The owner of a floor or slab sees the hardened surface as a platform on which to conduct business. What he wants is a uniform finish with minimal cracking, curling, and joints. There are no deep dark secrets to achieving such a floor—it all starts with a pre-installation conference.
Successful floors, slabs, and toppings (FSTs), begin with a thorough discussion between the design professionals and the owner over the requirements for the floor, the owner’s expectations, and the anticipated life cycle. These discussions should include visits to the client’s previous floor, slab, or topping installations or to other projects the design professionals feel exhibit the qualities desired by this owner.
After the FST requirements are agreed upon, the design professional can proceed with plans and specifications. Successful FSTs are rarely accomplished by sending out the project for open bid—successful projects require a team approach rather than an adversarial model. The general contractor and concrete contractor selected must have a history of success with similar projects. The personnel assigned to this project must also have a history of similar successful installations.
After selecting the prequalified contractors, the project will require a preconstruction meeting. Discussions with the early contractors will include the construction process and schedules.
After the project has progressed to within about 2 weeks of the FST installation, a pre-installation conference should be held. The owner, architect, engineer, and construction manager/general contractor (CM/GC) will have prepared an agenda that thoroughly covers all aspects of the FST installation. This agenda should be mailed to all attendees at least 10 days before the meeting, giving them enough time to review it. Each attendee can prepare thoroughly for the meeting by outlining the methods and procedures they intend to use on the project. They should compile a list of questions regarding coordination, schedules, and any uncertainties about the intent of the plans or specifications.
Properly prepared agendas are the basis on which successful installation conferences proceed. If all of the attendees are not prepared, agreements cannot be reached at that meeting, and often a second meeting must be scheduled. When that happens, all floor decisions may not have been made before the beginning of the floor installation. A poor pre-FST meeting can cause those early concrete placements to become practice slabs, to the chagrin of all, particularly the owner.
A typical pre-installation conference includes representatives of the owner, architect, structural engineer, construction manager, general contractor, concrete contractor and finisher foreman, concrete producer, testing laboratories, admixture manufacturer, and dry-shake hardener manufacturer. If the project is going to be pumped, a representative of the pumping contractor must also be present. Everyone who will have a role in the proper execution of the concrete work should be included in the pre-FST conference.
Here is a complete list of the discussion items for a pre-installation conference.
Base and Subbase Composition, Preparation, Compaction, Testing, and Tolerances: ACI 302.1R-04, “Guide for Concrete Floor and Slab Construction” (Section 4.1.3), recommends a subbase tolerance of +0/-3/4 inch for Class 4 and higher floors. Meeting this tolerance is extremely important since it affects the concrete thickness tolerance.
Vapor Barrier/Vapor Retarder: If one is required, it is generally placed directly under the slab. Its installation must be approved prior to concrete placement.
Concrete Mix Design: A floor subjected to vehicular traffic requires smoothness, surface density, and minimal cracking and curling. The mix design must therefore have workability, finishability, and setting time characteristics that will permit the qualified contractor to provide the floor required. Mix designs are prepared on the basis of compressive strength accordance to ACI 318, “Building Code for Reinforced Concrete,” Section 5.3, or ACI 301, “Specifications for Structural Concrete for Buildings,” Section 3.9. These mixes satisfy strength requirements but often do not satisfy the requirements mentioned above for a concrete floor. These mixes must always have a maximum shrinkage (see below). Many mixes will contain 4 to 5 pounds/cubic yard of a macro-synthetic fiber.
Window of Finishability: The selected mix design must have a wide enough “window of finishability” (ACI 302.1, Chapter 8). This enables the concrete contractor to perform the various finishing techniques necessary to incorporate dry-shake hardener, if required, and to equal or exceed the specified FF / FL tolerances Many mixes do not provide the needed window of finishability—the mix sets too quickly or too slowly, causing unnecessary overtime work. Retarded concrete can lead to delayed bleeding and delamination. A successful test placement onsite is necessary to verify proper workability, pumpability, setting time, and finishability. The concrete contractor should state at that time, that the proposed mix design will enable him to achieve the specified concrete qualities.
Flexural Strength: The compressive strength of slabs on grade is of secondary importance to the flexural strength. Many floor and slab specifications require a flexural strength of 700 psi at 28 days.
Shrinkage Limits: The maximum shrinkage to achieve the desired floor characteristics is 0.04 percent at 28 days as measured in accordance with ASTM C 157 modified (7 days of moist curing); 0.02 percent at 28 days is increasingly being required to minimize joints (increased joint spacing). The 28-day test results are required at the pre-installation meeting.
Climatic Conditions: Work on any large project will probably occur across two seasons. A single mix design cannot be optimum for the anticipated range of air temperatures, so the mix should be adjusted to offset the expected hot or cold weather conditions. All possible mix designs should be submitted, reviewed, and approved in advance. If climatic conditions do not require mix design adjustments, the project does not suffer, but if a mix design change is required, it can be done quickly since the mix will have already been approved.
Schedule: The concrete contractor should outline a placing schedule for the FSTs. This allows all other subcontractors to be fully aware of when their work must be accomplished and in what areas. It also lets subcontractors know when they can put their equipment and material on the slabs.
Placement Methods: The concrete contractor should present a general placing procedure and give clear details of the placing methods in broken-up areas, pits, slabs adjacent to walls, and any floor area that presents a change from the standard placing procedures, such as pumping. Also, will the change in placing methods necessitate a mix design change? If so, that change in mix should also be submitted for approval.
Joints: The maximum joint spacing and type of joints should be clearly indicated on the plans. Saw-cut timing is key to successful contraction joints that minimize mid-bay cracking. An early entry saw should be used.
