August 28, 2009

Concrete Matters #1 in a series: Placement

by Kevin Morris

Concrete appears at first glance to be one of the easier substrates to prepare and coat. The surface in most cases is rough and porous, and should accept coatings with no problems. However, under the surface lies a sleeping beast that can create problems most people never consider. This series of five posts will describe some of the pitfalls with placement, surface preparation, repairs, coating and inspection.

Concrete in a sense is a raw material when it leaves the batch facility; it can be altered on site by the installer, affecting its long-term durability and performance.  It may be altered by the addition of too much water, leaving it very porous and causing problems with coating adhesion due to Moisture Vapor Emissions (MVE).  It may leave a weak surface layer called laitance that must be removed to ensure proper adhesion. Poorly consolidated concrete will leave a surface full of voids that will require repair prior to coatings application. Proper design and placement of concrete should follow the guidelines set forth by the American Concrete Institute (ACI).

Problems with concrete can begin with the placement of this material. When placing concrete on grade it is imperative to specify a waterproofing membrane under the slab. The membrane should be more substantial than polyethylene or sheet plastic membranes, which are too easily disturbed during placement of rebar or concrete. As concrete is placed it will receive a rough finish until the bleed water evaporates and then it will receive the final specified finish. The contractor, in an effort to speed his production and profit, can puncture holes in these types of membranes.  This results in the bleed water exiting from both sides, allowing faster finish troweling, but long-term, also causing ground water to enter the slab. This creates issues with MVE.

Cast-In-Place concrete walls that are below grade also need to have a waterproofing membrane installed.  Careful consideration of proper water drainage should be evaluated to eliminate potential problems.  Cast-In-Place walls may also exhibit issues with surface imperfections due to poor consolidation of the concrete during placement.  If the material is being pumped into the forms, there is the potential issue of having a laitance layer at the bond line between pours, due to heavy water content.

The durability of concrete will be affected by the water / cement ratio. Concrete takes very little water by volume to properly hydrate.  Anything in excess of this is considered water of convenience and aids in the placement of the concrete. The ideal mix should provide some of the following characteristics and more, depending on the project requirements:

  • Good flow and encapsulation of reinforcing members
  • Properly covered aggregate with the cement paste
  • Minimal shrinkage during cure and other related cracking issues
  • Economics of the mix

Next up:  Surface preparation

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