Name: concrete fatigue under repeated loads
Uploaded: 2024-11-07T14:48:31.000Z
Duration: 1 min 22 s
Channel: JoVE

コンクリートでは、疲労は2つの異なる形態の破壊につながる可能性があります。1つ目は、一貫した負荷または徐々に増加する負荷の下で発生します。これは、静的疲労またはクリープ破断と呼ばれます。
2番目の形態の破壊は、周期的または反復的な荷重の下で発生し、周期的疲労と呼ばれます。
圧縮強度は通常、2〜4分の短いスパンで測定されます。負荷率が低下するか、試験時間が長くなると、測定された強度は標準試験で観察された強度を下回ります。
逆に、荷重が急速またはほぼ瞬時に加えられると、結果として得られる強度は高くなりますが、破壊時の材料のひずみまたはひずみ容量は減少します。
低荷重率では、応力が短期強度の約70〜80%を超えると、コンクリートの静的疲労が発生します。このレベルは、急速なマイクロクラック形成の始まりを示し、失敗につながります。
コンクリートは経年劣化とともに強度が増し、繰り返し荷重に耐える能力、つまり疲労強度が比例して増加します。これは、コンクリートが、年齢に関係なく、最終強度に比例する応力レベルで疲労下で破損することを意味します。

コンクリートの疲労強度

Fatigue Strength of Concrete

材料科学および工学の文脈における疲労とは、繰り返し加えられた荷重によって、材料が強度限界未満であっても弱くなったり、破損したりすることを指します。コンクリートの疲労強度は、耐久性と寿命に影響を与える重要な特性です。コンクリートは、疲労によって 2 つの方法で破損する可能性があります。静的疲労またはクリープ破壊は、一定の荷重またはゆっくりと増加する荷重の下で発生します。もう 1 つは、疲労破壊と呼ばれる周期的または反復的な荷重によるものです。


 興味深いことに、コンクリートの圧縮強度は、通常数分という短い時間で測定されますが、荷重が加えられる速度や試験の継続時間によって変化します。たとえば、荷重速度が遅い場合や試験時間が長い場合は、測定される強度は低くなります。同時に、荷重を急速に加えると強度は高まりますが、材料のひずみ容量は低下します。静的疲労は、応力が短期強度の約 70 ～ 80 ％を超えると、低荷重速度で発生し、急速に微小亀裂が形成されて破損につながります。コンクリートが古くなると、強度と疲労強度が連動して増加します。これは、周期数に関係なく、最大強度の一定の割合で疲労破損が発生することを示しています。

コンクリートでは、疲労は2つの異なる形態の破壊につながる可能性があります。1つ目は、一貫した負荷または徐々に増加する負荷の下で発生します。これは参照されます...

Education

JoVE Core

Civil Engineering

土木工学

Unit 1

ユニット1

Strength of Concrete

コンクリートの強度

KEY TERMS AND CONCEPTS

主要用語と概念

concrete fatigue under repeated loads

Concrete fatigue describes how concrete can weaken or fail when loads are applied again and again, even when each load is below the material’s strength limit. This property matters because it affects the durability and service life of concrete structures. In materials science and engineering, fatigue is a key reason concrete may not last as long as expected under real use.

Concrete can fail in two main ways through fatigue. Static fatigue, also called creep rupture, happens under a constant load or a load that rises slowly. The other type comes from cyclical or repeated loading, which is simply called fatigue.

The measured compressive strength of concrete depends on how fast the load is applied and how long the test lasts. A slower loading rate or a longer test time gives a lower strength value. A rapid load can increase the measured strength, but it also reduces the strain capacity of the material.

Static fatigue usually begins when stress is above about 70 to 80 percent of the short-term strength. At this level, microcracks form quickly and can lead to failure. As concrete ages, both its strength and its fatigue strength increase together. This shows that fatigue failure tends to occur at a consistent percentage of the maximum strength, no matter how many loading cycles are involved.

