Green Umbrella has assembled the instructions and advice given here to create an exposed concrete floor that will look its best.
These guidelines will help ensure expectations of both Owner and Architect are met and, if specified, will positively impact a Grind, Honed, and Polished finish.
As a recommended best practice the Flatwork contractor and GHP contractor should be included in the pre-con, and pre-pour meetings to discuss the procedures outlined below:
Getting Started with the Green Umbrella Polished Concrete Flatwork Guide
Sub Grade (Laser Grading Recommended)
To minimize cracking caused by restricting movement, care should be taken to compact and fine grade the sub-grade as flat as the finished concrete on the surface to insure uniform thickness and the least resistance to friction as the slab contracts and expands during thermal temperature changes. This will reduce possible reflective cracking cause by differential sub grade restraints. This will also reduce surface defects caused by bleed water pooling in low spots on the vapor barrier during the plastic state and excessive water rising to the surface during hydration stage causing craze cracking and surface blemishes that cannot be fixed by the polishing contractor
An Under-Slab Vapor or Moisture Barrier may be used to prevent rising moisture from affecting your floors and maintaining a healthier establishment. Under-Slab Vapor Barrier material in 15 mil Polyethylene may be placed with the seams and penetrations taped before the concrete slab is poured and turned down in the footing. Moisture affects the polished surface appearance. If a vapor retarder is specified, the slab should also be reinforced with steel or welded wire that has a cross-sectional area of steel less than 10 percent of the concrete cross-sectional area, and continuous below saw cuts to minimize dominant joint activation. Concrete slabs on vapor retarders slip, causing some joints to not open. This can allow the concrete shrinkage shortening to accumulate at one wide dominant joint.
The concrete mix itself is critical to the quality of the concrete and ultimately of your surface. These are the recommended guidelines for an ideal ‘Mix Design’ to give you the optimal polished concrete floor.
- Concrete Mixture shall be non-air entrained and Minimum 3500 PSI or higher.
- Mix should be designed at around a .45 water cement ratio
- The mix shall be designed as a total gradation mix and not using gap-graded aggregates.
- Any admixtures, plasticizers, slag, fly ash or anything taking the place of
- Portland-based cement shall not exceed 20%. A straight cement mix is recommended.
- The cement shall be Portland cement type I/II conforming to ASTM C 150.
- Maintain concrete temperature below 85 degrees preferably below 75 degrees.
- Keep concrete cool and moist as long as possible.
- Decrease the rate of hydration and drying to minimize cracking.
- Architect must approve all mix designs.
- If integral color is used the minimum batch size should never be smaller than 3 cubic yards.
- Use same source for cement, aggregates, and pozzolans throughout the job.
- Monitor and control incoming material consistency. Do not use calcium chloride based admixtures. Non-chloride admixtures may be used.
- Wash out all drums before loading. Ready-mix supplier shall maintain a 5” inch slump +/- 1/2”.
- Obtain approval from Owner’s Authorized Representative if slump is outside these parameters. Minimize driver added water to maintain a 5” slump at point of discharge without compromising the water content ratio. Utilize plasticizers for slump adjustment.
- Place concrete with 5” slump at point of discharge, *strike off with laser screed, bull float 90 degrees to screeds pull direction.
- The bull float shall be 6’ in length or larger, smaller bull floats can to be used on sloped areas.
- When the concrete is dumped into piles and then raked out, coarse aggregate can ‘stack,’ distribution will not be uniform, and the non-uniform coarse aggregate distribution will show through the polished surface. You will have uneven exposure for aggregate.
- When placing edges use a 3’ to 6’ metal or wooden 2x 4 screed and run perpendicular with form or edge after initial screed and before floating.
- Re-screed edges, as possible, after the initial set. This will help to reduce the typical dip along edges.
- Machine Floating with pans shall begin after placement when concretes psi is sufficient to operate without causing excessive mortar or ridges and little or no bleed water is present.
- Slab shall be checked and re-straightened with 10’ or larger highway straight edge or bump cutter to ensure FF’s are met.
- When using pans it is best to over run the formed edge by 5”
- Machined edges (walk behind) if walk behinds are used it is preferred to have pans or float blades for the initial passes.
- The first pass along the edges shall be with the left side of the machine which is referred to as the cutting side. This will pull the high concrete away from the edges first.
- The second pass shall be on the right side of the machine which is referred to as the fill side to fill the low spots.
- Riding trowels are to be used in the same manner as the walk behind.
