Name: how concrete mixes separate during placement
Uploaded: 2024-11-07T14:48:28.000Z
Duration: 1 min 16 s
Channel: JoVE

يمكن أن تؤدي الحركة الأفقية المفرطة للخرسانة داخل الأشكال ، أو إسقاطها من ارتفاع كبير أو من خلال المساحات الضيقة ذات التغيرات الاتجاهية ، إلى فصل مكوناتها.
الفصل هو ظاهرة تميل فيها جزيئات الركام الخشنة الأثقل إلى الاستقرار في الجزء السفلي من النموذج. في الوقت نفسه ، تهاجر المكونات الأخف وزنا والدقيقة لأعلى ، مما يؤدي إلى توزيع غير متساو داخل خليط الخرسانة.
يحدث نوع آخر من الفصل عندما يميل الجص ، الذي يتكون من الأسمنت والماء ، إلى الانفصال عن بقية المزيج الخرساني.
يجب تحضير الخرسانة في أقرب وقت ممكن من موقعها النهائي ، مما يقلل من مسافة النقل لمنع الفصل.
إذا تم تنفيذ الكثير من العمل على الخرسانة ، مثل استخدام هزاز لفترة طويلة لتوزيع كومة من الخرسانة على مساحة شاسعة ، فقد يفصل المكونات الخرسانية.
لمراقبة الفصل المحتمل بسبب الاهتزاز المفرط ، يمكن اهتزاز الخرسانة في المكعب لمدة 10 دقائق تقريبا. ستسمح إزالة القالب بفحص توزيع الركام الخشن.

الانفصال الحبيبي في الخرسانة الطازجة

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إن الانفصال الحبيبي في الخرسانة الطازجة هو ظاهرة تنفصل فيها مكونات خليط الخرسانة، مما يؤدي إلى توزيع غير متساوٍ وتهديد سلامة البنية. ويحدث هذا الانفصال عادةً عندما تتعرض الخرسانة لحركة أفقية مفرطة داخل القوالب، أو عندما يتم إسقاطها من ارتفاعات كبيرة أو دفعها عبر مسارات ضيقة ومتعرجة. ونتيجة لذلك، تستقر جزيئات الركام الخشن الثقيلة في القاع، بينما ترتفع المواد الأخف والأدق مثل الأسمنت والماء إلى الأعلى.
هناك شكل آخر من أشكال الانفصال، وهو انفصال المونة (المكون من الأسمنت والماء) عن الخليط. ولا يؤثر هذا الانفصال على مظهر الخرسانة فحسب، بل ويضعف أيضًا من قدراتها البنيوية. وللحد من خطر الانفصال، يجب إعداد الخرسانة في أقرب مكان ممكن من موقع وضعها النهائي، وبالتالي تقليل الحاجة إلى النقل المكثف الذي قد يخلخل الخليط.
بالإضافة إلى ذلك، فإن الإفراط في العمل على الخرسانة، مثل الاستخدام المطول للاهتزازات لتوزيع الخرسانة على مساحة كبيرة، يمكن أن يؤدي إلى الانفصال. تتضمن إحدى الطرق العملية لاختبار الانفصال المحتمل اهتزاز عينة خرسانية في قالب مكعب لمدة 10 دقائق تقريبًا. عند إزالة القالب، يمكن فحص توزيع الكتل الخشنة لتقييم درجة الانفصال. تعد هذه التدابير الوقائية والاختبارات ضرورية لضمان اتساق وقوة هيكل الخرسانة.

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يمكن أن تؤدي الحركة الأفقية المفرطة للخرسانة داخل الأشكال ، أو إسقاطها من ارتفاع كبير أو من خلال مساحات ضيقة مع تغيرات اتجاهية ، إلى ...

Education

JoVE Core

Civil Engineering

الهندسة المدنية

Unit 1

الوحدة الأولى

Fresh Concrete

خرسانة حديثة الصب

KEY TERMS AND CONCEPTS

المصطلحات والمفاهيم الأساسية

Segregation in Fresh Concrete

how concrete mixes separate during placement

Fresh concrete can separate during placement, which changes how the mix is spread and can weaken the finished structure. This separation is called segregation. It happens when the parts of the mix no longer stay evenly blended.

Segregation often occurs when concrete moves too much inside a form. It can also happen when the mix is dropped from a great height or pushed through narrow paths with sharp turns. In these cases, the heavier coarse aggregate settles at the bottom, while lighter materials such as cement and water move upward.

