Ask BJ
Frequently asked questions about concrete driveways

Can I do concrete work myself ?
Concrete work is very demanding and requires many tools of the trade to complete the job correctly, but yes you can do-it-yourself if you follow the steps of the trade.  Always remember that concrete placing and finishing is very hard work and enough help is very important. Working with concrete is also a trade or specialty and the timing of finishing operations is critical. Always try to have one professional on the job when working on a concrete project.

Should I apply a sealer to my concrete ?
A concrete Cure & Seal product MUST be applied to your concrete as soon as possible after the concrete is placed. Today’s products serve two purposes. First to cover the concrete during its 28 day curing cycle with a membrane to allow the concrete, and more specifically the concrete surface, to cure slowly which will make the concrete achieve a higher strength. Second the sealer in the product will help keep water and salt out of the concrete surface which will help prevent surface deterioration during the freeze thaw cycles of Wisconsin’s harsh winters.
Please refer to Cure & Seal

When can I drive on my new concrete ?
Concrete cures to required strengths in 28 days. Driveway concrete is recommended to have 4000 PSI. The concrete will achieve approximately 70% of its required strength in 7 days, which would be about 2800 to 3000 PSI. That is sufficient strength for you to drive a standard family vehicle on the concrete. Always use caution if you are driving off a corner or edge of new concrete.

Where must I use expansion joint ?
An expansion joint or the black felt strips should always be used when concrete is going to be put up against a stationary structure. Example; if you are putting in a patio against the back of your house you must use an expansion joint. This will prevent the concrete from pushing against the house and damaging your house foundation if the frost moves the slab.
You must use an expansion joint against your garage foundation when putting a driveway against your garage floor.

What type of concrete mix should be used?
Your local ready mix producer and your contractor will assist you in ensuring the proper concrete mix is pre-ordered.  The American Concrete Institute (ACI) recommends a .45 water/cementious ratio for concrete that will be subject to freeze / thaw cycles and exposed to de-icing chemicals.

What does cementious mean?
There are materials other than cement that are used within concrete that the American Concrete Institute (ACI) and Wisconsin Department of Transportation recognize as cementious materials such as fly ash and granulated ground blast furnace slag.  Because these materials have similar characteristics to Portland cement they are recognized as cementious materials and may be considered part of the overall cementious weight in a concrete mix.  Therefore, make sure you understand what you are ordering and what cementious materials , if any, are in your concrete. 

What is a six bag mix?
ASTM C94 refers to a bag of cement weighing 94 lbs.  Therefore, a six bag mix would be a mix that has 6 x 94 lbs = 564 lbs of cement in the mix per cubic yard.

What does water/cementious ratio mean?
Water/ cementious ratio is expressed by the pounds of water in the mix divided into the pounds of cementious materials in the mix.  Cementious materials include Portland cement, fly ash, and ground blast furnace slag.  Fly ash and slag are recycled materials which, when used properly, enhance the concrete's durability while helping the environment.

What is air-entrainment?
All exterior concrete subjected to freeze/thaw environments must be air-entrained concrete.  Simply put, air entrainment puts microscopic voids into the concrete which serves as relief valves when water in the concrete freezes allowing the moisture to expand without damaging the concrete.

What does PSI stand for?
Pounds per square inch, this measurement is used to describe the compressive strength of concrete.

Should I use "low chert quarry stone" in my concrete?
Chert is a light porous stone found in many aggregates.  If this stone is within one inch of the concrete surface, absorbs moisture and freezes, it is possible for an area of up to 1 1/2" diameter to break out of the surface of your concrete and as deep as the stone.  ASTM C33 specifies that aggregate for concrete should have no more than 5% deleterious materials within the aggregate.  If you want to minimize your odds of getting chert pop outs in your concrete, you should explore what aggregates are available in your market.  In southeastern Wisconsin quarry stone is used as low chert stone.  Quarry stone is not a washed stone and thereby has the possibility of having friable or clay products (deleterious materials) blended in the material when it is fun through the crushing process.

A chert pop is much larger and far more visible than a friable material bug hole.  With the quality control practices in place by your ready mix producer, friable material is minimal and below the ASTM specifications.  Therefore, "low chert quarry stone" can be considered as an alternative to standard coarse aggregate for exterior concrete.

Resealing your concrete every couple of years also helps to protect from water penetrating chert stones and potential pop outs.

How thick should a concrete driveway be?
A minimum of 4 inches is recommended.  Keep in mind that the 2 x 4's often used to form driveways are only 3 inches wide, so the ground inside the 2 x 4 forming needs to  be removed at least  an  inch below the bottom of the form.  Thickness is the major factor (even more than the strength of the concrete) in determining the driveway structural capacity.  In other words, thickening your concrete slab carries more load.  Increasing your driveway's thickness from 4 inches to 5 inches will add 20% to your concrete cost, but the additional inch of concrete will add almost 50% to the load -carrying capacity of your driveway.  If the driveway will be used by both cars and light trucks or SUVs, then a 4" thickness should be adequate.  If garbage trucks or other heavy vehicles will be traveling on the slab 5 or more inches may be necessary.

What kind of sub grade preparation needs to be done?
Sub grade is the material directly underneath the concrete.  This material should be compacted gravel that allows drainage.  Clay is not recommended.  All sub grades should be uniform in firmness, grade, and dampness.  The sub base is the material below your sub grade.  This material should be solid.  Make sure all the soft pockets of sub base are removed before placing the sub grade materials.  The driveway should be sloped away from the home for proper drainage, with a minimum slope of  " per foot.

