concrete slab question

[Cat_Driver]

The best thing anyone can do when pouring a slab is add FIBERS. Your choice of fibers either stainless if fiberglass.

Reinforcement throughout the entire concrete section: By adding millions of fibers to the concrete mix, the concrete achieves three-dimensional reinforcement.
Increased construction speed: By adding reinforcement to the concrete mix, construction is faster and labor costs are reduced.
No special equipment required: By adding fiber reinforcement in commonly used concrete mix designs and conventional concrete pumps, no additional equipment expenses are needed.
Fiber reinforcement also inhibits plastic shrinkage crack formation, reduces plastic settlement cracking, increases green strength, and improves the cohesion of the mix, all of which give you durable, low maintenance concrete.
Fibers enhance both the fresh and hardened properties of concrete. In fresh concrete, fibers help reduce bleeding, settlement and the cracking associated with settlement. Fibers also help minimize the development of plastic shrinkage cracks that have a tendency to form during adverse weather such as hot, dry, and windy conditions.
In hardened concrete, the primary advantage of fibers is their ability to improve the post-crack load capacity or flexural toughness of the concrete.

Carbon & Steel Fiber Reinforced Concrete - CEMEX USA - CEMEX


I poured 4" on beach sand 10 years ago and not a crack while drining heavt tractors and fork lifts on it.

 

[nateisw]

Are these slabs to be poured directly on the soil? How much to get the second guy to dig down 6 inches and and lay down a 6 inch pad of base course and compact it to minimum of 95%? That would be my preferred route.

 

[nateisw]

For a slab on grade; what is under it is more important than what's in it. I am not famaliar with your type subgrade material. A 6" layer of 3/4 minus crusher run gravel base will always help stabilize. If an unstable sub-grade is not economical to correct then you need the structural slab with re-bar. Slab thickness and re-bar design are matched to the static and dynamic loading downward. For admin type vehicle parking quote #2 appears more than adequate unless building on mud. For size of the slab it will need to be divided in four sections with control joints sawed in right after slab is finished (not the next day). I have done saw cutting in the dark many times.

Ron

Ron

This dude concretes.

 

[3Ts]

When I lived over there in the Blackland soil area, the concrete slabs all had 1' wide x 2' deep beams. The barn i built was 60 x 80 clear span and had beams every 20'. As you know, the clay really causes problems with getting a good foundation but, since you're doing "just" a parking spot I don't see that much really needs to be done. If you were going to put heavy equipment on it, or put a building on it in the future, I'd go with the beams.

 

[EddieWalker]

Is your black clay the type that has cracks in it several inches wide in summer? With that amount of movement, you either have to have beams to hold it all together, or it needs to float on the surface so it's not pulled apart. This is a highly localized issue that you will have to rely on your local experts.

What I really wanted to comment on is that the most common reason for cracks in concrete is because of too much water in the mix. A lazy crew, or one that's in a hurry will add as much water as they can get away with to the mix when the truck is pouring it so it flows easier. Water adds volume to the concrete mix, and when that water evaporates, you are left with less volume covering the same amount of area. This forces the concrete to crack. Rebar helps hold it together to it's less obvious, unless it was a lot of water, then you will have huge cracks no matter how much rebar is in there. You will know if too much water was used in less then a week. 95% of the time when I'm looking at cracks in concrete, it's because of water.

Cracking from the soil not being compacted is very rare. Most of the time, the concrete will span the void and nobody ever knows about it. This takes decades to happen. In the few cases where it was from poor soil prep, the most obvious issues is drainage that undercuts the foundation and the weight of the building collapses the concrete.

Rebar on 12 inch centers is going to be hard to walk through. 16 inches will allow you to use chairs and give everyone enough space to walk through the rebar. The second biggest lie told by concrete contractors is that they will lift it as they work. This is impossible and in most cases, they only do it for the first few minutes for pictures, and then abandon that lie completely to get the mud spread and screed. Number one lie is that they will use as little water as possible. You absolutely have to have chairs under rebar. If they do not have chairs, do not let them pour the concrete.

 

[sandman2234]

Is your black clay the type that has cracks in it several inches wide in summer? With that amount of movement, you either have to have beams to hold it all together, or it needs to float on the surface so it's not pulled apart. This is a highly localized issue that you will have to rely on your local experts.

What I really wanted to comment on is that the most common reason for cracks in concrete is because of too much water in the mix. A lazy crew, or one that's in a hurry will add as much water as they can get away with to the mix when the truck is pouring it so it flows easier. Water adds volume to the concrete mix, and when that water evaporates, you are left with less volume covering the same amount of area. This forces the concrete to crack. Rebar helps hold it together to it's less obvious, unless it was a lot of water, then you will have huge cracks no matter how much rebar is in there. You will know if too much water was used in less then a week. 95% of the time when I'm looking at cracks in concrete, it's because of water.

Cracking from the soil not being compacted is very rare. Most of the time, the concrete will span the void and nobody ever knows about it. This takes decades to happen. In the few cases where it was from poor soil prep, the most obvious issues is drainage that undercuts the foundation and the weight of the building collapses the concrete.

Rebar on 12 inch centers is going to be hard to walk through. 16 inches will allow you to use chairs and give everyone enough space to walk through the rebar. The second biggest lie told by concrete contractors is that they will lift it as they work. This is impossible and in most cases, they only do it for the first few minutes for pictures, and then abandon that lie completely to get the mud spread and screed. Number one lie is that they will use as little water as possible. You absolutely have to have chairs under rebar. If they do not have chairs, do not let them pour the concrete.

Good post Eddie, I agree especially with the excess water. When I have concrete poured, I bring them a five gallon bucket of water and turn off the water to the house. That then adds another issue, where they start over using vibrators. Those will screw up concrete almost as fast as too much water. You have to keep on top of them, no doubt!
David from jax

 

[joeu235]

Thanks for the input everyone.

 

[RomeK2]

There are only 2 kinds of concrete. There is cracked concrete and concrete that isn't cracked yet.

 

[Tractor Seabee]

There are only 2 kinds of concrete. There is cracked concrete and concrete that isn't cracked yet.

All concrete cracks; control joints in slabs put cracks where you want them, not where they want to go. I have built many slabs that after many years show no cracks outside the control joints. Use of fiber as previously mentioned reduces stress and shrinkage cracks. Low slump concrete (finishers will hate you) and water reducing additives also are key to minimize cracks. I never allow water to be added at the site unless the delivery can prove the water to be added is part of the mix design (only needed when haul time is close to maximum). Concrete batch plants will accurately apportion the ingredients as you order it. The finisher(s) are your enemy on water addition.

There are concrete guys and then there are concrete guys. A lot of science, engineering, and experimentation go into mix designs. C-94 "ready mix concrete" is a very standard product.

Prior to placing concrete everybody needs "The Contractor's Guide to Quality Concrete Construction" fourth edition. Available on Amazon. Well worth the $100.

Ron

 

[fried1765]

Diggin It: post#7:
Is your experience with concrete in Florida?
Is it in Alaska?
Since you refuse to provide a reference for where you live,..... your generic response about concrete is useless to the OP.
Perhaps you are not aware, but there are major concrete mix/placement differences/requirements across the US.