Aggregates

Aggregate

• Inert, granular, and inorganic materials that normally consist of stone or stone like solids

• Can be used alone (in road bases and various types of fill) or with cementing materials (Portland cement or asphalt cement) to form composite materials or concrete

• Occupies approximately three-fourths of the volume of Portland cement concrete hence strongly affects the properties of both fresh and hardened products.

Fine Aggregate

• Aggregate material such as natural gravel, crushed gravel, or crushed rock

• Most particles pass through the 5 mm BS sieve and predominantly are retained on the 75 um sieve.

Coarse Aggregate

• Aggregate material such as natural gravel, crushed gravel, or crushed rock

• Most particles are retained on the 5 mm BS sieve

• For mass concrete, the maximum size can be as large as 150 mm.

Natural Aggregate

• Extracted from larger rock formations through an open excavation (quarry)

• Either in the form of rock which is crushed or simply screened (large size removed) before use

• Hong Kong most commonly uses granite.

Igneous Rocks

• Rocks that were at one time molten

• Examples include granite and basalt.

Sedimentary Rocks

• Rocks formed when fine particles become cemented together over a period of time

• Examples include limestone and sandstone.

Metamorphic Rocks

• Rocks formed or modified by heat and pressure.

Artificial Aggregate

• Man-made materials produced as a main product or an industrial by-product

• Examples include expanded clay or shale (such as Leca) and foamed slag (such as Lytag), which are used in the UK and China.

Normal Weight Aggregate

• Includes river sand and stone fines as fine aggregates, and crushed granite as coarse aggregate

• Fine aggregates have sizes of 75 um to 5mm diameter, relative densities of 2550-2750 kg/m³, producing concrete densities of 1800-2400 kg/m³

• Coarse aggregates have sizes of 5 - 40 diameter, relative densities of 2550- 2750 kg/m³, producing concrete densities of 2000-2500 kg/m³.

Lightweight Aggregate

• Includes Lightweight Expanded Clay Aggregate (Leca) and Lytag aggregate

• During kiln firing, expanding gases within the clay create the porous internal texture

• Leca has a size of 5-20mm diameter and relative densities of 300-899 kg/m³, producing concrete densities of 1400 - 1800kg/m³

• Lytag has a size of 5-10 mm diameter and relative densities of 700-800kg/m³, producing concrete densities of less than 1400 - 1800 kg/m³.

Ultra Lightweight Aggregate

• Includes Polystyrene Aggregate, Expanded Vermiculite, and Perlite

• Polystyrene aggregate has a brown colour coating to improve bonding and prevent segregation, size of 1 - 6 mm diameter, relative densities of 55 - 160 kg/m³, and produces concrete densities of 300-500kg/m³ ,

• Produces lightweight concrete which can be sawed, nailed, and used for its insulating properties

• Expanded Vermiculite has a size of 0.5 - 16mm diameter, relative densities of 55 - 160 kg/m³, and produces concrete densities under 500 kg/m³

• Perlite has a size of 0.5 - 16mm diameter, relative densities of 30-400 kg/m³ and produces concrete densities under 500 kg/m³

Heavyweight Aggregate

• Barytes are also known as Barite (Barium Sulfate)

• Particle size is 1-6 mm diameter

• Densities reach up to 3500 - 4500 kg/m³

• Used where high density of concrete is required, such as in radiation reactor shielding.

Oven Dry (OD) Condition

• Obtained by keeping aggregates at a temperature of 110 dc for a period of time long enough to reach a constant weight

• All internal and surface moisture is completely removed.

Air Dry (AD) Condition

• Obtained by keeping aggregates under room temperature and humidity

• Pores inside the aggregate are partly filled with water.

Saturated Surface Dry (SSD) Condition

• Pores of the aggregate are fully filled with water and the outer surface is dry

• Obtained by immersion in water for 24 hours, followed by drying of the surface with a wet cloth

• Most desirable moisture condition because it is an equilibrium state where aggregates neither absorb water nor give up water to the paste.

Wet (W) Condition

• Aggregate pores are fully filled with water and the surface is covered with a film of free water.

