Name: aggregate durability in freeze-thaw cycles
Uploaded: 2024-11-07T14:48:25.000Z
Duration: 1 min 26 s
Channel: JoVE

Considera un aggregato leggero con una struttura porosa. L'acqua che entra in questi pori si espande di volume dopo il congelamento, generando pressione idraulica all'interno dei pori dell'aggregato.
Questa pressione alla fine disintegra l'aggregato, rendendolo insano a causa di variazioni di volume permanenti.
La dimensione e la continuità dei pori negli aggregati sono cruciali nei danni da gelo in quanto controllano il movimento dell'acqua all'interno dell'aggregato.
Allo stesso modo, anche la cristallizzazione del sale all'interno dei pori dell'aggregato dovuta ai cicli di bagnatura e asciugatura provoca variazioni permanenti del volume.
La solidità degli aggregati viene verificata sottoponendo gli aggregati calibrati e pesati a cicli di immersione in una soluzione di solfato di sodio o magnesio seguiti da essiccazione in forno.
I sali cristallizzano all'interno dei pori dell'aggregato in questo processo, creando sollecitazioni che potrebbero disintegrare l'aggregato. Ciò si traduce in una riduzione delle dimensioni delle particelle e nella perdita di peso, che indica la sua solidità.
Altri test di laboratorio per la solidità prevedono di sottoporre gli aggregati sciolti e gli aggregati incorporati nel calcestruzzo a cicli di congelamento e scongelamento che possono essere incontrati nella pratica.
Generalmente, si verificano fuoriuscite, incrostazioni e fessurazioni superficiali nel calcestruzzo con aggregati non solidi.

inaffidabilità dell'aggregato dovuta a variazione di volume

L'inaffidabilità degli aggregati dovuta a variazioni di volume è causata principalmente dalle alterazioni fisiche a cui sono sottoposti gli aggregati, come congelamento e scongelamento, variazioni termiche, bagnatura e asciugatura. Gli aggregati instabili, quando sottoposti a queste variazioni, subiscono una variazione di volume che porta alla disintegrazione. Ciò, a sua volta, contribuisce al deterioramento del calcestruzzo, tra cui desquamazione, scagliature e screpolature. Particolari tipi di aggregati, come selci porose, selci e quelli contenenti minerali argillosi, sono particolarmente sensibili a queste variazioni.


 Per valutare il danno potenziale di tali cambiamenti, sono stati stabiliti diversi test standardizzati, come l'esposizione degli aggregati a una soluzione di solfato di magnesio e cicli di essiccazione. Queste procedure mirano a simulare la formazione di cristalli di sale e la conseguente rottura delle strutture delle particelle attraverso ripetute espansioni e restringimenti. Inoltre, la suscettibilità al gelo degli aggregati viene analizzata specificamente concentrandosi sulla reazione degli aggregati ai cicli di congelamento e scongelamento. Questi test vengono eseguiti esclusivamente su aggregati o campioni di calcestruzzo che incorporano aggregati.


 La forma e la continuità dei pori dell'aggregato, che influenzano la velocità e la quantità di acqua assorbita e rilasciata dall'aggregato, sono spesso più importanti del volume totale dei pori nel determinare la durabilità dell'aggregato in diverse condizioni ambientali.

Inaffidabilità dell'aggregato dovuta a variazione di volume

Considera un aggregato leggero con una struttura porosa. L'acqua che entra in questi pori si espande di volume dopo il congelamento, generando pressione idraulica ...

unsoundness-of-aggregate-due-to-volume-change

Education

Didattica

JoVE Core

Civil Engineering

Ingegneria Civile

Unit 1

Unità 1

Aggregates and Water

Aggregati e acqua

KEY TERMS AND CONCEPTS

TERMINI E CONCETTI CHIAVE

Unsoundness of Aggregate due to Volume Change

aggregate durability in freeze-thaw cycles

Aggregate durability can decline when volume changes damage the particles. Freezing and thawing, thermal change, and wetting and drying all place stress on aggregates. When unsound aggregates break down under these conditions, they can contribute to scaling, pop-outs, and cracking in concrete.

