request recommendations for rebar spacing for concrete project

disney · Nov 2, 2014

mikim said:
you need to drill the existing and install rebar in the holes to tie the existing to the new slab. There's where #5 could really be useful. I would want those holes to be at least 6" deep into the existing....

I was planning on doing this, but while we are on that topic, two more questions. how close would you space the holes? 6" o/c, 12", etc. Also, should I keep the holes 'tight' so I have to hammer the rebar in, or loose so they just/almost slide in?

Streetcar · Nov 2, 2014

A thickened edge will help tie the two slabs together., be sure to slope the subgrade from thickened edge to your normal driveway thickness. The fiber wil help control microcracks in the new concrete. I have used fiber if I am worried about cracking. 4000psi is a good mix for a driveway. Evaporation of the water from the setting concrete is your enemy. Make sure to get a curing compound as soon as you can on the finished concrete, or use burlap that is continuously wetted. If you are using rebar use a 2 " chair. For rebar to be effective it must be in the tensile stress area of the concrete. For your purpose, the smaller rebar(#4) on twelfth inch grid will help,with loading and crack control. Good luck

Streetcar · Nov 2, 2014

I would never drill an existing slab if it is only 3.5" thick. The chance of shattering slab is too great. For the rebar to be effective in tying slabs together it must be anchored into the existing slab, either by mortar or an epoxy . This requires the hole to be oversized. the depth of the hole is based on the strength of the material used to anchor the rebar

/pine · Nov 2, 2014

disney said:
I was planning on doing this, but while we are on that topic, two more questions. how close would you space the holes? 6" o/c, 12", etc. Also, should I keep the holes 'tight' so I have to hammer the rebar in, or loose so they just/almost slide in?

Many architects say to use epoxy but I've seen some spec. sheets say to use a slurry of portland cement and a bonding agent...

SPIKER · Nov 2, 2014

lots of info:

I would also look at maybe driving that motor home across the dirt a few dozen times to make sure it is well flattened

The footer under existing drive should be a good way to go and the holes (drilled into existing slab) they need to be slightly over sized. distance of 16" or up two 24" apart & they make an epoxy to coat the hole/rebar to hold it tight. If you go hammering the rebar into a tight hole it can split/crack the existing slab thus weakening it. Digging back under a couple inches (UNDER) the existing slab & pushing wet crete under that lip will help tie the two together, be sure to get the wet concrete competely under filling al created voids. Also get the sand/mud/dirt off the edge of the existing crete and use an adhesion promoter on it. Running a chipping air hammer or needle scaler to knock a bit of etch into it is a good method but having CLEAN surface is needed (even scrub brush and muratic acid to clean it as long as it is neutralized afterwards) done in advance enough that it drys out a bit.

Rebar spacing (if the soil is good and solid) can be wider & mesh on top of a wide spacing is good too. I'm a fan of Fiber too as it helps to keep micro cracks at bay from my research. I had my barn slab done with 6000PSI mix, fiber and used Mesh on 2" chairs every 18" there has been one crack outside with a really deep frost/freeze this year. The slab was done in 2011 in September (dry for most of July/Aug that year so the clay rich soil was very solid.)

Mark

/pine · Nov 2, 2014

FWIW...If the OP has not exposed the edge of the existing driveway he may find that the pour was thickened to prevent break-offs...

dodge man · Nov 2, 2014

A couple of points, fiber in the concrete is a good thing, it adds some strength and if you do get a crack it tends to keep the concrete intact. The problem is you end up with a furry surface, but it will wear off.

Sand is actually not that bad of a sub grade material. It settles easier and quicker than clay. The problem is as you've discovered it tends to squirm around when you compact it.

Eddie also has a good point about concrete slump. I work for a civil engineering firm and have tested a lot of concrete and something near a 4" slump is ideal but you do need to be able to finish it also. As the concrete gets wetter, the strength goes down. It's also common to add air to concrete which makes it more resistant to freeze thaw but if your far south that's not an issue.

disney · Nov 2, 2014

Thanks again for all of the replies. The existing driveway/ concrete is exposed. It is only the height of a 2x4. In my neighborhood not a single driveway is thicker than that anywhere. I have had to repair a section on another neighbor's driveway, and under mine when adding a sidewalk, and I also drilled mine for hurricane protection just off of the garage slab, and it was likely not more than three inches. When the development was built the builder must have treated concrete like gold!

disney · Nov 2, 2014

I will use air hammer/chisel and rough up the edge on the old piece. The motor home has been driven/parked over where the pour will be for several years, so it is likely ok. I had not thought of intentionally digging under existing concrete, but I like the idea, provided I can get it filled with little to no voids.... I was planning on using mist hoses to keep a continual mist on the concrete for at least a week. What is everyone's opinion about driving on it after it has had three weeks to cure? As for the appearance of fibers on the surface, the driveway is kind of at the back/rear of the house, and either the motorhome will be on top of it, or they will be gone (in the rv), so the visual aspect will not be critical...