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Blinding and sulfate resistance

Blinding is a non-structural concrete to form a level and more stable surface on which further works can be more readily undertaken and is typically of 50 to 100mm depth.

It is usual to ignore blinding when considering the sulfate environment, the structural concrete mix design being proportioned to resist the potential of sulfate attack. BRE Special Digest 1, Section D6.5 Provide a sacrificial layer (APM4) notes that if the blinding is to be adopted as an Additional Protective Measure, the blinding concrete should be at least 50mm thick and of the same quality as the foundation construction. Therefore, if the blinding is being used as an APM it needs to be of the same quality as the foundation, but if this is not the case and the blinding is only used as a working platform, it does not have to be sulfate resistant.

Similarly, BS 8500-1:2015 cl A.4.7 notes that ‘other than blinding or similar applications, standardized prescribed concretes (ST) should not be used where the presence of sulfates or other aggressive chemicals in the groundwater, the ground or any adjacent material give a ACEC classification higher than AC-1.´ Again this implies that blinding (ST or equivalent designated concrete) does not need to be sulfate resistant.

Sulfates react with concrete resulting in an expansive formation of ettringite or gypsum in hardened concrete causing cracking and exfoliation. If there is a continuous supply of sulfate, through movement of groundwater, the cracks will allow contact with more concrete surface and the reaction can lead to a softening and further disintegration of the concrete.

Lea’s Chemistry of cement and concrete (4th edition) section 7.6.3 Volume change notes that the conversion of calcium hydroxide to gypsum more than doubles the solid volume (of the cement paste). However, there is difficulty in ascribing expansion directly to increased volume of the solids produced. Various theories exist but in simple terms, provided the water can diffuse freely out of the capillaries or cavities, it is not possible for the volume of growing crystals to exceed the space available, hence swelling pressure is limited. For a weak non-structural blinding concrete this is probably the case and this is reflected in the standards/guidance as mentioned above.

In summary, the replenishment of sulfates to the blinding surface by mobile ground water is the worst case scenario. If this actually happens the products of expansion are unlikely to fill the capillaries and voids in the low-quality product and therefore unlikely to exert pressure by expansion.

Acknowledgement: The Concrete Society