Grinding 2100 sq ft

[Majeztic]

I'm ready to prep my shop floor for epoxy. It is about 2100 square feet of 1 year old concrete. I already ground all the edges with a 7" handheld grinder prior to framing and drywall. I laid down Ramboard around the perimeter to protect it from drywall and paint.

The floor is not completely flat and all along I have been planning on renting a planetary grinder for it's speed and it's ability to smooth out the high spots. However, my usual rental place does not carry the 3 head machine and it looks like it is going to cost me over $2,000 to rent the machine from Sunbelt (which includes the machine, vacuum, inserts, insurance, delivery, pickup, and taxes). That's for a one day rental. That's a hard pass.

My next option is to rent one of the Edco machines. My usual rental place has a variety of the Edco machines available for a much more reasonable price and, if I rent it on Friday afternoon, I can keep it until Monday morning for the one day rate. However, I've read that they are slow and don't do a great job. I was hoping to get this all completed on a weekend, but if it's going to take just as long as it would with the handheld grinder, I'll just save the money and do it with the equipment I already have.

So, now I'm considered grinding the entire floor with the 7" grinder. I can work on it a couple hours in the evenings and finish up on the weekend. Is that a ridiculous idea?

Thoughts?

 

[FJ4FUN]

Saying "floor is not completely flat" and "smooth out the high spots" is very subjective.. No floor is perfectly flat which means that all floors have "high" and "low" areas. For the most part, grinders will not flatten out floors. They are used to remove the thin "skim", "cream" or laitance coat which opens up the porous structure of the slab that, in turn, promotes absorption of the coating creating a stronger bond line between concrete and coating. If you are dealing with "high spots" such as those created by a broom finish or, to a lesser degree, swirls that are often a result of a hand troweled finish , then grinding will flatten those out but the subtle hills and valleys common, to some degree, in all slabs cannot be corrected by diamond grinding (and don't need to be...). The diamond grinders, whether it be the large three head planetary or the simple single or twin head Edco types, will simply follow the existing contour of the floor.

As far as grinders go, I'm not sure which Sunbelt setup you're looking at but typically the 21" electric Lavina with a compatible dust extractor and tooling will run you closer to $800 a day (which is still a lot of $... I get it) a bit more if you have it delivered, and would knock out 2100sf in one good day of somewhat "relaxed" grinding. A twin head Edco would take you roughly twice as long and would beat you up a bit more but would still get the job done just fine. The final finish of the planetary unit would be more uniform but for high build epoxy systems such as ours that's not a concern (and, in fact, can be beneficial). As far as hand grinding 2100sf with a 7" grinder goes.... let's just say, my back, shoulders, knees hurt just thinking about it...!

If it were me, I'd rent the twin head Edco, count on two moderate days of grinding, and spend the money I "saved" on chiropractors and grinders on more LiquaTile 1184 to help fill in / level out some of the low areas.

IF it were me....

 

[Majeztic]

Here's the best picture I can find that shows the unevenness of the concrete:


I'm not too worried about it but I figured the planetary machine would flatten it more than a single or double head grinder. Overall, I think it will look fine once coated.

I just checked the quote I got from Sunbelt and it is for the 30" machine. I will check on the 21" Lavina.

Is there a DIY test to determine the hardness of the concrete? The concrete is 5000psi and it was power troweled (poorly) but Edco tells me I should use the hard bond inserts because most likely my concrete is soft.

 

[930dreamer]

That area looks like it might need some filler?

 

[Majeztic]

That area looks like it might need some filler?

Possibly, but I think a 1/32" grind would take care of the majority of that...the picture makes it look worse than what it is.

 

[FJ4FUN]

That is pretty lumpy... I'd work it hard with the Edco and count on having to go back over some of the areas with the hand grinder just to remove the laitance from the low spots that the Edco will skip over (not to correct the profile).

Begin by filling the saw cuts with IntegraFlex 1921 (+ backer rod) to prevent the cuts from siphoning off epoxies.

