grinding, and shot blasting. Acid etching is sometimes used for thin mil resinous systems, but should be avoided (it is often done incorrectly;it can be hazardous; it is ineffective if there is a sealer/curing agent on the floor.) Mechanical methods provide a consistently better profile. Most profiles are specified by the manufacturer of the product to be installed and are described by the International Concrete Repair Institute guide 03732 and graded from CSP1-9. The CSP level and approved methods of achieving the profile should be specified independently as some manufacturers still list acid etching as an approved method (to appeal to lesser equipped installers.) See also ASTM D4259-88. Surface profiling is one of the easiest places for a contractor to cut corners because no one will see it until the floor fails. DMA can help you determine the proper CSP for your specified system and show you what the profile should look like.

Overlays, Patches, and Levelers

Small surface imperfections can be treated with patching materials, which typically include white patch (which is not suitable), Portland cement, calcium aluminate, and various resinous bases. Levelers and overlays are typically made from Portland cement or calcium aluminate and are used to cover a large area in order to provide a smooth consistent surface for a floor covering or resinous system. There are some overlays that can be used in polished concrete applications as well. Most standard patching and overlay materials should not be used under moisture mitigation systems as they will fail.

Floor Prep Standards

• Cast in Place Concrete Specification – ACI 318 and ACI 306R
• Concrete Mix Specifications – ACI 211.1
• Slab Moisture Vapor Retarders Standard – ASTM E1745-11
• Joints in Concrete Construction – ACI 224.3R-22
• Floor Flatness Standard – ASTM E1155
• Calcium Chloride Moisture Vapor Standard – ASTM F1869
• In Situ RH Moisture Testing Standard – ASTM F2170
• Surface Testing Moisture Vapor Emissions Standard (Impedance Meter) – ASTM F2659
• Std. Test for Measuring Moisture in Concrete by Plastic Sheet – ASTM D-4263
• SSPC-SP 13/NACE No. 6 – Surface Preparation of Concrete
• Concrete Surface Profile – ICRI 03732
• Standard Practice for Abrading Concrete – ASTM D4259-88
• CRI Carpet Installation Standard 2011
• Practice for Preparing Concrete Floors to Receive Resilient Flooring – ASTM F710