Site preparation for concrete slab


  #1  
Old 08-14-05, 10:55 AM
W
WFO
WFO is offline
Member
Thread Starter
Join Date: Feb 2004
Location: usa
Posts: 226
Upvotes: 0
Received 0 Upvotes on 0 Posts
Site preparation for concrete slab

I'm building a small shop/carport; the shop being approx. 14' x 30' and the carport 24' x 30'. It will all be covered by a common tin roof.

We live on soil commonly referred to as "sugar sand" in east Texas. Very sandy, but it will pack. Common procedure in this area is to bring in several yards of red clay to supplement the sand. Yet I read that clay is a poor base for concrete.

Everything I'm reading says to compact the soil as much as possible prior to pouring to prevent having voids under the slab as the soil settles. "They" say voids will be prime spots for cracking.

However, at work we are building a major addition. The construction crews placed 4" tall cardboard boxes under the beams prior to pouring. Additional cardboard boxes were then laid in approx. 12' x 12' squares (4" tall) and covered with plastic before pouring the remainder of the slab.

I realize the cardboard is to prevent heaving, but I know they will eventually deteriorate, leaving a void under the slab (if not the beams). This seems entirely contrary to everything else I've read.

Which is correct? Clay or sand? Compacting or voids?
 
  #2  
Old 08-14-05, 07:33 PM
C
Member
Join Date: Nov 2001
Location: Taylors, SC
Posts: 9,261
Upvotes: 0
Received 0 Upvotes on 0 Posts
Ideally, concrete would be laid over undisturbed earth. Otherwise, any fill should be compacted to 95% Proctor in 4 to 6 inch lifts in order to avoid movement after the pour. Proper reinforcement wire for the slab and rebar for the footers will help. Some surface cracking is inevitable. Stress relief cuts are usually made in a large slab to allow the cracks to happen there rather than elsewhere in the slab.

Cardboard boxes? Interesting application.
 
  #3  
Old 08-15-05, 04:22 AM
W
WFO
WFO is offline
Member
Thread Starter
Join Date: Feb 2004
Location: usa
Posts: 226
Upvotes: 0
Received 0 Upvotes on 0 Posts
I'm not sure I follow your terminollogy. I assume "Proctor" is a standard of some sort for compaction. And "4 to 6 inch lifts" means pack thin layers each time to assure uniformity?

What I'm doing is driving my 30 hp tractor back and forth over it to pack it, followed up by my car. Given that I don't have access to an engineer, is this sufficient, or am I wasting my time?
 
  #4  
Old 08-15-05, 06:06 AM
Join Date: Mar 2005
Location: USA
Posts: 5,650
Received 1 Upvote on 1 Post
Site preparation for concrete slab

A "Proctor" - regular or modified refers to the maximum compaction obtained by testing in a specified manner. It is usually in specifications to denote the level of compaction. It is rarely done except on very large projects.

95% of proctor for for base material just means compact it some. From your application, what you are doing just fine. Soil and base compact better if it is slightly moist.

Dick
 
  #5  
Old 08-15-05, 09:52 AM
W
WFO
WFO is offline
Member
Thread Starter
Join Date: Feb 2004
Location: usa
Posts: 226
Upvotes: 0
Received 0 Upvotes on 0 Posts
Thanks to all.

I spoke to a local contractor about the cardboard. He said its purpose was to intentionally create a void under the slab to give space for heaving of the soil.

....in case anyone was curious......
 
 

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