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Concrete construction joints 3

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JETSconcrete

JETSconcrete

Structural
Joined
Feb 23, 2020
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AU
Hi, I’m a concrete contractor, regarding dowels for construction joints in pathway/ driveway slabs etc, I had an idea I wanted to float by some professionals... how effective would m12x200mm hex head bolts be at locking a new slab into existing, ie drill 12mm hole into existing and then use impact driver to drive bolt in 100mm deep. Thinking of possible alternative to traditional rebar and chemical anchoring. Thanks for your time.
 
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Could work, but 100mm is fairly shallow embedment. It doesn’t give you much in terms of pull out depth when the slab wants to contract. I suspect you’ll get cracking at the bolts.

Bolts threads probably arent ideal either. Concrete screws generally have more widely spaced more pointy threads to bite into the concrete. Sort of like coach screws.

What’s the aim here? Eliminating the glue?

Have you done any tests?
 
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The goal is to save money, time and effort. Bolts should be around 40% cheaper per hole, time is saved by only having one person putting in dowels rather than one squeezing glue and the other hammering dowels in, also holes don’t need to be cleaned out, and it’s much easier using a impact driver to push a bolt in than it is swinging lump hammer around.

The idea only has value if I can get the bolts for cheaper or the same price as glue. Custom made 400mm concrete screw bolts won’t be cheaper...

 
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What was the original detail that you're replacing? Was it a 200mm dowel that's meant to allow shrinkage movement parallel to the bar?
 
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Sometimes the detail specifies dowels ie one side of the bar unbonded, other times it’s a tied joint with epoxy, without clear indication of why the difference.

Doing mainly residential renovations, I figure a tied joint will perform better in the long run, if installed correctly?
 
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I assume you meant "contraction joint" when calling "construction joint". A properly installed contraction joint should allow the free movement of concrete (shrink or expand), a smooth dowel is essential for such movement, and maintaining levelness of the slabs. For other type of joint, I suggest to use Hilti drill in dowel. But no matter use any method, make sure the person in charge agrees such application.
 
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@JETSconcrete....You have two distinctly different applications. Doweling to tie two sections of concrete together requires bonding on both sides. Dowels for pavement application (driveways, roadways) require that only one side be bonded and the other side allowed to slip.

Dowels for tieing together two sections of concrete should be deformed bars. You have proposed replacing these with headed bolts. While that will suffice for vertical loads it is not appropriate for lateral loads. The bolts will only penetrate into the concrete on both sides by about 100mm. This is insufficient on the headed side to resist shrinkage stresses as a stress concentration will develop at the bolt head causing a crack. Alternately, the threaded side may have limited friction resistance because of the thread pitch and limited embedment. In either case, you have a failed joint.

I'm not sure why you described hammering rebar into a hole filled with epoxy. That's an improper procedure. You should have a drilled and cleaned hole that is about 3mm larger than the rebar diameter so that the epoxy can properly distribute and bond around the rebar. Further, the rebar should penetrate into each side of the concrete at least 250mm to provide proper stress distribution in the lateral direction.

If there is a problem in the future with failed joints, you will likely bear the responsibility as such a failure determination is relatively easy (I do failure investigations for a living). Don't expose you or your company with unnecessary liability when you can do it right without much more effort and a huge savings rather than tearing out a failed concrete structure.


 
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I have never seen 500mm dowel bars, 400mm at most so a 250mm embedment is pretty rare. The hammer comes into play because forcing a dowel home into epoxy isn’t like putting a glove on, there is resistance, and it usually needs a few taps, although you mentioned a hole 3mm larger hole, opposed to the 2mm hole I’m used to. Regardless I’m sure if you watched a guy try to push 50 dowels in 200mm with his hands you would understand why a hammer is needed.

So two questions now, 1. would 400mm concrete screws be acceptable to resists lateral movement? 2. what situations is a tied joint required?

An example: existing path around a house, client wants extended into patio area, contraction joint with one unbonded side would eventually pull apart creating a space for water to run, weeds to grow through and other dirt to collect, it tied correctly is it possible to resist shrinkage, and what would be the effect of this on the new and existing slabs?
 
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Why to connect these two areas together. I'll separate the areas with preformed joint filler, and seal the joint with epoxy or flexible sealant. If the subgrade is properly prepared/compacted, and concrete is well cared of during curing period, you shouldn't have any problem. Delay the application of the sealant as long as possible, the best timing is in the Autumn, when temperature is not too cold, nor too hot.
 
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If you are using rebar to tie two sections of concrete together, you can cut it to any length you want. As I mentioned before, deformed bar is necessary in this application as you need to develop and distribute shrinkage stresses so that you don't get a crack at the end of the dowel. You wouldn't use a slip dowel in your application.

As for seeing dowels installed, I've seen thousands and have installed quite a few myself.

To answer your two questions....

1. First, you should not be using smooth dowels (the kind you buy already cut and made of hardened steel) for a tied joint, so there is no limitation on length. Cut rebar to whatever length you need. I recommend a minimum of 250mm and even that can be a bit short if your concrete shrinks a lot.

2. A tied joint is required when you want two sections of concrete to act as one section and loaded both vertically and horizontally. If you only need load capacity in the vertical direction, use a slip doweled joint.

 
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A word from this old guy. Take a look at the background of us here by clicking on the name. You will see that the experience of Ron can't be beat. Follow his advice and you can't go wrong. Don't try to reinvent these things.
 
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Thank you OG...That's very kind of you.

 
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