Name: how moisture cracks alkali-reactive concrete
Uploaded: 2024-11-07T14:48:46.000Z
Duration: 1 min 26 s
Channel: JoVE

骨材には、セメントアルカリからのアルカリ水酸化物と反応する珪質鉱物が自然に含まれています。
形成されたアルカリシリカゲルは水を引き付けて膨潤し、それは周囲のセメントペーストによって制約され、内圧につながり、セメントペーストをひび割れて破壊し、膨張によって証明されます。
膨張は、アルカリシリカ反応の浸透圧油圧と固体生成物の膨潤圧力に一部起因しています。
珪質粒子のサイズは反応速度に影響を与え、微粒子は数週間で膨張を引き起こしますが、大きな粒子は損傷を引き起こすのに数年かかる場合があります。
アルカリ-骨材反応の進行は、骨材の多孔性、セメントアルカリ含有量、ペースト中の水、セメントペーストの透過性などの要因の影響を受けます。
この反応は、主にコンクリートの表面で、連続的な水分下で、または特に華氏50度から100度の温度で湿潤および乾燥サイクル中に発生します。
有害なセメントと骨材の組み合わせを避けるのが難しい場合は、ポゾランと質量比で少なくとも30%の石灰岩粗骨材を組み込むことをお勧めします。低アルカリセメントを使用すると、アルカリ骨材反応を減らすのにも役立ちます。

コンクリートのアルカリ骨材反応

Alkali Aggregate Reaction in Concrete

コンクリートのアルカリ骨材反応では、骨材に含まれる天然のシリカ鉱物がセメントアルカリから得られるアルカリ水酸化物と反応します。この反応により、水を吸収して膨張し、体積が増加するアルカリシリカゲルが形成されます。このゲルは周囲のセメントペーストによって閉じ込められ、内部圧力が生じてコンクリートに亀裂や破壊が生じます。膨張と損傷の程度は、アルカリシリカ反応の浸透圧と、形成された固体生成物の膨張圧に一部起因します。


 反応の速度は、シリカ粒子のサイズによって左右されます。粒子が細かいと数週間で膨張する可能性がありますが、粒子が大きいと、同様の損傷が現れるまでには数年かかることがあります。骨材の空隙率、セメントのアルカリ含有量、ペースト内の水分量、セメントペーストの透水性などの要因はすべて、この反応の進行に影響します。この反応は通常、継続的な湿気の下、または湿潤と乾燥のサイクル中にコンクリートの表面で発生し、特に 10 ～ 37.78 ℃ の温度で発生します。これらの反応を緩和するには、低アルカリセメントを使用し、ポゾランを混ぜ込み、コンクリート混合物に質量比で少なくとも 30 ％の石灰岩粗骨材を使用することをお勧めします。

骨材には、セメントアルカリからのアルカリ水酸化物と反応する珪質鉱物が自然に含まれています。
形成されたアルカリシリカゲルは水を引き付けます...

Education

JoVE Core

Civil Engineering

土木工学

Unit 1

ユニット1

Permeability of Concrete

コンクリートの透水性

KEY TERMS AND CONCEPTS

主要用語と概念

how moisture cracks alkali-reactive concrete

Alkali-aggregate reaction in concrete can weaken a structure when siliceous minerals in the aggregate react with alkaline hydroxides from cement. The reaction makes an alkali-silica gel, which pulls in water and swells. As the gel expands inside the rigid cement paste, it creates pressure that can crack and disrupt the concrete.

The damage comes from both osmotic hydraulic pressure and the swelling of the solid reaction products. Particle size also matters. Fine siliceous particles can cause expansion in weeks, while larger particles may take years before the same kind of damage appears.

Several factors can speed or slow the reaction. These include the porosity of the aggregate, the alkali content of the cement, the amount of water in the paste, and the permeability of the cement paste. The reaction often shows up near the surface of concrete when moisture is present all the time or when the material goes through wetting and drying cycles, especially at temperatures between 50 and 100 degrees Fahrenheit.

Concrete can be protected by using low-alkali cement, adding a pozzolan, and including at least 30 percent limestone coarse aggregate by mass in the mix. These steps help limit the formation of the damaging gel and reduce the chance of cracking over time.

How Moisture Cracks Alkali-Reactive Concrete

how-moisture-cracks-alkali-reactive-concrete

permeability of concrete

Concrete Durability and Water Penetration

Concrete durability depends partly on how easily water and gases can move through it. This property, called permeability, is important for judging water-tightness and resistance to chemical attack in concrete structures. Permeability is measured with comparative lab tests. In a typical test, the sides of a concrete specimen are sealed, water pressure is applied to the top surface, and the amount of water that passes through is measured. The test is often run for about ten days until a steady...

sulfate attack on concrete

Concrete Damage from Sulfate Exposure

Concrete damage from sulfate exposure starts with a whitish discoloration, often first seen at the edges and corners. As the attack continues, the concrete can crack and spall. This deterioration happens when sulfates react with hardened concrete and create new compounds that take up more space than the materials they replace. Sulfates may enter concrete from soil, groundwater, or industrial effluents. Inside the concrete, they can form calcium sulfate and calcium sulfoaluminate. The reaction...

effect of sea water on concrete

Seawater Damage and Concrete Durability

Seawater can damage concrete by changing its chemistry, increasing porosity, and lowering strength. In marine settings, concrete is exposed to dissolved salts and repeated wetting, so its durability depends on how much water can enter the material. One major form of damage is the breakdown of ettringite and gypsum, two concrete minerals that can dissolve in seawater. As these materials are lost, the concrete becomes more porous. Salt can also crystallize inside pores, especially above the...

acid attack on concrete

Concrete Damage from Low pH Water

Concrete damage from low pH water happens when acids react with the hydrated cement paste. This reaction dissolves the paste, so the concrete becomes softer and weaker. The problem is often seen in chimneys, sewers, and industrial settings. The amount of damage depends on how acidic the water is. When pH falls below 6.5, the attack becomes more severe. A pH below 4.5 can cause major concrete damage. The rate of damage also depends on how fast hydrogen ions move through the cement gel. This...

alkali aggregate reaction in concrete

Alkali-aggregate reaction in concrete can weaken a structure when siliceous minerals in the aggregate react with alkaline hydroxides from cement. The reaction makes an alkali-silica gel, which pulls in water and swells. As the gel expands inside the rigid cement paste, it creates pressure that can crack and disrupt the concrete. The damage comes from both osmotic hydraulic pressure and the swelling of the solid reaction products. Particle size also matters. Fine siliceous particles can cause...

corrosion of reinforcement

Why Concrete Can Fail Around Rebar

Steel reinforcement in concrete can corrode when its protective environment changes. Fresh concrete is highly alkaline because of calcium hydroxide, with a pH of about 13. That high pH helps form a passive iron oxide layer on the steel surface, which slows corrosion. Over time, carbonation, chloride ingress, and cracking can weaken this protection. Concrete contains hydrated cement paste with salts, and it acts as an electrolyte. Different areas of the steel then behave as anodic and cathodic...

key terms and concepts

Learn how alkali-silica gel forms in concrete, why moisture makes it swell, and how low-alkali cement and pozzolans help reduce cracking.

How Moisture Cracks Alkali-Reactive Concrete | 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 Moisture Cracks Alkali-Reactive Concrete

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