Habitat homeowner approached me last night with complaint that his driveway is crumbling. Checked it out and he’s right: the surface is turning to dust, exposing aggregate, which with little friction, loosens and falls away as well. His drive was poured last fall with temps. averaging around 50. I want to go to bat for this homeowner and ask the Habitat chapter to bust up and replace driveway. My fear is they, in an effort to say $, will suggest a quick-fix that will eventually fail.
Any opinions as to how concrete failed, and good remedies to repair it would be greatly appreciated as I need facts when I suggest a fix to Habitat.
Thanks in advance, Carl
Replies
It sounds to me one of three things -- either:-
1. The mix was too wet
2. The mix was too weak or
3. Water was added to the surface during trowelling up to improve plasticity.
If it was 3, then only the top surface should be affected -- if 1 or 2, the problem should be evident through the entire thickness of the slab.
I don't know how much trouble you want to go to on this, but having ruled out 3, then you can hire a testing company to take a core sample.
IanDG
IanDG, in this case, the top surface is failing and it appears that by just scuffing your shoe, it breaks up even further. Is there a lesser remedy e.g. applying a certain mix on top of damaged surface? It seems it would not bond properly. I've seen concrete fixes whereby cement w/o aggregate was used to resurface, but it didn't last.
Thanks
The remedy is to scarify the surface to remove all the loose and then re-surface.A cementitious re-surface is tricky -- it needs to be bonded securely with the existing slab or it will crack -- it shouldn't be too thin or it will de-laminate and crack -- it shouldn't be too strong a mix or it will curl and crack.
If it were mine, I'd scarify it, prime with bitumen and re-surface with blacktop unless the whole thickness of the slab is affected, in which case I'd re-do it.
It's a perfect example of how you only get one shot at a concrete slab!
IanDG
IanDG's three (wet, weak, and water added to the surface) are possibilities as are stonefever's thought about the mix being old.
But I discard weak as a likely scenerio because what yard is going to send out 2-sack mix?
Too wet can definitely happen. Either at the plant or because some moron on site doesn't understand concrete and wants some mystical self-leveling compound.
Water added to the surface would likely be splotchy in occurance - hard to screw it up uniformly. Also requires a high level of on-site concrete stupidity.
Too old is a definite possibility. One traffic jam and you're pooched. And the driver would be disinclined to tell you, "Hey, you should refuse this load and I'll just drive back through rush-hour traffic and use a jack hammer for 5 hours to get it out of the truck". Can you check if the requested time and actual time of delivery are far apart? Better yet, when did the truck leave the plant versus when was the pour finished?David Thomas Overlooking Cook Inlet in Kenai, Alaska
"Too old" concrete is concrete in which the cement has hydrated so much that the mix has started to set.
It is impossible to work in this state so water is added to make it plastic and the remaining unhydrated cement is insufficient to form a strong bond -- hence a weakened mix.
IanDG
"Too old" concrete is concrete in which the cement has hydrated so much that the mix has started to set.It is impossible to work in this state so water is added to make it plastic and the remaining unhydrated cement is insufficient to form a strong bond -- hence a weakened mix
Ian, whilst I agree with your analysis, I must disagree with your "It is impossible to work in this state"
Whilst the most common response to the initial set problem in concrete is to add, the proper response is to vigourously remix the concrete. It's amazing how the workability returns when the mix is reagitated. I've seen good work where the concrete was kept alive for over 8 hours. Wouldn't want to make a habit of it though.
Ian
ian, I gotta agree with IanDG here.
Concrete starts hydrating (curing) the moment it contacts moisture. The longer it is mixed, breaking the crystals that are forming, the weaker it's cured strength will be.
Mixing for 8 hours (with no set preventing admix) will lose about 50% (SWAG) of potential strength.
SamT
Sam is correct. The hydration of cement is a process where a lattice of crystals is formed that make a mechanical bond. It starts as soon as water is added and continues until all the cement is hydrated but it is a finite process -- each cement granule produces a set number of crystals. If you continuously agitate, these bonds are broken and won't be replaced by others so the longer you go on the weaker the resulting concrete will be.
