Too often lately, we don’t finish concrete as well as we used to. Why is that? Does the surface shown below look a little too familiar? When our concrete flatwork develops scaling, mortar flaking, and pop-outs early in its life, the easy retort is that deicing salts are the culprit.
The truth is that when a proper concrete mix is designed and it is mixed, placed within its design water/cement ratio, consolidated, struck off, allowed to free its bleed water, and then finished, broomed, and cured without blessing the surface with water, it should stand up against prudent levels of deicing salts and be a durable surface.
If you are over-applying deicing salts…stop it. The Delaware T2/LTAP Center offers a free winter maintenance training workshop each fall that talks about deicing materials and strategies so you can effectively control ice without wasting material and adversely impacting the environment in the process. Keep an eye out for this most-excellent workshop, attend, and stop blaming poorly performing concrete on deicing salts.
That said, long-lasting, durable concrete requires that we eat our Wheaties. Best construction practices are key, and it begins with selecting the correct mix. The Delaware T2/LTAP Center also offers a free workshop that walks through the practices that will ensure the best concrete results, recognizing that we won’t always have the ideal controls over every aspect of the project. That is fine, as long as we understand how each compromise will impact the product.
Scaling and mortar flaking in particular are a dead giveaway that we probably compromised the surface during finishing. Specifically, it is likely that the surface was finished before the bleed water was removed and/or water was added to the surface during finishing in a variety of clever ways. The most blatant finishers will simply dip their tool brush into the bucket to soak it and then sling water across the surface before continuing with floating the surface (so-called “blessing” of the surface). When a watchful inspector is nearby, they may resort to continually dipping the float in the water bucket to “clean” it or they will pop a hole in their water bottle to use it for broadcasting water. There are other tricks, of course, and that presupposes that the bleed water isn’t just being worked back into the surface.
All of this weakens the wearing surface, right where the worst beating takes place. No wonder the surface scales and flakes. Hey finisher, we know what you’re up to; knock it off.
The other big water mistake we make is allowing too much water to be added in the first place. Speaking broadly, cement needs about 0.35 pounds of water per pound of cement for full hydration (the process that makes our cement magically durable). But that is under ideal control conditions and so our mix designs are usually based on somewhere between 0.45 and 0.60 for its water to cement ratio (w/c). For many of our common mixes, the specification is a maximum of 0.45 w/c.
So, the batch plant prepares the concrete according to the mix and often, they will “hold back” a certain amount to keep the mix below the maximum water to cement ratio. Let’s say they hold back 8 gallons for the nine cubic yard load. The truck arrives at the site and before the chutes are even unfolded, the finishing foreman tells the driver to add ten gallons. As you can imagine, we’re already off to a bad start.
We can do better.
The Delaware Department of Transportation decided in its 2016 revisions to add a certification requirement to its Standard Specifications for Road and Bridge Construction. In relevant sections, the requirement is to:
“Provide a flatwork technician with an ACI or NRMCA certification, or a certification from another program pre-approved by the Department, to supervise all concrete finishing work. Provide the flatwork certification to the engineer before placing concrete.”
The Delaware T2/LTAP Center worked with DelDOT and Henry Prenger (LaFargeHolcim and the National Ready Mix Concrete Association) to develop a certification program that could ensure a better understanding of best placement and finishing practices, particularly those that can result in scaling and other premature distresses. Since late 2017, Henry has been regularly leading sessions of the training course here in Delaware and many hundreds of finishers, inspectors, and even design engineers have completed it.
The NRMCA course consists of a morning session where the instructor walks through the fundamentals of concrete, particularly the role water plays in it. The afternoon session takes place at a local ready mix plant, where all students in the class must participate in the placement of fresh concrete, along with consolidation, screeding, sealing, waiting for the bleed water to come off, and then finishing, jointing, edging, brooming, and curing the surface. For non-finishers, it is sometimes the first time they have actively finished concrete and it gives them a new-found appreciation for the craft. You can see a little of what the course consists of in this video compilation from some of the first training workshops.
The NRMCA course is available to all local agency personnel as well. It is frequently hosted now by the Delaware Contractors Association (DCA) and currently costs $350 per student. This investment in your personnel can pay dividends when they are repairing sidewalk and curb or are overseeing the work of contractors.
Jamie Chambers is the Director of Workforce Development for DCA and she schedules upcoming course sessions. The next one will likely be scheduled for when there is a break in the weather, perhaps March, to hopefully allow that patient period needed while the bleed water comes off and yet still get done before the sun sets (stupid daylight savings time). Jamie can be reached at 302-985-7988 or at email@example.com. Touch base with her now if you think you may be interested in sending one or more of your personnel this year so that she can ensure that there are a sufficient number of sessions offered.
Meanwhile, consider whether a requirement similar to DelDOT’s belongs in your contract specifications. If you look around at concrete flatwork from the past few years and see a lot of surface distress (or worse), maybe it’s time to push your concrete finishers and your inspectors to a higher standard, and the NRMCA flatwork finishing course can be a good way to prepare them to succeed.
We truly have to take a more active role to limit poor finishing practices so that we have enduring flatwork. As public agencies, we have an obligation to be good consumers with the taxpayer dollars allocated to us. When it comes to concrete work, that means taking aggressive steps to ensure that concrete work we or our contractors construct will be as durable as the materials that we still see around from 50, 60, and 70 years ago.
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