Joint Filler: In areas subjected to heavy or hard-wheeled traffic, the construction and contraction joints should be filled with semi-rigid joint filler (ACI 302.1, Section 3.2.6). These joints should not be filled until the majority of slab shrinkage has occurred, and not sooner than 90 days after concrete placement. On many fast-track jobs, however, the joints must be filled much earlier. The consequences of early filling (separation of the joint filler from the joint edge, ACI 302, Section 9.10.1) must be discussed with the owner and an agreement reached regarding the timing and the repair of the expected separations.
Test Floor, Slab, or Topping: How large should the test area be? It must be large enough to represent the placing and finishing techniques required on the FST. Ideally, it would be in an area that will ultimately be used for offices and will therefore be covered. If this is not possible, the test panel can be in an area in the facility that will be subjected to minimal traffic. A successful test panel assures the owner that the contractor is capable of fulfilling the contract requirements. It also allows the mix design to be fine-tuned to the contractor’s needs. For example, if an aggregate gradation problem reveals itself through excessive bleeding, the gradation can be modified to eliminate the problem. If the mix is setting faster or slower than desired, the setting time can be normalized by adjustments in slump or with appropriate dosages of a water-reducing/retarding admixture or a noncorrosive accelerating admixture. If the slab is being placed in very hot conditions, the slump may be increased with an increase in a HRWR admixture dosage. This permits a longer period of placeability.
Dry-Shake Hardener: Proper application of a dry-shake hardener can be accomplished in several ways, but the result must be that the hardener is properly incorporated into the top surface of the concrete. The top 1/8 inch of the surface is toughened by the use of a metallic or mineral aggregate hardener. Application of a factory pre-mixed hardener should always be done using a mechanical spreader, although it may be applied by hand against walls and in areas with pits. This application must be coordinated with the specified floor flatness (FF) tolerances. The higher the flatness number, the more restraightening operations are required. The hardener should be applied to the slab at a time when the floor profile is close to that specified, so that the restraightening operations will not remove the hardener from high spots. These application and finishing procedures must be discussed in detail and agreed upon at the pre-installation conference.
Curing Methods: The project specification will outline the curing methods, which could include a variety of moist-curing materials as well as curing compound applications. If moist curing is selected, the timing and the method must be agreed upon. Curing should start immediately after final finishing. If sheet covering is to be used for 7 days of moist curing, the floor flatness measurements must be made after the final finishing operations so that the curing process can follow immediately.
Cracking and Curling: All parties want to minimize cracking and curling. As concrete shrinks it tends to crack. The question then is which cracks must be repaired and which are cosmetic in nature and need not be repaired? Repair guidelines may state that the crack must be located in an area subject to traffic. Added requirements could be that it must be over 1/16-inch wide or be breaking down under traffic. Any crack wide enough to lead to a loss of aggregate interlock, so that the slab rocks under a normally loaded vehicle, must be repaired. Curling joints cause a decrease in rideability and lead to joint breakdown. Minimal cracking and curling require proper slab design, low shrinkage mixes (less than 0.04 percent or 0.02 percent at 28 days), proper installation, and proper joint spacing. The proposed mix design’s 28-day shrinkage results should be known at the meeting.
Floor Tolerances: The overall FF / FL tolerance should be clearly specified. Minimum local values should also be specified. Acceptable remedial measures must be discussed and clearly understood before the project begins. If there is an area, such as a superflat surface, where no portion of that floor can be below a specified minimum, everyone should be made aware of these special requirements in advance. Floor tolerances should be measured within 72 hours after finishing, but remember that the owner is really only interested in the FF / FL numbers of his slab in operation. In general, the floor profile at 12 months is the floor profile for the next 20 years.
Test Reports: Prompt reports should be furnished to all members of the concrete team. Test reports indicating that concrete does not meet the specified requirements should be emailed to the distribution list and hard copies sent on colored stationery to immediately alert the recipients to this potential problem.
QA/QC Program: The owner requires a uniform finish and minimal cracking and curling. The surface profile at 12 months should not be more than 15 percent below the initial profile. The concrete producer should be consistently delivering his concrete within a 2-inch slump envelope agreed to by the concrete contractor.
There are many other topics that may be included in the conference agenda, depending on the type of FSTs being installed and the owner’s requirements.
Plan for Success
Proper planning is an important ingredient in any successful concrete project. A pre-installation conference enables the concrete team to meet prior to the installation, discuss all aspects of the project, and have everyone in agreement and marching to the same tune. The meeting also allows a thorough discussion of potential problems beforehand that have occurred on previous projects undertaken by the assembled group. Minutes of the conference should be prepared promptly and sent to all attendees.
I have attended some pre-installation conferences where team members were very aware of the project requirements and prepared to execute them faithfully. These meetings almost always result in an excellent floor installation and a satisfied owner. I have also attended meetings with a poor agenda or with key players missing or both, and meetings where there was poor preparation and uncertainty about the methods and procedures to be used. The consequence of this type of meeting is an installation that results in an unsatisfactory floor and project relationships that quickly became adversarial.
Today, sophisticated owners have very definite requirements for their FSTs. Experienced design professionals are capable of preparing proper plans and specifications in accordance with ACI standards and recommendations. There are concrete contractors who install successful FSTs every day. Qualified people producing unsuccessful floors, slabs, and toppings is a tragedy. A well-prepared, detailed agenda, which thoroughly addresses all aspects of the installation, and accurate minutes distributed soon after the meeting, minimize conflicts and greatly enhance the likelihood of success.
William S. Phelan is senior vice president of marketing and technical services for The Euclid Chemical Company, where he has worked for over 37 years. He is an Honorary Member of ACI.