Absorption Capacity (AC)

• (Wssd / Wod) / Wod x 100%

Effective Absorption Capacity (AC eff)

• Calculated relative to air-dry weight

• (Wssd - Wad) / Wad x 100%

Density (D)

• Weight per unit volume excluding internal pores inside a single aggregate

Bulk Density (BD)

• Weight per unit volume where the volume includes internal pores inside a single aggregate particle

• BD = W/ (Vsolid + V pores)

• Can be BDssd or BDad depending on the moisture condition when weighed.

Specific Gravity (SG)

• Mass of a given substance divided by unit mass of an equal volume of water, representing the density ratio of a substance to water.

Absolute Specific Gravity (ASG)

- Calculated using solid volume excluding pores

ASG = weight / (Vsolid x Pw) = D / Pw

Bulk Specific Gravity (BSG)

• Calculated using bulk volume including internal pores

• BSG = weight / (Vsolid+Vpores) = BD / Pw

• Realistic value to use in practice since the effective volume occupied by aggregate in concrete includes internal pores

• Range for most rocks is 2.5 to 2.8, and can be BSGssd or BSGad

Particle Shape and Surface Texture

• Important for proper compaction, load resistance, and workability

• Cubic angular-shaped particles with a rough surface texture are best

• Spherical aggregates have the smallest surface/volume ratio, need less water, and are desirable, while flat, needle-shaped, and elongated particles should be avoided due to high water demand and segregation risk

• Rough aggregates need more water for the same flow properties but provide better mechanical bond and higher strength in hardened concrete.

Sieve Analysis

• The process of dividing a sample of aggregate into fractions of the same particle size using a series of progressive sieves to determine particle size distribution and paste requirement.

Fineness Modulus (FM)

• The sum of cumulative percentages retained on standard series of sieves divided by 100

• Usually calculated for fine aggregate rather than coarse aggregate

• Typical values range between 2.3 and 3.0, with higher values indicating coarser grading

• Used to detect slight variations in aggregate from the same source that could affect fresh concrete workability.

Well-Graded Aggregate

• Gradation that is near to maximum density.

Gap-Graded Aggregate

• Gradation containing only a small percentage of aggregate particles in the mid-size range, resulting in a flat curve in the mid-size range and potential vulnerability to segregation.

Open-Graded Aggregate

• Gradation containing only a small percentage of aggregates in the small size range, resulting in more air voids and a flat, near-zero curve in the small-size range.

Uniformly Graded Aggregate

• Gradation containing most particles in a very narrow size range (all particles same size), represented by a steep curve.

Maximum Size of Aggregate

• Governed by the type of work to be done

• For reinforced concrete, should allow placement without difficulty around reinforcement and formwork corners; normal size is 20mm

• Larger aggregates (20-40mm) lower specific surface area and water demand, increasing strength, but very large particles lower bond area and introduce discontinuities.

Chemical Admixtures

• Materials added to concrete when produced or transported with the aim of developing and changing properties according to needs

• Should be added to mixing water and thoroughly dispersed. Typical dosage is by weight of cement.

Accelerator

• Purpose is to accelerate hardening or early strength development of concrete

• Most common example is calcium chloride (CaCl2)

• Applied for low-temp placing (2 - 4 dc) or urgent repair work

• Influences concrete properties by

  • increasing rate of loss in workability

  • increasing 1-day strength

  • decreasing ultimate strength

  • increasing drying shrinkage/ creep

  • decreasing durability (sulfate attack and ASR).

Set Retarder

• To delay concrete setting by clinging to calcium hydroxide to inhibit crystal growth (lengthening Stage 2)

• Acts as water reducers for improved long-term strength

• Products normally based on sugars, tartaric acid, and carbohydrate derivatives?

• Applied in hot weather, large pours, and to avoid cold joints. Dosage is 0.2 - 2 % by weight of cement; overdose can completely destroy setting properties

• Influence

  • Decreases rate of loss of workability

  • Decreases 1-day strength

  • Increases ultimate strength.

