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Cold Joints: Avoiding Costly Repairs

By Scott Cohen

ScottCohenColdJointsIn pool-remodeling work, it's very common to raise a bond beam to meet the needs of a new deck or edge detail — or simply to make the pool level again.

As ordinary a step as this may seem, it can be trickier than you might think because, in applying new, poured concrete atop the old gunite or shotcrete, you create what is known as a cold joint — that is, a space where you need to be certain you've created a good, reliable bond between the new material and the old.

Scenario: I inspected a project where a floated bond-beam addition had cracked at a point behind the tile line that was just below the water's new level. This caused a leak that oversaturated a surrounding clay soil with moderately expansive properties. When that clay started to swell, it heaved the concrete all the way around the pool, damaged the skimmer and created a number of trip hazards.

To take care of the problem, a contractor had to come in, remove the concrete decking, pull up the coping and cut away all of the tile before basically redoing the original renovation contractor's work. All told, it was a $60,000 repair.

The Fix: All of this hassle might have been avoided had the original renovation contractor followed a few simple steps.

First, after a preliminary cleaning, he or she should have roughed up the surface using hammers and chisels. This increases the contact surface between old and new material and offers the best possible chance of subsequently establishing a good mechanical bond.

Next, the contractor should have made the top surface of the original bond beam as clean as possible — meaning free of all dirt, dust and debris. We accomplish this in our own projects by pressure-washing the old material, which not only cleans things up but also thoroughly wets the concrete, thereby helping with the next step.

Now the contractor should have applied a thin film of cementitious paste over the surface of the existing material. For this purpose, we brush a high-performance, non-shrinking concrete onto the surface. (We know a number of contractors who also add some concrete glue to the mix to increase flexibility.) Note that this is not regular cement, which will shrink and pull itself away from the bond and cause the cold joint to fail.

Within minutes of brushing on the cementitious paste, we pour the new concrete — again using non-shrinking, high-performance concrete. Yes, it's more expensive, but it will not shrink or fail the way regular concrete will in this sort of critical application.

In using this high-performance material (or any concrete material, for that matter), it is crucial to follow manufacturer instructions to the letter with respect to water-to-cement ratios and overall measurements. Put aside the Big Gulp cups and shovels: Always use marked measuring containers so you can be sure the mix is correct.

Once this concrete is placed, it's important to consolidate the material by vibrating it, either mechanically or manually. That's all there is to it.

Lesson Learned: Whenever you're creating a cold joint, be sure to follow the basic steps needed to establish a reliable bond. Otherwise, the damage to the pool and adjoining structures can be severe — and costly!

 

Scott Cohen is president and supervising designer of The Green Scene, an outdoor design and construction firm based in Canoga Park, Calif. He provides consultation for clients nationwide and gives seminars on designing landscapes, swimming pools and outdoor kitchens.  For more information, visit www.greenscenelandscape.com.

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  • Guest - Jeff Miller

    On almost all of my beam repair projects, I like to call the gunite company (dry mix) to reapply the beam. We still remove all unsatisfactory material and properly prepare the old gunite before applying the new. I like to soak the old beam for at least 12 hours before application, and use a bonding agent. I like the idea that we are trying to bond like materials, gunite-to-gunite. It may seem too messy for a beam repair, but a professional pool builder will take the steps to make sure the area is properly prepared for gunite.

    I have repaired beams both ways, and found I can use the time wasted on clever ways to re-form the beam on cure time. Not only does this let me take on more jobs, but the jobs are also completed faster. Everyone knows how long it takes to properly demo a beam, form, re-pour and let the new stuff cure. Take out the hours of forming and think of all the time you save — and because of that I think it puts more in your pocket. I would like to know what the other readers think.

    Jeff Miller
    Michigan Pool Design, LLC

  • Really great article... Love the tips and tricks, and love contractors that are not afraid to share real information. I see delaminated beams all the time in my area. Of course cleaning is imperative for good adhesion, but I never thought of using a pressure washer... Love it

    from Lowell, MA 01852, USA
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