CEM200 chapter 8

CHAPTER 8 – AGGREGATES (EXAM-FOCUSED, POWERPOINT-ALIGNED NOTES)

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Role of Aggregates in Concrete

Aggregates are the primary component of concrete, occupying approximately 60 to 80 percent of the total volume. Because they make up the majority of the material, they control many key properties of concrete, including durability, strength, workability, and cost. According to the lecture slides, aggregates are the most important factor influencing the durability of Portland Cement Concrete (PCC) . While cement paste binds the material together, the performance of the concrete largely depends on the quality and characteristics of the aggregates used.

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Strength and Performance Factors of Aggregates

The strength and durability of concrete are influenced by several aggregate-related factors. These concepts are emphasized both in Chapter 8 and in the lecture slides.

• Mineralogical composition of aggregates affects strength because different rock types have different hardness, durability, and chemical stability. Some aggregates may react with cement, causing expansion and cracking.

• Aggregates are generally considered inert, but certain types can be chemically reactive and lead to deterioration of concrete under specific conditions.

• The maximum size of aggregate (MSA) plays a major role in performance:

• Larger aggregates reduce cement and water demand, making the mix more economical.

• Smaller aggregates increase surface area and improve bonding, resulting in higher strength concrete.

• Medium-strength concrete can use a wide range of aggregate sizes.

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Key Aggregate Characteristics (Critical for Exam)

Your PowerPoint identifies six main characteristics, and Chapter 8 expands on them. These must be fully understood.

Cleanness

Cleanness refers to the absence of contaminants such as clay, silt, organic materials, coal, and lightweight particles.

• Clean aggregates produce strong concrete because they allow proper bonding between the cement paste and the aggregate.

• Contaminated aggregates weaken the bond, reduce strength, increase water demand, and negatively affect appearance.

• Deleterious materials such as clay and silt can coat particles, preventing proper adhesion and increasing shrinkage.

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Gradation (Particle Size Distribution)

Gradation refers to how aggregate particles are distributed across different sizes and is determined through sieve analysis.

• Well-graded aggregates contain a variety of particle sizes that fit together efficiently, reducing voids.

• Poorly graded aggregates have uniform sizes, creating more void space and requiring more cement paste.

• Excess voids increase water demand and reduce strength.

Important details from Chapter 8 and lab material:

• Gradation is measured using standard sieves and expressed as percent passing or retained.

• A good gradation curve is smooth and continuous, typically described as an “S-shaped” curve.

• Poor gradation results in segregation, poor workability, and inconsistent concrete.

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Fineness Modulus (FM)

Fineness modulus is a numerical index that represents the fineness or coarseness of fine aggregate.

• It is calculated by summing cumulative percentages retained on a standard set of sieves and dividing by 100.

• A higher fineness modulus indicates coarser sand.

• A lower fineness modulus indicates finer sand.

• Typical values range from about 1 to 4.

Important concept:

• Fineness modulus does not fully describe gradation but is useful for comparing materials.

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Particle Shape

Aggregate particles may be rounded, subrounded, angular, or subangular.

• Rounded particles improve workability because they move easily within the mix.

• Angular particles improve bonding and increase strength but reduce workability.

• Flat and elongated particles are undesirable because they reduce strength and create weak planes in the concrete.

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Texture

Texture refers to the surface condition of aggregate particles.

• Rough-textured aggregates improve bonding with cement paste and increase strength.

• Smooth-textured aggregates reduce bond strength.

• Coatings such as dust or flaky materials interfere with bonding and should be avoided.

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Specific Gravity and Unit Weight

Specific gravity is the ratio of the density of aggregate to the density of water.

• Typical aggregates have a specific gravity of about 2.65.

• Unit weight is the weight per cubic foot, often around 165 pounds per cubic foot for aggregates.

These values are important because:

• They are used in mix design calculations.

• Lower specific gravity may indicate porous or weak aggregates.

• Higher specific gravity indicates denser and typically stronger materials.

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Absorption and Moisture Content

Absorption is the amount of water that aggregate can absorb into its pores.

• Coarse aggregate typically absorbs about 1 to 2 percent.

• Fine aggregate may absorb between 0.5 and 8 percent.

Moisture condition is critical and directly affects mix design. Aggregates exist in four conditions:

• Oven dry: no moisture; absorbs water from the mix.

• Air dry: partially dry; still absorbs water.

• Saturated surface dry (SSD): ideal condition; does not affect water content.

• Wet: contains free moisture; adds water to the mix.

Key concept:

• Moisture content must be accounted for because it changes the effective water-cement ratio, which controls strength.

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Grading and Sieve Analysis (Lab 6 Connection)

Sieve analysis is the method used to determine aggregate gradation.

• Aggregates are passed through a series of standard sieves.

• The amount retained on each sieve is measured.

• Results are expressed as:

• Percent retained

• Cumulative percent retained

• Percent passing

Standard sieve sizes:

• Coarse aggregate: 2”, 1½”, 1”, ¾”, ½”, 3/8”, #4

• Fine aggregate: #4, #8, #16, #30, #50, #100

Key concepts:

• Material that passes all sieves is collected in the pan.

• Proper gradation reduces voids and improves strength.

• Poor gradation leads to segregation and inconsistent concrete.

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Relationship Between Aggregates and ACI Mix Design

Chapter 8 directly supports the ACI mix design process. The following relationships are critical.

• Gradation affects workability and water demand. Poor grading requires more water and cement.

• Particle shape and texture influence bonding and strength.

• Maximum aggregate size affects water demand, strength, and cost.

• Moisture content must be accounted for when calculating mix proportions.

From your ACI worksheet and slides:

• Aggregate properties such as specific gravity, moisture content, and fineness modulus are used in calculations .

• ACI tables use aggregate size and gradation to determine water content and aggregate quantities .

• Moisture corrections are required to adjust actual water content in the mix.

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Processing, Stockpiling, and Quality Control

Chapter 8 emphasizes that aggregates must be properly handled to maintain quality.

• Processing includes crushing, screening, washing, and blending to achieve proper grading and cleanliness.

• Stockpiling must prevent segregation and contamination.

• Segregation occurs when different particle sizes separate, leading to inconsistent concrete.

• Proper sampling is required to ensure test results represent the entire material.

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Testing Aggregates

Testing is necessary to ensure aggregates meet ASTM specifications.

Common tests include:

• Sieve analysis for gradation

• Moisture content determination

• Specific gravity measurement

• Absorption testing

Testing is typically done at the processing plant and batching plant to ensure consistency.