How is concrete made?
Concrete is a proportioned mix of sand, limestone or river run gravel, water, Portland cement, and various additives depending on your region and the time of year. Concrete is perishable - once it is mixed together, concrete will become unworkable in 3 to 6 hours without any accelerators or retarders. Adding water at the jobsite to make the concrete more workable is a standard practice in the industry; however, excessive water will weaken the concrete.
How long should concrete last?
In our northern climate, a standard residential 4000 PSI (6-bag) exterior concrete driveway, under normal wear and tear in our extreme climate, should last 25-35 years is a reasonable service life for a concrete driveway or patio. We've seen 50-year old concrete driveways still in serviceable shape. Concrete's life span is usually shortened not by its inherent PSI strength, but by environmental factors such as tree roots, excessive loads, erosion, and our annual freeze-thaw cyles. The freeze-thaw cycles can swell or shrink the soil beneath, particularly in clay soils, where swell volume changes can reach 30%. Proper drainage and reinforcing measures in the newly laid pavement are imperative.
Why does concrete crack?
Concrete, like all other materials, will slightly change in volume when it dries out causing shrinkage cracking. These types of cracks can be avoided. Shrinkage cracks are the result of the concrete volume decreasing as water is lost from within the concrete mixture. These cracks can still occur some months after the slab has been poured. However, the more rapid the drying process the more likely it is that shrinkage cracks will develop. Other factors may cause concrete to crack. Brown & Sons Services understands and promotes the ACI (American Concrete Institute) standard specifications to should reduce or deter the likelihood of various concrete cracking possibilities.
Here are the most common reasons that concrete will crack:
- DRYING SHRINKAGE. New poured concrete shrinks just a little bit as it dries (cures) A typical concrete slab will shrink about 1/16 of an inch in 10 feet. The reason that contractors put joints in concrete pavements and floors is to allow the concrete to crack in a neat, straight line at the joint when the volume of the concrete changes due to shrinkage.
- CONCRETE TOO WEAK OR TOO THIN. If not poured with the correct strength (PSI) or at the correct depth, concrete might develop unexpected cracks due to its sheer weight.
- TOO MUCH WATER. Too much water will overhydrate and weaken the concrete mix by creating excess voids. In concrete mix design terms, this is the water/cement ratio. Strength improves with lower water cement ratios. A .45 water cement ratio most likely will hit 4500 psi (pounds per square inch) or greater. A .50 water cement ratio will likely reach 4000 psi or greater. Only put as much water in the concrete as you need to accomplish its proper placement.
- INSUFFICIENT CONTROL/EXPANSION JOINTS. You will know how much your contractor knows about basic concrete construction by asking him or her about control and expansion joints. They should be spaced about the same as the width of the pour up to about a maximum of twelve feet in width. This rule is just a guideline, and would change if your concrete pour has footings, rebar or wire, or is thicker than 4 inches.
- JOINTS IN THE WRONG PLACE. An experienced concrete installer knows where to put expansion and control joints. The placement of both types of joints is both empirical & an art. Contraction Joints (tooled or sawed) need to be placed at building corners and turns, and obvios weak spots such as over a culvert pipe. “ ACI 302 recommends that contraction joints be provided at 24 to 36 times the slab thickness in both directions, unless intermediate cracks are acceptable. Therefore a 4” (0.33’) driveway slab would have spacing between 8 ft.-12 ft. Furthermore, PCA (1983) recommended adjustments of the multiplier, depending on the likely shrinkage, as represented by the amount of mix water in the concrete and the aggregate size”. Expansion joints are to installed where the new concrete will meet another existing concrete pavement or structure or between pours.
- EXCESSIVE WEIGHT ON CONCRETE. Most residential driveways are not designed for weight exceeding a light truck.
Why does concrete surfaces flake and spall?
Concrete surfaces can flake or spall due to repeated freeze and thaw cycles in our region of NW Indiana.. The concrete should be air-entrained to resist flaking and scaling of the surface. If air-entrained concrete is not used, there will be subsequent damage to the surface. During installation, the design mixes water/cement ratio (usually .45-.50) should be kept as low as possible to improve durability of the surface. Too much water added during installation to make more workable may weaken the surface contributing to early flaking and spalling of the surface. The finishing operations should not begin until the water sheen on the surface is gone and excess bleed water on the surface has had a chance to evaporate. If this excess water is worked into the concrete because the finishing operations are begun too soon, the concrete on the surface will have too high a water content and will be weaker and less durable.
What should I do about cracks in my old concrete?
First, get professional advice as to why it cracked by an experienced eye. A crack can be filled with a silicone based (rubberized) caulk or an epoxy-based crackfiller. The former is least expensive but lasts only a few years before it moves with the exopansion & contraction of the crack casing the caulk to separate and loosen up. Therefore, regular application of this type of crackfiller is required. A much more permanent solution is to apply a 2-part epoxy-based or polyurethane-based crakfiller. These crackfillers fuses the two slabs together and its bond strength is greater than (> 4,000 psi) the concrete itself.
Another procedure that may need to be done is to saw cut additional contraction joints to relieve a slabs (patio, driveway, etc.) internal or external forces if the original installation had limited number or insufficient joint placement. There are even more corrective measures to rehab a cracked slab. Consult Brown & Sons Services-Restoration Div. for details.
What is the purpose of welded wire mesh (WWM), or fiber mesh in concrete?
Using steel reinforcement will provide some additional structural capacity for your driveway and is especially important if the slab will be exposed to heavy traffic. Reinforcement won't prevent cracks, but it will help hold them together if they do occur. Reinforcement can be either wire mesh or ½-inch (#4) steel rebar placed in a grid pattern with a spacing between bars of approximately 12”-24” inches. The problem with welded wire mesh is that it often ends up on the ground from being stepped on as the concrete is being placed (particularly if no support blocks are used). Another problem is that mesh does not prevent or minimize cracking-it simply holds cracks that have already occurred together.
