Fresh Concrete Operations PDF

Fresh Concrete Operations 1 FRESH CONCRETE OPERATIONS Batching Mixing Transporting/Handling Placing Compacting Curing 2 Batching Preparing or measuring quantities of the concrete constituents Two methods: Batching by w...

Fresh Concrete Operations 1 FRESH CONCRETE OPERATIONS Batching Mixing Transporting/Handling Placing Compacting Curing 2 Batching Preparing or measuring quantities of the concrete constituents Two methods: Batching by weight Batching by volume 3 Batching (by volume) Mixture Cement in Bags Sand Gravel Proportion Class cu.m. cu.m. 40kg 50kg AA 1:1.5:3 12.0 9.5 0.50 1.0 A 1:2:4 9.0 7.0 0.50 1.0 B 1:2.5:5 7.5 6.0 0.50 1.0 C 1:3:6 6.0 5.0 0.50 1.0 4 Mixing Objectives: To coat the surface of aggregate particles with cement paste To blend concrete ingredients into a uniform mass Can be manual or thru use of equipment/machines 5 Manual Mixing Mixing by Hand Spread aggregate out on a hard, clean, and non- porous base Cement is spread over the aggregate Dry materials are mixed by turning over three times with a shovel until the mix looks uniform Water is gradually added Mix is turned over until the mix appears uniform 6 Machine Mixing Tilting Drum Mixer Either one-bagger or two-bagger Chamber or the drum is TILTED for discharging (even during the mix) Good discharge action, unsegregated Perfect for mixes with: Low workability Large aggregates 7 8 Machine Mixing Non-Tilting Drum Mixer (Reversing drum) Axis of the drum is ALWAYS horizontal Chute is used during discharge Rotation can also be reversed to discharge Discharge rate is slow  segregation Susceptible to segregation Essential to check the mix at the start and towards the end 9 10 Machine Mixing Pan-Type Mixer Efficient with stiff and cohesive mixes Doesn’t rely on free fall for mixing, similar to electric cake mixers  mixing action is electrically forced Scraping action at the sides of the mixer (due to its revolving star of blade (scrapes or paddles) Drum type mixers have no scraping action thus some adheres on the walls of the drum 11 12 Machine Mixing Terminologies: Charging the mixer - putting the ingredients in the mixer Buttering - coating the sides of the mixer with an initial amount of mortar 13 Machine Mixing Mixing Time Dependent on: Size and type of mixer Speed of mixer rotation Quality of blending of the ingredients 14 Mixing Time The exact value of mixing time is a function of size of mixture. A minimum mixing time of 1 minute for mixer size of ¾ cu.m and 15 additional seconds for each addition ¾ cu.m. giving satisfactory uniformity of mixing. Typical minimum values: One-bagger mixer: 0.09 m3 = 1 minute Transit mixer: 4.60 m3 = 2 ¼ minutes generally, around 2 minutes Prolonged mixing would lead to: Evaporation of the water from the mix Grinding of the aggregates 15 Mixing Time 16 Mixing Intermittent Mixing Does not affect strength and durability Workability decreases with time unless loss of moisture is prevented in the mixer Retempering – adding water to restore workability 17 Transporting Dump trucks for short distances Agitator trucks Water is added at central plant and mixing occurs during transportation Transit mixers Water is added while being transported 18 19 Handling Buckets Portalifts Wheelbarrow Pumping Concrete buggies Crane and bucket Belt conveyors 20 Handling Pumpcrete Concrete pumped under pressure Types of pumps: Direct-acting concrete pump Squeeze-type concrete pump 21 Handling Pumpcrete Pipe diameters > three times the maximum aggregate size Recommended slump: 40-100mm (medium workability) 22 Handling Pumpcrete Advantageous since it can be delivered to points over a wide area otherwise not easily accessible Note that: too dry  blockage too wet  segregation 23 24 25 Handling Shotcrete Pneumatically applied mortar Mortar pumped under pressure at high velocity unto a backup surface Also called gunite 26 27 Placing Deposit concrete as close as possible to its final position to avoid segregation and obtain full compaction Methods: Chute Tremie Flexible drop chute 28 29 Compacting and Vibrating Purpose of vibrating: To remove entrapped air Filing up the forms Ensure uniform mix Methods: Manual and mechanical means 30 Vibrating By Hand By rodding or ramming Internal Vibrators Poker or immersion vibrators 31 Vibrating Internal Vibrators 12,000 cycles per minute 0.50 to 1m on center for about 5-30 seconds depending on consistency 32 Vibrating External Vibrators For precast or thin in-situ sections Rigidly clamped onto the formwork resting in an elastic support so both form and concrete are vibrated 3,000 to 6,000 cycles per minute 33 External Vibrators 34 Vibrating Vibrating Tables Formwork is clamped to a vibrator in precast concrete or in testing 1,500 to 7,000 cycles per minute 35 Vibrating Tables 36 Vibrating Revibration For concrete placed in layers Purposes: Reduce settlement cracks Reduce internal effects of bleeding 37 Curing procedures used for promoting hydration of cement Consists of: Control of temperature Moisture movement to and from the concrete 38 Curing Purpose: Enhance hydration Prevent further evaporation of water Replace any amount of water that already evaporated 39 Curing Methods: Ponding Wet sand, earth, Plastic sheets sawdust, straw Waterproof paper Spraying Wet burlaps Steam curing 40 Ready-Mixed Concrete 41 Ready-Mixed Concrete Types: Central-mixed Batched and mixed at central plant Transit-mixed Batched at central plant and mixed in mixer trucks 42 Ready-Mixed Concrete Advantages: Close quality control of batching Use on congested site Use of agitator trucks to ensure care of transportation 43 Ready-Mixed Concrete Advantages: Convenience when small quantities of concrete or intermittent mixing is required 44 Ready-Mixed Concrete Disadvantages: Maintaining workability up to the time of placing Cost is relatively higher 45