Hairline cracks in porch slab

[Inspectorjoe]

I could use some opinions about some cracks in a concrete porch slab. The house is four years old. It's currently the model home for an upscale development. The development isn't built out yet, but I suspect the builder is down to eating his seed corn. Overall, the quality and attention to detail was pretty good.

There is a long crack that runs parallel to the front edge of the slab, and several that run perpendicular. At these, you can see that the cracks run close to the full depth of the slab.

The slab has a roof over it, but it's still pretty much exposed.

Does it need to be fixed? Is it possible to fix it without looking like crap (the buyer is buying a 'new' house)?

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[kurt]

How would you "fix" them? It's a cracked lab.

My first guess would be too much water in the mix because that seems pretty much standard nowadays.

Are there frost grade footings under the porch roof support columns, or is there a stem wall footing under the porch? If there's footings, it's just ugly.

[Inspectorjoe]

How would you "fix" them? It's a cracked lab.

I should have said "fix or replace". I'm not sure if it's likely to get worse over time. As for a fix, I was thinking something that would keep water out and make it look like it wasn't cracked. I've seen acrylic, urethane and epoxy coatings used and they look pretty decent and seem durable.

Not that I would specify a repair, but I'm just not sure what to tell the buyer: It should be replaced; It can be sealed, but you still might want to have it replaced; It's not a problem, but if it bothers you, you might want to have it repaired or replaced.

Are there frost grade footings under the porch roof support columns, or is there a stem wall footing under the porch? If there's footings, it's just ugly.

I don't really know, but the odds are that it's just a stem wall.

[Chad Fabry]

I'm not sure if it's likely to get worse over time.

It will.

As Kurt said, wet mix: a 12 slump. Soup. Wet= no aggregate lock, no cementious bond, so the stuff gets hard but doesn't stick together.

[Jeremy]

In the first picture it looks like the brick is under the edge of the slab. If this is the case, then frost in the soil will heave and cause the slab to crack as seen. When a slab is overhanging the stem wall it is improtant that soil is not placed up tight to the bottom of the slab.

Cracks exposed to freeze thaw cycle with water present will get worse.

[Bain]

I'm not sure if it's likely to get worse over time.

It will.

As Kurt said, wet mix: a 12 slump. Soup. Wet= no aggregate lock, no cementious bond, so the stuff gets hard but doesn't stick together.

Who do you think's responsible? The concrete plant or the truck driver?

I used to manage a company that had an asphalt plant in its complex. The state would randomly send inspectors to the plant to sample the asphalt mixes to make certain the ingredient proportions were correct for highway projects. What they didn't take into consideration was that the idiot truck drivers would spray several gallons of diesel fuel onto their truck beds to make certain the asphalt slid out smoothly when dumped and so cleaning the bed at the end of a day was much easier.

I wonder if it's the same with concrete-truck drivers? Do you think they add water to prevent their mixers from getting gunked up? The machinery that controlled the mix at the asphalt plant was computer controlled and exceedingly precise. I imagine the set-up at a concrete plant is pretty similar.

[inspector57]

Who do you think's responsible? The concrete plant or the truck driver?

The person placing the concrete has the ultimate responsibility.

[kurt]

That's probably right.

Could be anything. I've seen moron truck drivers on residential jobs that stop @ McDonalds for a burger. They soup up the mix so they can eat.

Commercial stuff doesn't fly that way. There's an engineer at the site that slumps each load before it's tossed in the pumper.

Residential stuff is a can o' worms. I don't think I've ever seen a residential concrete job that wasn't a can of soup.

Fabry's description is accurate; it gets hard, but it doesn't bond. Cracks happen real easy with soup. Can't be fixed, really. Just gooped up with some other additive or coating.

If you look at enough concrete placements, you can almost tell the soup stuff by looking at the finish; it's kinda white, and chalky, just like those photos.

I have one of those "learning jobs" going on right now. I did a job 4 years ago that I pegged as bad due to all sorts of incorrect flashing details. The builder made it "work" by soaking it in sealer. Well, this last winter took out the last of the sealer, and the Spring rains have this sucker running water like a hose. I don't think I'm getting sued, because they're asking me to provide additional analysis, which I'm glad to do; if you're on the team, they aren't pissed. There's real subtle touches to the finish of good mortar and cement; I can see those now, largely due to "learning" the conditions I'm seeing in the problem house.

I've taken to carrying a water bottle and Rilem tubes with me on jobs. It's way beyond SOP, but I Rilem and soak brick or concrete I'm suspicious of. It's amazing how the funky stuff just soaks up water due to poor mix. Lousy concrete just soaks up water.

[Inspectorjoe]

Good catch Jeremy. The slab overhung the stem wall by several inches. I don't know how far the brick extended under the slab because the mulch was heaped up at the sides.

Thanks everybody.

[hausdok]

Hi,

You could try injected epoxy. Once it sets up and the surface is ground flat and painted with some concrete paint you probably wouldn't see it. That's no guaranty it won't crack somewhere else at the next extreme weather though.

Check out the emecole site at: http://www.emecole.com/crawl-space/index.html

ONE TEAM - ONE FIGHT!!!

Mike

[randynavarro]

I've not heard mention of any control joints.

Seems like that might have helped.

[Tom Raymond]

Who do you think's responsible? The concrete plant or the truck driver?

The contractor. Batches get refused all the time, but it's expensive to pay guys to stand around and wait for a fresh batch. He made a bad call.

I wonder if it's the same with concrete-truck drivers? Do you think they add water to prevent their mixers from getting gunked up? The machinery that controlled the mix at the asphalt plant was computer controlled and exceedingly precise. I imagine the set-up at a concrete plant is pretty similar.

Every batch plant I've ever seen is computer controlled but even then not all the components are that presice. Color, fiber, and less common addmixtures are very often hand loaded in bags that are designed to break down in the drum. The drivers carry water for lots of reasons, not the least of which is that it is far less expensive to scrap a load than a drum. They also batch loads "dry" if they are going far from the plant and "finish" them on route so they don't come off the truck too hot.

Tom

My wife has a PT gig batching...the softer side of concrete[]

[kurt]

I agree. I don't remember ever meeting a residential contractor that understood concrete at all. It's just another step they don't pay as much attention to as they should.

I've found this paper to be illuminating.

http://www.vernonhills.org/UserFiles/Fi ... ncrete.pdf

Lots of possibilities, but I'm betting high slump concrete. Sure, it could be settlement, or heaving due to the overhanging slab, but that stuff probably wouldn't happen if the mud was right in the first place. It's only a front porch. Thre's barely a load, and it's hard to imagine much heave in recently placed fill under a porch slab. It's probably mostly mulch.