Name: how concrete gains strength faster
Uploaded: 2024-11-07T14:48:35.000Z
Duration: 1 min 25 s
Channel: JoVE

コンクリートの硬化は、熱を加えて水分を加えることで加速され、より初期の強度を持つコンクリートが得られます。
プレキャスト製品によく使用される蒸気硬化は、チャンバー内のコンベヤー上で製品を移動させるか、プラスチックで覆うため、大気圧で蒸気を循環させることができます。
コンクリートの典型的な蒸気硬化サイクルは、湿潤硬化の最初の遅延期間から始まり、その後に加熱期間が続き、蒸気温度が華氏40〜60度で上昇し、華氏150〜180度に達するまで最大になります。
このピーク温度は、追加の熱を加えることなく維持され、この期間中、コンクリートは残留熱と水分を吸収します。
蒸気硬化サイクルは、冷却期間で終了し、その間に温度が下がります。
電気硬化技術は、外部電極、コンクリート内の鉄筋、または電気毛布を介してコンクリート部材を交流電流で加熱し、硬化プロセスを加速します。
あるいは、赤外線ヒーターを使用してコンクリートを加熱したり、鋼製フォームの周りに高温の油を循環させたり、低温にさらされたコンクリートを断熱ブランケットで覆ったりすると、硬化が促進されます。

コンクリートの養生

Accelerated Curing of Concrete

コンクリートの養生は、熱と水分を追加することで実現します。このプロセスによりセメントの水和が促進され、コンクリートの強度が早く高まります。蒸気養生は、コンクリート製品をコンベアベルトでチャンバー内を輸送するか、プラスチックで覆い、大気圧の蒸気がコンクリート製品の周囲を自由に循環できるようにする方法です。このプロセスは、通常 3 ～ 5 時間続く湿潤養生の段階から始まり、その後、温度が 1 時間あたり 4.44 ～ 15.56 ℃ の割合で徐々に上昇し、最高 65 ～ 82 ℃ に達します。その後、コンクリートには熱を加えませんが、残留熱と水分を吸収します。続いて、冷却期間が続き、温度が低下します。高圧蒸気養生は、主に中空コンクリートブロックなどの小型プレキャストコンクリート製品の養生に使用されます。あるいは、外部電極、鉄筋、またはコンクリートに巻き付けた電気毛布を介して交流電流を流すことでコンクリートを加熱します。その他の技術としては、赤外線ヒーターを使用したり、鉄骨の周囲に熱い油を循環させたりします。凍結温度にさらされるコンクリートの場合、断熱毛布、干し草や藁などの多孔質材料の層を使用して、熱損失を防ぎ、硬化を促進します。

コンクリートの硬化は、熱を加えて水分を加えることで加速され、より初期の強度を持つコンクリートが得られます。
蒸気硬化、プレキャストによく使用されます...

Education

JoVE Core

Civil Engineering

土木工学

Unit 1

ユニット1

Mixing, Placing and Curing Concrete 

コンクリートの練混ぜ、打設および養生

KEY TERMS AND CONCEPTS

主要用語および概念

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 hours. The temperature is then raised slowly, usually by 40 to 60 degrees Fahrenheit per hour, until it reaches about 150 to 180 degrees Fahrenheit.

After the target temperature is reached, the concrete is held so it can absorb the remaining heat and moisture without more added heat. A cooling period follows, and the temperature is allowed to drop. High-pressure steam curing is mainly used for smaller precast products, such as hollow concrete masonry blocks.

Other heating methods can also speed curing. Concrete may be warmed by alternating current through external electrodes, reinforcing steel, or electric blankets wrapped around it. Infrared heaters and hot oil around steel forms are other options. When concrete is exposed to freezing temperatures, insulating blankets or porous materials like hay or straw help reduce heat loss and support curing.

How Concrete Gains Strength Faster

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

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

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 heat, steam, and insulation speed concrete curing, from moist curing and temperature control to precast blocks and winter protection.

How Concrete Gains Strength Faster | 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 Gains Strength Faster

Description

Transcript