Concrete Driveways- dos and don'ts

Published by Millennial1 on August 5, 2007 - 2:00am
Concrete driveways should last you many years if they are properly installed and spec'd. Here are some recommendations about what to put in the contract and what to avoid. The existing driveway was about 50 years old and had some huge cracks in it. Last summer we installed a new 24-foot wide x 44-foot long driveway in the Washington DC area. We hired a concrete contractor to do the job. Our driveway also slopes down sharply from the street. During heavy rains, their is water in the garage. So we also wanted to install a 4-inch wide drain in front of the garage door to take of the problem. We decided not to replace the concrete driveway apron. In our area, concrete driveways are not reviewed by building inspectors, although concrete driveway aprons are. The concrete driveway you end up with really depends on specifying what you want in the contract and then holding the concrete contractor to it. What do Concrete Experts Recommend? We did our home work and visited various websites of concrete trade groups such as the Concrete Network. Armed with this knowledge, we talked to many concrete contractors and some who professed to be. The Concrete Network specified the following for driveways: a) A concrete mix of 4,000 psi (pounds per square inch) .50 water to cement ratio for driveway construction. According to them, this provides better wearability and a "denser" concrete than the typical 2500-psi mix, b) The sub grade should be compacted and have an even thickness. A standard driveway is 4" thick you want 4" thick continuously, not a 3" to 4" varying thickness. In the west it's even more important to pay attention to this since soils expand, c) Reinforcement with either wire mesh, or steel bars (1/2 inch rebar) placed in a grid pattern. In either case blocks should be used to keep the reinforcement in the center of the concrete, d) Joints should be at least 25% of the thickness of the concrete- so a 1" deep joint should be used in a 4" thick concrete driveway. Joints should also be spaced 2-3 times in feet the thickness of the concrete: so a 4" thick driveway should have joints no farther than 8- 12 feet, and e) A simple broom finish for sufficient traction We also found out that concrete can now be made to match any almost any color desired. It also can be stamped to make it look nicer. We opted for the plain vanilla color and just a broom swept surface for traction. What most Washington DC metro area Concrete Contractors say We were very concerned and amused that most of the concrete contractors don't follow the above recommendations. Even the contractors that have been in business over 20 years. We purposefully showed our ignorance to see what concrete contractors would tell us. Most said that: 1) The standard 2,500 psi mix was fine. Some contractors said they would use 3,500 psi, but only when we asked about the mixture. When we asked why a 2,500 psi mixture was OK, they just said it's typically used in our area. That's not a good enough reason in our opinion, 2) When we insisted on a 4,000 psi concrete mix, they went along stating that it would cost a little more. All they have to do is request it and it costs only a $100 to $300 more, 3) Most contractors said a 4-inch thick driveway was needed. The concrete thickness should be 4-5 Inches, we went with the 5 inches thick driveway, 4) When it came to reinforcing with rebar, most contractors balked at doing it. They preferred wire mess, because it is very easy to work with. Rebar will take time to install. We ended up using heavy gage wire mesh, 5) Most contractors were knowledgeable about where to install pressure joints and when to make cuts to prevent cracks from being seen, and 6) No one we talked to had a problems with installing a 4 inch drain across the driveway, We'll be writing a separate article on that soon] So, we contracted for the above and work began. Because this is a big job we decided to keep an eye on things. We are glad we did. Lessons learned Constructing a concrete driveway involves preparing the surface adequately with crushed stone, assembling forms, and reinforcing mesh or rebar. That part of the project goes slowly. The actual concrete pour goes very quickly and everyone is a bit nervous because the concrete can't stay in the concrete mixer too long. Processing the concrete as it is poured is hard work work that can be compounded by hot weather. Make sure that your contractor has plenty of people on hand so when the concrete is poured they can distribute it and start to smooth it immediately. Here are the lessons learned from our experience 1) You must be on top of the contractor or have someone who is to make sure that the concrete you ordered is the proper mix, in our case it was 4,000 psi. We asked the cement truck driver for the slip. If it had said 2,500 psi, we would have sent the driver home, 2) No problems with the grading or compacting rocks, 3) We opted for heavy wire mesh. The concrete contractor said that there was no need of blocks to keep the wire mesh in the middle of the concrete. They usually pick it up right after they pour. This was a mistake on ourpart. It is impossible to pull it up consistently and half the time we hadto insist that they do it. We should have just used the rebar, 4) Make sure they install pressure joints between the new concrete and any solid surface (a stone wall, other concrete, or brick). If they don't that where the concrete is going to crack, 5) You need more than 5 people when you pour 4,000 psi concrete. Why?Because it is harder to work with then the standard 2,500 psi mix anddries faster. My heart went out to our crew who were really understaffedin very high temperatures, 6) Don't pour concrete when the temperature is nearing 90 degrees F. Its hard on the crew and the concrete as well. If you have to, schedule an early pour early in the morning. It's best though to choose a nice day in the Fall instead. 7) Make sure that the contractor cuts the lines in the concrete the very next day to prevent cracking from showing. All concrete cracks, but at the bottom of the cuts where you can't see them, and 8) Bring a copy of the contract with you. Many times, what you specify in the contract is not always adhered to or communicated to the people who actually do the work. Given that, you must speak up and point this out to the contractor and there staff. You can expect to meet some resistance at this point, but you should hold your ground. We found that being able to speak a little Spanish and providing refreshments especially if it is hot, will go a long way in getting the work done the way you ant it.

