Attention Foundation Guys!
My father is building a house on old farmland and had a slab foundation put in. A couple people recommended putting in a pad before the foundation but he chose not to as others in the area have not. The foundation is 2 months old and already cracking, one crack goes all the way from the front of the house to the back right down the middle. So since we are about to put in a 2nd foundation for my house I am wondering if I should put a pad in. Its about $3000 to do this. When they dug the hole for the septic tank they never hit rock. I think its about 8 feet deep.
So opionions? Do I need a pad or not? What do you guys think?
what is a pad?
You have not provided a lot of information. What kind of soil do you have? What kind of compaction was done prior to placing the foundation? Does the slab have rebar? If so, how much? Does the slab have contraction joints?
Not sure how to answer some of those questions. As far as type of soil I don't know, we live in Southern N.M. The land is an old alphalpha field that has not been used in years, but there are irragation benches still present, we are building between them. They did no compaction prior to digging. The footings were about 18" deep. There is rebar in all the footings only, footings go aroun dthe perimeter and I think there was 3 or 4 going down the middle of the house. They used fiber cement, 3000 psi. I don't know what contraction points are. Hope this clarify some.Edited 9/23/2005 5:42 pm ET by Johnny1985489
Edited 9/23/2005 5:43 pm ET by Johnny1985489
Johnny, you still need to tell us what you mean by a 'pad'.
Do you mean excavating the building site and filling with more stable material?
The heck, you say?
I'm not 100% sure what a pad is either. I was hoping you all could tell me. I had never heard the term til I moved out here to N.M. I don't do construction for a living we are just trying to build a couple houses for our family. I've had many contractors tell me I need to have a pad put in before the foundation but when I ask them why they don't really know. My understanding is they bring in some type of rock and then pack it down. Then put the foundation on top of that. Its what they do at all the oil rigs out here and part of me thinks some of these folks have misinformation. Most people out here don't put a pad under the house but I thought I would check here. From the way you all are talking it sounds like something I probably don't need.Let me ask you this, is some cracking in the foundation acceptable under normal conditions?
Catskinner is giving you good info. Your local contractors should know what works in your area, as long as there is nothing unusual enough about your site that would warrant an engineered foundation.
is some cracking in the foundation acceptable under normal conditions?
The answer is, it depends: Shrinkage cracking and movement cracking are completely different conditions.
Consult your local building department for more info.
HeckThe heck, you say?
I'll try to post up some pictures of the cracks later and you guys can tell me what you think. Thanks for clearing up what a pad is. I may consider it.
By a building pad what they mean is site prep. We're talking about preparing a subgrade to support your foundation and slab.
Before you put a slab down it is important to make sure that the soil is stable and capable of bearing the load.
As long as there are no unusual site conditions, here's how it goes;
1) all the organics get scraped off, roots come out to at least a foot below the slab. On agricutural soil you might have a lot of organics.
2) If native soil is suitable, it gets processed to optimal moisture content and compacted in 8" lifts to 95% of theoretical max. up to the level of the bottom of your slab.
If native soil is not strong enough, you can import fill.
3)If you are going with stemwalls on footings, you can size the footings according to native soil conditions.
If it's a monopour, you need to be really careful about site prep. If it's a monopour, tell us and I'll write more.
But that's all a pad is -- site prep.
On some soils it is not very involved, on some sites it gets pretty involved. I'm doing one right now that involved overex and replace native soils up to 4' below the footings.
How wide are the cracks in the slab?
Here is a basic concept for concrete slabs: You need either uniform support, or need to make provisions to span weak areas in the substrate. So, when you said you had a cracked foundation with footings around the perimeter and some interior footings, what you had is weak areas in the substrate between the footings and hard spots where the footings are. Regular concrete has trouble spanning these weak areas with out cracking. The way to strengthen the slab is to add steel, etc. It's either that, or as discussed above build a pad, which basically means preparing the area by removing the bad stuff, adding gravel or compacted earth, etc.
BTW the "fiber mesh" as we call it, I believe is more for preventing small surface cracking.
Above, when he said mono-pour, (which means monolithic pour) what he means is to pour the footings and slab all at the same time. The alternative being to pour the footings one day, wait several days, and then pour the slab.
My advice would be to use WWF (welded wire fabric) (it looks like fencing with a maybe 6'x6' holes) and rebar in at least a 4'x4' grid, if not a 2'x2', probably #5s (5/8" in diameter bars) and 3500 PSI fiber mesh concrete. Chairs are normally used for holdig up the wire and steel off the bottom of the slab. After the slab has set for 2 days, have the concrete guy come back and cut control joints in the concrete, which basically means 1" deep (min) saw cuts on maybe a 10' grid. When cutting the control joints you have to be careful not to expose any of the steel - which goes back to placing the steel at the right height. Making the slab an inch or so thicker helps with this. What the control joints do is simply make it so if the slab does crack, it cracks in a controlled fashion, rather than all hap-hazardly.
There is an old construction saying - which I have learned the hard way, customers don't want to hear :-) "there is concrete that is cracked, and there is concrete that is gonna crack..."
You also need polyethylene plastic sheeting between the substrate and the slab, and possibly rigid foam board insulation too depending on your climate.
On the other hand, any specific advice that is given here, like I just did, is really null and void since we don't know your site conditions. BTW - foundations are rarely dug down to bed rock.
Ok well I think we are going to pay someone to come in and pack the dirt down prior to pouring cement. Should that be a big help?Heres a picture of my fathers crack and what the dirt looks like where we live, maybe these will be helpful.http://us.f3.yahoofs.com/users/42b1c8e2zba15431f/1310/__sr_/ed50.jpg?ph5MHODBbcfKj84ihttp://us.f3.yahoofs.com/users/42b1c8e2zba15431f/1310/__sr_/5861.jpg?ph5MHODB3GYz.RcX
that crack pretty normal with slabs. look like it wasnt cure properly
I can't open the pics - never seen that .rcx file type - although I also have been having some security related browser issues lately...
hmm, try this link.http://pg.photos.yahoo.com/ph/mrdrunkpenguin/my_photosthen click on richards house
I looked at the pics...
It's a little hard to tell - plus remember that I live clear across the country and only know dirt in my state... I'd start by poking it with a probe rod... Really, I can't help you from this point. If we have questionable soils, we get a soils engineer. Costs about $300 for him to come out, do some test borings, and then give us a letter with his professional opinion.
As far as the concrete pic, it's a little hard to tell since there is nothing in the pic to give it scale. Is that pic of ~1 sq ft of concrete? or is does the pic cover an area several feet wide/long. I will say that the concrete looks gaud awful rough in the foreground for a slab... As long as the crack is not more than about 1/16 - 1/8" wide and is not displaced - one edge higher than the other, I wouldn't get too excited - but you are right - you would definately want to try and prevent that from happening on your house...
the crack pic is about a sqaure foot. they really did a pretty bad job on it actually. they put in the wall anchors at random, where doors needed to be etc. thats why i'm asking as many questions as i can here, we plan to use a different foundation guy for my house as a result of the other house and I just want to know what to look out for and how to best prepare for a good foundation.so far you've all been very helpful! thank you!