Engineering materials

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Florence Atkinson
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1 1 Engineering materials Lecture 9 Aggregates

2 2 Composition and structure Natural aggregates are derived from rocks. Classification: Igneous( 火成岩 ), sedimentary ( 沉積岩 ) and metamorphic ( 變質岩 ) Fine or coarse Heavy weight, normal weight and light weight

3 3 Igneous, Sedimentary and Metamorphic Igneous 火成岩 (basalt 玄武岩 or granite 花崗岩 ): The rate of cooling determines the crystal size and hence the rock properties -> fine-grained rocks with smaller grain size are generally stronger. Sedimentary 沈積岩 (shale 頁岩, limestone or sandstone): Formed by the consolidation of deposits of the products of weathering and erosion of existing rocks at the earth surface -> weaker than Igneous surface -> anisotropic behavior. Metamorphic 變質岩 (slate 板岩, marble or quartzite): Formed by the formation of intense heat and pressure to sedimentary rock deposits -> recrystallization and grain growth can occur -> less porous, stringer and less anisotropic than sedimentary rocks.

4 4 Geometric properties Particle shape Round is good for workability Particle size A series of sieves or screen with different size openings is used to obtain some estimate of the particle size distribution

5 5 Standard sieves The standard sieve sizes are 3 3/2 3/4 3/8 No.4 No.8 No.16 No.3 No.5 No.1 76.mm 38.1mm 19.mm 9.5mm 4.75mm 2.36mm 1.18mm.6mm.3mm.15mm

6 6 Sieve analysis To obtain Maximum-Size-Aggregate (MSA) The smallest sieve opening through which the entire aggregate sample passes Purposes of MSA Make sure concrete paste can go through reinforcing steel bars Together with MSA, the gradation determines the specific surface area (surface area per unit volume) of the particles in an aggregate sample. For a continuously graded aggregate, the specific surface area increases as the MSA decreases.

7 7 Grading (gradation) Grading: the distribution of particle size in an aggregate sample, expressed as the cumulative percentage of particles that are smaller or larger than each of a series of sieve opening Grading curve: the cumulative percentage passing or retained on a particle sieve is plotted against the sieve size

8 8 Continuous vs. uniform gradation Continuous (dense) gradation: The aggregate contains every size of fraction between the maximum and minimum particle size Uniform (open) gradation: Aggregates contain particles of only one size fraction Gap gradation: Aggregates are missing particles of one or more size fraction

9 9 Continuous vs. uniform gradation Continuous Uniform Gap Ref: Young et al., 1998

10 1 Fine vs. Coarse aggregate Fine aggregate: particle size smaller than No. 4 or 5 mm. Coarse aggregate: particle size larger than No. 4 or 5mm.

11 11 Fineness Modulus (FM) The coarseness or fineness of an aggregate gradation By adding the total percentages of a sample of the aggregate cumulatively retained on each of a specified size of sieve and dividing the sum by 1 Aggregates having the same fineness modulus may not necessarily be identical in their grading curve

12 12 Fineness Modulus (FM), cont d An example of a sieve analysis and how to calculate FM:

13 13 Fineness Modulus, example One sample of different sands was found to have the following amounts retained on each sieve: 3in=g, 3/2 in=5g, 3/4 in=1g, 3/8 in=2g, No. 4=25g, No. 8=2g, No. 16=15g, No.3=5g. (a) Plot the aggregate grading curve. (b) Calculate the Fineness Modulus of the sample. sieve size weight retained weight retained % Cumulative % retained Cumulative % passing in 3/2 in 3/4 in 3/8 in Cumulative % Passing Sieve size (mm) pan 1 F.M. 5.9 F.M.=5.9

14 14 Properties Physical Properties Porosity: the volume inside individual particles that is not occupied by solid material. Voids content: the volume of the space between the individual aggregate particles, i.e. interparticular volume. VMA: voids in mineral aggregates. Cause strength variations. (same as Voids content) Permeability: the susceptibility to passage or penetration by fluids. It s a function of the porosity of the aggregate, and has a significant effect on the resistance of the aggregate to deteriorating influences. Cause durability variations.

15 15 Moisture states: Properties, cont d 1. Oven-Dry (OD) : All moisture is removed from the aggregate by heating in an oven. 2. Air-Dry (AD) : All moisture is removed from the surface of the aggregate, but the internal pores are partially full. 3. Saturated-Surface-Dry (SSD): All of the aggregate pores are filled with water, but there is no film of water on the surface. 4. Wet : All of the aggregate pores are filled with water, and there is a film of water on the surface.

16 16 Absorption Absorption capacity = W SSD -W OD W OD x1 Surface moisture = W WET -W SSD W SSD x1 Moisture content = W AGG -W OD W OD x1 If Moisture content > Absorption capacity, then the aggregate is wet. Otherwise, It is air-dry.

17 17 Durability Unstable volume changes: expansion due to water expansion, oxidation, hydration or carboration Freeze - thaw deterioration: the volume change of water undergoing solid-liquid phase transformation breaks aggregates. Alkali - aggregate reaction ( 鹼骨材反應 ): Na and K ions replace Si ions, causing fracture. Breakdown of aggregates during handling and stockpiling

18 18 Deleterious substances Absorbent particle: shale, porous flint; frost susceptible Clay lumps and friable particles easily crumble, especially when saturated Coal or wood particles act as weak inclusions Organic impurities High aspect ratio particles

Aggregates for Concrete

Aggregates for Concrete Fine Aggregate Sand and/or crushed stone < 5 mm (0.2 in.) F.A. content usually 35% to 45% by mass or volume of total aggregate Coarse Aggregate Gravel and crushed stone 5 mm (0.2 in.) typically between