Name: concrete mixer charging and timing
Uploaded: 2024-11-07T14:48:34.000Z
Duration: 1 min 19 s
Channel: JoVE

Les bétonnières sont initialement chargées de 10 % de l’eau totale, suivies d’ajouts constants de matériaux solides et de 80 % de l’eau totale.
La durée du mélange commence dès que tous les composants sont dans le mélangeur. Les 10 % restants de l’eau sont incorporés dans le premier quart de la durée totale du mélange.
Un mélange pendant moins de 1 à 1,5 minute peut conduire à un mélange de béton non uniforme avec une résistance plus faible.
Le temps de malaxage optimal pour produire un béton uniforme dépend de la capacité du malaxeur ; un minimum d’une minute de mélange est nécessaire pour un malaxeur d’une capacité allant jusqu’à un mètre cube, qui augmente de quinze secondes pour chaque mètre cube supplémentaire.
De plus, le type de mélangeur, la vitesse de rotation et la qualité du mélange pendant le chargement des ingrédients sont essentiels pour déterminer le temps de mélange optimal.
Un mélange prolongé peut entraîner l’évaporation de l’eau du mélange, le broyage des agrégats mous, une diminution de la maniabilité du mélange, une augmentation de la température du mélange due aux effets de frottement et une diminution de la teneur en air pour le béton entraîné à l’air.

temps de mélange

Mixing Time

Le temps de mélange est un élément important pour obtenir un mélange de béton uniforme avec la résistance requise. La période de mélange commence une fois que tous les composants sont dans le malaxeur. Au départ, le malaxeur est chargé avec 10 % d’eau, suivi de l’ajout constant de solides, puis de 80 % d’eau. L’eau restante est ajoutée plus tard, dans le premier quart de la période de mélange. Le temps de mélange minimum varie en fonction de la capacité du malaxeur ; par exemple, les malaxeurs d’une capacité allant jusqu’à 0,76 mètre cube nécessitent au moins 1 minute, avec un incrément de 15 secondes pour chaque 0,76 mètre cube supplémentaire. Il est également essentiel de prendre en compte le type de malaxeur, sa vitesse de rotation et la qualité du mélange pendant le chargement pour déterminer les temps de mélange optimaux. Alors que les malaxeurs standard nécessitent souvent un temps de mélange d’au moins 1 à 1,5 minute, les malaxeurs modernes sont efficaces avec des périodes plus courtes. Si des granulats légers sont utilisés, la durée de mélange ne doit pas descendre en dessous de 5 minutes, et pour le béton à air occlus, elle doit être d’au moins 2 ou 3 minutes. Un mélange prolongé peut entraîner une évaporation de l’eau, des changements dans la granulométrie des granulats, une réduction de l’ouvrabilité, une augmentation de la température et, pour le béton à air occlus, une diminution de la teneur en air.

Temps de mélange

Les bétonnières sont initialement chargées de 10 % de l’eau totale, suivies d’ajouts constants de matériaux solides et de 80 % du total ...

Education

Enseignement

JoVE Core

Civil Engineering

Génie civil

Unit 1

Unité 1

Mixing, Placing and Curing Concrete 

Mélange, mise en œuvre et cure du béton

KEY TERMS AND CONCEPTS

TERMES ET CONCEPTS CLÉS

concrete mixer charging and timing

Concrete mixer charging and timing are key to making a uniform concrete mix with the needed strength. Mixing begins only after all ingredients are inside the mixer. The usual charging order starts with 10% of the water, then the solid materials are added steadily, followed by 80% of the water. The last portion of water is added later, within the first quarter of the total mixing period.

The minimum mixing time depends on mixer capacity. Mixers with up to 1 cubic yard of capacity need at least 1 minute, and the time increases by 15 seconds for each extra cubic yard. Mixer type, rotation speed, and how well the materials blend during charging also affect the best mixing time. Standard mixers often need at least 1 to 1.5 minutes, while modern mixers can work well in less time.

Some concrete mixes need longer mixing periods. If lightweight aggregate is used, the mixing time should be at least 5 minutes. For air-entrained concrete, the time should be at least 2 or 3 minutes. Too much mixing can cause water to evaporate, change aggregate grading, lower workability, and raise the temperature. In air-entrained concrete, prolonged mixing can also reduce air content.

Concrete Mixer Charging and Timing

mixing concrete

Concrete Mixer Types and Capacity

Concrete mixing creates a uniform blend of cement, aggregates, and paste. The goal is to coat the aggregates evenly with cement paste. Mixers used for this job are usually grouped into two main types: batch mixers and continuous mixers. Batch mixers handle one load at a time. They mix the materials thoroughly, discharge the batch, and then receive the next one. Continuous mixers work differently. They take in a steady flow of ingredients, mix them at a constant rate, and discharge the concrete...

mixing time

Concrete mixer charging and timing are key to making a uniform concrete mix with the needed strength. Mixing begins only after all ingredients are inside the mixer. The usual charging order starts with 10% of the water, then the solid materials are added steadily, followed by 80% of the water. The last portion of water is added later, within the first quarter of the total mixing period. The minimum mixing time depends on mixer capacity. Mixers with up to 1 cubic yard of capacity need at least...