On the other hand, Synthetic fibers (fiber mesh) have proven to be beneficial in driveways as a way to reduce micro cracks and shrinkage cracks by intersecting with the fibers thereby blocking their growth and providing a higher tensile strength capacity at this crucial time. Fibers will not provide structural reinforcement, however. Fibers can be added to the concrete mix in lieu of welded wire mesh.
What is concrete resurfacing and would it work on my project?
Concrete resurfacing is the process by which a thin coat of specialized cement (polymer cements) is laid on top of an existing concrete surface in an attempt to restore the old surface. Resurfacing, if done correctly (usually with proper crack repair), can last several years and can theoretically provide a durable new surface. Unfortunately resurfacing does not prevent, hide, or erase cracks, and any concrete cracks that lay below the newly resurfaced area are likely to reappear. Only an expert can tell you if resurfacing is right for you. Brown & Sons Services-Restoration Div. provides our customers both expert advice and experienced applications of a variety of polymer cement coatings.
How long does concrete take to harden or cure?
In general, after concrete leaves the mixing truck, concrete will harden to the touch in a few hours, although the hardening time will vary with concrete's slump, ambient temperature, moisture content of the sub-grade, finishing method, and entrained admixtures. Within 8-24 hours you should be able to walk on your new concrete, although dogs should be kept off the concrete for another day or two.
We recommend to our customers not to drive on a newly laid concrete driveway for a minimum of seven days since concrete gains two-thirds (2/3’s) its design strength (4,000 psi) after one week thru proper hydration. New concrete obtains its design strength (4,000 psi) in one month namely called its "28 day strength".
What is the best time of year to pour concrete?
Concrete is best poured when the ambient temperature is between 45 degrees F and 85 degrees F. The closer you are to 45 degrees F the day of the concrete pour the longer concrete will take to dry (hydrate). In winter, most concrete installers use an accelerator such as calcium chloride that will build heat in the concrete mix and speed up the hydrating process. Non-chloride accelerators are used for colored and stamped concrete as well, although they are usually more expensive. In the summer, many concrete installers use a retarder to slow the concrete mix as it hydrates.
What is a concrete accelerator and a concrete retarder?
Concrete admixtures such as accelerators and retarders can both speed up and slow down the process of hydration (curing) in the concrete. Concrete accelerators, namely Calcium Chloride or Non-Chloride Accelerator (NCA), achieves this by building heat when it is added to the mix, and this heat acts as a catalyst to speed up hydration. Typically, an accelerators are used in cold weather conditions.
Concrete retarders do the opposite. Retarders slow the hydration process (hardening) and is used in hot weather conditions.
What maintenance does concrete require?
A plain concrete driveway, patio or walkway requires minimal maintenance. Over time, cracks may appear and the concrete surface can wear, flake or spall due to salt intrusion from melting snow and feeeze-thaw cycles. To minimize this kind of deterioration concrete slabs should be sealed at the time of installation and re-sealed every 2 to 3 years surface with a penetrating-type sealer. Sealers offer resistance to rain, sun, freezing temperatures, petroleum products, deicing salts, and debris such as leaves laying on the surface.
Colored, textured, and stamped concrete require a protective coat of sealer to protect and maintain its color & beauty using a topical-type of sealer, in a flat, semi-gloss, or high gloss appearance.
Brown & Sons Services-Restoration Div. can advise you with the proper sealer application.
Should You Expect a Warranty for your Concrete Work?
Yes. Most qualified concrete contractors should provide a one year warranty. Why just one year? Well, concrete is dependent on its sub-grade. What's happening underneath the ground? Is it shifting? Is there a sinkhole? Is there erosion? Are there underground springs? These are all problems that concrete installers can only attempt to address in the short time they are there. Suppose a person parks too much weight on top of their driveway, or a tree falls on it; you get the picture. Most installers, knowing the difficulty of proving negligence on the part of the homeowner, and aware of the unpredictability of mother nature, are only willing to extend themselves so far, and most have decided on one year.
How wide should my driveway be?
A minimum width for your concrete driveway would be 9 -10 feet as a standard width. A comfortable one-car width is 12 feet, especially if your driveway curves. A two-car with is typically 16 ft.-17ft. as a standard width. . A comfortable one-car width is 18-20 feet. Remember to consider not just the width of the vehicle, but the width you will need to get out of the vehicle and walk around it.
What questions should I ask my contractor before I hire them?
- Do you self-perform the work or subcontract it? (Most, if not all should be self-performed for a higher likelihood of success)
- Is there on-site supervision of the workers that demand a quality product?
- Are you licensed? (Most states require a license to install concrete)
- What insurance do you carry? How much do you carry? (You want to see a business liability policy)
- Do you take checks? (If not, that is a sign of a hobbyist or fly-by-night, not a real business with a stake in there community)
- Can I see some pictures and references? (They should be able to produce them)
Stamped Concrete FAQ’s Go to the following link: www.concretenetwork.com/stamped-concrete/faq.html
Stained Concrete FAQ’s Go to the following link: www.concretenetwork.com/stained-concrete/faq.html
Brown & Sons Services, LLC endeavors to consistently provide the highest quality of concrete products and services. Our "customer first" approach, understanding of the technical aspects of the concrete construction, coupled with our company’s diverse & years of practical experience and experienced concrete workers has resulted in many, many happy customers. If you have questions about concrete or a project you have in mind, don't hesitate to give our friendly concrete contractors in Crown Point, IN a call today. We'll be happy to provide you with the information you need.