Comments

Thanks for this article. It was very informative for a concrete layman like myself. Worked a deal with a contractor to replace my driveway in the southern end of the San Joaquin valley. It was caved-in due to ground water erosion. We have dry heat temperatures in the high 90's to 100's and this effects the concrete as well. Many of the driveways in the 'hood have some signs of sinking and we want to avoid problems as far into the future as possible.
It held up under the autos, but finally gave way when a forklift moving replacement tiles snapped it. Bad luck it was the last load of tiles going up the drive.
The house value and low income require that we do this inexpensively. We are using 4,000 psi and steel wire. We'll see how this holds up over the years.

There is a much better reason for not using rebar in your driveway, Your salesman did not know the answer.

Rebar is an excellent form of reinforcement for concrete... When it is vertical ie your homes foundation walls, Your wide welled Exit to get out of the basement, And maybe even a Basement slab since it is below the frost line and the earths pressures are on the walls more than the slab.

1) You live in the DC area and by the nice brick house I would guess NOVA maybe Arlington or Alexandria who knows maybe even fairfax. This means you have a lot of water in the ground and yet temperatures still cold enough for freezing in the winter. I am sure you have seen the settling in your home near your ceiling.

2) I believe the strength of the rebar most builders in that area use is about 60-65,000.00PSI and although I would not suggest 2500PSI concrete I believe concrete network may be shooting high with the 4000PSI mix. With the usual conditions 3500PSI will be strong enough to handle what you throw at it and and you can take the 20% savings and put it toward the stone patio in the back.

Now, because of how wet your ground is you are going to experience a good bit of settling over the years. Your driveway or patio lays horizontally on the earth and is going to feel these effects. Now if your concrete is rated at 3500PSI or even 4500PSI and the reinforcement holding it together is almost 20 times stronger at 60-65,000.00PSI its going to be the reinforcement that causes or compounds your cracking. When you hit your freeze thaw sessions the concrete is going to want to move a little like the earth below it. Concrete is not flexible by any means but it will have some give to it with the forces spread out over the surface area. Your super strong 60k+ PSI rebar is going to say "no" and it wont budge. If the concrete can give just a little But your reinforcement will not this is going to cause a crack. Rebar is too strong to use in a horizontal application like this. You will be much better off with the wire mesh.

As for needing 4000 PSI and absolutely having 5" to double your load capacity....I would bet money your basement slab, and your garage slab are both 4" thick and 3-3,500 PSI and no more. If you trust your garage floor to park your car on it and leave it there for hours, maybe days, maybe months at a time... Don't you think the concrete you drive over just to get to that floor is ok with the same exact specs?

Unless you think a delivery truck might come into your driveway, but not your garage

Going with 4000psi was a good choice in rainy northern Va. Overtime, rain pounding down on concrete slabs, driveways, sidewalks, and patios, will over the years pit and expose the gravel embedded in the mix. 4000psi will resist mother nature years longer than 2500 or even 3500psi.

This comment above about using wire mesh vs rebar doesn't sit well with me.

First, The grade of steel required by Canadian codes in wire mesh is actually higher than that used for standard rebar (550MPa yield vs. 400 MPa). Based on the above statment, it would be worse to use mesh than bars.

Second, concrete will only crack if one of two conditions occurs. The first, is that it is crushed beyond its capacity, the second is if it is pulled by tensile forces. Concrete's tensile strength is roughly 10% of its compressive strength. It is because of this weak tensile strength that steel is added. When water freezes, it expands, hence the requirement for air-entrained concretes in cold climates (like Canada). This expansion will induce a tensile stress on the concrete, and without reinforcement (which is good in tension), the concrete would shatter and break up.

The ultimate strength of the reinforcement can not be discussed in isolation. One must also include the spacing between reinforcing strands, and the size of the strands as well. The reinforcement ratio (area of steel/area of concrete) is an independent value which can be compared between designs to show how much give (ductility) there is in the slab.

Reinforcement in driveways (and other concrete slabs) are spaced (or should be spaced) to control the cracking that inevitably happens in concrete. Cracks can propagate through unreinforced concrete with wild abandon. Closely spaced wires/bars will provide barriers that cracks can't get through. The steel-concrete bond allows the steel to hold together concrete that wants to crack.

Anyway, there is so much more to the selection of rebar than I can give here. If you want to know more, talk to a structural engineer, or take a course on concrete design.

Please recommend your contractor, if you feel comfortable doing so.
Thanks!

The reason is that the concrete pour was done in very hot weather (95 degree F) contrary to our requests that they delay the pour. Add to that the concrete dried very quickly when they charged ahead plus the fact that they didn't have enough people on hand during the pour added up to a not so great job.

Hey friend I liked ideas which you have suggested here. As layman like us we do trust on our contractor but what he does for what reason we can't understand whether it will good or bad. I think you have given better tips by this we can ourselves try to implements it in our driveways. Anyways thanks for sharing these dos & don'ts.
Regards,
emilio
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