Concrete Strength

Q: I have heard that concrete can be made to a wide variety of strengths. What are typical strength ranges of concrete and where is it appropriate to use them? Sterling Fence Inc is very aware of the importance fo concrete strength.

Flexural strength testing

A: Concrete can be proportioned to meet a wide variety of strength requirements. It is important to note that there is more than one type of strength property used to design concrete projects. The most commonly used design properties are:

1. Flexural strength, used for design of pavements (slab-on-grade).
   
2. Compressive strength, used for design of foundations, building elements (walls, columns, slabs), bridges (abutments, columns, decks), etc.

Flexural Strength

Flexural strength increases proportionally with compressive strength (as the compressive strength goes up, so does the flexural strength). This property is used specifically for pavement design the flexural strengths of interest fall in a range of 3.9 MPa (570 psi) to 5.1 MPa (750 psi). These flexural strengths correspond approximately to compressive strengths of 28 MPa (4000 psi) to 48 MPa (7000psi). While concrete can attain much higher flexural strengths, it is not required for pavements, and use of higher strengths would have an adverse effect on the economics of the project with little benefit in performance.

Compressive Strength

Compressive strength testing

The compressive strength of structural concrete begins at 17 MPa (2500 psi) and can be produced commercially at 138 MPa (20,000 psi) or more. With such a wide range of strengths to choose from, the following guidance is provided to assist in making an appropriate choice for specific projects:

• Residential and light commercial building projects typically use concrete strengths ranging from 17 MPa (2500 psi) to 34 MPa (5000 psi). Keep in mind that the lower strength concrete is only appropriate for mild environmental exposures, and interior concrete protected from the elements. Severe environmental exposures (freezing and thawing cycles and deicer chemical exposure) require a minimum strength of 4000 psi to assure durability. Local codes commonly provide guidance for the minimum requirements, but in many cases do not address long term durability issues.

• Heavy commercial and special structures (high rise buildings, long span bridges, slabs exposed to heavy abrasion, etc.) typically require concrete strengths of 28 MPa (4000 psi) or more. The actual required strength may be controlled by the structural loading, durability requirements, special property requirements (low permeability, high abrasion resistance, etc.) or a combination of these factors. Concrete design professionals should always be consulted for guidance regarding these important structures.

References:

Kosmatka, Steven H., Kerkhoff, Beatrix, Panarese, William C., Design and Control of Concrete Mixtures, EB001, Portland Cement Association, 2002, 372 pages.

Tarr, Scott M., Farny, James A., Concrete Floors on Ground, EB075, Fourth Edition, Portland Cement Association, 2008, 252 pages