Critical threshold for concrete moisture content

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by Paul Potts
This article is about prescriptive specifications and focuses on 100-mm (4-in.) interior concrete slab on grade for schools, offices, hospitals, libraries, and other commercial buildings that will receive moisture-sensitive or bonded floorcoverings.

A basic specification stating the requirements for strength and slump is the simplest way to specify concrete for interior slab on grade—for example, 24-MPa (3500-psi) compressive-strength concrete with a 100-mm (4-in.) slump. This simple performance specification serves well for many applications. The strength satisfies the structural engineer’s requirements and the slump reduces the cost of placement. It is called a performance specification because it states how the concrete is to perform, but says nothing about the mix design, proportions, or ingredients.

Choosing to write a performance specification is a policy decision. It leaves the actual mix proportions up to batch plant operators, who will select a formula from their library of previously proven mixes. By writing a performance specification, the architect-engineer or specifier gives up any refinements to the design, but shifts liability for performance of the product away from themselves to the contractor and batch plant operator.

When the required performance of concrete becomes more complex than the simple requirements used in the previous example, a design professional may prefer to write a prescriptive specification with all the details of how they would like the concrete designed, proportioned, and perform in actual applications. A prescriptive specification may include:

  • water-cement (w/c) ratio (i.e. the weight of mixing water divided by weight of cementitious materials);
  • minimum cement content;
  • maximum allowable (and type of) cementitious materials substitute;
  • maximum size and minimum quantity of coarse aggregate;
  • air content;
  • strength;
  • slump;
  • fiber; and
  • any allowable admixtures.

Some batch-plant operators and concrete contractors are opposed to the designer writing a complete prescriptive specification, because it limits their options and overlooks their experience in proportioning concrete. This author would recommend the designer write a complete prescriptive specification when needed and collaborate with the concrete company during post-bid interviews to discuss any suggestions they might have—ideally, this is done at post-bid because contracts are not yet signed, giving the architect or engineer more leverage.

In addition to the basic properties of workability, strength, and economy, the desired characteristics of hardened interior slabs-on-grade for commercial buildings are:

  • flatness (i.e. minimal curling);
  • minimal long-term shrink cracking; and
  • moisture content that will not unduly delay the start of moisture-sensitive floor coverings. (For more, see page 24 of the American Concrete Institute’s (ACI’s) Guide for Concrete Slabs that Receive Moisture-sensitive Flooring Materials, reported by ACI Committee 302.)

A low w/c ratio mix design and proper quantity, sizing, and grading of coarse aggregates and proper curing methods are the basics to achieving these results. (This comes from the Portland Cement Association’s [PCA’s] Volume Changes of Concrete [1967], specifically the Robert F. Ytterberg article, “Control of Shrinkage and Curling in Slabs on Grade.”)

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