How to order/specify concrete
Here’s how most people order concrete: Call the concrete contractor and tell him they want a new driveway/porch/sidewalk.
Here is the problem: You put a great amount of faith in your concrete contractor to order the right concrete, place it properly, and finish it correctly. Unfortunately, MOST concrete contractors don’t really know how to do any of those steps correctly. So, how do you know that you are getting the job done right? (Especially if you are not around to oversee the placement of your concrete?)
Well, a little basic concrete knowledge will help you a great deal. When ordering concrete, there are a few terms you should know that will help you insure that you will get a quality product.
Strength:
Most contractors know that they can specify the STRENGTH of the concrete. Strength of concrete is measured in PSI, or Pounds per Square Inch. A normal strength of concrete for driveways is between 3000 and 4000psi. This strength is the compressive strength of the concrete. Higher and lower strengths can be ordered, but unless you expect a lot of heavy traffic, it is not necessary
Concrete is strong in compression, but very weak in tensional or torsional strength, meaning it will break or crack when bent (tension) or twisted (torsion). Adding steel to the concrete (rebar) gives the concrete tensional and torsional strength. The amount and size of the rebar you need depends upon how much weight and movement you expect. For most driveways, 1/2 inch (#4) rebar placed and tied together on a 2 foot grid is a good way to go. If you expect a large truck (such as a moving van) or something of similar size and weight, you may want to have the rebar placed in a 16 inch grid. The rebar needs to be elevated off the ground so that it sits in the middle of the slab. It won’t do any good if it just sits on the ground.
The addition of rebar gives your driveway structural support, and will be the best thing you can do to prevent cracking. The other thing to do to prevent cracking is to make sure that the ground underneath the driveway is thoroughly compacted. The use of a rolling vibrating compactor or a “sheep’s foot” compactor are the best ways to compact the sub-soil. On most driveways, contractors simply scrape the turf off the area, put up the forms, and start pouring. Guaranteed, you will have cracks if they don’t compact or put rebar in the concrete.
What about wire mesh, you may ask. Well, it is less expensive, and is a great improvement over no reinforcement, but if you ever expect a vehicle larger than a car or passenger van, or didn’t have the sub-soil compacted, or you have any seismic activity, you will likely have cracks. Once again, the reinforcement needs to be in the middle of the concrete, not at the bottom or top. Fiber reinforcement sounds like a good idea, but really doesn’t help tensional or torsional strength. Our advice on fiber and wire mesh is to put your money into rebar. Rebar simply works.
Aggregate:
After specifying the strength of the concrete, the next important factor is the aggregate, or rocks, that are used. Unless you want to have a washed or exposed aggregate slab, you will want to make sure that you specify 3/4 inch aggregate in any slab of 4 inches in thickness. The thicker the slab, the bigger the aggregate. Most contractors leave the aggregate decision up to the concrete plant, and they are usually ok to do so. But it is always good to ask what size rock the plant uses on driveway slabs.
Water:
The next, and probably most important, specification is the water to cement ratio. This is usually specified in terms of “Slump.” Slump is simply the amount the concrete will sag when concrete is put into a device called a slump cone, turned upside down, properly consolidated, and the cone removed. Under most circumstances, a slump of 4 inches is the MAXIMUM slump that will have the proper water to cement ratio. HOWEVER, if you tell the concrete plant to include a Super-Plasticizer, you will get a slump of around 8 or 9 inches with the same amount of water as a standard 4 inch slump concrete.
A concrete contractor usually gets the concrete delivered to the site at the proper water to cement ratio (because the plant won’t send out a knowingly defective product), and then the contractor adds water to achieve a more plastic or workable mix. THIS PRACTICE SEVERELY DAMAGES YOUR CONCRETE AND IS UN-FIXABLE. It makes it easy for him to work, but is the most common problem with concrete placement. The use of a Super Plasticizer will allow the contractor to have a workable mix without exceeding a proper water to cement ratio. ALWAYS use a Super Plasticizer on slab placement, and you will stand a much better chance at getting a quality result.
Super Plasticizers are a very inexpensive addition to the concrete, usually less than $5 per cubic yard. It will add about $40-50 per truckload of concrete. The cost of replacing your concrete is much higher, and most concrete contractors will not stand behind their work if there is spalling or a surface failure (They will claim it is “salt damage”). Specifying a super plasticizer in your mix will provide help you insure that your concrete pour will be a quality pour.
Finishing:
The last mistake concrete contractors commonly make is to finish the concrete before it is ready. Like most people, concrete contractors understand that time is money. If they can get done quickly, they can do more projects and make more money. However, premature finishing seals the surface of the concrete and prevents air (which is a normal by-product of the setting process) from escaping the concrete. These bubbles get trapped just beneath the surface, and create a wonderful place for water and salts to collect. When the water freezes, the pressure from the expansion of water into ice will pop off the surface of the concrete.
It is critical that the contractor wait to float out the concrete until the off-gassing process is complete. Leveling it with a board or “screeding” device is ok as the concrete comes off the truck, but floating it out needs to wait. It is hard to know when this state occurs, but usually it is just after the water gets sucked back into the concrete. He’ll have to work fast. If his crew is big enough, that won’t be a problem.
Time:
Time is the other thing that can really hurt a concrete job. If you are far away from the concrete plant, and the total time from the time the concrete was put on the truck and mixed until it comes off the chute is ONE HOUR or greater, the concrete will not be good because it has already begun to set, and you should not accept it. Don’t be shy about this. However, if you are close to or over an hour away, you can have the plant add a RETARDER to the mix, which will keep it in good working condition. The length of the retarder can be specified (e.g. 1 hour, 2 hours, 3 hours). If you don’t know where the plant is, ask your contractor and then call the plant to see what the normal delivery time is from that location. Don’t forget to include the amount of time the truck will sit at the job site waiting for the contractor to be ready for it.
Summary:
That’s it. Now just to summarize: When placing a new driveway (or any concrete slab):
- Compact the sub-soil
- Use rebar to provide structural strength to the concrete, and make sure it is elevated to the center of the slab
- Order a 3000 or 4000 psi mix
- Make sure the aggregate is 3/4 inch, and not more than 1 inch
- Order the mix with a Super Plasticizer
- Make sure the slump is not greater than 4 inches without the Plasticizer, 9 inches with the Plasticizer
- Don’t take any concrete that has been on the truck for more than an hour without a retarder in it.
- Wait until the concrete is ready before floating and finishing the concrete.
Follow these steps and you will have the best opportunity for a quality concrete slab.
