Volume 28, Issue 3 (May 2000)
An Improved Capping Technique for Excavatable Controlled Low Strength Material Compressive Strength Cylinders
ASTM D 4832: Standard Test Method for Preparation and Testing of Controlled Low Strength Material (CLSM) Test Cylinders currently permits three types of capping for CLSM cylinders: sulfur mortar (ASTM C 617), gypsum plaster (ASTM C 617), and unbonded neoprene restrained by rigid retainers (ASTM C 1231). These cappings were developed for portland cement concrete (PCC) cylinders and all produce good estimates of PCC potential strength. However, these cappings are inappropriate for excavatable CLSM cylinders. Sulfur mortar and gypsum plaster methods often result in cylinder damage during capping due to the low strength of excavatable CLSM cylinders. The recommended 50 to 70 durometer neoprene pads are far too stiff at low stress levels, which are common in testing excavatable CLSM cylinders. The neoprene pads essentially act as a rigid plate rather than a capping material. The authors developed an unbonded capping system using a much softer neoprene that is more malleable at lower stress levels.
To evaluate the effectiveness of the new capping system, ten batches of an air-entrained and ten batches of a non-air-entrained excavatable CLSM were produced. Each batch was approximately 0.0453-m3, allowing 20 100-by-200 mm cylinders as well as flow consistency (ASTM D 6103) and gravimetric air tests (ASTM D 6023). Five pairs of cylinders were tested at 7 days and five pairs at 28 days. At each age, one pair was capped with each of the following materials: sulfur mortar, gypsum plaster, 50-durometer neoprene, nothing, and the new softer neoprene unbonded capping system.
The average 28-day compressive strength of the cylinders using the proposed capping system was 275 kPa (39.83 psi) for non-airentrained and 345 kPa (50.01 psi) for the air-entrained mix design. The proposed capping system was not significantly different than the sulfur mortar or gypsum plaster. It was significantly superior to 50-plaster durometer neoprene and the no-cap technique.