Civil Engineering Materials Aggregates PDF

Aggregates Although mixtures consisting only of hydraulic KNA1402 cements and water will harden in the shape of any mould...

Aggregates Although mixtures consisting only of hydraulic KNA1402 cements and water will harden in the shape of any mould in which they are placed, they find few practical uses because they are Civil Engineering Materials relatively costly and may shrink unacceptably Aggregates on drying. In order to reduce cost, and to modify such properties as the strength and drying shrinkage of the hardened mass, it is usual to introduce into the mix insoluble non- cementing particles described as aggregates. 1 2 Aggregates Aggregates Should not contain any constituent that might Igneous rocks affect the hardening of the cement and the granite, gabbro (cooling of molten rock) durability of the hardened mass. Sedimentary rocks Organic matter reduce hydraulic activity of cement Limestone, sandstone, shale, coal Dust or clayey matter on the surface reduce the bond between them Metamorphic rocks Free from constituent that decompose or change Originally sedimentary, but under high pressure and in volume heat they convert similar to igneous rocks e.g. slate, quartzite, marble 3 4 Igneous rocks Sedimentary rocks Intrusive Mechanically deposited Coarse grained Consolidated shale, siltstone, sandstones  Granite, diorite, gabbro Chemically deposited Extrusive Calcarous: Limestone, dolomite Fine grained  Rholite, andesite, basalt Silicious: Chert, Opal Volcanic Ash, tuff, agglomerate 5 6 1  Relevant Properties Potential problems Maximum size Grading Organic impurities (affect cement hydration) Texture – crushed or rounded Coatings (clay, silt affect bond) Shape – flakiness, % misshapen, elongation Unsound particles (shale, wood, etc) Abrasion resistance (LA test) Alkali-aggregates reaction (AAR) Aggregate crushing value Specific gravity (ssd) Bulk density Porosity and absorption Moisture content 7 8 Potential problems Types of aggregates Alkali-aggregates reaction (AAR) Normal density aggregates Alkalis in the cement (Na2O, K2O) form hydroxides s.g. 2.5-3.0 during cement hydration If aggregate contain active silica (amorphous opal, Bulk density 1450-1750 kg/m3 chalcedony, etc) in cherts, siliceous, limestone, Widely used: gravel, crushed rocks, slag rhyolites, dacites, tuffs, etc), then alkali silicate gel Coarse: retained on a 5mm test sieve, uncrushed forms in C-S-H gel at aggregate/paste interface gravel (natural disintegration of rocks) and The alkali silicate gel is expansive, and disrupts bond crushed rock or gravel and C-S-H gel, cracking the concrete months or years later Fine: passing 5 mm sieve, uncrushed sand and crushed rock or gravel fines All-in aggregates + mixture of coarse and fine 9 10 Types of aggregates Types of aggregates Lightweight aggregates High density aggregates Porous solids, both natural and man-made For making high density concrete eg screening Higher the porosity – lower the thermal radioactive sources conductivity, density and strength Eg. barytes (barium sulphate rock) ferrous High porosity: vermiculite – excellent metal granules (shot and punchings) and thermal insulating but little resistance to stress ferrous ores (magnetite, haematite and limonite) Density 4800 kg/m3 11 12 2  Types of aggregates Types of aggregates Fibres Gaseous Naturally occurring fibrous rock - asbestos Small sopherical bubbles of gases (eg. Air) can For making asbestos-cement goods such as be formed in fresh cement paste by using roofing sheets and thin panels appropriate foaming agents Wood fibre – bonded with cement to form low- If a suitable stabilizing agent is also mass thermal insulating blocks and panels incorporated, the bubbles remain uniformly (lightly stressed applications) dispersed throughout the cement paste, forming Steel, glass and polymers fibres also used discrete spherical air voids which persist until the cement has hardened Used to make lightweight blocks and panels 13 (aerated cement products) 14 Choice of aggregates Choice of aggregates 1. There is every incentive to use aggregate from a 5. Durability – should be hard and not decompose source close to hand or change in volume (eg coal and lignite may 2. Properties influence outcome of concrete: but swell and decompose leaving small holes on the mostly dependent on the shape and size of surface, pyrites may decompose causing iron particles, rather than type of rock oxide stains on surface) 3. Adequate quality can often be made, provided it 6. Cleanliness – free from organic impurities is available and uniform quality 4. Unless some aggregate require special quality eg. Hard wearing granite is good for flooring, also precaution against corrosive environment 15 16 CHARACTERISTICS AND TEST CHARACTERISTICS AND TEST 1. Resistance to abrasion and 8. Absorption and surface moisture degradation 9. Compressive and flexural strength 2. Resistance to freezing and thawing 10.Definitions of constituents 3. Resistance to disintegration by sulfates 11.Aggregates constituents 4. Particle shape and surface texture 12.Resistance to alkali reactivity and 5. Grading volume change 6. Bulk unit weight or density 7. Specific gravity 17 18 3  Particle Shape and Surface Aggregate size and distribution Texture Rough textured, angular, elongated aggregates Sieve analysis require more water to produce workable concrete Maximum size than smooth, rounded, compact aggregates Nominal maximum size Angular aggregates require more cement for the For reinforced concrete – placement without same w/c. However, bond is better as particles difficulty, surrounding all reinforcement change from smooth to rough thoroughly, particularly in the cover zone and Do not use long, blade shaped aggregate pieces, filling the corners of the formwork increased