Step 6 - Final Product Quality Assurance
The production process should result in a high-quality product. It's important for a producer to establish a stringent product testing program.
For CMUs, the product should meet the current minimum requirements of ASTM C90 for Loadbearing Concrete Masonry Units. It is suitable for loadbearing or non-loadbearing units.
At delivery, the units should have at least 1900 psi net compressive strength (average of three units, with no individual break less than 1700 psi) and absorption rates less than 13 lb/ft3 for normal weight block (125 lb/ft3 or higher), lb/ft3 for medium weight block (105-125 lb/ft3), or 18 lb/ft3 for lightweight block (less than 105 lb/ft3).
ASTM C140 covers the sampling and testing methods for dimensions, compressive strength, absorption, unit weight (density), and moisture content of the units. There are also dimensional tolerances and a maximum linear shrinkage requirement of 0.065% (ASTM C426).
There may be occasions where specifications call for more stringent requirements than the corresponding ASTM standards.
For Segmental Retaining Wall Units (SRWs), the product should meet ASTM C1372, the Standard Specification for Segmental Retaining Wall Units, requirements. While the water absorption requirements are the same as CMUs, the minimum strength requirement is higher: 3000 psi average of three units with no individual break less than 2500 psi.
For markets where repeated freezing and thawing under saturated conditions occurs, proven field performance or test must demonstrate durability. If a test is specified, ASTM C1262 is required. The weight loss of five specimens (also cut coupons) should be no more than 1% over 100 cycles (or 1.5% over 150 cycles in four of the five specimens).
Some state transportation departments require testing in saline solution versus water, which is much more severe. This method is being investigated for its high degree of variability. It has been proven that cement content, aggregate durability, absorption rate, and density will have the greatest impact on freeze/thaw performance.
Aggregates can be analyzed for soundness under ASTM C88, although this is still no assurance that the SRW will pass the stringent freeze/thaw test. The best practice for making durable units includes increased cement content (a minimum of 13-15%), mixing a little wetter and longer than normal, and increasing compaction (increased density and lower absorption). Some admixtures also improve freeze/thaw resistance.
— Garry Culton is a 25-year veteran in the concrete industry and is an MCP technical service manager for Cemex. He has specialized in manufactured concrete products and is a Certified Consultant of Concrete Masonry (National Concrete Masonry Association) and Construction Documents Technologist (CSI and Construction Specifications Institute). He serves on several technical committees for NCMA and ICPI. You can e-mail him email@example.com.Visit the following manufacturers of block-making equipment at World of Concrete:
- ANMOPyC, Booth #C3564
- Brecon Inc. Vibration Technology, #N1074
- CTI Inc., #N126
- Hess Machinery Ltd., #N313
- Pathfinder Systems/Tiger Machine, #N2941
- WEHRHAHN, #N1269