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

מערבלי בטון נטענים בתחילה ב-10% מכלל המים, ולאחר מכן מתווספים באופן עקבי חומרים מוצקים ו-80% מכלל המים.
משך הערבוב מתחיל ברגע שכל הרכיבים נמצאים במיקסר. 10 האחוזים הנותרים של המים משולבים בתוך הרבעון הראשון של משך הערבוב הכולל.
ערבוב של פחות מדקה עד דקה וחצי יכול להוביל לתערובת בטון לא אחידה עם חוזק נמוך יותר.
זמן הערבוב האופטימלי לייצור בטון אחיד תלוי בקיבולת המיקסר; מינימום דקה אחת של ערבוב נדרשת למיקסר בעל קיבולת של עד מטר מעוקב אחד, אשר גדל בחמש עשרה שניות עבור כל מטר מעוקב נוסף.
יתר על כן, סוג המיקסר, מהירות הסיבוב ואיכות הערבול במהלך טעינת המרכיבים הם המפתח לקביעת זמן הערבוב האופטימלי.
ערבוב ממושך עלול להוביל לאידוי מים מהתערובת, טחינה של אגרגטים רכים, ירידה ביכולת העבודה של התערובת, עלייה בטמפרטורת התערובת עקב השפעות חיכוך וירידה בתכולת האוויר של בטון מאומן באוויר.

זמן ערבול

Mixing Time

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מושג זמן הערבול הוא בעל חשיבות רבה בהפקת תערובת בטון אחידה עם החוזק הנדרש. זמן הערבול מתחילה ברגע שכל המרכיבים נמצאים במערבל. בתחילה, נטענים למערבל 10% מהמים, לאחר מכן מוסיפים בצורה עקבית את המוצקים, ולאחריהם 80% נוספים מהמים. יתרת המים מתווספת מאוחר יותר, בתוך הרבע הראשון של זמן הערבול הכולל.

זמן הערבול המינימלי משתנה בהתאם לקיבולת המערבל; לדוגמה, מערבלים עם קיבולת של עד 0.76 מ"ק דורשים לפחות דקה אחת לערבול, עם תוספת של 15 שניות לכל 0.76 מ”ק נוסף. חשוב גם לקחת בחשבון את סוג המערבל, מהירות הסיבוב שלו ואיכות הערבוב בזמן טעינת החומרים, כדי לקבוע את זמן הערבול האופטימלי.

בעוד שמערבלים סטנדרטיים דורשים לעיתים זמן ערבול שלא יפחת מ-1 עד 1.5 דקות, מערבלים מודרניים יעילים יותר ויכולים לעבוד עם פרקי זמן קצרים יותר. כאשר משתמשים באגרגטים קלים, זמן הערבול לא צריך לרדת מתחת ל-5 דקות, ובמקרה של בטון הכולל אוויר כלוא, יש להקפיד על זמן ערבוב של לפחות 2 עד 3 דקות.

ערבול ממושך מדי עלול להוביל לאידוי מים, לשינויים בדרגת האגרגטים, להפחתה בעבידות, לעליית הטמפרטורה, ובמקרה של בטון הכולל אוויר כלוא, לירידה בתכולת האוויר. לכן, חשוב להתאים את זמן הערבול הנכון כדי להשיג תערובת בטון איכותית התואמת את דרישות הפרויקט.
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מערבלי בטון טעונים בתחילה עם 10 אחוזים מכלל המים, ואחריו תוספות עקביות של חומרים מוצקים ו 80 אחוזים מכלל ...

Education

JoVE Core

Civil Engineering

הנדסה אזרחית

Unit 1

יחידה 1

Mixing, Placing and Curing Concrete 

ערבוב, יציקה וריפוי בטון

KEY TERMS AND CONCEPTS

מונחי מפתח ומושגים

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

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