CIVE 320 Chapter 3.2, Spring 2016

Faculty of Engineering and Architecture American University of Beirut Dept. of Civil and Environmental Engineering Concrete Mixing-Placing-Handling • Proper Batching – Batching is measuring and introducing the concrete ingredients into the mixer. Batching by wt. is more accurate than by volume • Mixing – Concrete should be mixed thoroughly either in a mixer or by hand, until it becomes uniform in appearance. Mechanical mixers include on-site mixers and central mixers in ready-mix plants. Mixing time and number of revolutions vary with the size and type of the mixer. Mixers are charged with 10% of the water, followed by uniform additions of solids and 80% of water, finally the remainder of water is added. The mixers capacity varies from 1.5 to 9 m3. Faculty of Engineering and Architecture American University of Beirut Dept. of Civil and Environmental Engineering Concrete Mixing-Placing-Handling • Ready-Mixed Concrete (3 methods) 1. Central-mixed concrete is mixed completely in a stationary mixer and delivered in an agitator truck (2 to 6 rpm) 2. Shrink-mixed concrete is partially mixed in a stationary mixer and completed in a mixer truck (4 to 16 rpm) 3. Truck-mixed concrete is mixed completely in a mixer truck (4 to 16 rpm) The number of revolutions in a truck is limited in order to avoid segregation. And the concrete should be discharged on site within 60 minutes even if retarders are used (ASTM C94). • Mobile Batch Mixer: concrete mixed in a mobile batch mixer on site • Depositing Concrete: concrete should be deposited continuously as close as possible to its final position Faculty of Engineering and Architecture American University of Beirut Dept. of Civil and Environmental Engineering Concrete Mixing-Placing-Handling Placing concrete using mixers, pumps. Finishing using long-hand straightedge. Ready-mix plant and mixer. Continuous concrete placement for pavements. Faculty of Engineering and Architecture American University of Beirut Dept. of Civil and Environmental Engineering Concrete Mixing-Placing-Handling • Pumped Concrete: frequently used for large construction projects. Careful attention must be exercised to ensure well-mixed concrete with proper workability. The slump should be between 40 to 100mm. • Vibration of concrete: Quality concrete requires thorough consolidation to reduce entrapped air in the mix. For small jobs, consolidation can be accomplished manually by ramming and tamping the concrete. For large jobs, vibrators are used . Several types of vibrators are available. Internal vibrators are most commonly used. They consist of an eccentric wt. housed in a spud. The wt. is rotated at high speed. Vibrations is applied between 5 sec and 2 minutes. Over vibration causes segregation. Faculty of Engineering and Architecture American University of Beirut Dept. of Civil and Environmental Engineering Concrete Mixing-Placing-Handling • Precautions for Mixing water: water-cement ratio plays an important role in the concrete quality. Water content must be carefully controlled on site. Water should not be added during transport. The amount of water must not exceed the mix design specifications. • Measuring air content: Mixing and handling can significantly alter the air content of fresh concrete. Thus, field tests are used to ensure the proper air entrainment. Air content can be measured using pressure, volumetric, gravimetric, or Chace air indicator methods Faculty of Engineering and Architecture American University of Beirut Dept. of Civil and Environmental Engineering Concrete Mixing-Placing-Handling • Spreading and Finishing Concrete: Different methods are available to spread and finish concrete, depending on the nature and equipment availability. Tools and equipment used include hand floats, power floats, darbies, straightedges, trowels, vibratory screed, and slip forms. Hand Floats Slip Form Faculty of Engineering and Architecture American University of Beirut Dept. of Civil and Environmental Engineering Concrete Mixing-Placing-Handling • Curing: is the process of maintaining satisfactory moisture content and temperature in the concrete for a definite period of time. Hydration of cement is a long-term process and requires water and proper temperature. Therefore, curing allows continuous hydration and, consequently, continued gain in concrete strength. Increasing temperature increases the rate of hydration and, consequently, the rate of strength development. Temperatures below 10oC are unfavorable for hydration and should be avoided. Curing should start after the final set of the cement. If not cured, concrete will shrink, causing cracks. • Curing can be performed by any of the following methods: – Maintain the presence of water in the concrete during early ages by: pounding or immersion, spraying or fogging, and wet coverings. – Preventing loss of mixing water from concrete by sealing the surface by: impervious papers or plastic sheets, membrane-forming compounds, and leaving the forms in place. – Accelerating the strength gain by supplying heat and additional moisture to the concrete by: steam curing, insulating blankets or covers, and various heating techniques Faculty of Engineering and Architecture American University of Beirut Dept. of Civil and Environmental Engineering Concrete Mixing-Placing-Handling Compressive Strength of Concrete at different Ages and Curing Levels Faculty of Engineering and Architecture American University of Beirut Dept. of Civil and Environmental Engineering Concrete Mixing-Placing-Handling Some Curing Techniques Spraying Liquids Covers sheets, membranes Water immersion/spray Use Straw & Hay at icing temperatures