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Key points

  • ASTM C90 (load-bearing concrete block): minimum average net-area compressive strength 2,000 psi (13.8 MPa).
  • The limit was raised in the 2014 revision from 1,900 to 2,000 psi and applies to all density classes (normal, medium, light).
  • Insulated block contains an insulating layer/insert that reduces heat transfer through the wall, counted in the wall's U-value.
  • Ask the manufacturer for strength and dimension test certificates.
Written for:OwnerContractorSupplier

Concrete block is the most widely used material in residential walls. Its quality affects wall durability, load-carrying ability (in load-bearing walls) and thermal and acoustic insulation.

1) The ASTM C90 strength requirement

The American standard ASTM C90 covers load-bearing concrete masonry units. The US Concrete Masonry & Hardscapes Association (CMHA) states12:

  • Minimum compressive strength on the net area = 2,000 psi (13.8 MPa).
  • The limit was raised in the 2014 revision from 1,900 psi (13.1 MPa) to 2,000 psi.
  • The standard defines three density classes (lightweight, medium and normal weight), and the stated minimum is one figure for all ASTM C90 units.
What is "net area"? The area of actual material in the block's cross-section after excluding the voids. So don't compare a result calculated on gross area with one calculated on net area.

2) Common block types

TypeDescriptionNotes
HollowVertical coresLighter; cores are filled with concrete and steel in some designs.
SolidNo cores or small coresHeavier; used where higher strength is required or in specific locations.
InsulatedWith an insulating layer or insert (e.g. polystyrene)Improves the wall's thermal insulation; its performance counts in envelope design under the energy code.
LightweightWith lightweight aggregateReduces dead loads and improves insulation somewhat. See also AAC blocks.

3) Loadbearing and infill walls: what differs?

ItemLoadbearing wallInfill (non-loadbearing) wall
FunctionCarries the roof and floors aboveDivides space and encloses the concrete frame
BlocksNeeds loadbearing blocks meeting strength requirements, such as ASTM C901Blocks per the design (standard, insulated, lightweight)
Later changesNever demolished or opened for a door without a structural engineerEasier to alter, but watch for services inside
Tie to the frameTying walls to columns and beams and movement joints follow the structural details; a vertical crack where a wall meets a column recurs when the tie or joint is missing (practical advice)

4) Insulation, sound and damp

  • Heat: insulated blocks improve wall insulation and count in the design U-value. See thermal insulation.
  • Sound: wall mass matters most; a single block wall with a sealed surface (plaster or paint) gives about STC 45–55, and joints must be fully filled with no gaps3. See sound insulation.
  • Damp: rising damp is usually linked to a missing or weak damp-proof layer4. See damp walls.

5) What you can check by eye and what needs testing

By eye (owner)Needs a test or design review
Wall straightness and plumb, full joints, breakages, match with the drawingBlock strength (lab certificate), loadbearing capacity, structural ties, required acoustic or thermal performance

6) On delivery

  1. Ask for a recent strength test certificate from an accredited lab, and dimensional compliance.
  2. Inspect random samples: broken edges, cracks, uniform colour and texture.
  3. Check the type (load-bearing/non-load-bearing, insulated) matches the drawings.
  4. Store blocks on level, clean ground away from contamination.
  5. Don't lay very wet or very dry blocks without following the supervisor's wetting advice.

Sources

The number next to a statement points to its source in this list. Sources back the general information; anything marked “practical advice” is editorial experience, not a quoted source. Anything that affects safety, structural design or electrical installations needs a licensed professional, and in Saudi Arabia the binding reference is the Saudi Building Code.

  1. 1What is the Minimum Required Compressive Strength for Concrete Masonry? (ASTM C90) — Concrete Masonry & Hardscapes Association (CMHA)
  2. 2Concrete Masonry Unit Shapes, Sizes, Properties, and Specifications (CMU-TEC-001) — Concrete Masonry & Hardscapes Association (CMHA) — technical note
  3. 3Controlling Sound Transmission through Concrete Block Walls, Construction Technology Update No. 13 (Warnock) — National Research Council Canada
  4. 4Diagnosing the causes of dampness in buildings — BRE (Building Research Establishment)
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This guide is for general information and planning, based on standards and technical guides. Standards are revised periodically; in Saudi Arabia the binding reference is the Saudi Building Code and the instructions of the competent authorities. For design and execution, use a licensed engineer or technician. Found an error? Tell us.