Concrete Is Not A Perfect Material

I'm in Las Vegas at the World Of Concrete taking classes for my Master Certificate. The first class was titled "Reinforcement for Slabs on Ground".

The instructer Jery Holland is a structual engeneer who's been in the cement business for over 40 years. The first thing he say's to the class of almost 1,000 cement men and women is "concrete isn't a perfect material". Not what I thought I was going to hear at a convention titled, The World of Concrete. He went on to explain.

• All Concrete Shrinks
• It shrinks more on top than the bottom
• All slabs curl
• All concrete cracks
• That's the nature of concrete

At this point of his presentation which was only 2 minuets I already knew I really liked this guy. His next point was again outsatnding.

• A joint or saw cut in a Garage Slab is a straight crack where we want it to be.
• A hiarline crack in a Garage Slab is a crack where the concrete wants it to be.

At this point in the presentation I thought to myself, what ever it cost me to come to Las Vegas was worth it.

Someone in the class (me)raised their hand and asked the question,

Can Steel Reinforcing Prevent Cracks in a MN Garage Slab?

• No! Steel inside the Garage Slab is typically taking a nap until a crack actually occurs-then it will start taking tension and doing its job.
• However, if it's in the proper amount and location, it can keep the cracks tight and servicable.

What Type of Reinforcing is recomended for a MN Garage Slab?

• Welded wire fabric or what we call wire mesh.
• Reinforcing bars or re-bar
• Polypropylene or steel fibers

How much can I expect my Twin Cities Garage Slab to shrink?

• A typical Minnesota garage slab will shrink about 1/8" every 20'.

How Do WE Mininize Garage Slab Shrinkage?

• Use Quality Aggregate
• Mininize Paste Quality
• Increase aggregate size
• Optimize aggregate gradation
• Minimize cement content
• Maximize Pase Quality
• Reasonable water/cement ratio
• Just enough water for a workable slump
•  Mix design must be low shrinkage
• Keep cement content as low as possible without compromising strength
• Stay away from chloride and Superplasticizers containing admixtures

Are Contraction Joints needed In a MN Garage Slab?

• Yes, the American Concrete Instute (ACI) recomends 36 times slab thikness
• The Portland Cement Association (PCA) recomends 2'-2.5' per inch of garage slab thickness

The rest of the class got pretty technical so I wont bore you with the detail. If you'd like more information about Garages or Garage Slabs contact Western Construction.