Concrete Fatigue Under Repeated Loads

tensile strength considerations of concrete

tensile-strength-considerations-of-concrete

How Cracks Limit Concrete Strength

Concrete tensile strength is limited by tiny cracks inside the material. The cement paste itself can be much stronger in theory than it is in practice. In real concrete, the measured strength can be up to a thousand times lower than the theoretical value. These small cracks help explain that gap. When a load is applied, stress rises at the crack tips. This idea is described by Griffith's theory of brittle fracture. In brittle materials, cracks can grow quickly once the stress at their tips...

behavior of concrete under compressive load

behavior-of-concrete-under-compressive-load

Concrete Failure Patterns Under Compression

Concrete failure patterns under compression show how the material responds to different kinds of direct stress. These patterns help explain concrete’s structural integrity. Under uniaxial compression, where force acts in one direction, cracks often form parallel to the load. These parallel cracks come from local tensile stress, which is pulling stress that appears perpendicular to the compression direction. As uniaxial loading continues, angled cracks may also appear. These cracks are linked...

porosity in cement paste

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Cement curing changes porosity in the hardened paste. Porosity means the void spaces inside concrete. These spaces affect both strength and durability. When water and cement react, hydration forms a semi-solid paste that traps water in different ways. Some of that water becomes combined water. It makes up about 23% of the cement's dry mass. Other water stays as gel water, which fills very small voids called gel pores. Gel pores account for about 28% of the cement gel volume. The...

total voids in concrete

Concrete Voids and Strength

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pore size distribution

Concrete Porosity and Durability

Concrete porosity and durability depend strongly on pore size distribution in hardened cement paste. Capillary pores are much larger than gel pores, and in partially hydrated cement paste they form a connected network that lowers strength and raises permeability. Higher permeability makes concrete more vulnerable to freeze-thaw damage and chemical attack. The risk also depends on the water-to-cement ratio. A higher ratio can leave more space for capillary pores to remain open. As cement...

microcracking in concrete

How Concrete Cracks Before Failure

Concrete can develop tiny cracks before any outside load is applied. These microcracks usually form at the boundary between coarse aggregate and hydrated cement paste. They often result from small volume changes caused by stress-strain differences, heat, and moisture movement. At first, these microcracks stay stable. They usually do not grow much until the concrete reaches about 30 percent of its ultimate strength. As the load rises, the material can begin to change more quickly. When...

water cement ratio

Concrete Strength and Water Balance

Concrete strength depends on the balance between water and cement in the mix. This water-cement ratio is the weight of water compared with the weight of cement. It affects strength, durability, and how well concrete resists environmental stress. Abrams’ law shows that less water usually makes stronger concrete. Water is still needed for hydration, which is the chemical process that binds water and cement. It also helps the mix stay workable and compactable. Some water must be absorbed into the...

aggregate cement ratio

How Mix Ratio Changes Concrete Strength

The aggregate cement ratio changes how concrete performs. It is the weight of aggregate divided by the weight of cement in a concrete mix. This ratio affects strength, workability, and durability, so it is an important part of concrete design. A higher aggregate cement ratio makes the mix leaner. Lean mixes have less cement paste to coat and lubricate the aggregate, so they can be harder to work with. At the same time, they may increase strength because the concrete has less total porosity, or...

transition zone

Concrete Transition Zone and Its Porosity

The concrete transition zone is the thin region where aggregate meets cement paste. It has more pores than the surrounding material, so it is usually weaker at first. The bond around the aggregate comes mainly from Van der Waals forces, which are weak attraction forces between particles. When concrete is compacted, this zone often has a higher water-cement ratio near the aggregate. That extra water can form thin water films around the coarse aggregate. These films increase porosity and help...

