Concrete Facts and Frequently Asked Questions

These are general guidelines taken from ACI and ASTM.  For your specific requirements go to the applicable ACI and ASTM standards (see links).

CURING TIME FOR CONCRETE

Q.  How long should I cure (keep the plastic, straw, blankets, water, etc) on my concrete (driveway, sidewalk, slab, etc.) 

A.  This question can only be answered by the professionals associated with the project. Concrete cures at different rates depending on the constituent ingredients and the ambient conditions it is exposed to. Your contractor (concrete supplier) should know what sort of curing is required for the particular mix being used. Also, by following their recommendation, you maintain whatever warranty that might be associated with the work performed. If there is a problem that occurs later on, they can't blame you for not curing it properly.

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CRACKS IN CONCRETE

Q.  I just had a new patio poured last month and now I see some cracks.  What happened?

A.  All Concrete Cracks.

So, you want to put in a patio, driveway or sidewalk, and you are going to use concrete. A very wise choice, we can all agree. One thing to know before you put in your concrete - - all concrete cracks. You say, "Wait a minute, I've seen concrete that doesn't have any cracks. How can you say all concrete cracks?" Concrete typically consists of cement, rock, sand and water. In the fresh, or plastic stage, concrete is fluid. As it hardens, the cement and water begin to shrink, and the stresses created by this shrinking cannot be overcome by the small amount of strength developed by the young concrete. If you place the concrete on a windy day, the top may start to harden before the bottom, which will cause the concrete to shrink unevenly (plastic shrinkage cracks.) Also, if the ground underneath the concrete is not level, there will be an unequal dragging force while the concrete shrinks, also causing stresses the new concrete cannot withstand. So, how do you get concrete with no VISIBLE cracks in it? By following a few simple steps before and after you place the concrete, you will have a very nice looking structure that will require very little maintenance, and give you years of enjoyment.

Before you place the concrete, make sure your subgrade (ground beneath the concrete) is thoroughly compacted and level. The absolute best thing to do is get a garden tiller, till the soil to a depth of 6 inches, then rent a hand operated compactor and compact the soil vigorously. This will help ensure there are no soft spots. You can apply a layer of cushion sand if you want. This will help achieve a totally level surface and allow a consistent friction to the shrinking concrete. Four inches of washed sand ought to be plenty for the cushion. If you use a wire mesh for reinforcement, use panels and not rolls. The rolled wire mesh is extremely difficult to keep in the top half of the concrete, where it HAS to be in order to do its job. You can also use reinforcement bars (rebar) tied together with steel wire, but spacing and size requirements vary based on load and soil conditions, so it is hard to recommend a standard set up for that. If you do use rebar, it is essential that you keep it in the top half of the concrete. You can use stones, broken brick or you can buy plastic chairs that the steel will sit on to keep it in the proper position when you place the concrete. You can also have the ready mix concrete company supply fibers to the mix. These fibers are usually nylon or polypropylene. They help keep the cracking of the concrete on a micro level instead of a macro level (where you can see the cracks with your naked eye.) Steel reinforcement also helps keep cracking in check, but if cracking does occur, the steel, when properly placed in the concrete, will hold the concrete together, whereas fibers will not do that.

Okay, you've got your subgrade ready, you have placed a plastic vapor barrier on the subgrade for slabs that will support dwellings, your steel is sitting nicely on your plastic chairs in the proper position, and you now have 14 of your closest friends on their way over to help you place the concrete you have coming. When the concrete arrives, if you don't have a vapor barrier, wet the subgrade without puddling the water so that the water in the concrete will not be absorbed by the dry subgrade, thus causing uneven drying and the dreaded plastic shrinkage cracks. Once the concrete is placed, make sure to protect it from high winds and direct sunlight so the concrete will dry evenly from top to bottom. You are now ready to perform the most important step in preventing noticeable cracking. Contraction joints are the secret to no cracking! By placing contraction joints that are at least 1/4th the depth of the concrete and on intervals of 25 to 30 times the depth of the concrete (usually easiest with a jointing trowel or tool while the concrete is still fresh), you will almost ensure there will be no visible cracking in your concrete. If your slab is 4 inches thick, the joints must be at least 1 inch deep and placed every 100 to 120 inches. If you cannot use a jointing tool to put the joints in, you can hire a concrete sawing contractor to do this for you. Make sure he cuts the joints a minimum of 1/4th the slab depth. This jointing method helps the concrete crack at the weakest point. This is why it is so important for the joints to be deep enough. Variations in subgrade levels could cause greater stress in the concrete in an area where the joint isn't deep enough, and the concrete will crack outside the joint. Once your joints are in place, and the concrete has cured for about two weeks, you are ready to seal the joints. This will prevent water from migrating into the subgrade and expanding and contracting, or getting into the joints and freezing, causing the water to expand and breaking out the concrete around the joints. You now have a concrete structure that will serve you well.

