最新土木工程材料第四章1PPT课件.ppt

A Definition of Portland Cement Concrete （PCC） A mixture of: Portland Cement Fine Aggregate Coarse Aggregate Water Air Cement and water combine, changing from a moist, plastic consistency to a strong, durable rock-like construction material by means of a chemical reaction called “hydration”.Slump testSlump test Mold: slump cone坍落度筒坍落度筒 Bottom diameter of 200mm top diameter of 100mm height of 300mmProcedure of Slump testa. Fill the cone using standard procedureb. Strike off excessc. Raise the mold and measure the displacement of the original center of the specimenSlump In the slump test, the number of mm the original center of the cone sinks or settles uniformly is measured to the nearest 5 mm. The measurement is called slump. E.g. slump can be 75, 80, 85mm, etc.Types of slump Near-zero slump Very dry mix Normal slump Properly proportioned concrete Shear slump Concrete lacks plasticity and cohesion Collapse slump Lean, harsh or extremely wet concreteRecommended slumps for various type of constructionType of constructionSlump (mm)MaxMinFootings, caissons, foundation walls and substructure walls7525Beams, columns and walls10025Pavement and slabs7525Mass concrete50254.1.2 Factors affecting Consistency and Workability Internal Factors Cement propertiesProportion of cement, Types of cement, Fineness Aggregate propertiesSurface area of aggregate, shape, gradation, maximum size of aggregate Water contentWater-to-Cement ratio 水灰比水灰比 AdmixturesWater-reducing admixture, air-entraining admixture减水剂、引气剂减水剂、引气剂4.1.2 Factors affecting Consistency and Workability External Factors EnvironmentTemperature, humidity, wind speed TimeDelay in placingThe relationship of slump and temperatureslumpTemperatureThe relationship of slump and timeTimeSlump4.1.3 Segregation and Bleeding Segregation 离析离析 and Bleeding 泌水泌水 results in Nonhomogeneity 不均匀性不均匀性 Excessive bleeding - Weak top layerSegregation Segregation is the tendency for separation between large and fine particles of fresh concrete. Segregation results in Rock pocket /honeycomb 蜂窝蜂窝 Sand streaks 砂条痕砂条痕 Crazing 龟裂龟裂 Surface scaling 表面剥落表面剥落Segregation Reasons of segregation Very wet mixtures, lack of fine particles, high slump concrete, Overvibration Controlling segregation Air-entainment Finely divided materials Dropped verticallyBleeding Bleeding-A concrete mix that does not possess proper consistency is unable to hold the mix water, which slowly gets displaced and then rises to the top of the form. Reasons of Excessive bleeding Overvibration, overtroweling, lean mixes, lack of fine sand, premature finishing Controlling bleeding Air-entainment Increase the fineness of cement Decrease water-to-cement ratio Rich mixesBleeding26 结束语结束语