water demand for a given slump Usual for nominal size of 20 mm 19 20 Aggregate size and distribution Aggregate size and distribution Aggregate of 40mm max. size can be used for foundations and mass concrete and similar Significance of grading sections where there are no restrictions to the Economy flow of concrete – large size, reduced water demand, reduced cement (strength, workability) Consistency Smaller aggregates (10 mm) for placing in Strength congested reinforcement – cement content Shrinkage increased 10-20% to achieve strength and Finishability workability, fine and water content also increased 21 22 Grading Grading The proportions of the different siezes of particles 100 open making up the aggregate are found by sieving and Percent finer by weight are known as the “grading” of the aggregate 75 Sieve analysis uniform We are interested in 50 Gap- Well- Size largest particle present graded Coarse graded Size distribution 25 dense 10 1 0.1 0.01 0.001 Fine Grain diameter, mm 23 24 4  Grading Schematic representation Continuously graded aggregate contain particles Uniform size Continuous grading ranging in size from the largest to the smallest In gap-graded aggregates, some of the intermediate sizes are absent – necessary to achieve certain surface finishes. Gap-graded No-fines 25 26 Sieve sizes Fineness Modulus The sieve sizes in general use are 50,37.5, 20, 14, Sieve size % Cum. % Cum. % 10 and 5mm for coarse aggregates retained passing by retained by 5, 2.26, 1.18, 600micro, 300 micro and 150micro mass mass meters for fine aggregates 5mm 2 98 2 In gap-graded aggregates, some of the 2.5mm 113 85 15 intermediate sizes are absent – necessary to 1.25mm 20 65 35 achieve certain surface finishes 630microns 20 45 55 Aggregates containing high proportion of large particles – coarsely graded 315microns 24 21 79 One containing high proportion of small particles 160microns 18 3 97 – finely graded Fineness modulus = 283/100=2.83 283 27 28 Fineness Modulus Significance of grading FM – a measure of fineness of the sand The coarse aggregates A single parameter useful in checking the Not enough sand voids are overfilled uniformity of grading with fine aggregates Cement content For concrete sand, FM range is 2.3-3.1 (fine to coarse) % passing Economical 100% FM=2.3 FM=3.1 0% % sand Sieve size 29 30 5  Maximum aggregate size Maximum aggregate size Max. agg. size is the smallest sieve opening The choice of maximum aggregate size is through which the entire sample passes determined on job conditions It is usual to use nominal maximum size, which is Max. size not greater than 1/5 of the smallest the smallest sieve opening through which the dimension of the member – to guard against entire sample is permitted to pass, but need not do misrepresentation of concrete mix at any so. A percentage (usually 5-15%) of the sample section weight may be retained on this sieve For slabs on ground, max. size = 1/3 thickness In other words, if only a small amount of For reinforcements, nominal max. size = ¾ of aggregate is retained on a sieve it will not min. clear spacing affect the properties of the concrete 31 32 Moisture contents Moisture states of aggregates Four moisture states of the aggregates 1. Oven-dry (OD) – all moisture removed from Absorption the aggregate by heating in an oven at 105 capacity degrees Celsius to constant weight Oven dry 2. Air-dry (AD) – all moisture removed from the surface, but internal pores partially full Air dry 3. Saturated-surface-dry (SSD) – all pores filled Saturated surface dry with water, but film of water on the surface 4. Wet – all pores completely filled with water wet Effective absorption with a film of water on the surface Surface moisture 33 34 Absorption capacity Effective absorption Represents the maximum amount of water the Represents the amount of water required to bring aggregate can absorb an aggregate from AD state to SSD state AC= (WSSD – WOD) 100% AC= (WSSD – WAD) 100% WOD WOD Most aggregates have absorption capacity 1-2% Used to calculate the weight of water absorbed by the weight of aggregates in the concrete mix Wabs = EA (Wagg ) 35 36 6  Surface moisture Moisture content Represents the amount of water in excess of the Represent the water in excess of the SSD state SSD state Total water content of a moist aggregate SM= (Wwet – WSSD) = absorption + moisture content 100% WSSD MC= (Wstockpiled – WSSD) 100% Used to calculate the additional water added to WSSD the concrete with the aggregate To enable an estimate to be made of the quantity Wadd = SM (Wagg ) of water contained within it so that the water added at the mixer can be adjusted to control the required total in the mix 37 38 Moisture content Bulking of sand Coarse aggregates generally in the AD state MC Surface moisture with effective absorption BSGSSD>BSGOD vol. of aggregate (solid +pores)  BSG of most rocks 2.5-2.8 (value well = wt of aggregate (solid+pores) below this indicates high porosity) wt of water displaced 43 44 Unit Weight Measurement Can be defined as the weight of a given By filling a container of known volume volume of graded aggregate (known as and weighting it bulk density) Compaction affect the volume of void, Effectively measured the volume that thus, standard method involves the graded aggregate will occupy in compaction by rodding concrete and includes both the solid Since moisture content affects weight, aggregate particles and the voids oven-dry aggregates are used between them 45 46 Optimum blend Most economical concrete Unit weight of both fine and coarse 2000 normal-weight aggregates falling within a grading limit are in the range 1450- 1800 1750kg/m3 A plot of unit weights for various blends of two particular aggregates shows a 1600 maximum unit weight when the fine aggregate content is 35-40% 1400 0 20 40 60 80 100 47 48 8