May 30, 2017
Every spring, we experienced a number of call-backs from owners complaining about scaled surfaces on exterior, slabs-on-grade (mainly driveways). In fact, this problem is common among ready mix producers in the Des Moines area, Iowa, and the entire Midwest. This is disappointing to us at Liberty Ready Mix, and eliminating scaling has become a top priority.
There are many contributing factors to why the winters are so severe in terms of scaling. The main reason is the weather. From mid-November through January, we have prolonged cycles of rain/freezing rain/sleet followed by hard freezes. The freezing rain and slush accumulate so quickly that concrete which normally has quick run-off, now has ponding water hard frozen to the surface. As the saturated surface freezes, the water expands, causing stresses in the surface of the slab. This is scaling.
We know how to produce concrete which is resistant to scaling, and, for the most part, we do produce scale resistant concrete. What we have found, however, is that we have concrete that meets specifications as it was placed, exceeded specifications when it hardened, yet suffered moderate scaling. Independent testing of cores is pointing to two major problems; excessive jobsite added water (water added to the mixer and to the slab surface), and lack of curing.
Construction practices that were “good enough” in the past are causing us (concrete producers and contractors) problems and costing us money today. Because of the changes in the materials and environment and what new concrete is subjected to today, it is imperative that we go back to the basics of placing and curing.
Recent changes in our industry:
- Today’s cement is ground finer than in years past. Fast-track construction schedules demand that concrete mixes achieve design strengths at earlier ages, as well as contractors want quick setting concrete to get their finishers off the job (and clock) as soon as possible. Cement producers react to these demands the cheapest way possible…. by grinding cements finer. However, quick setting concrete does not always necessarily mean more durable concrete.
- Unwillingness of contractors to properly cure new concrete. There is nothing that Liberty Ready Mix can put into new concrete that will have a greater, positive impact than curing. The benefits of curing are most evident at the surface of the slab. A properly cured slab will have more complete hydration of the cement, will be less porous, have better abrasion resistance, and will maintain the good air void system necessary for freeze-thaw resistance. This has to be done!!
- Iowa Department of Transportation’s expanded use of salt brine. Applying this material to roadways makes snow and ice removal easier and cheaper. However, it appears to be at the cost of new concrete surfaces. An easy solution would be to quit using this type of de-icer. However, when it comes to public safety and shrinking snow removal budgets, you quickly realize that this is not likely.
Today’s concrete IS DIFFERENT than concrete produced in years past and it is being placed into harsh service conditions at a much sooner time frame. It is up to us to adapt to the negative changes which are not going to go away.
We recommend that all exterior slabs on grade, including residential, be properly cured. Curing is maintaining an environment favorable for strength development, which includes moisture retention AND protection from temperature extremes (mainly cold temps).
- Wet curing is the best cure, but is rarely possible.
- Plastic sheet curing works well when done properly. Plastic sheeting should extend beyond the slab edge and be secured so that air does not blow under the plastic. This is a good cure method when a post treatment will be later applied.
- Membrane-forming cures, which are applied to the slab (at or above the recommended coverage rate) after finishing are the most cost effective type cures. If a sealer is to be applied later, however, this material usually must be removed.
- Penetrating sealers work very well in preventing scaling, particularly when heavy salt use is anticipated. Salt is carried into the slab by water. Penetrating sealers prevent water penetration, and doing so prevent salt from getting into the surface. These types, however, need to be applied after a cure period of weeks to months.
We have researched effective cure products, and found these locally available products to offer cure benefits and/or sealer which will protect from scaling:
- Any membrane forming cure which conforms to ASTM C-309, available from any of our local construction materials supply stores. Applied to newly finished concrete
- Sikagard 701W, available from Liberty Ready Mix. Penetrating sealer applied to concrete after 10 to 14 day cure.
- Chem-Crete Pavix CCC100 available from Logan Contractor Supply. Penetrating sealer applied to cured concrete.
- Saltguard, available from Stetson Building Products. Penetrating sealer applied to cured concrete.
- One-Step, available from Liberty Ready Mix. Combination cure and penetrating sealer applied after final set. Offers some curing benefits and sealer benefits. New product not locally tested, but sounds promising.