Later we will talk about repairing cracks in existing concrete. Until then, NO CRACKS!!!

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REMOVING SPOTS FROM CONCRETE

Q.  I have an unsightly rust spot and I have not been successful in removing it. Will you please advise me how to remove this?

A.  If the stain is fresh, use a heavy duty cleaning compound containing a large proportion of tri-sodium phosphate or TSP, available at any hardware store. Apply with a damp cloth or pad. Rub the discolored surface until the stain is removed and rinse thoroughly.

If that doesn’t work, make a paste of 1 part sodium citrate crystals to 6 parts water and enough whiting or talcum powder to make a paste. Spread the paste on the rust stains. Scrape the dried paste. The rust should be removed. Repeat if necessary.

Another rust removing product is naval jelly, which is available from paint stores. Apply a small amount of the jelly with a soft paintbrush or cloth. Allow the jelly to cover the stain for 30 minutes to an hour or until the stain disappears. Then wash with clear water.

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CONCRETE PRICING

Q.  Is there an average going price per cubic yard of concrete?

A.  Not really; it varies from town to town and region to region. If you are estimating a project, call 1-800-CONCRETE, and our Construction Consultants will help you with your questions and help determine what the best solution to your project is.

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DUSTING, PEELING, SPALLING, ETC.

Q.  The top of my concrete is dusting, peeling up, spalling, or coming up in little spots. What causes this and how can I repair it?

A.  Several situations cause spalling, delamination, dusting or peeling. Water sealed under the surface of the concrete during the finishing process, dirty coarse aggregate, or applying too much water to the surface before the concrete hardens. To repair, make sure all loose material is removed, using a stiff broom or even a high-pressure sprayer. Remember, always use eye protection in those type situations. Once you have exposed the good concrete, you can apply a topping to cover the old, ugly concrete. The best material to use and which many companies manufacture, is epoxy grout. It is usually stronger than the in-place concrete, it bonds very well with the existing concrete, and can be applied as thick as several inches down to a feather’s edge.

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WHY ARE THERE CRACKS IN MY CONCRETE

Q.  My concrete is cracking after only a short period. Is there something wrong with it and can it be repaired?

A.  All concrete cracks. It has to crack because it contracts during the drying, curing, hardening process, and the bond between the cement paste and the aggregates is not strong enough to withstand that stress. The best way to prevent unsightly cracking is to put joints in your concrete every 2 ½ to 3 times in feet the depth of your structure in inches (4" slab should have joints every 10 to 12 feet). Uneven shifting of the substructure or subgrade can also cause cracking. This is a structural failure, as opposed to improper curing or jointing as mentioned above. Before repairing any concrete cracking, determine the source of the cracking and remedy that first. Epoxy grout is an excellent crack repair agent.

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COMMON QUESTIONS BEFORE SIGNING A CONTRACT FOR CONCRETE WORK

Q.  What standard specifications should I know or understand regarding information received from a contractor in anticipation of signing a contract to have my concrete driveway replaced? What is better rebar, reinforcement mesh (both?), fiberglass reinforced cement? Depth of the driveway poured, 2inches 4 or more? How far should expansion joints be put in to handle heat and cold?

