Guidelines for the Use of Dry Lean Concrete as Sub-base for Rigid Pavement (First Revision)
2014 Edition

The preferred cement for Dry Lean Concrete (DLC) sub-base is Ordinary Portland Cement (OPC), with options for higher grades like 43 or 53 depending on strength needs. Mix proportions are determined through trial mixes with moisture contents ranging between 5.0% and 7.0%. Compressive strength tests on cubes at 3 and 7 days confirm suitability. If required strength is not achieved, cement content should be increased or a higher grade cement used, followed by repeat trials to finalize the mix.

Optimum moisture content is identified by preparing trial mixes at moisture levels of 5.0%, 5.5%, 6.0%, 6.5%, and 7.0%, compacted in three layers using a vibratory hammer. The moisture level yielding maximum dry density is selected as optimum. Compressive strength of cubes cast at this moisture content is tested at 3 and 7 days. During construction, moisture content is adjusted based on natural moisture in aggregates and cube strength results to avoid sticking or ravelling, ensuring consistent compaction and durability.

Batching must be done on plants capable of separate weight measurement for each material with capacity exceeding 125% of laying rate. Forced action central batching and mixing plants with automated controls are preferred. Monthly calibration ensures accuracy. Trial mixes at varied moisture contents confirm mix suitability. Compaction is performed in three layers using a vibratory hammer with a square or rectangular foot, ensuring the mix neither sticks to rollers nor is too dry, preventing surface defects.

Trial mixes are prepared to finalize mix proportions ensuring target strength and workability. A trial length of minimum 60 meters, covering full pavement width, is constructed at least 14 days before the main work over a maximum of 2 days. This section includes at least one transverse joint with hardened concrete and is located outside the main work zone. After approval, no changes are permitted to materials, mix, moisture content, or construction methods, ensuring consistent quality for subsequent works.

Post-placement, the DLC sub-base must be cured immediately using either continuous water curing with two layers of moist hessian cloth for 7 days or by applying a white pigmented curing compound with at least 90% water retention index. If water curing is not feasible, the compound should be sprayed right after rolling, and once the compound sets, the surface should be covered with moist hessian for an additional 3 days. Heavy vehicles are prohibited on the sub-base immediately after construction, while light vehicles may be allowed only after 7 days with engineer approval, ensuring adequate strength development before traffic exposure.