Then I'd plan on going heavy on epoxy. I'd go with minimum of 7.5 gal BondTite 1101 followed with 21gal of LiquaTile 1184 + full broadcast of flake + 14gal DecoFX 1110 + 5.625gal EnduraShield 2254.... The BondTite 1101 primer is amazing stuff and as long as there's not gross contamination of petroleum or silicone products it is very forgiving to surface prep. Because it is a 100% solids material you do want to do your best to promote absorption but it doesn't take much to build an indestructible bond line. The thicker body coat + flake + flood coat will smooth out that floor really well.

As far as testing concrete hardness the only dependable way I know of on a budget is a Mohs scratch test but that may be overkill for you. Hard diamonds are for soft concrete, soft diamonds are for hard concrete... A good rental shop will be able to tell you what is typical for the area, start with that but be prepared to make an exchange if needed. Some yards will be more cooperative with you in this matter, it all depends on who you're dealing with.

 

[jack stand]

I gotta ask.....why?
Unless that picture in no way represents your garage/shop floor I just don't understand the concern.

 

[Majeztic]

I followed back up with Sunbelt and one of their reps from the flooring division came out to look at the project. He did a scratch test and determined that it is hard concrete.

The quote for the 20" Lavina propane machine with the vacuum and appropriate tooling is about $1600 and I am told that I could get it done in about 5 or 6 hours.

The other rental place has the two head Edco for about $550 with the vacuum and tooling.

All the saw cuts have been filled and ground and all the edge work has already been completed. I also v-grooved all the cracks, filled, and ground them smooth. All I have left is to grind the field.

Current condition:

 

[Majeztic]

The final decision is to go with the two-head Edco machine with the red 30-grit Dyma-dots for hard concrete.

I have a Ridgid shop vac fitted with a bag and fine filter and connected to a Dust-deputy. It did a great job with my 7" grinder. Will that be sufficient with the two-head Edco machine or should I rent the Edco Hepa filter vacuum also?

 

[930dreamer]

I'd take a hepa filter over anything else.

 

[Armorpoxy]

This floor looks pretty good to us and you could get away with just a light grind with a rented Home Depot Diamabrush if you want to grind at all, and then use the Armorpoxy No-Prep Primer. Would save a lot of time and aggravation and the money would be comparable.

 

[Majeztic]

Thank you everyone for your input and suggestions. I finished grinding the floor and figured I'd post back with my experience.

I used the two-head Edco machine. It is a very solid and heavy piece of equipment, however it has a 1-1/2 hp electric motor and I feel it is underpowered. The machine bogged down and tripped the breaker a couple times. The thermal overload on the motor also tripped requiring a long break to allow the motor to cool down to allow it to be reset.

The vacuum worked well and was easy to empty. There was almost no dust in the air.

The machine required a lot of effort to maneuver. When I hit a high spot it would pull and shake and it took all of my body weight and effort to straighten it out and keep it moving forward. On very smooth and flat areas, the machine operated really smoothly and didn't require much effort.

I purchased both the single dot and the double dot tooling so that I had both available just in case. I started with the double dot and did about 100 square feet but they were not aggressive enough so I switched to the single dyma-dots and used them for the rest of the project. The dots were worn down about 60-70% so I would guesstimate that I could get another 1000 square feet out of the tooling, for a total of 3000-3500 square feet.

I had to go over each area 3 times to get a decent scratch and profile. To achieve that, I overlapped each pass by about 2/3. I was advancing across the room about 6" for each pass. I did have to go back over some areas a couple more times. The job took about 18 hours including breaks.

The machine did knock some of the humps and bumps down but it did not make the floor as flat as I feel a better machine would have done. However, overall, I think the finished surface is decent enough for a shop.

I would use the machine again for future projects under 1000 sq ft. The price is right and it did the job. For anything larger, that would require multiple days of grinding, I will get a larger propane powered planetary machine like the Lavina.

 

[FJ4FUN]

OP "I would use the machine again for future projects under 1000 sq ft. The price is right and it did the job. For anything larger, that would require multiple days of grinding, I will get a larger propane powered planetary machine like the Lavina."

...... Exactly. Looking good