Formwork Deflection and Lift Delamination in Rammed Earth
A 2.4 m high rammed earth wall is built in a 6 m long formwork panel. The formwork is 18 mm plywood on 50 x 100 mm timber studs at 600 mm centres, with through-ties at 900 mm vertically and 1.2 m horizontally. Lifts are 150 mm loose thickness, compacted with a 5 kg plate compactor; each lift is compacted for 20 seconds. At mid-height, the formwork bows 12 mm outward; after stripping, horizontal delamination planes are visible at 900 mm and 1.5 m, and the corners are porous. The mix is at optimum moisture according to a Proctor test, but the crew adds water to the top of each lift to 'help it bind'.
1. Diagnose the causes of deflection, delamination, and porous corners.
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The 12 mm mid-height deflection indicates formwork stiffness is inadequate for 2.4 m wet earth pressure. Studs at 600 mm and ties at 900 x 1200 mm are too flexible; plywood bows, joints open, and fines wash out, causing porous corners. Delamination at 900 and 1500 mm aligns with tie levels and lift joints. Adding water on top of each lift creates a wet, weak interface; the next lift compacts on slurry, producing a plane. A 150 mm loose lift compacted for only 20 seconds may be under-compacted at edges and corners.
2. Recommend changes to formwork and placement and compaction, giving specific numbers.
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Formwork: reduce stud spacing to 300-400 mm, add double walers, ties at 600 x 600 mm, and brace plumb; use 18 mm ply with sealed edges and corner fillets. Place 100-125 mm loose lifts, compact each with overlapping plate-compactor passes for 20-30 seconds per position, and hand-tamp corners. Scarify the surface if a lift dries; never add water between lifts. Control moisture by a drop test. These changes reduce pressure, close joints, and create a mechanical bond between lifts.
3. What quality checks would you require during the next pour?
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During pour: check formwork plumb before, during, and after; mark lift levels; measure moisture each batch; test density on a trial cube; inspect lift bond; record compaction time; check corners with a probe. After stripping, tap-test for voids and measure permanent deflection. Compare results with the trial panel and stop the pour if deflection exceeds 5 mm or a lift joint is visibly wet.