In the picture below, there are several issues that will cause long term problems with the performance of this newly constructed retaining wall. As constructed, this wall will deform and the concrete patio behind the wall will crack. Blocks have already started to shift in the top row, and the concrete is still wet! Let’s take a look why.

What Was Done Wrong:

  1. The concrete was allowed to flow between the blocks. This is a big no-no. Segmental block walls are intended to be a flexible system to allow the wall to handle settlement and movement from freeze and thaw cycles. Filling these gaps with concrete removes that flexibility which will cause the concrete patio to crack over time.
  2. In talking with the owner, no construction adhesive was placed under the top row of blocks. The block to the right in the picture above has pushed out a couple inches due to no adhesive. Wet concrete creates an extremely high load on the back of the wall before it cures. To put it in perspective, water has a unit weight of 62.4 lbs per ft^3 while concrete has a unit weight of 150 lbs per ft^3. Freshly poured concrete can create a load 2.5X’s that of water. Even with a 4-inch thick patio, wet concrete still adds a large temporary load to the back of the wall. The top row of blocks has no benefit of the weight of additional blocks above it to resist this load. That is why the top row of blocks require adhesive.
  3. There is no separation between the concrete and the blocks. Concrete will shrink as it cures. In addition, the concrete will shrink and swell as the temperature fluctuates. Also, the flexible retaining wall will shift and flex, but the concrete will not. Because there is no separation, the concrete will begin to crack behind the blocks.

What Should Have Been Done:

Wall with capstones

Let me start by saying every block wall should have capstones. This gives the wall a clean look at the top, and it creates a straight edge to place a concrete or paver deck up against. The top row of blocks and the capstones should always be attached to the blocks below using construction adhesive. For construction adhesive, I recommend PL 500* by Loctite. Construction adhesive is flexible, which allows the blocks to move. Never use grout between blocks, and never allow concrete to be installed in direct contact with the wall.

Typical Section Through Wall at Concrete Patio

After adhesive is placed under the top row of blocks and capstones, allow the adhesive to fully cure. You can now start planning your patio behind the wall. Before pouring any concrete, install expansion joint material* along the back of the capstones. Use the construction adhesive to attach the expansion joint material to the back of the capstone to hold it in place. Concrete can be poured directly up against the expansion joint material. The blocks will not move because of the construction adhesive.

This is how to properly prepare your segmental block retaining wall for a concrete patio behind the wall. If you have additional questions about retaining walls, leave a comment below or send us an email.

Everything required to construction the concrete patio, such as concrete thickness, steel rebar requirements, and concrete mix is beyond the limits of this post.

*Amazon Affiliate – If you use these links and make a purchase, I may be compensated.