Another form of segregation happens when the grout pulls away from the rest of the mix. Grout is the cement and water portion of concrete. When this separation occurs, the concrete may look uneven and lose strength.

To reduce segregation, concrete should be placed as close as possible to its final location. This lowers the need for transport that can disturb the mix. Overworking the concrete can also cause problems. For example, using vibrators for too long to spread concrete over a large area may lead to segregation.

A simple test can show whether segregation may occur. A sample of concrete is vibrated in a cube mold for about 10 minutes. After the mold is removed, the coarse aggregate is checked to see how evenly it is distributed. This helps judge the level of segregation and supports stronger, more uniform concrete.

How Concrete Mixes Separate During Placement

how-concrete-mixes-separate-during-placement

workability of concrete

Concrete Flow and Compaction Factors

Concrete workability is the ease with which fresh concrete can be mixed, placed, compacted, and finished. It is an important property during construction because it affects how well the material moves into position and how easily it can be shaped. Workability depends on the internal friction within the mix and the resistance it meets from molds and reinforcements. When the ingredients resist one another, concrete moves less easily. The same is true when it has to pass around barriers in the...

factors affecting workability

Concrete Workability and Mix Design Factors

Concrete workability depends on mix design factors that change how easily the material can be mixed, handled, and finished. The mix needs enough flow so it can move and be placed without losing quality. Water is a key part of this because it acts as a lubricant between the ingredients. The amount of water needed can change with the aggregates, or the solid particles in the mix. Finer particles and angular, rough-textured aggregates need more water than smooth, rounded ones because they have...

segregation in fresh concrete

Fresh concrete can separate during placement, which changes how the mix is spread and can weaken the finished structure. This separation is called segregation. It happens when the parts of the mix no longer stay evenly blended. Segregation often occurs when concrete moves too much inside a form. It can also happen when the mix is dropped from a great height or pushed through narrow paths with sharp turns. In these cases, the heavier coarse aggregate settles at the bottom, while lighter...

bleeding in fresh concrete

Bleeding Water and Laitance in Fresh Concrete

Bleeding in fresh concrete happens when water from the mix rises to the surface after placement. The solid parts of the mix do not hold all the water as they settle, so separation occurs and water collects on top. Engineers can measure the severity of bleeding by checking total settlement or by noting how much the concrete height drops per unit height. Bleeding can create weak spots inside the concrete. Rising water may get trapped under large aggregate particles or reinforcement bars, which...

slump test

Concrete Workability Check with Slump

The slump test checks the workability of fresh concrete. Workability means how easy the mix is to place and handle. The test uses a 12-inch-high truncated cone mold that narrows from eight inches at the base to four inches at the top. Before testing, the mold is fixed to a flat base and dampened. Concrete is added in three layers. Each layer is compacted 25 times with a steel tamping rod that is five-eighths of an inch in diameter and has a rounded end. This helps spread the concrete evenly...

compacting factor test

Concrete Workability by Compacting Factor

The compacting factor test measures the workability of concrete. Workability describes how easily concrete can move and fill a mold or form during construction. This test is especially suitable for concrete mixes with aggregates up to one and a half inches in size. The setup uses specialized equipment with two truncated cone-shaped hoppers and a cylinder. All of the inside surfaces are polished to reduce friction. Concrete is first placed into the upper hopper without any compaction. Then the...

vebe test

Concrete Workability with the Vebe Test

The Vebe test measures the workability of concrete, especially for dry mixes. Workability means how easy concrete is to handle and compact. The test uses a cylindrical chamber, a standard slump cone, and a transparent disc-shaped rider mounted on a vibrating table. The cylindrical chamber measures nine and a half inches in diameter and eight inches in height. Concrete is placed into the slump cone in layers and compacted in the standard way. After the cone is removed, the concrete stays in the...

flow table test

Concrete Spread Measured by Flow Table

The flow table test measures how far fresh concrete spreads, which helps assess workability. Workability means how easily concrete can flow and settle under its own weight. The method is useful for highly flowable concrete mixes. The test uses a flow table made from a wooden board with a steel plate on top. Together, the parts weigh 35 pounds. The board is hinged to a base and measures 27.6 inches on each side. A truncated cone mold is also used. It is 8 inches high, with an 8-inch base...

key terms and concepts

Learn how fresh concrete can segregate during placement, why aggregate and grout separate, and how a cube mold test helps check mix uniformity.

How Concrete Mixes Separate During Placement | 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

How Concrete Mixes Separate During Placement

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