Should there be reinforcement in my driveway?
Reinforcement is optional.  Types of reinforcement are wire mesh, rebar, or fiber reinforcement.  For many years wire mesh and rebar have been used in slabs to hold cracks together after a crack has appeared.  However, in the past 20 years the advancement of fiber reinforcements offer crack containment and much more.  There are any types of fibers that offer different benefits to the concrete.  Make sure you discuss this with your contractor or ready mix supplier to understand what fiber reinforcement can do for you.

What should the surface texture be on the driveway?
A broom finish for exterior concrete is ideal.  It provides a slip resistant surface and helps to ensure the contractor is finishing the slab properly and not overworking it.  A smooth "basement" finish on exterior concrete can greatly decrease durability.  The surface may also have an exposed aggregate look or be colored and stamped.  Visit www.wrmca.com for additional information on colored and stamped concrete.

What provisions can be made to avoid cracks?
There are many reason why concrete cracks.  Cracks rarely affect the structural integrity.  The majority of concrete cracks usually occur due to improper design and construction practices such as: omission of isolation & control joints, improper jointing practices, improper sub grade preparation, the use of high slump (very wet) concrete or addition of water on the job, improper finishing and inadequate or no curing.  Concrete "shrinks" slightly as it hardens, therefore; proper spacing of joints is important.

Should there be joints in a concrete driveway?
Install control joints at a spacing of no more than 10' x 10'.  Spacing joints at wider intervals incites random cracking.  While such cracks are generally not a structural problem, and will not reduce the service life of the driveway, they are unsightly.  Also avoid joint patterns that produce rectangular or triangular sections.  Lay out  your joints to form square sections.  If in doubt, make the sections smaller, not larger.  Ask you contractor to provide a jointing plan as part of his written proposal.

Why should concrete driveways be cured?
Curing of concrete is the final step of the construction process, and should be done as soon as the concrete is finished.  Curing is essential in making a concrete project durable.  Unfortunately, it is also one of the most neglected steps of proper concrete finishing practices.  In extreme cases, failure to cure concrete can result in reduction of the "surface" strength (and durability) up to 50 percent.  Curing or keeping the surface moist to assure greatest hydration of cement and highest potential strength is easily done through spray membranes or other readily available products and procedures.  Curing compounds should be applied after the final finishing operations to increase the concrete's resistance to the effects of weather, and to decrease the occurrence of surface defects. 

Is it necessary to seal a concrete driveway?
Periodically sealing a concrete driveway will protect it from water or chemical absorption.  This will make it much easier to clean accidental spills and will also help prevent the occurrence of surface defects.  Always follow manufacturer's recommendations for sealing concrete. Some sealers, primarily designed for industrial applications, darken or yellow the concrete.  Ask if your sealer discolors the concrete and follow the manufacturer's directions.  Your local ready-mixed producer is an outstanding resource for sealers and further information.

How should I care for the concrete in the winter?  /  Can I salt my driveway?
Prevent snow and ice from accumulating on the concrete especially during the first winter.  There are many materials referred to as "salt" these contain de-icing chemicals.  The purpose of deicing chemicals is to melt ice during freezing temperatures.  Freezing and thawing with moisture present on your concrete is one of the harshest conditions for your concrete surface.  It is highly recommended that you avoid freezing and thawing with any type of de-icing agents, especially the first winter.  NEVER use de-icers containing fertilizer ingredients such as ammonium sulfate, ammonium nitrate, calcium chloride, and magnesium chloride.  These chemicals WILL ATTACK AND DESTROY CONCRETE BY CHEMICAL REACTION.  NEVER USE THESE PRODUCTS!

How important is the role of the concrete contractor?
The concrete contractor has a major role in the outcome of the project.  The contractor is the "installer".  You can order the strongest and most durable mix available, however; if it is not installed (finished, handled, and cured) properly, the result will be unnecessary surface defects.

How should one select a concrete driveway contractor?
A good place to start is to call your local WRMCA ready mix producer or go to www.wrmca.com for a list of members in your area.  Once you come up with a list of contractors,  it is important to find out some information about each one.

Find out how long they have been in business and ask for references or ACI Certifications.  The American Concrete Institute and the WRMCA offer educational programs and certifications that contractors should attend.  These programs educate contractors on the installation of concrete with regard to ACI codes and recommendations.  If a contractor has the correct knowledge this will help assure you that your concrete project will be professionally installed.  Ask for the location of work they have recently completed, as well as additional projects completed  3-5 years ago.  Check out these projects and determine whether or not the workmanship exhibited in these projects is what you expect.  Note whether or not the quality has endured in the older projects.  Ask if the contractors offer any type of warranty on their work.  When obtaining bids on the project, remember that the lowest price is often not the best quality. 

Make sure that the contractor that you chose to work with understands your expectations and that you expectations are reasonable.  You will then be better served by the entire construction team. 

Other important questions to ask  your contractors before you make your choice include:

~ What mix design will be used and the water/cement ratio of that design.
~ If they cure the concrete and what products are used.
~ If a compatible sealer is used with the curing compound.
~ All surface expectations.
~ How thick the driveway will be.

If you have any questions concerning your concrete driveway or contractor always ask your ready mix concrete supplier.