What Did I Do Wrong Here?

slpj

Hi Everyone:

I could use some help here trying to figure out what I might have done wrong on the new porch of this older block fixer upper that will be rented.

***The cracks are along the top course of block and did not show up until a couple of weeks after we poured the top slab.***

Here is what was done:

(1) Poured footer to spec. and code.

(2) 7 course 8" Block wall using Brick Lok Type S. I did not have a mixer and used 8 shovels of sand to 1/2 of 75lb bag of mortar. The water was measured and added on each mix. *Note: I did not fill the block wall with concrete*.

(3) Parged the wall a week later and filled the pit with #57 gravel.

(4) I used left over roofing shingles and pushed them down into the top course of block so that the top slab would go down into the top course about 4" to tie everything together.

(5) Formed and poured the top concrete slab.

(6) Removed the forms a week later with no cracks present.

Did the top slab pull on the block while it was curing? I realize that it would have been stronger to pour the block wall with concrete, but I thought that it would be fine with 7 courses and at least 2 in the ground. The temperature was not freezing when the job was done.

Here is a picture: What do you all think?

Attached Thumbnails

 

lukachuki

Disclaimer!
Now i'm no expert. Their are many here who are so trust their diagnosis above mine! I would say that the cracking has to do with the micro expansion and contraction of the slab due to temperature fluctuation. Since the slab is tied into the top course of block that is where you see the stress cracks due to this movement.

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concretemasonry

The cause is probably due to the shrinkage from the concrete curing and the temperature shrinkage of the slab due to the temperature change.

You tied the slab to the top course of block, but there was really little tying the top course to the course below.

You should have "L" shaped rebars every 4' from the or so going down from the slab into a couple of courses of block. A bond beam on the top course is always good construction.

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kc4mqm

Yep, and anytime something is weight bearing, fill the cells. A couple of extra bucks saves a load of time and grief.

TurnkeyConst

When we do slab porches instead of backfilling and pouring on top. We do our porches with pipe and decking. you could have run a course or 6" block against the foundation, then layed the outer block with a row of 6" on the interior to hold the pipe from front to back, cut your decking to fit. Form your reveal up, then pour. but temperature definately will affect a porch though!! dont know if these type porches pass where you are anyways if not just ignore it.

slpj

Thanks for the replies everyone...I appreciate it!

Bud

savant

What did you back fill under the slab with? I assume you are saying that you backfilled the footing trench w/stone.
Are the top blocks still vertically aligned with the lower courses?
Did you cover the green concrete with a membrane, to control water loss?
How soon after the pour date did you first have temperatures dipping below freezing? was the ground frozen before you poured?

Did you compact every 6" at least while backfilling?
Is the center of the slab sagging?

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