If you mix for a long enough period you'll end up with something that will never set.A further consideration is that the longer the hydration process continues, the less free water is available to maintain the plasticity of the mix -- concrete traeted in the way you suggest would only be plastic enough to be placed correctly if the mix had been far too wet in the first place.
IanDG
Edited 4/18/2004 8:39 am ET by IanDG
I'm not disagreeing with you (and appoligise if my earlier post appeared to). However, my recollection from when I studied this some years ago is that whilst the cement begins to hydrate as soon as you add water, the cement gel remains plastic for at least 4 hours, and that workability during this time is best achieved by further agitation rather than the addition of water. As to the example I gave, I saw the result of the pour many years after it was placed and got the storey from one of those on site at the time. The story went that the on-site batch plant broke down partway though the pour and the gang had to keep the early part of the pour alive (using immersion vibratotors) for 6 hours whilst the plant was repaired. Given the when the work was done, the concrete was probably spec'd to be about 20MPa with a 80-90 mm slump, what I would think of as a pretty wet mix. (I'm more used to hand placed paving concrete, 35MPa, 45mm slump)
Ian
You forgot to mention drying too fast as one possible cause for spalling.
Yes, that's quite right, although it's more likely to give a dusty surface than a spalling one.
Concrete doesn't need much water to hydrate the cement -- most of what's added is to make it workable so I prefer to use a plasticiser as a matter of course in flatwork.
IanDG
i think we should start a new thread and keep the scarification definition debate going! surely we can get a few more to weigh in on this issue.
i can't help but wonder if there isn't some way to use the existing drive as a base for a paver or brick drive to minimize cost of repair? just the first thing i thought of, i was hoping to give direction to a possible cure.
Pavers would be a good solution but they'd also be affected if the slab developed major cracking -- which it will if the concrete is simply too weak.
IanDG
That's exactly the best way to describe the crete in question-------------dust. The top emultion is crumbling into a pretty fine dust.
yo
Take a screwdriver (the one you use for prying and scraping anyway) and see how big a hole you can dig in the concrete. If you can easily pry up chunks and dig a half-inch deep or deeper, then the concrete is rotten. If not then it has some potential to be OK, with a surface treatment.
FWIW, the driveway across the street (nearly 30 years old) has always had a bad spalling problem (about 30 feet of the 40 foot length is badly spalled). About 20 years ago, the guy that owned it at the time wondered if anything could be done to fix it, so I went over with a left-over half of bag of some patching cement (the kind of stuff that came with a gallon of latex-like stuff to mix in). I troweled over about a 2-foot-square area with the stuff, to show what could be done. He decided he didn't want to spend the money, but that area is still good now, 20 years later.
However, there's another driveway about 3 houses up (and maybe 5 years older) that has totally disintegrated into gravel. All sorts of different outcomes are possible.
Great suggestion. If, at the tribunal, I am unable to convince my colleagues by showing them above posts, a demonstration may be in order.
tftp, Carl
depending on your elevations in the area a possible bituminus cap could be placed at a cheaper rate than doing any kind of removal or fixing of the concrete. Sounds like the concrete should be torn out and redone for a quality job.
You'll still have to scarify the concrete as the bituminous cap won't adhere to a loose surface.
IanDG
IanDG, thanks for the feedback - this helps as I have no experience in flat work and will print your reply and take hard copy to the meeting I've already called. Just for the sake of clarification, what exactly is "scarify?"
Thanks, Carl
It means to take off all the loose or weak concrete until you get to a sound surface.
The methods you can use vary from a steel wire brush on a buffing machine to a multi-headed, air-driven bush hammer.
If the contractor argues, suggest that a testing company be called in; their fee to be paid by the contractor if the concrete proves to be faulty in any way.
