Reinforced concrete durability is only achieved by protecting the steel reinforcement from environmental attack. The level of the protection depends not only upon the quality of the cover concrete but also upon the type and quality of the concrete spacers that are used. The focus of standards/ specification on improving concrete quality and durability has not been matched by improved spacer requirements.
Eurocode 2 and the British Standard and AS/NZS standard have clearly specified rebar spacer strength and dimensional tolerance and their adoption improves concrete performance. Reflecting the recognition of how spacers can affect concrete durability, standard/specification are now also including spacer durability properties that required them to match or exceed those of the host concrete.
British Standard: Structural use of concrete — BS 8110-1:1997

Part 1: Code of practice for design and construction.

7.3 Fixing

The mix used for spacer blocks made from cement, sand and small aggregate should be comparable in strength, durability, porosity and appearance to the surrounding concrete as far as is practicable. Concrete spacer blocks made on the construction site should not be used.

LDED, CCB: Supplying concrete cover block as specified size conforming to BS 8110-1, satisfying a compressive strength fc=35 Mpa (5076 psi) Min ensuring perfect clear cover to rebar in concrete structure. The mix used for rebar spacer blocks made from cement, sand and small aggregate should be comparable in strength, durability, porosity and appearance to the surrounding concrete as far as is practicable. Concrete spacer blocks made on the construction site should not be used.