- The cut side of the machine is the rear.
- The fill side of the machine is the front.
- Troweling shall begin after the surface has received a 2 pan float finish.
- Concrete finish floors shall have a minimum of 3 passes resulting in a non-burned steel troweled finish. Do not over trowel and burn the slab. The use of plastic trowel blades will help prevent burning the aggregate but are not required.
- The slab drying must proceed naturally and must not be hastened by the dusting on of dry cement or sand.
- Lightly tool or machine all edges at construction joints and exercise care that slab edges are not depressed or chattered along bulkheads, formed edges, columns, and pipes during finishing operations, particularly hand troweling.
Local Flatness/ Levelness
Except as set forth below, the Random Traffic Floor shall conform to the following minimum F-number requirements:
Floor Flatness Number: FF
Specified Overall Value = 50
Minimum Local Value = 35 Floor Levelness Number: FL
Specified Overall Value = 30
Minimum Local Value = 20
* Preferred strike off method: Somero S 240 or SXP Laser Screed.
** If hand screed is used: Strike off with vibrating screed, using a rail system with set elevations, Slab must be bump cut with 10’ or 12’ Hwy straight edge during pan float finish in 2 directions to increase FF numbers and reduce wave index.
General Conformity to Design Grade
Except as set forth below, the entire Random Traffic Floor shall fall within +/- 3/4″ of its specified elevation.
Both the overall and minimum local FL levelness tolerances set forth in Local Flatness/Levelness above shall not apply to any Random Traffic Slab that is to be inclined or cambered. Likewise, no FL levelness tolerances will be applied to any un-shored elevated construction.
The general conformity to design grade tolerance set forth in General Conformity to Design Grade above will apply to un-shored elevated slab constructions, but in all such cases, the tolerance will be increased to +/- 1-1/2″.
All floor flatness, levelness, and grade conformity tests shall be made (at the Owner’s expense) on each newly installed Random Traffic Slab within 8 hours after completion of the final troweling operation. FF and FL tests shall be made by a factory certified technician in accordance with ASTM E1155 (latest revision) using an “F-Meter”.
Grade conformity tests shall be made using an optical or laser level. Results of all floor tolerance tests – including a formal notice of acceptance or rejection of the work shall be provided to the polishing contractor within 12 hours after testing. Failure to adhere to the testing and reporting requirements set forth in this paragraph shall constitute de-facto acceptance of the work.
*NOTE: Weekends and holidays shall be ignored when computing specified testing and reporting deadlines.
Remedy for Out-of-Tolerance Work
- The entire Random Traffic Floor shall be subdivided into Minimum Local Floor Sections bounded either by the column and half-column lines, or the construction and control joints, whichever subdivision yields the smaller areas.
- All Minimum Local Floor Sections measuring at or above the specified MLFF and MLFL numbers shall be accepted for F-number compliance as constructed.
- All Minimum Local Floor Sections which fail to meet or exceed both specified minimum local F-numbers shall be removed and replaced (in the case of slabs-ongrade), or ground and/or re-topped (in the case of elevated slabs).
- No remedies for defective Minimum Local Floor Sections other than removal and replacement of slabs-on-grade, and grinding and/or re-topping of elevated slabs will be permitted.
- Finish needs to be uniform in color and levelness with no trowel marks, foot prints or depressions from hand tools, knee board ETC…
- Concrete to be placed in accordance with ACI 302 1R 04 Class 5 floor. Concrete to be finished in accordance with ACI 117
- Floor shall be tested in accordance with ASTM E 1155 The polished concrete.
- contractor shall be included in distribution of reports and receive one copy of the
FF/FL test results within 12 days of actual testing.
- Based on independent studies we recommend saw cutting with a Soff Cut Saw ASAP without raveling edges or dislodging aggregates.
- We recommend the following based on slab thickness to help minimize slab curling and panel cracking:
4” thick to be cut at 10’ OC EW
6” thick to be cut at 12’ OC EW
8” thick to be cut at 15’ OC EW
- All re-entrant corners should have rebar inset & be cut 50% of the slab depth to reduce cracks from internal pressure.
- Panels should be kept as square as possible and should be approved by architect.
- Wet curing is preferred to dam exterior edges and flood slab or run sprinkler hoses to overlap so slab has no dry spots, but if this cannot be achieved, the use of blankets is preferred; blankets must be pulled 5 days before polishing use a dissipating or water based cure and seal.
