The Importance of Subgrade Prep

01-12-17

 

When it comes to concrete installation or replacement, one of the most important aspects of the project that is often overlooked is subgrade prep.  What do I mean by subgrade prep?  This is the time and care taken to ensure your subgrade is suitable for concrete to be poured on.

Material type

The type of material used as concrete subgrade will vary depending on the job.  Some projects will require an 8” – 10” base prep before the final grade can be added.  Subbase prep is often comprised of aggregate between 1.5” and 2.5”.  When installing final grade material on a suitable subbase, I generally go with either 1” with fines, or 1.5” with fines, while crushed asphalt is also suitable in many cases; both are generally accepted per City specifications.  When compacted, the fines added to the aggregate will create a tight, uniform surface that is essential before pouring concrete.

 

 

Concrete driveway replacement before and after subgrade was added and compacted.

 

Compaction

Without getting into too much detail about in-depth specifics like compaction percentages and moisture content, just take my word that it is worth your time to rent a tamper to ‘pack’ or ‘tamp’ your subgrade.  This will increase the longevity of your concrete, mainly by preventing soft spots in the grade that will cause unwanted cracking down the road. 

That was a pun. I know, I’m lame. 

Anyways, a jumping jack or plate tamper works best, and the industry standard around these parts seems to be the Wacker Neuson 1550.  It costs around $35 to rent for a half day from most rental companies.  Here’s a tip – spray your grade with a garden hose just before tamping to add moisture and increase compaction.  Here’s another tip - on hot, sunny days during the summer, spray the already compacted grade down again just before pouring the concrete.  This will hold the moisture in the concrete longer and further prevent early cracking.

 

 

 

Uniformity

The grade and concrete both need to be of a uniform thickness.  This reduces stress points and prevents early cracking on your project.  When concrete cracks, water and debris get in.  When water freezes it will expand and cause the cracks to get larger, which allows more debris to get in.  This, paired with vehicle traffic, will cause your driveway to ‘heave’ and forms large voids or humps.  So, when grading, use a concrete rake or yard rake to uniformly smooth out your aggregate.  The same goes for the concrete.  Another good reason for this is that a uniform base and correctly set forms makes measuring and ordering the correct amount of concrete much easier.

These are just a few simple tips and techniques to follow to ensure your concrete project will stand the test of time.  If you have any questions or ideas for other construction topics email me at tommy@cmcsolutionsia.com