Expert: Dan Griffin - 10/4/2008

Question
QUESTION: We moved into a newly constructed home 9 months ago.  Before occupancy we notice a growing crack in the suspended concrete slab on the surface of the veranda straight through to our ceiling of our cold room.  To date we now have four similar cracks that each allow water penetration.  The cracks go from the outside of the house "straight" to edge of veranda slab.  1) How do we best repair the cracks i)expanding rubber or ii) expoxy?  I have heard that expoxy is just a temporary fix and will eventually get brittle and need repairing again.  Three cracks have been repaired with the expanding rubber but now leak again.  Also 2) How do we check to see if the builder even used rebar in the construction of the slab?  We have never experienced rust or rusty water through any of the the 4 cracks nor have our neighbours with the same problem/same builder.  (The builder used particle board to form the slab.)

ANSWER: Mary,  I am having a hard time following the issues.  I'm not exactly sure what you mean by a cold room.

Number 1.  Concrete is not water proof, it can be made more water resistant, but never water proof.  As such, it would never be used as a ceiling over an occupied space exposed to the outside.

Nuber 2.  Concrete should never exceed 12 feet in any one direction without some form of movement joint to prevent cracking.  Concrete does not perform well as a rectangle, it does do well as a square.  A movement joint would need to be waterproofed, it exists to allow the concrete to crack in nice straight line.

Number 3.  I have the feeling that the particle board forms are still on your ceiling and were not stripped.  If it really is paricle board and it has been getting moist or wet, it will be coming down at some point.

Number 4.  I can't think of an easy way to determined size, depth, or existence of rebar.  You never tell what size the slab is or its thickness.  To be structurally sound to walk on, it would be at least  6 - 7 inches thick.  It would take quite a while to see rust unless you have lots of water and or salt during the winter.

The concrete should have had a roof type system installed either above or below the concrete.  YOur choice now would be limited to applying a roofing membrane to the top of the concrete.  YOu can talk to a waterproofing contractor in you area.  Pressuered epoxy injection of concrete cracks can be structurally sound.  This is not something that you can do as a home owner.  The crack(s) can be routed larger wtih backer rod and a urethane sealant.  This method is quite sound to seal the crack, but will need to be reperformed as movement and UV rays exceed its design perameters.


---------- FOLLOW-UP ----------

QUESTION: Thanks for your response.  The slab is 5" thick.  It is L-shaped.  Measurements are 35' 7" long by 7' wide at the front of house and then at the side it is 9'2" by 5'2".  When you put those two rectangles together you form the L-shape of the veranda.  Below the entire suspended slab is a cold "cellar".  The cold cellar is 7'7" tall and is the same L-shape as the slab.  The walls of the cold cellar (with 4 vents to outside) are poured concrete.  The slab sits on top of the poured concrete footings.  The slab was formed by placing particle board over the footings and then pouring cement onto the board inside a frame.  I had most of the particle board removed but it is still between the slab and the top of the footings.  There are no movement joints.  Above the veranda is a roof overhang covering the entire perimeter.
1) I really don't trust the builder at this point and would love to know how to check if rebar was used at all/ or if they did use it - how much did they use?  Do you recommend x-rays, scanning or just a metal detector?    
2)  Once the cracks are repaired, what kind of material do you suggest would best waterproof the entire surface?  Do you know of any product that I can apply myself?
Thanks for your kind assistance,
Mary

Answer
Hilti and Zircon make tools meant to find rebar in concrete.  I have not used them, they are meant to try to avoid rebar when installing some types of equipment and have been recommended.  Here is information on the Zircon:
http://www.professionalequipment.com/zircon-electronic-metal-rebar-locator-mt-6/

The Journal of Light Construction just had a cover article on a deck membrane that might be worth considering.
http://www.jlconline.com/cgi-bin/jlconline.storefront/48e817081070b99527170a3210

There is a product called Xypex that excels at waterproofing concrete.  I would think you would still need to create movement joints in the concrte, but it may have finished finding the joints it needs, though you've not been there a year.  Freeze/thaw and high summer heat will decide what joints the concrete needs.  Concentrated heavy loads will find and create their own set of cracks.  Xypes is expensive, but good:
http://www.xypex.com/products/products.php
I have used it for treatment of vertical surfaces, not your horizontal problem, but it may well be worth consideration.