IanDG
Ian I respectfully disagree on your definition of scarify. In other areas of construction, you would scarify old glossy paint with sandpaper to give it tooth for the new to adhere. Or scarify a formica counter so the new finish (more formica or maybe thinset for tile) could adhere properly. What yoiu are suggesting here is to remove all the loose flaky unstable concrete so there is a good base for the new topping. Similar to removing all the rotten wood back to sound wood before pathcing. What you are suggesting is correct, but I think there's a more appropriate word for the process.
Whenever you are asked if you can do a job, tell'em "Certainly, I can!" Then get busy and find out how to do it. T. Roosevelt
Like scale, or de-scale?
Don't forget that Ian is from that island where they use the language differently.
Actually, Scarify haaas more than one meaning and to loosen up the top layer is one. legitimate
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"definition of scarify"
Ian's def sounds less vigorous than what civils mean by scarify. They mean 3-foot deep rippers on a D-8. So the depth of "scarification" may only be a function of your media, not of the term "scarify".David Thomas Overlooking Cook Inlet in Kenai, Alaska
Piffin is correct -- it's a term commonly used here, as in ..... the surface of the construction joint shall be thoroughly scarified to remove latents and expose the aggregate.
I have a feeling that Dave Crosby's suggestion will be the only feasible one -- the problem seems as though it's been caused by a mix that's started to set and been re-worked. If the aggregate itself is loose in the surface of the slab it sounds more than just surface failure.
IanDG
It sounds like it is common practice to add water to the mix at the plant.
The jobs I have been involved with, the order was that water not be added more than 3 minutes from the job site. Sometimes not until the site foreman tells the driver to add. In critical applications how long the mix is agitated is important.
The concrete starts hydrating as soon as water touches it and mixing does not prevent hydration.
SamT
The reason I'm thinking it would make more sense to rip it out and do it again is that I can get most residential driveways out in less time than it takes to drive to the job and take the machine off of the trailer.
The resulting chunks of concrete waste makes a great stacked wall if theres no wire, if there's wire we use it for erosion control.
Being a Habitat house, I suppose the labor to form the new driveway is free, with the concrete at cost and paying a professional flatwork crew, I'd think the cost would still be under a couple bucks a square foot.
Kinda hard to resurface anything for that price I would think.
DRC
Have seen spaling due to Rock salt,(ice melt) placed on the concrete during the winter. That stuff will eat up concrete in a big way. Cheap fix I have seen is to blacktop over the concrete.
You're right - the labor is free and the mix is discounted. So my initial inclination to rip and replace may not be an unreasonable request.
thanks
How much did this habitat guy pay for his driveway?
He probably paid the same thing any of us do who invest our sweat equity in owning our homes -
blood
sweat
tears
and the firm handshakes of a few good friends
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Seems like this guys friends wern't concrete finishers.
Actually, at our Habitat homes, volunteers do all the drive and sidewalk pours. We usually do a pretty good job as the same crew is always involved at this point. Our flat work doesn't look as good as a pro job but it's acceptable; and in the 10 years I've been with habitat, I've never seen one of our pours fail like this. The information you've all provided is a very good starting point from which to approach this problem. Given what I've garnered here, I can now go to my meeting Monday, armed with (concrete) information. I really appreciate the solid answers, guys. At my initial post, I was convinced a bust-out and replace was the answer. But now I can present an argument for repair with an informed opinion.
Thank you all for responding,
Carl
I suspect you are on to the real issue.
I bet that mix was left over from the morning pour somewhere else and kept wet throughout the day. And I'll bet if confronted, they'll say they donated it without warranty...
You get what you pay for.
If it's only surface spalling (which can occur for a number of reasons) then there are resurfacing treatments that can be used. Mostly these are done by contractors that specialize in concrete resurfacing.
It would probably be best to get a core drilled from the driveway first, and tested, to see what the strength of the concrete is farther down.
How big is the driveway? If it isn't real big, wouldn't it be cheaper and easier to pull it up and try again?
DRC