Another question for the concrete experts

[Stan in Oly, WA]

I'm getting ready to pour three garden planter boxes. Each one will be 10' long, 30" wide, 22" high and 3.5" thick. They are going onto concrete perimeter footings I poured a few months ago. I don't care about the cold joints because I want them not to be watertight, anyway.

Can I get away with pouring them without re-bar. I have plenty of re-bar but it would take me a few hours to go get it. What I haven't got so much of is summer weather, and too, I'm definitely running out of steam.

Thanks, Stan

 

[ken in texas]

Will hold up for a while but will eventually crack and fall apart.I would use rebar.

 

[NcNE]

I second the rebar. As much as concrete costs, it would be worth the little extra time as you are likely to have problems later on. Especially if its in an area that can be hit with a lawnmower or something like that. Another option would be using fiberglass reinforcement. Upon last check its only a few dollars per yard more. The sparkles from the fibers will weather off after a short time. I would also vibrate the concrete to make sure its placed uniformally. Be careful not to over vibe it and blow out the form though.

 

[Anonymous-0]

If not rebar, how about some woven wire or concrete wire? Even some quarter inch rod from a hardware store/welding shop. wired together. I don't trust fiberglas in our weather.

 

[Anonymous-0]

I replace concrete for a living (for the past 35 years) Fibermesh, its not meant to replace rebar. When I demo concrete, I can't even tell if fiber mesh was used or not. Spend a little time, get your rebar and your expensive concrete job will last a lot longer.

 

[Dug]

Your concrete will not last 5 years without rebar, or at least wire mesh. Trust me, I learned the hard way...

 

[VaTom]

Stan, you've already heard about the need for reinforcing. That concrete's going to encounter some tension, where it needs your rebar. Make sure you get it centered, 2" clearance to the outside of the concrete is what I've always seen spec'd. You can get 1½".

BTW, making cold joints water-tight is simple if you ever need.

I'm using ferro-cement for my raised beds, twice your size. Still need rebar for the frame, but no forming.

 

[Anonymous-0]

you only want some strips of thin mesh,same as used on footpaths. its only 3.5" Better concrete if it cures slowly. did you leave some exposed mesh on the perimeter otherwise it may move eventually.

 

[Stan in Oly, WA]

Hi Tom,

Can you make any cold joint watertight, or only those you plan for in advance? I'd be grateful for the information either way.

Watertightness aside, there's an issue concerning cold joints that I've always wondered about. Since sheer is one of the forces that unreinforced concrete is not ideally suited to withstand, how much difference will a cold joint make in the construction of a heavily reinforced structure that will be subject mainly to sheer or lateral loads? Years ago I watched the ongoing construction of a freeway bridge in Seattle where the work at one point progressed from the ends of the portions extending toward each other over a river. I don't know what the maximum extension was before the ends of the span joined, but during the process I once estimated the extension to be at least 600 feet, with plenty of heavy equipment (transit mix trucks, etc.) out on the working end. If common sense wasn't enough to tell me that it couldn't possibly be done in one, continuous pour, there was a point at which they had to stop and remove a sizable section of the recently poured concrete because it was discovered that some of the earlier concrete had contained an unacceptable level of fly ash.

So, to bring this back to everyday human scale, how much more resistant to the lateral forces of an earthquake, for instance, would a heavily reinforced foundation wall be if it was poured as a single unit with the footing than if it was poured days or weeks after the footing? For purposes of this question assume that the configuration of the rebar reinforcement would be identical in either case.

It's only a matter of idle curiosity to me, so don't go to any trouble if you don't know the answer, even in general terms, off the top of your head. (But I would still like to find out how to make cold joints watertight.)

Thanks, Stan

 

[VaTom]

Stan, the traditional way to make a cold joint watertight is with a waterstop, often vinyl, inserted into the first pour. The second pour encompasses the protrusion, consisting of repeated lips. Redundancy is good.

Simpler alternative is to use bentonite rope installed just before the second pour. Bentonite's an expansive clay. Often a bane to builders, but an excellent water-proofing. You nail the rope centered in your next pour. When water hits it, it expands, plugging the gap. Used it between my swimming pool floor and walls.

Ropes are sized according to wall thickness, probably not down to 3½" though. You don't want it breaking your wall, which sometimes happens in areas with "expansive soils" (bentonite). Bentonite's the preferred well casing waterproofing here, pellets rather than ropes.

Seismic design I know little about, other than concrete columns.

In general, you lose no strength in a cold joint. Remember what concrete's good for: only compression, weak in tension. You want steel for tension, going across a cold joint (with adequate overlap) has no strength compromise according to the engineers I've hired.

We're assuming you don't have flex at the joint. For my underground concrete houses, we ignore cold joints, like they weren't there. Never a problem, just keep the rebar running.

4.2 was the largest quake I've experience here. Couple hundred tons overhead, gets your attention when it shakes. No damage.

 

[Stan in Oly, WA]

Hi Tom,

Thanks for the useful and interesting advice.

I've been through 2 major earthquakes and a handful of lower magnitude ones here in the Puget Sound region. I was less than 10 miles from the epicenter of the 6.8 Nisqually quake in 2001. There was no apparent damage where I was, but my wife, at home a few miles farther away, got to see pictures and mirrors fall off the walls and furniture fall over and be damaged. It was a long 45 seconds for her.

Realization that what's happening is an earthquake seems partly dependant on where you are when it happens. I couldn't figure out what was going on for at least half of that earthquake until I looked out the front window of the office and saw the parking lot rolling like ocean swell. But a few years earlier on a driving vacation, my wife and I were in a motel in Crescent City, CA (on the coast, close to the Oregon border) when a minor tremblor a few miles offshore shook the room for a couple of seconds. We looked at each other and both said, "Earthquake" at the same time. It was California.

All the best, Stan

 

[VaTom]

Stan, earthquakes are extremely rare here. Took me awhile to figure out what it was. Never occurred to me to ask any of the local engineers, who didn't volunteer anything on their own. Probably didn't know much.

Just recently got a note from my insurance agent offering earthquake homeowner's rider. No part of our building code.

Glad you found the info useful. Internet sure is handy, eh? Wish it'd been common when I was trying to figure out my place. Would've avoided several mistakes.