- Do not use a densifier or hardener material.
- Concrete shall be cured a minimum of 28 days before Diamond Polishing (*Unless use of Green Umbrella GreenCut and Green Umbrella WetCut systems are used)
- Application shall take place prior to fixture and trim installation and/or substantial completion.
- Polished area should be closed to traffic during Grind, Hone, Polish process.
- For day of placement cure and best results, use GreenIce Cure by Green Umbrella.
Concrete Slab Protection
The floor should be protected from the following:
- Silicone chalks should NOT be used if at all possible. The RED and yellow chalks are PERMANENT DYES. RED Chalk, black markers, wax pencils should NOT be used for framing. White or Blue chalks are OK. Do not over mark for the framing. Do NOT use silicone sprays to “Hold” the lines. The sprays repel the stain and leave harsh, permanent scars on the floor.
- Do not use , Tape, Glue, Solvents, Pine-Sol, Varnish, Non Breathing Plastics, Liquid Nail, Silicone, Plastics, Nails, Plumbers Glue, Foam Insulation, Bond Release Agents, Flux, Oils, Grease, Polyurethane, Paint, Markers (framers often write dimensions of doorways in marker on the slab. Ask them to make that note on the wood framing the doorway), Grease Sticks, Spray Paints, Crayons, Muriatic Acid, and other chemicals both before and after staining.
- Do not allow spills to sit on floor. Clean with Green Umbrella Green Clean™ It is important that wood, sheet goods, insulation boards, plywood, press board, drywall, sections of framing and the like not lay on the slab for extended periods of time. They can transfer resins and tannins into the slab. This will alter the moisture content in the slab which leaves a pattern in the finished floor. Cardboard should be placed between the slab and the stacked material to minimize any unwanted transfers. Also Food, Beverages, Oil, Glass, Metal, Paint, Caulk, or Primers.
- Be sure to check lifts tires for nails and screws, diaper all equipment from oil and grease drips, don’t allow pipe cutting equipment on slab.
After Grind, Hone, Polish Protection
- It is extremely important that you do not tape the floor! When covering the floor, overlap sheets of *Green Umbrella Build Board. Tape the first sheet to the wall then overlap the second sheet to the first and tape it to the paper. DO NOT TAPE TO THE FLOOR. Always use Breathable Tape.
- Duct Tape, Masking Tape, Packaging Tape, Strap Tape, Blue Tape, Green Tape, and Electrical Tape there are NO exceptions. The tape alters the natural curing process and transfers chemicals to and from the slab. Tape, Plastics and other Adhesives can contribute to Plasticizer Migration. This WILL SHOW in the finished product.
Polished Concrete Finishing Endnotes
*Figure 1 shows the wave index of a slab, both are within a 1/8” in 10’ therefore the ACI 117 and ASTM E1155 must be adhered to for polished concrete slabs.
- Screed slab surfaces with a vibratory straightedge and strike off to correct elevations.
A sufficient amount of paste is needed at the surface for polishing, and this is best accomplished by the use of a vibratory straightedge such as a hand-held vibratory bull float, a vibrating screed, a truss screed, or a laser screed. Hand-screeding will not provide enough surface paste. There are also new vibratory handles available.
- Do not leave the vibratory screed at any one location while the vibrator is running; move it continuously across the slab.
Too much vibration at one location drives the coarse aggregate deeper into the slab. This results in variations in coarse aggregate exposure after polishing. This effect of non-uniform vibration can be due to leaving a vibratory screed running when it is not advancing or, for self-propelled laser-guided screeds, lowering the vibrating head onto the surface before the machine is moving.
- Do not use a highway straightedge or bump cutter on the concrete surface during finishing operations.
This removes too much paste from high spots, leaving variations in the polished surface. Flatness for polished concrete is best achieved by the use of pan floats.
Polished Concrete Finishing
- Do not trowel over adjacent hardened concrete surfaces.
- Inspect trowel machines prior to each pass and remove any accumulated mortar.
Troweling over adjacent hardened concrete from previous placements scratches the concrete surface. Some of the worst scratches come from debris falling off an uncleaned trowel machine. These scratches will often be visible on the polished surface.
- Machine-trowel the slab edges and as close as possible to walls and columns.
- Minimize hand-finishing. Insist the contractor use a 2’diameter edge power trowel.
Machine-troweling and hand-troweling result in differing surface densities, with the former’s surface being denser and harder. Results of the differing densities will be obvious in a polished surface.