Sub-Base Movement

Why does concrete move?  Isn’t it supposed to be one of the strongest building materials?

Concrete, just like any other building material is only as good as the ground it’s places on. 

Concrete is strong, but the pads poured for your driveway, garage, patio, basement, sidewalk, etc. are not engineered to span large voids or ever move.  No matter the engineered strength of the concrete, it still has the potential to move depending on what the base below the concrete does.

The ground under your concrete can move which will cause your concrete to move.  Causes of that movement can be from:

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  • Frost heaving

  • Settlement

  • Erosion

  • Heavy Loads

  • Root growth

Frost heaving happens when the ground/soil under the concrete pad begins to expand lifting the concrete.  This can cause issues where one pad is beside another.  If the two pads are free flowing or have a break between them (buffalo board) one pad can become higher than the other and cause a tripping hazard.  If the two pads are connected the movement or friction can cause one or both of the pads to chip.  This can also cause the concrete to hump and crack.

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Settlement happens when the ground/soil beneath the concrete pad starts to compact over time.  The majority of settlement tends to happen in the first year after a house has been built but can continue to happen down in years to follow.  As the ground beneath the pad compacts it can leave void between it and the concrete.  These voids do not cause issues if they are not large, however, if the voids become too wide under the concrete even a void that is only a few inches deep can cause issue.  If there is nothing holding the concrete up it will fall.  This can cause cracking, displacement (one piece of the concrete higher than the other) and dips.  If cracking become very large water can enter below the pad which can increase the settlement.  Displacement can lead to tripping hazards and chipping of your concrete.  Low spots or dips can cause water to pool on your concrete.  Sometimes this can occur along the house or garage and sometime it can happen in the middle of the pad.  Excess water sitting on concrete can cause premature deterioration and other issues especially when those areas freeze. 

Erosion is another cause of settlement.  Water flowing under or by concrete pads can wash the dirt or soil away from under your pad, leaving a void.  Please ensure your drainage or grading is such that water does not flow near or around your concrete pad.  Issues from erosion are similar to the above, the concrete can fall, crack or dip. 

Your concrete driveway, garage, patio, sidewalk, etc. are not generally engineered to support heavy loads.  The basic rule is no vehicles or belongings over 4500 kgs or 10,000 lbs.  Things like busses, semi-trucks, hot tubs, etc. are not meant to be placed on these types of concrete pads unless otherwise engineered before construction.  Placing these items on your concrete pad can potentially lead to settlement of your pad, chipping or cracking, as well as drainage issues.

During the initial construction of your home tree and root placement may not be of concern, however, as homes and community’s age root growth from trees can begin to affect concrete.  We’ve all see older city sidewalks that have pushed up due to root growth below the surface.  Although it is not possible to control where tree roots will grow, understand the affects can help allieviate issues in the future.

Please refer to Alberta New Homeowner Warranty Program/AB Government guide for more information.