concrete crack..


  #1  
Old 10-18-05, 05:18 PM
big hack
Visiting Guest
Posts: n/a
concrete crack..

This is my first post so please bear with me. I am having a new home built and it is a patio home built on a concrete pad. There is a hairline crack starting at the outside of the garage and traveling the entire length of the pad. Where the bathroom drains are located, the crack gets to be about 1/16th. of an inch wide. I can insert a nickel in the crack. It continues this wide until it exits the pad, by the back door. It is probably 30 feet from the garage to the bathroom and another 30 feet to where it exits. Should this be considered normal. The contractor says it is, however, I have looked at other patio homes that are being built in this sub-division and all have cracks but not this large. Sorry for the long post
 
  #2  
Old 10-20-05, 02:31 PM
rdhamm's Avatar
Member
Join Date: Jun 2005
Location: USA
Posts: 160
Upvotes: 0
Received 0 Upvotes on 0 Posts
Unfortunately

All concrete cracks. Shouldn't happen soon after a pour though.
 
  #3  
Old 10-20-05, 02:56 PM
Join Date: Mar 2005
Location: USA
Posts: 5,650
Received 1 Upvote on 1 Post
concrete crack..

It sounds like you have a shrinkage crack. The fact that it is running the length of the slab is surprising. Usually, the first cracks in a slab are at the middle of the long side of the slab. Is the slab tensioned in one direction?

Most shrinkage of concrete occurs in the first 30 days. The mix and conditions will have an effect. If the slab was poured wet, which is common in residential construction, the shrinkage would be more than normal. You have warm days AND warm nights, so the accelerated curing will cause cracks to be larger than normal and show up quicker. The amount and spacing of reinforcement also has an effect. As you see there are many factors.

Is the concrete on one side of the crack even or level with the concrete on the other side?

Could be nothing major.

Dick
 
  #4  
Old 10-20-05, 04:15 PM
Doug Aleshire's Avatar
Member
Join Date: Feb 2002
Location: United States
Posts: 4,455
Upvotes: 0
Received 0 Upvotes on 0 Posts
big hack,

I agree with Concretemasonry.

Here is something that all should understand about basement slabs and what needs to be done for a good job.

1. When the basement is ready for a pour – it should be FLAT – I mean a tamper should be used to make it as flat on the bottom as it will be on top. Reason – just raking out soils to make it look good, removing high spots means nothing to concrete other than a poor subgrade means it will sink – sinking means voids and voids mean pockets for things to happen – Cracks! Many do not add sand nor do a good job at compacting, if at all. They rely solely on existing soils. This is wrong, IMHO.

2. Proper placement of reinforcement is critical. At the very least, reinforcing mesh, pipes, and cables should be suspended so that none lie on the bottom of the poured slab. Wire mesh should be in the middle of the slab if it is to do its job properly. Rebar placed proportionately is another aspect of a solid slab. Pipe or conduit laid directly on the ground will dramatically weaken the slab; it has the same effect as scoring a ceramic tile before snapping it. However, that same conduit suspended with 1 inch of concrete underneath will have no adverse effect.

3. Pouring the slab is another area that is of concern. We all call in an order, it comes out and we start pouring. Wrong! A slump test should be done. A slab mix should not have a slump greater than 3 to 4 inches. If smoother flow is required, add super plasticizer, which gives the effect of a 6- to 8-inch slump without weakening the mix. Measure the exact volume of concrete required, and then add 10% as a buffer. If you underestimate and have to call for a short load, not only do you waste time, but you may get “cracks”. Make sure the concrete supplier will have the trucks needed to supply your project without having to wait for an hour between loads. Excessive delay can result in a hot load. If the mix comes out of the chute hot, then send it back. Make sure concrete is screeded and bull-floated as it's placed, but don't allow finishing to begin until the bleed water has disappeared. If bleed water is worked into the concrete while finishing, the surface will be weakened. For the same reason, don't allow finishers to add water in an attempt to ease finishing. Weather conditions also effect the curing which can be hard on concrete. Again these are just issues I have learned. Others may have their own opinion.

4. Laying out contraction Joints - The shape of the slab can greatly affect cracking. An inside corner, for example, is a pressure point and, as far as the concrete is concerned, a great place to crack. No matter how you make them, all contraction joints work the same way. By creating an intentionally weakened point in the slab, stress from the inevitable shrinkage (as a result of drying or temperature changes) is relieved in a predictable, controlled manner. These should be done in squares, not rectangles. I have seen very few do this inside homes. Reason – time consuming but it’s worth it. It doesn’t affect any future floor finishes in basement either.

5. Installing expansion joints. Some call this isolation joints around the inside perimeter of the foundation to allow the slab to move independently of the wall. The same thing holds true for structural columns — if they are inside the area of the slab, they should be surrounded with expansion joints. Residential expansion joints are often made with 1/2-inch-thick fiberboard strips

Hope this helps!
 
  #5  
Old 10-20-05, 09:19 PM
big hack
Visiting Guest
Posts: n/a
Good info

Thank for the info..appreciate it. I talked to a local concrete "specialist' and he told me that it would be no problem. The slab was poured several months ago and the concrete is even, level, on both sides of the crack. In this area, the site is inspected before the concrete is poured and they are very strict. Lots of re-bar and enforcement. Talked to the building contractor today and he advised me of the same thing...so I should be OK. Again, thank you.
 
  #6  
Old 10-21-05, 04:13 AM
Doug Aleshire's Avatar
Member
Join Date: Feb 2002
Location: United States
Posts: 4,455
Upvotes: 0
Received 0 Upvotes on 0 Posts
big hack,

You're very welcome.

Glad you got some reassurances.

Have a great day!
 
 

Thread Tools
Search this Thread
 
Ask a Question
Question Title:
Description: