I. Mix Design
A. PSI Ratings
Ratings for compressive strength of a concrete mix represent the "maximum" PSI achievable when conditions and process are ideal.
But greater PSI ratings of mix do not by default translate to the density needed for a superb slab.
Adding water to the mixture to get a slump beyond 5 is not a good idea, and will dramatically reduce both the actual end PSI as well as the density of the matrix.
Excess water has to bleed and or evaporate from the slab. The more that is present, the more voids are left behind.
If you need to increase the slump for easier placement, use a mid-range plasticizer admixture to get the flowability you want, as opposed to more water. It will "behave" like a wetter mix without the corresponding loss of PSI and density. And this usually only adds a couple of dollars per yard to the cost of the mix.
B. Portland Content
Portland cement is the "glue" holding everything in the matrix together. This is why more
Portland content translates to greater PSI.
A minimum of 3500 PSI rating of the ready mix is preferable.
It isn't so much about "hardness" as it is about density, tensile strength and color.
One other important note about Portland is that it is what is responsible for the "gray" color in your slab. A 3,000 PSI mix (in this region of the country) will end up being a light pecan or tan color when polished. This is because the fine aggregates and sands in this area are mostly tan. Without a high portland content to offset this, the end result will be a tan slab.
C. Admixtures
As noted above, mid-range plasticizers are acceptable, and certainly preferable to additional water content.
II. Placement
A. Place & Screed
This is the most important process after proper mix design.
Flatness (NOT to be confused with "smoothness") is the surest way to ensure a good
palette for your polishing crew.
A FF >50 is great. If you are not able to measure your flatness, at least understand what is is, and is NOT:
Flatness is NOT "level."
Flatness is NOT "smoothness."
All slabs have variations in grade height. There is no such thing as perfectly flat. But the flatter it is, the better the results will be when polished.
Using a laser screed is ideal. But a normal hand screed can achieve similar results if care is taken to keep to grade stake heights and not leaving humps and waves during the screed.
B. Floating
"Bull-floating" or "mag-floating" after screeding is an essential practice.
The purpose is to close the pores in the surface of the wet mix, slowing down the water transpiration. This allows for maximum PSI and density.
Common mistakes which cause problems later include: overworking the float, and/or floating bleed water back into the surface.
A good rule of thumb is "float it early, float it tight, and leave it alone until the bleed water evaporates."
Rapid drying is NOT a good thing for slab strength and density. If you know that the goal is to help the slab retain its water content as long as possible, then don't place it when the wind is blowing 35 miles an hour.
Floating the surface properly will close the open pores and help the slab retain water content as it is plasticizing.
C. Bleed-off
Avoid floating or troweling bleed water back into the surface. This is equivalent to adding a lot of extra water to the ready mix (but right at the surface) and is the number 1 cause of spalling.
D. Troweling
Once your bleed has run its course, steel trowel to a tight finish.
Your polishing crew doesn't need "smoothness" for their process. They need "strength" and density at the surface. This comes from slow hydration. And a tightly troweled finish slows the hydration process during its critical stage.
E. Formed Edge Finishing
Edges at forms are typically floated and finished by hand. This can result in flatness issues in that 14" zone from form out into the field.
A little is normal. But severe dips and/or rises will result in inconsistencies in the final polish. Probably not a big deal for a mechanical room or janitor's closet. But pay attention to this in areas on the plan where they are going to be highly visible. Know your plans.
Try to plane them out well enough that your power trowel can ride all the way to the formed edge and still make contact with the surface.
F. Penetrations
Flat Placement around penetrations (plumbing, electrical and HVAC stub-outs) can be difficult.
Study your floor plan set to see where penetrations are likely to be exposed along a wall in a highly noticeable location (showroom, living room, foyers, etc).
Knowing that the walls which cover the penetration will hide ONLY the mechanicals, you can decide where to give extra attention to flattening out your placement around these.
G. Boot Prints, Screed Prints
In the placement process, it may be necessary for a guy to step out into wet mix after screed has passed. WHEN this is necessary, it is too common for another guy to float wet mud over the boot print.
This wet slurry does not contain all of the aggregates any longer.
If you must step into wet mix after screeding, take a mud rake or mag trowel and STIR the adjacent mix to get aggregates dispersed again (stepping in it pushed them all down, and the wet slurry from the surface doesn't replace them, so when polished, they show).
H. Aggregate Dispersion
IF the polish design calls for aggregate exposure, it is important to reduce steps into wet mix, and to maintain consistent slumps throughout placement.
III. Control Joints
A. Purpose
Control joints provide a "weak" spot for slab shrinkage. The slab WILL shrink. As it does, it WILL crack. Sawcutting the slab to an inch depth creates a joint at which the shrinkage cracking defaults. You are "controlling" the cracking.
B. Timing
As soon as your slab starts to set, it starts to shrink. Temperature, wind, water content and finishing all factor in to how quickly it starts to crack.
But rest assured that if you do not sawcut control joints within 36 hours, you are wasting your time. Shrinkage cracking has already begun and your sawcuts will be useless.
C. Planning
At a minimum you need to sketch out a layout for your cut man.
Standard Operating Procedure is 10-12' OC for a 4" pour, and 15-18' OC for a 6" pour (IF slump was 5 with water...more water means more shrinkage, and shorter intervals needed for sawcuts)
Plan for GREEN-CUT saw blades. Have multiples at the ready. They do not last long in green slabs, but they will ensure clean cut lines.
ALWAYS plan for cuts where shrinkage will occur around inside corners, piers, and penetrations.
IF adequately planned, your control joints can actually provide a nice pattern for the floor.
IV. Curing & Sealing
A. Purpose
Once again, the purpose of a curing sealer is to help the slab retain its moisture content for as long as possible.
B. Types
Most curing sealers are either acrylic or epoxy (water based) and are usually intended to be sprayed on.
C. Hazards
Spraying cure sealer is fine...BUT...its all about Consistency!
If your spray tip is dirty or clogged or improper for fine sprays, you are likely to get patterns in the slab. As the slab cures, it will cure slower where there is more sealer, and faster where there is less. This can result in DEEP variations in density, thus variations in the final polished color.