Some aggregates are more vulnerable than others. Porous flints, cherts, and aggregates that contain clay minerals are especially prone to this kind of damage. Their internal structure can make them easier to disintegrate when environmental conditions change.

Engineers use standardized tests to check whether an aggregate may be unsound. One common method exposes the material to magnesium sulfate solution and drying cycles. This helps simulate salt crystal growth and the repeated expansion and shrinking that can disrupt particle structure. Frost susceptibility is also tested by observing how aggregates respond to freezing and thawing cycles, and these tests may be done on aggregates alone or on concrete specimens that contain them.

The pore structure of an aggregate matters a great deal. The shape and continuity of the pores affect how much water the aggregate absorbs and releases, and how quickly it does so. In many cases, these pore features are more important than the total pore volume when judging durability in changing weather conditions.

Aggregate Durability in Freeze-Thaw Cycles

aggregate-durability-in-freeze-thaw-cycles

unsoundness of aggregate due to volume change

Aggregate durability can decline when volume changes damage the particles. Freezing and thawing, thermal change, and wetting and drying all place stress on aggregates. When unsound aggregates break down under these conditions, they can contribute to scaling, pop-outs, and cracking in concrete. Some aggregates are more vulnerable than others. Porous flints, cherts, and aggregates that contain clay minerals are especially prone to this kind of damage. Their internal structure can make them...

deleterious substances in aggregate

Testing Aggregate Impurities in Concrete

Aggregate impurities can lower the quality and durability of concrete. Organic matter, such as loam in sand, can interfere with cement hydration and weaken the bond between the aggregate and the cement paste. Organic impurities are often checked with a colorimetric test. In this test, the darkness of the solution after agitation shows the level of organic content. Clay and fine material can also coat aggregate particles and reduce the bond with the cement paste. The amount of clay and silt in...

sieve analysis and grading curves

Aggregate Size Testing with Sieve Grading

Aggregate size testing with sieve grading measures the particle size distribution of aggregate materials. It starts with a dried sample of aggregate that is weighed first. The sample is then sorted through a stack of square-opening sieves arranged from largest opening on top to smallest opening below. The sieve stack is shaken or vibrated so the particles move through the openings by size. After a set time, the material left on each sieve is weighed. That weight is then written as a percentage...

fineness modulus

Fineness Modulus from Sieve Analysis

Fineness modulus is a numerical index used to describe how coarse or fine aggregate particles are. For sand, it is found from sieve analysis using a standard set of ASTM sieves. The result helps show whether the sample is finer or more coarse. To calculate fineness modulus, first record the weight of aggregate retained on each sieve and in the pan at the bottom. Then find the cumulative weight retained on each sieve. Next, express each cumulative weight as a percentage of the total sample...

types of aggregate grading

Aggregate Grading for Strong Concrete Mixes

Aggregate grading describes the particle size distribution in a sample of aggregate. It is important for making a concrete mix that has enough strength, reasonable workability, and minimal segregation. The right grading also helps concrete stay economical. There are four main types of aggregate gradation: well-graded, uniformly graded, gap-graded, and open-graded. Each type has a different pattern on a gradation curve, which shows how particle sizes are spread across sieve sizes. These...

maximum size of aggregate

How Aggregate Size Affects Concrete Mix

Aggregate size affects how concrete is mixed, placed, and strengthened. The maximum size of aggregate is defined by the sieve aperture that retains 15 percent or more of the particles in the sample. This size choice helps determine the concrete’s water demand, workability, and final strength. Larger aggregate particles reduce the surface area that must be covered by cement paste. As a result, they can lower the water needed in the mix. When the desired workability and richness are kept, this...

quality of water

Water Standards for Concrete Mixing

Water standards for concrete mixing help control strength and durability. Potable water is usually preferred. Even so, it should not contain too much sodium or potassium, because these salts can weaken concrete integrity. Water quality is judged by impurities such as dissolved solids, chlorides, and sulfates. The pH value is ideally between 6 and 8. Slightly acidic natural water may still be acceptable if it does not contain harmful organic matter or algae. Limits for these impurities depend...

