Couple Concrete Pad Questions

[Beltzington]

I requested a quote for a 30x40 pad and told the contractor I would take care of the grading as I have a BB and tractor. After thinking about this I realize having a BB is not the same as knowing how to use one. The ground is gently sloping and I estimative is 1-foot higher on the uphill side to bring it close to level. Should I give it a go or hire it done? I have the time and patience but I do not want to destroy the building sight in the process.

Second question - anyone know what 1-cubic yard of fiber reinforce ready mix is going for delivered?

 

[sam5570]

one thing i can tell you from exprience is if you are moving the dirt to level your site make sure you compact it good or let it settle for several months, if not your concrete will crack as the ground settles. rebarb stell on top of the gravel will help along with the roll wire. if you want your floor to slope in one direction you need someone with transit to shoot it as it's graded so your concrete will be same depth thru out.

 

[paulharvey]

1 cy in north florida is maybe $135 Plus trip, and short order charge or abiut $150-165 in a "U tote" buggy.

You'll need to grade and compact the area for the pour, but I wouldn't hire it out uf you have a box blade. Just go slow, and compact it all.

Edit: I thought you asked about one yard, but after rereading I assume you mean a full load, $110-130 per yard with a 5+ cy min order, plus some sur charges

 

[Tractor Seabee]

It depends. What do you want from your concrete contractor, such as do you want a guaranteed crack free slab with no differential settlement? If so you will never win that argument if you do the subgrade and it happens. If so give him the whole job, get a contract, and make sure the contract has the guarantee language. Yes, you can do a slab that big and be crack free, but quality has its price. If you do the subgrade, then live with the results. In some cases you won't care about all that, again DEPENDS, what is the end use and how critical is the finished product to you. The cheap guy will not give you a contract or a guarantee in writing, talk is cheap. DO Not believe the old adage of "all concrete cracks"

OLD Contractor, Ron

 

[XSKIER]

Get started by pulling the top soil off with your BB. Then you can judge your operational ability, but not be too far into it that you wouldn't be able to hire it out if you get tired off dragging material around.

 

[Beltzington]

Thanks for your input. Due to all the rain I have not discussed the specifics with the contractor. Another question concerning the concrete. The quote was for 3000psi fiber enforced concrete however after more research I found 6000psi concrete was not much more than a couple hundred for my project. Other than the extra cost is there any reason I should not use the higher psi rating concrete?

 

[bcjmmac]

Hi. I did my own prep for the slab for my "barn" a few years ago (20'x26") - have a sandstone pit so lots of good fill, finished with 5-6 " 3/4 crushed rock on top. If you can get hold of a laser level you can get the prep area fairly level, or have the contractor adjust your work when they put the forms in. I'd recommend you put in drainage when doing the prep work - greatest danger for a slab is having water underneath the slab freeze and push the slab up unevenly. I asked a few builders and they all agreed that the most important step is getting the base uniformly thick with even compaction - that ensures the slab will "settle" evenly and move as a single unit. As such, I would recommend that you dig down on the high side and get the sub-soil level before you start to put in fill.
Anyhow - points to consider. There are basically 2 ways to put in a slab; 1. level the forms on the prepared area and fill to top/scrape it level, or 2. put forms on top of prepared area and use a laser level to level the concrete inside the forms. With either method, it is preferable to have the concrete thicker on the edges. My slab is 6" deep for about 1.5' all around, 3" deep in middle. Used rebar on 2' centers vice fibreglass and haven't had any issues The base of the slab is at ground level on the "high" side, about 6" above grade on the "low" side, perimeter drainage all around to avoid any water buildup.
Regarding concrete rating - I used 3500. Higher rated concrete is thicker so harder to work - check with your contractor to ensure they won't be any issues if you go with 6000 psi

good luck.

 

[Gary Fowler]

I wouldn't go with more than 4000 PSI concrete at the max with 3000 being more than adequate for normal slabs. More cement will mean hotter temps on curing and more chance of stress cracking. Also higher air temp and windy conditions tend to make the concrete dry out faster than it should which also will cause surface cracking. Pick a day when the temps are less than 70F and wind is fairly calm, I prefer to place concrete in the fall/spring or even in winter days before the nighttime temps go below freezing. The lower the temp as long as it is not freezing the better concrete cures. I would put the extra money into a bit thicker slab, like 6" rather than standard 4" thick. You do need the edges all around the slab to have at least 12" deep x minimum of 12 wide thickened area with some rebar reinforcement to prevent edge cracks when driving heavy equipment up on it. I also like to put in a grade beam of the same size at about every 20 feet, in your case one in the middle of the 40 foot width will work.You have to have the area really level to avoid thinned areas in the slab so a transit level is a must to check multiple spots in the site to make sure there isn't any high spots (slightly low is not a problem other than taking a bit more concrete). Lastly don't let the contractor tell you that they can "hook up the wire" when pouring. That doesn't work and usually leaves the wire reinforcement laying on the ground. Invest in some 4x4x2" thick concrete blocks that are specially made to hold up the reinforcement. I would put a #3 or #4 rebar grid on 4 foot centers resting on the blocks places at every corner and one in the middle, then roll out the 4"x4" wire reinforcement on top of the rebar and tie it in several places to hold it in place. This is plenty of reinforcement for a normal slab that wont see anything heavier than a 5 ton tractor. The concrete with the nylon fiber is less prone to cracking but is may leave some "whiskers" sticking up that burn off a bit leaving some black residue when steel power troweling. If this isn't a problem then I would go for the fiber reinforced concrete.
I leveled the area where I build my shop using my tractor with FEL and boxblade and it worked out well. I didn't use a transit but the contractor shot it in prior to forming and it was all level within 2 inches (had a bit of slope) which he said wasn't a problem, he could adjust the forms for that and just put a bit more concrete on the low end. I leveled the slab area but left in some natural slope for the low side so the eaves height would be more which turned out to be good planning (even though I didn't plan it) so my 70HP tractor will go under the shed on the low side but not on the other side

 

[orezok]

6000PSI concrete has double the COMPRESSIVE strength of 3000, but very little added tensile strength. Tensile strength is where cracks are developed. As others have said, it's harder to finish and if it goes off too soon, you will end up with a poor finish. I wouldn't recommend it.

Unfortunately, ALL concrete cracks. The cracks can be controlled by judicious use of control joints. Before retirement, I used to build State of California public works projects. Even with subgrade over-excavated to 6' and recompacted to 90% and tested, 4" of rock, Stego, 2" of sand, 4500PSI concrete poured at 4" slump it still cracks. The concrete was poured with a w/c ratio of .45. It's about the best that can be done.

The only concrete that does not crack is poured in heaven!

 

[Carl Bert]

Show me a contractor that will sign a guarantee that your concrete will not crack, and I will show you a guy that is very desperate to get the job and will pretty much do or say whatever it takes, but he could never stand behind it.

My family has been in the concrete business my whole life, and there is no way to GUARANTEE it will not crack. One thing for sure is, 6000 will crack before 3500.

If you follow what Gary said you will be fine.