5.5
Tipos de gradação de agregados
A gradação de agregados é crucial para obter economicamente uma mistura de concreto com resistência adequada, trabalhabilidade razoável e segregação m…
A classificação agregada significa a distribuição do tamanho das partículas dentro de uma amostra de agregado.
Agregados bem classificados contêm todas as frações de tamanho agregado necessárias nas proporções necessárias para formar uma matriz densa com vazios mínimos.
A curva de gradação correspondente é uma curva suave contínua, indicando uma ampla gama de tamanhos de partículas.
Quando a maioria dos agregados passa por uma peneira específica e é retida na próxima peneira mais fina, isso significa que as partículas são predominantemente do mesmo tamanho.
Isso representa agregados graduados uniformes ou de tamanho único, representados por uma linha quase vertical na curva de gradação.
Os agregados são classificados por lacuna se aproximadamente as mesmas frações de agregados passarem por duas peneiras diferentes consideravelmente diferentes em tamanho, indicando a ausência de agregados na faixa de tamanho intermediário e é representada por uma linha horizontal na curva de gradação.
Quando os agregados miúdos estão ausentes, a mistura é chamada de grau aberto, resultando em grandes vazios entre as partículas grossas. A curva de gradação correspondente é plana e quase zero na faixa de tamanho pequeno.
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Q1: What does aggregate grading measure in concrete construction?
Aggregate grading signifies the particle size distribution within a sample of aggregate. It determines how different sized particles are distributed throughout the material. Understanding grading is crucial for economically obtaining a concrete mix with adequate strength, reasonable workability, and minimal segregation.
Q2: How do well-graded aggregates differ from uniformly graded aggregates?
Well-graded aggregates contain all required size fractions in proper proportions, forming a dense matrix with minimum voids and a smooth, continuous gradation curve. Uniformly graded aggregates consist predominantly of particles of the same size, depicted by a nearly vertical gradation curve. While well-graded aggregates ensure good workability, uniformly graded aggregates provide good permeability but lack stability.
Q3: What characterizes gap-graded aggregates on a gradation curve?
Gap-graded aggregates are characterized by the absence of certain intermediate size fractions. Approximately the same fractions pass two different sieves that differ considerably in size, resulting in a gradation curve with a nearly horizontal section. This missing material can lead to issues with concrete stability.
Q4: Why are open-graded aggregates highly permeable?
Open-graded aggregates lack fine particles that would normally fill voids between coarse particles. When fine aggregates are absent, the mixture results in large voids between the coarse particles, creating high permeability. However, this also reduces stability compared to other grading types.
Q5: How is a gradation curve used to identify aggregate grading types?
Gradation curves visually represent particle size distribution. Well-graded aggregates show a smooth, continuous curve; uniformly graded aggregates display a nearly vertical line; gap-graded aggregates exhibit a horizontal section indicating missing sizes; and open-graded aggregates show a flat curve with minimal values in smaller size ranges. These distinct curve shapes help classify aggregate types.
Q6: What is the relationship between aggregate grading and concrete workability?
Well-graded aggregates ensure good workability and stability by creating a dense matrix with minimum voids through proper size fraction distribution. In contrast, uniformly graded and gap-graded aggregates lack the complete range of sizes needed for optimal workability. Open-graded aggregates, while permeable, also compromise workability due to excessive voids.
Q7: Why does the absence of intermediate particle sizes affect concrete quality?
Gap-graded aggregates lack intermediate size fractions, creating discontinuity in the particle matrix. This absence prevents efficient packing and can compromise concrete stability and strength. Similarly, open-graded aggregates missing fine particles create large voids that reduce overall concrete quality and performance.