Pouring Concrete 101

[lurker]

Shamelessly stolen from another board and posted here cuz Tee wants a slab and the owner of a former board prolly needs some learnin, too.

As to pouring small sections, I stay away from it. I never will pour with less than three guys. Too much can happen that MAY require help. If it aint there, you are stuck. The main reason I don't like pouring small sections is cosmetic. Try as you will, you just can't get the joints perfect. Most of my slabs require monolithic pours anyway due to the interior beams. If you don't have this, and it sounds like you won't, try to pour as much as possible. Small loads of concrete will cost more...even for me. Get the most you can handle and get some help. The fewer joints, the better. Try half and half. You can always find hired guns for finishing. You could even find a small crew and do it all at once maybe...with you supplying the labor. I pay my finishers $140 per day per man on small stuff and .30/sf for a crew on big stuff. Three experienced guys could easily handle the big size you mentioned. The trowel machine, like Carl said, can make a mess. In experienced hands, it will do the work of 10 finishers and give you the best results. If I was doing this and was unexperienced, I would get me a finisher. One experienced man with a machine could easily handle 1000SF without trouble in hot weather. I don't have as much experience as my finishers and I could do it easily. Cold weather or cool weather slows concrete down and therefore adds to the SF. I would expect to pay one guy with a machine about 180 per day or so around here. That would make a world of difference as to how much you could possibly do at one time. Hand finishing is a lot more work and yields so-so results in warm weather. With machine finishing, all the hard stuff is done when you place and set the machine on. If it gets away from you by hand, you are stuck and done for. With the machine, you have a good shot at recovery. Just some things to think about.

As to mesh.....I am not a big fan of fiber alone, esp on a slab. Fiber was made to augment steel reinforcement, never to replace. Along with the mesh, it will do wonders as to keeping the cracking down. By itself, I don't like it and won't do it.

As to mesh size, I don't like putting 10ga down when it will be driven on. That is what 6ga is for. It is getting extremely high priced now, so that may be a factor. If you go with mesh, I would suggest giving 6ga serious thought. Rebar is better, but the mesh, when chaired up properly can be just as strong. FWIW, 6ga wire mesh is the same as #3 rebar on 18" centers both ways. My shop slab now has the bars at 12" OC and the new one has it at 9"OC. Both with mesh in addition to steel. Overkill maybe, butI did it in preparation for heavy loads. The new one has already seen a heavy load...much heavier than the old one ever had and it will get even heavier loads soon.

And then came a reply:

Ok Don give us your take on slump. When the guys poured the 4-feet tall walls for my daughter’s foundation, I was shocked at the slump when it came down the chute. Later I picked up a ticket on the ground, I almost fell over, that truck was dispatched at 8 to 10-inch slump . Before that I would have bet a concrete plant would never add so much water. In my line of work 4-inch is the norm.

And then the followup:

For those that don't know what slump is, it is the rate the concrete will "slump" after it is removed from a slump test cylinder. They take a conical cylinder, fill 50% with concrete, rod 50 times, fill remaining and rod again and then flip it upside down on a flat board or steel plate. The concrete will fall after the cylinder is removed. The slump is the amount in inches that the concrete will "slump" from its original height. The test cylinders are all the same and are 12" tall.I have seen 10-11" batched slumps. If they were batched that way it doesn't matter what the slump is. The mix compooter will automatically add cement to make up for the extra water content. You will get the same strength mix as you would on a "normal" slump. You oughtta see what super plasticiser does. Super P will double the slump .............for about 45 minutes. After that hold onto your hat...its gonna set up fast. They poured the columns at Retama Racepark that way.....6" batched slump with Super P. Was like pouring water with rocks until the plasticiser dried up.....BAM...like a rock after that. They did that to ensure the concrete would flow around the steel. Tight quarters and lots of steel led to that design. They also used the smallest rock I had ever seen used on structural elements.


Slump is a very individual thing as to process, finish type, weather and specs. Generally, if I am finishing it, whether it is a slab or driveway, I want a 4.5-5.5" slump. I have a bad back, wonder why , and I don't like killing it before I am done working if I can help it. If there is a steep grade, a 3.5-4" is good. Walls, I'll start at a 3.5 if it is a single row of steel. When there is more steel, I'll wet it up a little. Water is the reason forms blow out most of the time. Wet mixes flow easier, weigh more and overload the bracing faster. Dry mixes don't flow as fast and will support themselves faster. On finished surfaces, like slabs, the wetter the mix ( within reason ) the easier it is to make smooth. The rock holes fill faster and there will be less of them. Too wet and it is a mess. Too much slush and shrinkage. That makes finishing much more difficult. Too much water in cold weather will exponentially extend your finishing time. Been there...don't like that.

In this case, I'd batch it at 4.5" and add more if necessary ( if it is cooler than 85° for the high, then maybe a 4" slump to start ). That should be fine for his application. Easy to work with, lots of strength left and all that good stuff. Most batch recipes allow for about 30% more water than will be used to batch.....mostly because they know WE WILL add water. That is why a typical 3000 psi mix will break in the 4200 to 4600 psi range. That is their mix water insurance.

That is my take in a nutshell. There are way more ins and outs as to what to use and what to consider. Besides, I am still learning on this stuff.... and hope to continue doing so. A wise old concrete man told me that you need to learn one thing or more on each job. Every mix is different, every job is different and they all can teach you something if you let them....stop learning and you are doomed to repeat the same mistakes and have the same troubles. I have found them to be wise words in other fields of endeavor as well. Welding falls in that range. Lots of little things to learn from if you pay attention.


Here is a helpful hint for anyone considering concrete......stay with a 3000 psi mix. Lots like using the cheaper 2500 psi mix. Don't do it. That mix has less cement and more rock generally. It is much harder to work with than the 3000 psi mixes. 3000 has more cement with to work. The cement allows you to work it to smooth it out, fill holes and generally make it nice and flat. Now, you may think that 3000 is good, so 4000 would be even better......NO!!!! 4000 psi mixes are at the start of what we call a hot mix. They have more cement and will get much hotter, faster. That means they set up faster and need to be finished faster, which takes skill and skilled people. You should see what 8000 psi sets up like.....is is scary!

A straight cement mix will be much nicer to finish, but it will also set up faster than a regular, fly ash mix. They use fly ash to slow down mixes and cut down on cement content. Fly ash is cheaper than cement and acts almost like cement. Some areas do not have access to fly ash, so all they have is a straight mix. Water is the key in that case. Pour it dry and you may live to regret it. Water addition will help slow it down enough to help you if you don't go overboard. Sadly, it takes experience to know exactly when that is.

Good luck with your pour, Tee.

Crete: You can thank DodgeBoard anytime you want to...

Madam Admin, you can move this into the Concrete Information Forum.