A.  Reinforcement steel in flatwork is strictly for crack control. The best crack control is affected by putting the steel in the upper half of the slab. If your contractor can guarantee he will keep the wire mesh in the upper half of the concrete, that will do as well as rebar. Make sure he gives at least 3/4" cover to whatever reinforcement he uses. The standard driveway depth is 4", but that doesn't mean that depth is right for you. It really depends on the soil conditions and the load you will be putting on the concrete. Thickened edges (beams) are highly recommended for the outsides of the driveway. This will enable the edges to take that occasional load on the edge or coming onto or off of the edge. Normal Portland cement concrete likes to crack every 10-12', so jointing in all directions no more than 10', with the joints being a minimum of 1/4 the slab depth, should help the expansion/contraction problem. There are two types of joints: contraction and construction. Contraction joints are usually installed with a concrete saw after the concrete gets hard or a jointing tool while the concrete is still fresh. Construction joints are preinstalled dividers such as redwood strips, aluminum keyways, or other physical barriers. Either type of joint will allow for expansion/contraction, so if you don't want redwood every 10', you don't have to have it. Make your joints as square as you can. Try to avoid rectangular jointing sections. Make sure the contractor orders concrete that has 4-6% entrained air, since you live in conditions that probably require salting of roads. The air will enhance the concrete's freeze/ thaw durability and make it less susceptible to salt damage. I could spend at least 8 hours going over things to look out for and things to avoid. The main thing is sitting down with the contractor before you sign a contract, and tell him how you want the thing to look when he gets finished. Discuss whether visible cracks are acceptable or not. Discuss a warranty. Tell them what you want in the end, put it in your contract, then get out of his way and let him give you what you want. If he doesn't give you what he says he would in the contract, you have legal standing for a remedy. Also, let him buy the concrete so
he can't blame you for buying an inferior product that he couldn't work with. 

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SELECTING THE PROPER CONCRETE MIX

Q.  What strength concrete is normally used for the slab under a home?

A.  Typical mixes range from 2500 psi to 4000 psi, depending on the area of the country and the quality of the raw materials. Should there be a different weight used for the garage area? Not really. Usually the biggest load on a slab is the heaving from the subgrade and not the structure on top of it (or the car.)

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A checklist to make sure you are ordering the right concrete

•         Has a professional confirmed how much concrete you need?

•         Do you need any admixtures?

•         Is the site accessible to Ready mix trucks? Confirm with 1-800-CONCRETE.

•         Confirm with 1-800-CONCRETE what you are trying to achieve and the best mix to order to reach that result. The items listed in the admixtures are meant to give you the information to ask the right questions, not make you an expert requiring no assistance from the professionals in the business.

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Determining if the concrete can be placed directly from the truck or will need to be pumped

Can you get a truck up close to where the concrete’s final destination is?  Trucks can’t get close enough to the pour for four reasons:

1. The ground surrounding the project is too wet and the truck will sink.
2. The slope leading to the concrete pour is too great for the truck.
3. There is too narrow of an opening for where the truck would need to pass to reach where the concrete is needed.
4. The pour is big and the trucks chute can not reach everywhere concrete will need to be poured.

Confirm with 1-800-CONCRETE that there is truck access. Each Ready Mix Company has their own criteria on what they feel is accessible for their trucks and should be consulted.

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Scheduling a site visit from your ready mix supplier

Reputable ready mix suppliers are usually pleased to have a representative come to your home to check your job several days before the concrete pour. Plan through 1-800-CONCRETE and schedule this visit ahead of time-do not expect the representative to be able to come out the same day you call.

Discuss with the representative at this time:

•         The amount of concrete you calculated, get their opinion.

•         Discuss any admixtures.

•         Clarify if the site is accessible to Ready mix trucks.

•         If you need more than one truck, discuss the spacing of the order.

Confirm with them what you are trying to achieve and the best mix to order to reach that result. The items listed in the admixtures are meant to give you the information to ask the right questions, not make you an expert requiring no assistance from the professionals in the business.

As straightforward as this information is, most dispatchers will say customers calling to order concrete don’t know for sure how much concrete they need, don’t know if they need any additives, had never considered if the concrete might need to be pumped, and need the concrete tomorrow!

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