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

型枠内でコンクリートを過度に水平に動かしたり、かなりの高さからコンクリートを落としたり、方向を変えて狭いスペースに落としたりすると、そのコンポーネントの分離につながる可能性があります。
偏合は、より重い粗い凝集粒子がフォームの底に沈降する傾向がある現象です。同時に、より軽く、より細かい成分が上方に移動し、コンクリート混合物内に不均一な分布をもたらします。
別のタイプの分離は、セメントと水からなるグラウトが他のコンクリート混合物から分離する傾向がある場合に発生します。
コンクリートは、最終的な場所にできるだけ近づけて準備し、輸送距離を短縮して分離を防ぐ必要があります。
バイブレーターを長時間使用してコンクリートの山を広大な領域に分散させるなど、コンクリートに過度の作業を行うと、コンクリートの成分が分離される可能性があります。
過度の振動による可能性のある分離を観察するために、立方体内のコンクリートを約10分間振動させることができます。金型を取り外すと、粗い骨材分布の検査が可能になります。

生コンクリートの分離

生コンクリートの分離とは、コンクリート混合物の成分が分離し、不均一な分布と構造的完全性の低下につながる現象です。この分離は、コンクリートが型枠内で過度に水平方向に移動した場合、かなりの高さから落下した場合、または狭く曲がりくねった経路を強制的に通過した場合によく発生します。その結果、重い粗骨材粒子が底に沈み、セメントや水などの軽くて細かい材料が上に浮かびます。


 分離のもう 1 つの形態は、セメントと水で構成されるグラウトが混合物から分離することです。この分離はコンクリートの外観に影響を与えるだけでなく、構造能力を弱めます。分離のリスクを最小限に抑えるには、コンクリートを最終配置場所にできるだけ近い場所で準備することが理想的です。これにより、混合物を乱す可能性のある長距離輸送の必要性が減ります。


 さらに、長時間バイブレータを使用してコンクリートを広範囲に散布するなど、コンクリートに過度の作業を行うと、分離が生じる可能性があります。分離の可能性をテストする実用的な方法は、立方体の型枠内でコンクリートサンプルを約 10 分間振動させることです。型枠を取り外した後、粗骨材の分布を検査して分離の程度を評価できます。これらの予防措置とテストは、コンクリート構造物の均一性と強度を確保するために不可欠です。

型枠内でのコンクリートの過度の水平移動、またはかなりの高さから、または方向が変わる狭いスペースからコンクリートを落とすと、...

Education

JoVE Core

Civil Engineering

土木工学

Unit 1

ユニット1

Fresh Concrete

生コンクリート

KEY TERMS AND CONCEPTS

主要用語および概念

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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