Frequently Asked Questions about Concrete Driveways
February 12, 2021

Categories | Ready-Mix

Frequently Asked Questions about Concrete Driveways

Concrete driveways are a simple, effective way to increase the value of your home. Concrete’s inherent strength protects the driveway from aging and inclement weather conditions and promotes a service life of at least 30 years. The unmatched benefits of concrete have gained the attention of homeowners looking for a reliable driveway solution; however, the preplanning research process may leave you with more questions than answers. Use this frequently asked questions guide as a starting point to help you gain a better understanding of industry terminology and concrete care.

What type of concrete mix should be used?

Consult with a sales representative at County Materials or your project contractor to determine which concrete mix is best suited for your application. The American Concrete Institute recommends a minimum 4500 psi mix with maximum 0.45 water to cementitious ratio (w/cm) that is air-entrained to resist the effects of freezing, thawing and exposure to deicing chemicals.

What does portland cement mean?
Portland cement is an industry term, defined by the American Society for Testing and Materials (ASTM) C 150, that refers to the type of cement used in the majority of concrete.

What does cementitious mean?
This term describes the materials used to bond the aggregates in concrete together. Portland cement, blended cement, hydraulic cement, fly ash, slag cement, silica fume and other supplementary materials combine to form cementitious material.

What are supplementary cementitious materials?
Also known as mineral admixtures, supplementary cementitious materials enhance the properties of hardened concrete through hydraulic or pozzolanic activity. Common materials include fly ash and ground granulated blast furnace slag.

What is a six-bag mix?
ASTM C94 refers to a bag of portland cement weighing 94 pounds. Therefore, a six-bag mix refers to 594 pounds of portland cement that meets the requirements under ASTM C 150. Mixes comprised of supplementary cementitious materials are considered performance-based and should not be referred to as bag or sack mixes. Discuss the intended usage of your driveway with a County Materials Sales Representative or your contractor to ensure the proper mix is supplied.

What is water/cementitious (w/cm) ratio?
This ratio is expressed by the pounds of free water that are not absorbed by the aggregate divided by the pounds of cementitious materials in the mix. Water/cementitious ratios are commonly used as an indicator of durability and strength. A maximum 0.45 w/cm should be used for exterior conditions that are exposed to freeze/thaw cycles.

What is air-entrained?
Air-entrainment is a chemical admixture added to exterior concrete that is subjected to freeze/thaw cycles and deicer chemicals. This process creates small, evenly spaced bubbles in the concrete that resist internal tension forces caused by freezing. For exterior concrete that is exposed to regular freeze/thaw cycles, a target air content of 6 percent +/- percent by volume is recommended.

What is psi?
Psi, or pounds per square inch, is a unit of measurement that indicates force per unit area and is used to measure the compressive strength of concrete. Exterior concrete in Wisconsin must have a minimum compressive strength of 4500 psi.

Should I use low chert stone?
Chert is light, porous stone that is found in many aggregates that can cause some aesthetic issues in concrete driveway applications. The risk of chert pop outs can be minimized with proper concrete driveway care, including cleaning and sealing. Luckily, these pop outs tend to be aesthetic issues and do not impact structural integrity.

How thick should a concrete driveway be?
Thickness is major factor in determining a driveway’s structural capability. A standard driveway intended to support cars and light trucks will require a minimum of 4 inches, whereas a slab supporting oversized loads should be 5 inches or more.

What kind of base and subgrade preparation needs to be done?
Base refers to the material underneath concrete. It typically is comprised of compacted gravel or crushed stone to allow for adequate drainage. A 6” – 12” base will achieve optimal drainage and support enhanced load capacities. Prior to placing base materials, remove organic material and soft pockets of subgrade to protect the structural integrity of the driveway. The base and subgrade should be uniform in firmness, grade, and dampness and must slope away from structures, allowing for proper drainage, with a minimum slope of ¼” per foot.

Should there be reinforcement in my driveway?
Steel reinforcement is often used to contain cracks. Discuss with a sales representative at County Materials or your contractor to determine if your project requires reinforcement.

What should the surface texture be on the driveway?
A broom finish for exterior concrete is ideal, as it provides a slip resistant surface. Colored and stamp concrete can also be used to achieve a specific aesthetic.

What provision can be made to avoid cracks?
Concrete can become susceptible to cracking for several reasons, some of which are inherent and unavoidable. This type of cracking rarely affects the driveway’s structural integrity or life cycle. The leading culprit of cracking is improper design and construction practices; however, proper joint spacing is also a contributing factor.

Should there be joints in a concrete driveway?
Install control joints at a spacing of no more than 10” x 10” and to a depth of no less than 1/4th the driveway’s depth. Spacing joints that exceed these parameters are more prone to random cracking. These cracks impact the driveway’s aesthetics more so than its structural integrity.

Why must driveways be cured?
Curing is an essential part of the construction process, as concrete gets stronger with time. Allow freshly poured concrete to cure as soon as the project is finished and continue for at least one week. Inadequate cure time results in surface strength reductions of up to 50 percent.

Is it necessary to seal a concrete driveway?
Periodically sealing a concrete driveway will protect it from water and chemical absorption, making it easier to clean accidental spills and prevent surface defects. For more information, contact County Materials.

How should I care for concrete during the winter months? Can I salt my driveway?
Prevent snow and ice from accumulating on concrete, especially during the first winter. Deicing materials, such as salt, should be avoided, as they react with moisture from freeze/thaw cycles and can diminish the concrete’s surface and structural integrity. Sand is a great alternative to deicers and may be used anytime.

How important is the role of the concrete contractor?
A contractor’s involvement plays a major role in the outcome of a project. The contractor is responsible for placing, finishing, curing, and sealing the driveway. Working with a reputable contractor is the best way to achieve your desired results.

How to find a concrete driveway contractor?
Locate a reputable contractor near you by contacting County Materials or visiting the Wisconsin Ready Mixed Concrete Association’s website.

The Wisconsin Ready Mixed Concrete Association offers valuable technical resources for designers and contractors who provide ready-mix solutions for customers. For more information, refer to the publication titled “Frequently Asked Questions (FAQ’s) about Concrete Driveways” on WRMCA’s website.

Use County Materials’ Ready-Mix for your next project by contacting our Central Ready-Mix Dispatch Team at (715) 769-5503.

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