5.6
Tamanho Máximo do Agregado
O tamanho máximo do agregado é definido como a abertura da peneira que retém 15 por cento ou mais das partículas presentes na amostra de agregado. O t…
Convencionalmente, o tamanho máximo agregado é determinado pelo tamanho da peneira que retém pelo menos 15% das partículas em um conjunto de amostras.
À medida que o tamanho máximo do agregado aumenta, a área de superfície por unidade de massa que deve ser umedecida ou revestida pela pasta de cimento diminui, melhorando a trabalhabilidade do concreto.
No entanto, além de um tamanho específico, agregados maiores oferecem menos área para colagem com argamassa de cimento.
Agregados maiores também tendem a produzir mais microfissuras na interface agregado graúdo-cimento e introduzir descontinuidades na massa de concreto, levando a um concreto heterogêneo com resistência reduzida.
O tamanho máximo do agregado depende da dimensãos do membro de concreto e do espaçamento livre entre as barras de reforço no membro de concreto.
Além disso, o custo associado ao armazenamento de agregados de diferentes tamanhos necessários para uma mistura estável e à prevenção da segregação durante o manuseio e colocação desses agregados é vital para decidir o tamanho máximo do agregado.
Geralmente, para concreto estrutural, o tamanho máximo do agregado é restrito a uma polegada ou uma polegada e meia.
View the full transcript and gain access to JoVE Core videos
Q1: How is the maximum size of aggregate determined?
Maximum aggregate size is determined by the sieve size that retains at least 15 percent of particles in a sample. This measurement defines the aperture through which smaller particles pass while larger ones are retained. The sieve analysis and grading curves method provides a standardized way to classify aggregates by size for concrete mix design.
Q2: Why does larger aggregate size improve concrete workability?
As maximum aggregate size increases, the surface area per unit mass requiring cement paste coverage decreases significantly. This reduction lowers water requirements and allows the water-to-cement ratio to decrease while maintaining desired workability and mix richness, ultimately enhancing concrete strength and durability performance.
Q3: What problems can occur when aggregate size is too large?
Oversized aggregates reduce the bonding and strength of aggregate with cement mortar, creating fewer contact points for adhesion. They also produce microcracks at the coarse aggregate-cement interface and introduce discontinuities, resulting in heterogeneous concrete with reduced strength, durability, and structural reliability.
Q4: What factors constrain the maximum aggregate size in concrete design?
Maximum aggregate size depends on concrete member dimensions and clear spacing between reinforcing bars. Additionally, the cost of stockpiling different aggregate sizes and preventing segregation during handling and placement are vital considerations in selecting optimal aggregate size for structural applications.
Q5: What are typical maximum aggregate sizes for structural concrete?
For most structural concrete applications, maximum aggregate size is typically limited to either 25 millimeters or 40 millimeters, though conventionally it may be restricted to one inch or one and a half inches. These limits balance workability, strength, and practical handling requirements for construction.
Q6: How does aggregate size affect water requirements in concrete?
Larger aggregates reduce the surface area needing cement paste coverage, significantly lowering overall water requirements in the concrete mix. This reduction allows engineers to decrease the water-to-cement ratio while maintaining desired workability and mix richness, resulting in enhanced concrete strength and improved long-term durability.
Q7: What role does aggregate size play in concrete homogeneity?
Oversized aggregates introduce discontinuities and heterogeneity in the concrete mass, weakening the overall structure significantly. Proper maximum aggregate size selection ensures a more uniform, homogeneous concrete with better strength distribution, reduced risk of localized failure points, and improved structural performance.