Air Entrainer

• To improve durability against moisture and freeze/thaw action by producing microscopic air bubbles evenly distributed throughout concrete. (ASTM C 260-81)

• Application: agricultural concrete

• Mechanism involves surface-active agents lowering surface tension to form and stabilize bubbles

• Influence

  • Increases workability

  • Decreases density and strength

  • Reduces bleeding and segregation

  • Increases durability (10x or more)

  • Allows reduction in water and fine aggregate content.

Plasticizer (Water Reducer)

• BS 5075: Part 1. Purposes include achieving higher strength by decreasing w/c ratio at same workability, increasing workability for placement in inaccessible locations, maintaining workability with reduced cement content in mass concrete, and improving long-term strength via uniform dispersion.

• Mechanism involves electrostatic charges attracting to cement grains and imparting negative surface charges so grains repel one another.

• Types (BS 5075: Part 3): Conventional (min 5% water reduction, 25 - 50 mm extra slump), Mid-range (8 -15% water reduction, 75 -100 mm extra slump), High-range/Superplasticizer (up to 40% water reduction, 100-200 mm extra slump), Advanced (self-compacting concrete).

Superplasticizer (High-Range Water Reducer)

• BS 5075: Part 3. High range water reducers to produce high workability concrete (slump >200 mm for pumping) or very high strength concrete with low w/c ratio.

• Made mainly from sulphonated melamine formaldehyde condensates or sulphonated naphthalene formaldehyde condensates.

• Mechanism: sulphonic acid adsorbs on cement particle surfaces, creating negative charges and mutual repulsion via the sterical separation effect.

• Examples include Daracem 100, Work 500, Melment.

• Dosage is 0.1-2% by weight of cement. Workability increase lasts 30 - 90 minutes; 2-minute re-mixing is essential for ready-mixed concrete.

• Advantages: flowing concrete, very high strength, optimized cement proportion. Disadvantages: higher cost, requires stronger formwork.

Advanced Superplasticizer (3rd Generation)

• Modifying property based on polycarboxylic polyether type polymer

• Purposes - produce self-compacting concrete (SCC), hot weather/extended transportation concrete, highest water reduction (up to 40%), very high strength concrete. Dosage is 0.25 - 2.5 % by weight of cement.

Self-Compacting Concrete (SCC)

• Highly flowable, cohesive concrete produced using advanced superplasticizers that flows and consolidates under its own weight without external vibration. Used in heavily congested reinforced concrete structures where vibrating pokers are inaccessible.

Mineral Admixtures (Pozzolans)

• Siliceous materials (SiO2: 55 - 90%) of very fine particle size that react chemically with lime (Ca(OH)2) at normal temperatures to form C-S-H gel.

• Can be used as an addition or replacement of cement.

• Densifies microstructure, increases strength, abrasion/erosion resistance, paste-aggregate bond, and chemical durability, while reducing segregation, bleeding, and thermal cracking.

Silica Fume

• By-product of silicon and ferrosilicon industry

• Chemical composition: 85 - 98% reactive microsilica with average particle size less than 1 um

• Ultra-fine powder that densifies concrete microstructure. Optimum content is 5 - 15% addition; increases workability (cohesive/homogenous mix), increases early and long-term strength, and provides excellent durability. Cost is 2xconventional concrete.

Pulverized Fly Ash (PFA)

• Residue of coal combustion in electric power generation

• Chemical composition: 49 - 55% microsilica, 25% aluminium dioxide

• Optimum content is 20-30% replacement; improves workability via "ball bearing effect" of round particles, reduces early strength, increases long-term strength, and provides excellent durability.

Slag

• By-product produced by the iron industry

• Chemical composition 45-70% microsilica

• Optimum content is 30-50% (MTR use up to 75%); produces a cohesive mix with early slump loss, reduces early strength, increases long-term strength, and provides excellent durability.

General properties of aggregates

1. Does not hydrate, no heat of hydration

2. Does not swell or shrink

3. High young modulus

4. Strong resistance against stress

5. Aesthetic requirement

Relative to oven dry condition

• MC (OD) = (Wstock - Wod) / Wod x 100%

Relative to saturated surface dry condition

• MC (SSD) = (W stock - Wssd) / Wssd x 100%