Aggregates Chapter 4

Aggregates Granular mineral particles used Various types of cementing material Road bases Backfill Subbases Railroad ballast Fill under floor slabs Concrete blocks Water filtration beds Drainage structures Riprap Gabion material

Sources Natural sand and gravel deposits Crushed rock Slag and mine refuse Rubble and refuse Artificial and processed materials Pulverized concrete and asphalt pavements Other recycled and waste materials

Sources Natural sand and gravel deposits – crushed rock – make up bulk of aggregates Although the use of recycled materials is growing Natural sand and gravel deposits are used for aggregates Consist of sand or gravel soils

Natural sand and gravel deposits Used for aggregate Have been naturally sorted to eliminate most silt and clay Mine in open pit mines – May be crushed further to produce different sizes

Properties of aggregates Three major classes of rock Igneous rock Original rock Formed from the cooling of molten material Coarse grained igneous rocks cool slowly Fine grained igneous rocks cooled more quickly Sedimentary rocks Formed from the solidification of chemical or mineral sediments deposited Metamorphic rocks Igneous or sedimentary rocks that have been changed due to intense heat and presure

Best Types for aggregates Igneous and metamorphic rocks are very hard Excellent aggregates Sedimentary rock is softer But limestone and dolomite are acceptable for aggregate choices Shale is not a good choice because it is made from clay Slag is being used as well Water cooled slag is a good choice for base courses and concrete Air cooled slag is not a good choice because it could contain sulphur Pulverized concrete is being used more and more Asphalt pavements can be recycled as well Limite to 30-50% of total for new installs Use of other recycled materials is being investigated as well Glass Rubber pellets Bricks Building ruble

Aggregate terms and types Fine aggregate (sand sizes) between no4 and no . 200 sieve Course aggregate larger then no. 4 Pit run – aggregate from a sand or gravel pit no processing Crushed gravel – aggregate from the crushing of bedrock – all particles are angular not rounded Screenings – chips and dust or powder that are produced in the crushing of bedrock Concrete sand – sand that has been washed Fines – silt clay or dust smaller then no. 200 – usually not desirable Aggregate and sieve sizes table 4-1 page 128

Nominal size Size determined by sieves Not necessary that 100 percent be within the specified range Small amount about 5 to 10% is allowed to be larger or smaller Ex. 3/4in (19mm) 90 percent of sample must be smaller than 19 and 100 % must be smaller then next sieve size which is 1in (25mm) Ex fine agg. Should pass through a no. 4 sieve (4.75mm) 100 percent passes through 9.5 (3/8) – 90 passes 4.75 Ex coarse aggregates – 19 -4.75 should pass 100 through 25 mm 90 passing the 19mm sieve and 10 passing the 4.75 sieve Single coarse aggregate is called clear Most particles are between the specified maximum size and minimum size Ex 19mm – 100 smaller than 25mm 90 smaller than 19 and 10% smaller than 9.5mm

Aggregate Strength Use to distribute imposed loads over surface of the soil Or as economical filler material The ability for aggregate particles to carry loads without moving is the most important factor Loads come from both horizontal and vertical forces The load is distributed over a large area Crushed material is much better then rounded Rough faces increase the friction between particles Reduce the risk of sliding between particles Densely graded also increase the strength Particles are more easily lock together

Properties Gradation Grain size analysis Specifications for highway bases, concrete and asphalt mixes require a grain size distribution that will provide a dense strong mix Voids between larger particles are filled with medium particels – the remaining voids are filled with still smaller particles Fuller maximum density curves P=(d/D).5 where p is the percent passing sieve size d, and D represents the maximum sieve size (100% passing) Revised maximum density relationship used by federal highway admin 4.5 is used in place of .5

The Problem with Fines Fines must be limited Silt and clay particles (no. 200 sieve) are relatively weak In concrete mixes to much cement is need to cover the fines – in addition they weaken the bond between the cement and those particles Fines in highway bases may lead to drainage and frost heaving problems Clay is more harmful the silt Maximum values for liquid limit and index of plasticity are often specified for aggregates Washed sieve analyses are required when the amount passing 75um is important (200 sieve) Sample is dried and washed Wash water is poured out over a 200 sieve Material retained in the sieve is returned to the sample It is then dried again and dry sieved Total amount passing 200 sieve is the sum of the amounts lost in washing and passing the 75 um sieve Example page 132 – 4-1

Relative density and absorption Are important especially in concrete and asphalt mixtures Necessary to measure accurately the volumes occupied by the aggregate and any water that might have seeped into the pores in the particles Relative density rd=m/(V *pw) Other formula exist for dealing with moisture in the agg. – page 133 Example 4-2 page 134 Example 4-3 page 134

Hardness Resistance to wear Important Pavement surfaces – agg. Not get rounded or polished due to traffic Floors – subject to heavy traffic Roadbeds – subjected to innumerable cycles of lad application and removal Most common test is the los Angeles abrasion test Agg. Is placed in a drum with a number of steel balls Drum is rotated a specified number of times Loss of agg. Or amount ground down is measured Example 4-4

Durability Resistance to disintegration due to cycles of wetting and drying, heating and cooling and especially freezing and thawing Especially dangerous with particles from sedimentary rocks Soundness test is used Sample of aggregate is saturated in a solution of magnesium sulphate or sodium sulphate and then removed and dried in a oven Repeated five cycles The percent lost or broken down is calculated Example 4-5 page 137

Other properties Particle shape and surface texture Affect the strength and bond with cementing materials Resistance to sliding of one particle over another Flat –thin or long needle shaped particles break more easily Rough faces allow a higher friction strength Deleterious substances Harmful or injurious materials Include weak or low quality particles and coatings that are found on the surface of aggregate particles Include Organic coating Dust Clay lumps Shale Coal particles Friable particles easyt o crumble Chert may break up when exposed to freezing and thawing Badly weathered particles Soft particles Lightweight particles

Specifications For each of the properties of aggregates Specification vary considerably Requirements for road base differ from concrete and asphalt Table 4-3 gives astm standard sizes of aggregates for highway work

Typical specifications for highway base Passing 1in 100% ¾ 90 to 100% 3/8 50 to 75% No. 4 35-55% No. 16 15-40% No. 50 5 to 22% No. 200 2-8% Abrasion loss up to 40% Soundness loss up to 18% Example 4-6 page 140

Sampling and testing Csa standard A23.2-1a and astm standard d75 Give methods to follow in sampling aggregates Page 143 list astm d75 standards In conducting lab test – its critical that the sample tested be representative of the material being used in the field Usually a sample splitter is used to obtain the test sample

Blending Aggregates Meet the gradation requirement for asphalt or concrete it is often necessary to blend two or more aggregates together Charts are available but normally done by trial and error Example 4-8 page 147