Plum Concrete

Plum concrete (also called boulder concrete or cyclopean concrete) is mass concrete with large stones (plums) of 150 mm diameter or larger embedded in the concrete to save cement. Per IS 456:2000 + IS 1199, plum concrete uses 25-30% by volume of large stones embedded in plain concrete (M10-M15 typical). The stones must be hard, non-porous, free of dust and laitance — typically granite, basalt, or hard limestone.

Key uses in Indian construction: (1) Mass concrete fills — retaining wall fills, gravity dam fills, large foundations where the structural function is bulk weight. (2) Mass foundations — under heavy column loads where concrete cost is significant. (3) Filling unwanted spaces — old wells, basements being abandoned. (4) Bulk concrete in archaeological / restoration work. (5) Site-cast concrete blocks — large structural elements.

Design: (a) Plums maximum 150-300 mm diameter. (b) Spacing between plums ≥ 100 mm to allow concrete flow. (c) Concrete grade M10-M15 (lower than typical RCC since plums carry the majority of compressive load). (d) Cement content 200-280 kg/m³ (vs 320-380 for ordinary M25). (e) Aggregate maximum size in concrete 20 mm or smaller (to flow around plums). (f) Layered placement — stones first, then concrete poured in layers of 200-300 mm. The most-overlooked aspect: bond. Stones must be clean and damp before placement; dirty or dry stones cause bond failure between concrete and stone. Site QC verifies stone preparation pre-pour.