testing water quality

Water Tests for Concrete Performance

Water tests for concrete performance help determine whether an uncertain water source is safe to use in cement and mortar. The main checks compare the water against de-ionized or distilled water benchmarks. Engineers look at both setting time and the compressive strength of mortar made with the test water. Standards set clear limits for these results. ASTM C1602 requires setting times to stay within 90 minutes of the control. BS 3146:1980 allows a 30-minute change in initial setting, while BS...

aggregates classification

Aggregate Types by Size, Weight, and Source

Aggregate types are grouped by size, petrographic characteristics, weight, and source. These categories help describe how different aggregates are identified and used in concrete. Size is one of the most common ways to classify them. Based on particle size, aggregates are either coarse or fine. Coarse aggregates are particles that do not pass through ASTM sieve No. 4. Fine aggregates are the particles that do pass through that sieve. Petrographic classification looks at shared mineral features.

shape and texture of coarse aggregate

How Aggregate Form Changes Concrete

Aggregate form affects how concrete mixes, packs, and bonds. Coarse aggregate is classified by the sharpness or roundness of its edges and corners. The main shapes are rounded, angular, elongated, and flaky. These shapes help show how the particles will behave in concrete. Rounded aggregates are fully shaped by attrition, which means they have been worn down by rubbing and impact. River gravel and seashore gravel are common examples. Angular aggregates, such as crushed rock, have clear edges...

bonding and strength of aggregate

How Aggregate Shape Affects Concrete Bond

Aggregate shape and surface texture affect how well concrete holds together. Rough, angular aggregates usually bond more strongly with the cement matrix than smooth, rounded ones. This stronger adhesion can help produce high-strength concrete. Angular aggregates also have a larger surface area. That gives the cement paste more area to grip, which improves the bond. Bond quality is hard to measure directly, and there is no single test that is accepted everywhere. A broken concrete sample can...

toughness and hardness of aggregate

Testing Aggregate Impact and Abrasion

Aggregate toughness and hardness are important for concrete used on pavement surfaces and industrial floors that must carry heavy loads. Toughness means an aggregate can resist failure from impact. Hardness means it can resist wear.

specific gravity of aggregate

Aggregate Pore Effects on Specific Gravity

Aggregate specific gravity depends on the pores inside the material. These pores may be permeable or impermeable, so the same aggregate can be described in more than one way. For this reason, specific gravity is grouped into bulk or gross specific gravity, apparent specific gravity, and absolute specific gravity. Bulk or gross specific gravity uses the mass of aggregate in the saturated surface-dry state. It is compared with the total volume of the aggregate, including both the solids and the...

bulk density of aggregate

How Aggregate Size Affects Bulk Density

Bulk density describes the mass of aggregate that fills a unit volume, including the empty spaces between particles. Those void spaces remain because aggregate particles cannot be packed so tightly that every gap disappears. In practice, bulk refers to the total volume of both the particles and the spaces around them. This measurement matters when aggregates are batched by volume. It also helps convert materials measured by mass into a volume-based quantity. Natural mineral aggregates such as...

porosity and absorption of aggregate

Aggregate Pore States and Water Uptake

Aggregate contains pores of different sizes, and those pores may be sealed inside the particle or open to the surface. Open pores let water enter the aggregate. Because aggregate makes up about three-quarters of concrete by volume, its porosity strongly affects the porosity of the finished concrete. Aggregate can also be described by its moisture state. When water fills every pore, the material is saturated and surface-dry. If it sits in dry air, some water evaporates and the aggregate becomes...

moisture content and bulking of aggregate

moisture-content-and-bulking-of-aggregate

How Aggregate Moisture Affects Concrete Mixes

Aggregate moisture affects concrete mixing by changing how much water must be added to the batch. Moisture content is the water coating the outside of aggregate that is in a saturated and surface-dry state. The total water in moist aggregate includes both moisture content and water absorption. When aggregates sit in stockpiles or are exposed to rain, they can take in moisture. That extra water must be counted during concrete batching so the water-to-cement ratio stays correct. The moisture...

key terms and concepts

Learn how freezing, thawing, wetting, and drying affect aggregate durability, and how tests like magnesium sulfate and frost cycles detect unsound aggregates.

Aggregate Durability in Freeze-Thaw Cycles | 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

Aggregate Durability in Freeze-Thaw Cycles

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