1 / 30

Aggregates

aira
Download Presentation

Aggregates

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

    1. Aggregates Week 5

    2. General Occupy 70-75% of the total volume of concrete Defined as an inert mineral filler for OPC – BS 882

    3. Sizes Coarse aggregate e.g. Gravel – 4.76mm or more (5mm) Fine aggregate e.g. sand – less than 4.76mm Silt – varies from 0.02mm – 0.06mm Clay – much finer than 0.02mm

    4. Classification A) Petrological – BS 812 B) Density C) Shape & texture

    5. A) Petrological 1.) Igneous - Granites, basalts dolerites, gabbros & porphyries – Hard, tough, dense – Excellent aggregates 2.) Sedimentary - Sandstones - When hard & dense suitable - Siliceous better than calcareous which are liable to acid attack Limestones - Sedimentary rocks chiefly composed of calcium carbonate Harder, denser types are suitable

    6. Cont’d Shales - Poor aggregates – weak, soft, laminated & absorptive 3.) Metamorphic - Variable character – marbles & quartzites usually massive, dense & adequately tough & strong Schists & slates are often thinly laminated & therefore unsuitable

    7. B) Density 1.) Heavy – SG>4.0 i) Magnetic, natural iron ore (Fe3O4) ii) Barytes (BaSO4)

    8. Cont’d 2.) Normal SG 2.5-3.0 i) Sands & gravels – division is arbitrarily on size – sands pass 4.76 mm BS sieve ii) Granites & basalts – hard & tough iii) Sandstone – hard & dense types iv) Limestone – hard & dense types v) Broken bricks – must be free of plaster – low sulphur content vi) Air cooled blast furnace slag

    9. Cont’d 3.) Lightweight - SG<2.5 i) Sintered PFA ii) Foamed slag iii) Vermiculite iv) Expanded polystyrene v) Pumice vi) Wood waste

    10. C) Shape & Texture Particle shape classified to BS 812 : 1975 Terms used: Well rounded, rounded, sub-rounded, sub-angular, angular

    11. Cont’d Surface texture classified to BS 812 : 1975 Relates to the degree of polish or dullness, smoothness or roughness of particles surfaces Depends on hardness, grain size and pore character Note: BS 812 & BS 882 have been replaced by BS EN 12620

    12. Bond of Aggregate Partly due to interlocking Partly due to physical & chemical characteristics of the aggregate Other factors include the cement paste

    13. Aggregate Properties Ideally should be chemically inert, durable, hard & tough Should be capable of being compacted to the appropriate density & provide good bonding with the cement paste

    14. A) Chemical Properties Soluble salts – sulphates - cause problems with hydrated cement or chloride which accelerate hydration & increase risk of corrosion of steel reinforcement. Reactive aggregates – ASR – ‘concrete cancer’ – not common now but has been a problem in the past

    15. Alkali-Silica Reaction

    16. Cont’d Clay & silt – defined as materials passing a 75 micron sieve – harmful to concrete in substantial amounts – increase specific surface & hence water requirements Weak (unsound particles) – lower the strength of the concrete

    17. B) Physical Properties Strength Porosity Thermal Bonding Moisture Density - relative (SSD) & solid (x1000) - bulk (packing air)

    18. Calculation of Relative Density Relative Density (SSD) = mass of sample of SSD aggregate Volume of water displaced SSD sample x 1000 Where, SSD = saturated surface dry – all accessible pores are full of water, but the aggregate surface is dry

    19. Bulk Density Stockpiles of loosely packed aggregates contain large volumes of air trapped between particles, usually many times that of the volume of air present within the particles – ‘Bulking’ Coarse aggregates – 30-50% of total space occupied Fine aggregate – approx 20% - varies with moisture content

    20. Batching of Aggregates Bulking of aggregates produce uncertainty in the solid content of aggregates batched by volume Batching by weight (mass) is therefore preferred, hence most concrete mixes are batched by weight

    21. Aggregate Durability Soundness – degradation through interstitial crystallisation Organic matter – act as retarders Mica – Muscovite or Biotite Chalk – affects strength (freeze/thaw) Shell – strength

    22. Cont’d Sulphates – limited to 4% overall Chlorides – corrosion (<0.06%) Metallic Pyrites – Lead, Zinc, Iron impurities

    23. Sand & Gravel Sources – pits or dredging from river or sea bed – screening & washing Marine aggregates – 1985 - >20M Tonnes/year 1995 - >300M Tonnes /year Goodwin Sands (off Kent coast) – 8000 Tonnes/day use for Channel Tunnel concrete – Suction hopper dredgers with leading & trailing pipes

    24. Ship Dredging Up to1968 – 800 tonnes/day 1970’s – 2500 – 10000 Tonnes/day

    25. Problems with Marine Aggregates Liable to contain impurities of shell & salt Southern, North Sea & English Channel granites, sandstones or limestones – generally unconsolidated sedimentary deposits Up to 1968 – 50% of aggregates came from sand & gravel deposits Emphasis has now shifted to hard rock sources

    26. Grading of Aggregates Refers to the size & distribution of the aggregate Sieve analysis – BS 410 : 1976 Grading limits – BS 882 : 1983 Note: The Euro-standard BS EN 12620 now covers testing & specification for aggregates

    27. Graded Aggregate Uniform Graded – equal weights of each particle size present Gap graded – several consecutive particle sizes missing Single graded – consist predominantly of a single particle size

    28. Fine & Coarse Aggregates Fine Aggregate - <4.76mm (5mm) Coarse Aggregate - >4.76mm Grading: Fine Coarse C-Coarse 40, 20, 14mm down M-Medium F-Fine

    29. Aggregate Uses Concrete Mortar Renders Screeds Unbound pavements

    30. Cont’d Bituminous Materials Railway ballast Filter media - biological

More Related