Shrinkage Reducing Admixture — Crack Control in Thin Sections

Primary uses

• Drying shrinkage reduction
• Crack risk reduction
• Dimensional stability support

Applications

• Large-format architectural panels prone to shrinkage cracking
• Thin cast sections and toppings
• Restrained elements where cracking is unacceptable
• Repair and overlay systems

Advantages

• Directly targets drying shrinkage, which is the mechanism behind much of the cracking in restrained elements
• Useful in thin architectural and cast stone elements where cracking is a rejection issue
• Can be combined with mix design and curing improvements as part of a crack-control strategy

Limitations to consider

• Does not eliminate cracking on its own; restraint, curing and detailing remain the dominant factors
• Can interact with air entrainment and with other admixtures, so the whole admixture set has to be re-verified
• Dosage and shrinkage reduction achieved are established by testing in your own mix
• Dosage, addition rate, water content and binder proportions are decided by your own mix design and trial batches, not by a catalogue figure. Supplier literature ranges are a starting point for that development work and are not reproduced here as a specification.
• Where the finished element goes into a building, the performance that matters is the performance of your element under the applicable code and project specification, not the property of a single ingredient. Testing and conformity evidence for the finished product are the responsibility of the producer and are established by project-specific testing.
• Mix ratio, pot life, working time, demould time, full-cure schedule, hardness, tear strength, elongation, shrinkage, viscosity and temperature resistance are properties of one specific supplier system. They are never quoted here from memory or by analogy with a similar product; they are read from the supplier data sheet for the exact material offered and confirmed at quotation.

How to choose the right grade

• Start from the problem in the mix — water demand, segregation, set timing, shrinkage or durability — rather than from an admixture category
• List every other admixture already in the mix, because interactions between admixtures are the most common cause of unexpected results
• State the cement and supplementary materials in use, since admixture response is cement-specific
• Investigate the actual cracking mechanism first, because restraint and curing problems are not solved by an admixture
• Re-verify air content and set behaviour after introducing it into an existing mix
• Mould-making and casting behaviour depends on the pattern, the shop temperature and humidity, the mixing method and the operator. A production-representative trial on your own pattern and in your own shop conditions is the only honest way to qualify a material before a run is committed.
• Exact specification is confirmed against supplier documentation at quotation.

Typical grades

• Products the supplier classifies by chemistry and intended dosage range
• Products intended for precast versus site-placed concrete
• Buyers search this category by grade name, and those names describe what the workshop needs rather than what is asserted here. Which grade a specific offer corresponds to, and what the supplier declares for it, is read from the technical documentation issued for the exact material quoted.
• Materials in this category interact: rubbers, releases, sealers, resins, pigments and fillers can inhibit cure, stain a surface or shorten mould life when they are combined outside the combinations the supplier has tested. State every material already in the process at enquiry so compatibility is checked against supplier documentation rather than discovered on the shop floor.
• Reactive materials age in the container, and moisture-sensitive systems age faster once opened. Order against real consumption rather than against a discount, and confirm the shelf life the supplier states for the exact product and pack.