Title: A Comparison of Mechanical Properties of Hong Kong Medium and High-Strength NPC and Fly Ash Concretes
Author(s): I.D. MacGregor
Publication: Symposium Paper
Appears on pages(s): 129-158
Keywords: admixtures; aggregates; concrete; creep; curing; elasticity; fly ash;
Elasticity, shrinkage, and creep to one year were measured on 150 mm diameter by 300 mm cylinders made from concrete with crushing strengths of 40 and 70 MPa and slumps of approximately 80 mm. Within each strength grade a 25% fly ash replacement concrete was designed to match the normal portland cement (NPC) slump and 28 day strength. Creep samples were loaded at 3, 7, 14 and 28 days after water curing at 27°C and then tested in air at 25 o C and 65% relative humidity to approximately match Hong Kong’s sub-tropical climate. The maximum interval between readings was 14 days. Interface creep cylinder surfaces were ground. Hong Kong materials were used. The ash has a cementing efficiency that varies from 74 to 114% and typically has a silica content of 52% and an alkali oxide content of 3%. The admixtures used were a lignosulphonate/carbohydrate retarding water reducer/plasticiser, and a napthalene sulphonate and modified lignosulphonate superplasticizer. Hong Kong elasticities can be low due to the aggregates used; a granite in this study. There was little difference between the NPC and fly ash pairs of concrete samples at the end of one year. The similarity is considered to be due to the high fly ash cementing efficiency and the relatively high humidity testing regime. There was little variation in the 12-month elasticities and shrinkages between the four curing ages but big decreases in the 12-month load induced specific creeps.