ready mixed concrete

How Ready-Mixed Concrete Is Delivered

Ready-mixed concrete is prepared at a centralized plant and then delivered by truck to a construction site. It arrives ready for placement, which makes it different from concrete mixed entirely on site. Ready-mixed concrete includes three main forms: central-mixed, truck-mixed, and shrink-mixed. Central-mixed concrete is fully mixed at the plant and carried to the site in agitator trucks. These trucks rotate at 2 to 6 rpm, which helps keep the concrete in motion during transport. Truck-mixed...

pumped concrete

Concrete Pumping for Hard-to-Reach Sites

Concrete pumping moves large amounts of concrete through pipes to places that are hard to reach. It is used for sites such as deep foundation pits and other inaccessible locations. The system includes a hopper, a pump, and pipelines that carry the concrete to the placement point. Direct-acting pumps use a piston to move the concrete. Concrete enters the pump through the inlet valve by gravity and by suction created as the piston moves. The forward movement of the piston then pushes the...

placing concrete

Concrete Placement Methods and Compaction

Concrete placement methods focus on keeping the mix in a uniform shape and position. The concrete is placed as close as possible to its final location to reduce segregation, which is the separation of ingredients. It is then fully compacted to remove trapped air. The next layer is added while the layer below is still plastic, or soft enough to shape. The rate of placing and compacting concrete is kept equal. Concrete is laid in uniform layers, and hand shoveling is avoided. Moving concrete...

vibrating concrete

Concrete Compaction with Vibrators

Concrete compaction with vibrators helps newly poured concrete settle inside formwork and around reinforcement bars. The goal is to remove trapped air pockets and create a dense concrete mass. Mechanical vibration is a key part of that process. One common method uses an internal vibrator, also called a poker vibrator or immersion vibrator. It is quickly inserted through the full depth of the fresh concrete and slightly into the layer below, which is still in a plastic state. The vibrator is...

shotcrete

Shotcrete for Vertical and Overhead Surfaces

Shotcrete is a concrete spraying method used on vertical and overhead surfaces. It uses mortar or small aggregate concrete with a high cement content. The material is sprayed at high velocity, and the impact helps compact it so it can cling without sagging. Builders place shotcrete in layers until it reaches about 4 inches thick. The quality depends strongly on the operator’s skill in managing the nozzle. Careful nozzle control helps the material land evenly and build up properly. Shotcrete...

preplaced aggregate concrete

Concrete Placement for Tight Spaces

Preplaced aggregate concrete is used when a construction site is hard to reach. The process starts by wetting gap-graded coarse aggregates. This helps remove dirt from the stones. The aggregates are then placed in the form and compacted. Next, a mortar mix is pumped under pressure through slotted pipes to fill the spaces between the aggregates. The mortar usually contains Portland cement, pozzolan, fine aggregates, water, and a fluidizing aid. The pozzolan reduces bleeding and segregation and...

finishing concrete

Concrete Surface Finishing Steps

Concrete surface finishing begins right after the concrete has been placed and consolidated. The first step is screeding, which levels the surface by removing extra material until it is even with the top of the formwork. A screed is a straight edge used for this leveling step. After screeding, a bull float or darby is used to smooth the surface further. These tools help lower high spots, fill low areas, and push larger aggregate particles deeper into the concrete. This preparation should...

curing of concrete

How Curing Improves Concrete Strength

Curing helps concrete gain strength after it has set. It keeps the concrete moist and at the right temperature so cement hydration can continue. Hydration is the process where cement reacts with water in the water-filled capillary pores and forms the products that fill spaces in the paste. Dry air can interrupt this process by pulling water from the concrete surface. Concrete with a water-cement ratio below 0.5 can also lose water through self-desiccation, which means internal water is used up...

curing methods

Concrete Curing for Moisture Retention

Concrete curing for moisture retention helps fresh concrete keep the water it needs as it hardens. For members with a small surface-to-volume ratio, the forms are often oiled and moistened before the concrete is placed. These forms may stay in place for a long time to reduce moisture loss. If the form material can be wetted, it can also be kept damp. When forms are removed early, the concrete member is moistened and then covered with polythene sheets. Large horizontal concrete surfaces in dry...

accelerated curing of concrete

How Concrete Gains Strength Faster

Concrete can gain strength faster when heat and extra moisture speed up cement hydration. This faster hydration helps the concrete build early strength sooner than normal curing. The process is used when quicker hardening is needed after casting. One common method is steam curing. Concrete products may move through a chamber on a conveyor belt, or they may be encased in plastic so steam at atmospheric pressure can circulate around them. Steam curing starts with moist curing for about 3 to 5...

key terms and concepts

Learn how concrete mixer charging order and mixing time affect mix quality, strength, workability, and air content in high school concrete science.

Concrete Mixer Charging and Timing | JoVE K12

JoVE Core Civil Engineering provides a foundational exploration of essential construction materials in civil engineering, focusing on bricks, stones, steel, wood, and concrete through easy-to-understand animated videos. This collection covers the properties, manufacturing processes, and practical applications of these materials. Additionally, the accompanied science-in-action videos demonstrate the research experiments performed in laboratories.

civil engineering

Concrete Mixer Charging and Timing

Description

Transcript

Tags