Cover Block (Spacer)

A cover block (spacer) is a small precast cement-mortar or factory-made plastic component tied to or clipped onto the reinforcement to hold it the correct distance from the formwork, guaranteeing the design clear cover during concreting. Correct cover is the single most important durability detail — it protects the steel from carbonation and chloride ingress and provides fire resistance and bond.

Cover blocks must have the same or higher grade/strength and similar permeability to the surrounding concrete (cement-mortar blocks are preferred for aggressive exposure over plastic, which can create a permeable interface). Their thickness equals the specified cover from IS 456 Table 16 (e.g. 20 mm slabs, 25-40 mm beams/columns, 50-75 mm footings, +5 mm tolerance per Cl. 26.4.2). They are placed at close spacing (typically ≤1 m grid, ≥4 per m² for slabs) and staggered, and 'chair' bars are used to maintain the gap between top and bottom mats. Site use of stone/aggregate or steel offcuts as makeshift spacers is a common, unacceptable malpractice.