Slipforming, part 14 – Cold seam repair

This post follows Slipforming, part 13 – Kitty’s take on it.  To see a complete index of slipforming posts, click here.  For an index of comical posts, click here.

If you have gotten this far in my posts, you have read about how to slipform, what the concrete looks like fresh out of the forms, how to scrape it off for a smooth joint and the dangers of using mixed sources of cement.  An area that I have not covered is how to repair cold seams.  Cold seams are the bane of a concrete slinger’s existence.  They are deceptive.  They are not forthright, they make promises they do not keep.  They should die.  And now I’ll tell you how I really feel.

Kitty, Dad and I were racing an unforgiving clock–Colorado weather.  We knew we would get done only what the weather would allow.  As luck had it, we had glorious weather until one week before Thanksgiving.  That was remarkable because often we have snow by Halloween, which is in late October for those of you reading this from other countries.  We decided to go to the nine foot mark because that would be where the second floor would be attached to the slipformed walls.  

I read material on Helen and Scott Nearing and how they suggested to leave exposed rock sticking out atop the cement for the next layer of cement to bond to.  (This photo, at right, is of our shed, so disregard the wire mesh and focus on the rock tops that I leave exposed.  Leaving a layer of rock exposed allows the next pouring of cement to grab onto those rocks and also allows a normal seam across the top edge of the rocks.  The downside is that when you leave rocks like this exposed over a winter, snow and rain can occasionally get behind the rocks and pop them from the wall.  Left for a season, the break in cement causes a cold seam where the old concrete has cured and the new concrete continues to cure at a different rate, thus encouraging cracking.)

Also, it is important to leave rebar exposed so that the new concrete has something to grip.  I did that.  The cement was left to cure from September until the next spring.  Snow fell.  Spring rains washed the walls leaving small puddles that froze at night in the crevaces of cement.  In at least one area, the rain got in valleys between the exposed rock and the level cement and popped the seal of the exposed rocks.  This, I was not to discover until this year – a full 10 years after pouring the initial cement.

For those of you wondering why it would take 10 years to discover a cement problem, this comes my accusation that cement lies.  When I first poured the second layers of cement, there was no indication that I was going to face any cold seam issues.  That came later, first with a hint of a problem.  A hairline crack barely visible to the naked eye.  Being an optimist, I hoped it was just settling.  Then, it grew to a wider crack.  As it grew, I worried.  Was the house settling?  Should I repair it immediately?  Should I wait until all settling stopped?  Was something structural going on?  

My decision was to wait and allow any settling to occur, then to repair the cold seam rather than repair it every year.  The cracking did slow, and now at 10 years, I believe it has stopped.  That made this the time for repair.  Now, others might advise differently.  Certainly, weather is a consideration and can complicate repairs.  If your winters are harsh, with a lot of moisture, you may want to seek other opinions.  Our winters are not that harsh, and there is less rainfall in our county than in Tucson, Arizona so I was not that worried.  The big issue is having water enter those gaps, freeze, and weaken the wall.  To avoid that, grouting helps considerably.

Filling the gaps with caulk, or with mortar glue before applying grout is one possibility.  Applying straight grout into the gaps works, too.  This photo at left shows glue applied in the crevaces around the rocks.  From here, grout will be applied over the top hiding any sign of the glue and giving a uniform appearance. 

Our cold seam repair is somewhat of an experiment, which I hope to update next year.  We chose to repair our cold seam with masonry glue, which is applied like caulking.  Then, over the top of that, we put a mortar mix of straight mortar mix and water.  We mixed a stiff batch adding only enough water to bind it, but not so much that it was soupy.  Before applying the grout, we washed the walls and wet them down.  This keeps the dry cement from absorbing the moisture in the mortar mix and weakening the grout. 

The photo above shows a grouting job in process.  Notice the rough edges of the ungrouted cement.  These rough edges work really well because the jagged edges allow the freshly applied grout to grip the surface.  When the entire surface is complete, the joints will all be smooth.  This will repel rain, snow, insects, small children and Ken’s secret notes.  (Just kidding.)

It goes without saying that one must remove all loose rocks, cement, or dirt before applying the grout.  Make repairs carefully and then the grout can be molded into place by hand.  We used a small pointed trowel, but often we only used it to transport small handfuls of cement to the gaps.  Then, we used our gloved hands to work the cement into the gaps.  Dragging a finger atop the grout leaves a smooth grout seam.  That is what you want.  Rough edges catch water. The water, in most cooler climates, freezes and it is that freezing process that is the cause of most problems.

Most people who have experience with cement advise to grout as soon as possible after installing the cement.  I agree.  The bond is easier to encourage.  We have more than one project and have handled them differently.  The new shed was grouted as it was poured.  The house was grouted 10 years after it was poured.  While it is preferable to do it early, it is better to get it up late than never.  Grouting protects your work and your investment. 

Now, with vertical cold seams, the cracks are sometimes more problematic.  This one is a cold seam in a raised garden wall.  It was made worse because of tree root growth which ran beneath the wall.  The gap grew as winter snow and rainfall froze inside, further spreading the crack despite rebar extensions from the sides of the cement.  To repair this gap, Ken filled it with expandable insulation, then will shave off the extra insulation and grout from there.  Extra foam can be removed with a muriatic acid/water mix when it is smeared across the surface, but the acid will eat the foam quickly, so applying it to the actual joint is not recommended, as it will eat the foam you intended to stay in place.

This crack can be covered with grout without a problem.  We completed a similar vertical crack a few feet away from this one a few years ago and it has given us no problems, nor did it continue to crack.

Final word on cold seams:  An ounce of prevention is worth a pound of cure.  If you can pour your project consistently without long gaps between stages, you will have less cold seam problems.  If, like us, you cannot finish the project in one season, then you will likely face cold seam cracks.  Fear not.  They are not that difficult to repair, but do not delay.  The sooner you repair them, the less damage future weather will have on your walls.

(See Slipforming, part 15 – Snow and the balcony for the next thread.)