relation between tensile strength and compressive strength of concrete

relation-between-tensile-strength-and-compressive-strength-of-concrete

Concrete Strength Testing: Tension vs. Compression

Concrete strength testing compares tensile strength and compressive strength. These two properties are related, but they do not rise at the same rate. Tensile strength usually grows more slowly than compressive strength, so the balance between them changes as concrete gets stronger. Several factors affect tensile strength measurements. The test method, specimen size and shape, aggregate texture, and moisture content can all change the result. Direct tension tests are difficult to perform...

fatigue strength of concrete

Concrete fatigue describes how concrete can weaken or fail when loads are applied again and again, even when each load is below the material’s strength limit. This property matters because it affects the durability and service life of concrete structures. In materials science and engineering, fatigue is a key reason concrete may not last as long as expected under real use. Concrete can fail in two main ways through fatigue. Static fatigue, also called creep rupture, happens under a constant...

impact strength of concrete

Concrete Impact Resistance in Real Use

Concrete impact resistance matters when a material must handle repeated blows and sudden loads. It is important during pile driving, where concrete piles are driven into the ground. It also matters for machinery foundations that experience impulsive loads and for precast concrete parts that can be damaged during handling. Impact strength is measured by checking how well concrete absorbs energy from repeated impacts. A major sign of serious damage is when the concrete does not rebound after...

abrasion resistance of concrete

Concrete Wear Tests and Surface Durability

Concrete wear tests show how well a surface can stand up to abrasion and long-term use. Abrasion is the wearing away of a material by rubbing, scraping, or contact with moving solids. This property matters because concrete surfaces can lose material under vehicle traffic or when solids carried in water brush against them. The transcript describes three tests used to measure concrete abrasion resistance. In the revolving disc test, three plates rotate against the concrete, and silicon carbide...

reinforcements in concrete

Steel Bars in Concrete Structures

Steel bars in concrete structures help the material carry both compression and tension. Concrete is strong when it is squeezed, but it is weak when pulled. Rebar, or reinforcing steel bars, is placed inside the concrete to handle those tensile forces. Steel is a practical choice because it has a similar coefficient of thermal expansion to concrete. That means both materials expand and contract in a similar way as temperature changes. The steel used for reinforcement is usually made by a...

fiber reinforced concrete

Concrete Strength with Added Fibers

Fiber-reinforced concrete is concrete that includes added fibers to improve strength and durability. It uses hydraulic cement with fine or coarse aggregates, plus separated, non-continuous fibers mixed through the material. These fibers can come from steel, glass, and polymers, as well as natural sources such as sisal and cellulose or manufactured sources such as polypropylene and Kevlar. Steel fibers are often preferred because they are strong and tough. They help concrete resist cracking and...

prestressed concrete

How Concrete Is Preloaded for Strength

Prestressed concrete is a construction method that preloads concrete so it can handle more stress. High-strength steel strands are stretched before or after the concrete is loaded in service. The goal is to put the concrete into compression. That compression helps balance the tension the structure will face later. By reducing tension, prestressing helps limit cracks in the concrete. It can also let a structure carry greater loads or span longer distances than conventional reinforced concrete.

ferrocement

Ferrocement Mesh and Mortar Design

Ferrocement is a construction material made from dense layers of mesh or fine rods covered fully in cement mortar. It is a form of reinforced concrete, but it uses mesh instead of the larger steel bars found in standard reinforced concrete. This structure gives ferrocement its thin, flexible form. The material is built from wire meshes and a mortar made of cement and sand. The mortar is usually mixed in a 1:2 or 1:3 ratio. The water-to-cement ratio is kept between 0.40 and 0.45. This helps the...

non-destructive tests for concrete strength

non-destructive-tests-for-concrete-strength

Concrete Strength Checks Without Breaking It

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key terms and concepts

Learn how concrete can fail under constant or repeated loads, why loading rate changes measured strength, and how aging affects fatigue strength.

Concrete Fatigue Under Repeated Loads | 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 Fatigue Under Repeated Loads

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