Saw Cut “Control Joint”

There are ways to help avoid new concrete from cracking (in visible locations). How?? As bad as it sounds, EVERY contractor hides them in what is called a “control joint”. This is basically a grove tooled or cut into the concrete to help relieve the stress of the concrete. There are basic rules to follow when placing these control joints regarding spacing and at corners. When a contractor has been finishing concrete long enough, they know where to place these joint and maintain a pleasing appearance. 

The next problem that sidewalks, patios, driveways, and other slab on grade concrete faces in Northern climates is freeze/thaw cycles. Every winter the ground freezes and heaves the ground upwards. When the sun comes out, it thaws the ground, and it does not always do it evenly. This means that a portion of the slab in the sun may thaw and start to drop while the section it is attached to is in a shadow and stuck on higher frozen ground. This stress will also cause the concrete to crack, “hopefully” at the control joints which were placed there for just that reason.

As silly as it may sound, try looking a little closer to those joints and see if you can spot the cracks. Hopefully this article will help you understand some of the behaviors of concrete a bit better and will allow you to interview your Concrete Contractor more carefully.   

In the earlier days of ordering concrete, the concrete contractor would simply ask the ready-mix supplier for a mix based on the number of bags of Portland cement added, i.e. 5 bag, 6 bag, ect. which would dictate the compressive strength of concrete. Today with all the additives and mix designs for different applications, the deciding factors after all the ingredients are added is still the compressive strength measured today in PSI (Pounds per square inch) that the concrete can withstand before failing. So does that mean the stronger the better? Not necessarily, there is a point of diminishing return when the average homeowner or concrete contractor gets too smart for their own good. High amounts of Portland help contribute to shrinkage cracks along with too much water. These are just the beginning of many problems that most contractors do not consider when placing an order for their clients concrete.

So what is a good mix to use? It all depends on the application. Foundation footings and walls are usually poured using a 3,000 PSI mix and is usually more than sufficient for most foundation purposes. For interior flatwork which is not exposed to freeze thaw cycles, a 3,000 PSI mix without air entrainment should work fine in most cases. Where things start getting a little tricky are in the outdoor arena where freezing and thawing cycles will take its toll. We recommend a 6-1/2 bag mix or 4,000 PSI concrete with a 6-8% air entrainment and low slump in the 3-4“range to avoid Spalling of the surface from the winter. Then depending on the weather conditions, one might add fiber-mesh and a retarder to help the workability of the concrete and avoid shrinkage cracks. Also depending on the application, there are many other additives that the run of the mill concrete contractor is unaware that exist to make the job easier and provide a lasting product for the homeowner. Buyers beware; check that your contractor has a “Concrete Masons” License and not a “Home Repair” license.

So, why is stamped concrete so expensive? Well, to start off with, it takes a bit more planning to pull everything off for the big day. The client needs to pick out three things; the stamp design, the integral color of the concrete, and the color of the release which gives the highlights. The next step for the contractor depends on if the contractor owns their own stamps or needs to rent them. Chicagoland Concrete, Inc. does own their own stamps and makes this step easier as the alternative is renting them. Renting becomes a problem for contractors because the stamps are not always available when the job requires them. It can also be cost prohibitive to purchase a set of stamps as they can run upwards of $2,000-$3,000 for a complete set. The next issue at hand is ordering the concrete and either coloring it in the truck on-site, or ordering it straight from a ready-mix supplier if the supplier stocks the required color and then paying a hefty premium. There is also an additional charge that the ready-mix supplier charges for a special mix that reduces certain additives that won’t react to the integral coloring, on top of a charge to wash out the truck. If you mix the integral color on site, you will be adding waiting time to the ticket which in the Chicagoland market runs about $3/min. When the colored concrete is placed, it is finished smooth just like normal concrete then a release color (think of powdered sugar) is spread on the surface of the concrete serving a dual purpose. The release allows the stamp to peel off the surface while not sticking to the wet concrete and also impregnates the concrete with the secondary color. Once the concrete has cured, usually the next day, the crew returns and washes off the excess release from the concrete. Then next step is to wait till the water evaporates completely off. Then a special sealer is used to protect the surface and bring out the color, usually two to three coats is needed to ensure proper coverage.

To sum everything up, the coordination of renting the stamps, purchasing the coloring, the release, paying for the extra waiting time, paying a washout charge, extra labor to stamp the concrete after finishing, the second day to wash off the release, and then applying the sealer add up quickly. It typically adds up to the tune of about 50% over the cost of traditionally placed concrete. Is the additional cost worth it? In my personal opinion, Yes, It adds value and sets your driveway, sidewalk, or patio apart from the miles of other plain concrete in your typical neighborhood. It also is a lot less maintenance and is cheaper than brick pavers which most people see as the alternative.

We hope this article helps you to understand some of the logistics involved to bring you the completed stamped concrete product.

-Chicagoland Concrete, Inc.

So what is fiber mesh and what’s it for? Fiber mesh is an additive of synthetic fibers usually added at the mixing plant. The fibers are used to solve a very particular problem called shrinkage cracking. In the first few days of freshly poured slab life, the water in the initial mixing is evaporating from the concrete and it’s actually shrinking ever so slightly. The problem is that concrete does not shrink evenly as a whole, think of it more like a pie cut into four or more pieces. The concrete is shrinking away from each individual cut and in on itself. This is why we put in control joints (the lines in you see either tooled or cut into the concrete) to help dictate where the concrete will crack.

The fiber mesh’s job is to help hold the concrete together in that initial drying phase only, and when it cures, does little to NOTHING to add to the strength of the concrete. So why use is it that contractors will steer you towards substituting fiber mesh for WWF, because the contractor either is pulling the wool over the customers eyes or doesn’t know any better themselves. It sounds good, like a high tech answer to an age old problem, but it isn’t.

So what does the WWF do? That is another good question, and the answer depends on where the contractor places the WWF during the pour. If the WWF is lying on the ground and is never lifted into the pour, it then is not very effective. If it is lifted and suspended in the concrete, it then serves two purposes. First, when it does crack and it WILL CRACK (any contractor who says the other is someone to stay clear of) the WWF helps keep the two now separated cracks from spreading apart from one another. The second thing WWF does is to keep two sections of concrete that are divided by a crack in the same plane (meaning that one won’t sink while the other sticks up). These two attributes along with properly placed control joints will help give your new concrete a chance at long life.

Please feel free leave any comments or use the Contact Us tab and ask any other questions that you may have.

-ChicagolandConcrete.com

Frost lines through the United States can vary broadly.  For example, some of the Northern states have a frost line that can reach as deep as 60 inches while parts of Florida have no frost line at all.  The Midwest portion of the country has a frost line that goes between 30 and 40 inches on average.  Currently, the frost line in the Chicago area is recorded at 40 inches, which means that the water in the soil has not frozen at levels deeper than 40 inches below ground.  In the Chicagoland area, building codes require that supporting structures are poured at least 42 inches below the ground; however, many architectural plans dictate digging to a deeper level to accommodate any future changes in the average frost line.  When it comes to the foundation of your home, it is better to be safe than sorry.

We hope that this article has helped with some basic questions you may have regarding frost lines, frost heave and how it can affect your home or concrete project.  If you have any further in depth questions, please feel free to contact the professionals at Chicagoland Concrete, Inc. in the office or via the website through the “Contact Us” tab.  As always, please feel free to leave your comments on this post.