Notes for UNIT-II (Aggregates used for concrete making) Concrete The Extensively used material after water because of the following reasons  Versatile  Pliable when mixed  Strong & Durable  Does not Rust or Rot  Does Not Need a Coating  Resists Fire  Almost Suitable for any Environmental Exposure Conditions Concrete is a Rocklike Material  Ingredients – Portland Cement – Coarse Aggregate – Fine Aggregate – Water Admixtures (optional) Table Showing the diverse nature of the ingredients of the concrete Ingredient Size Shape Specific Gravity Texture Cement 50 microns Nearly Spherical Average Particle size 3.00 to 3.20 Smooth Coarse Aggregtes 80mm - 4.75mm Round, Angular, 2.6-2.8 cuboidal, rounded, flaky, elongated Glassy, Smooth, Granular, Crystalline, Honeycombed and porous Fine Aggregates 4.75mm-150µ Angular or rounded 2.5-2.6 Smooth,Granular Water - - 1.0 - Mineral Admixtures <40 microns Average Size Nearly Spherical Varies Glassy, smooth 1 Aggregates generally occupy 65- 80% of a concrete’s volume. Aggregates are inert fillers floating in the cement paste matrix for concretes of low strength. The strength of aggregates do not contribute to the strength of concrete for low strength concrete.The characteristics of aggregates impact performance of fresh and hardened concrete. Why use aggregate • • • • • • Reduce the cost of the concrete – 1/4 - 1/8 of the cement price Reduce thermal cracking – 100 kg of OPC produces about 12oC temperature rise Reduces shrinkage – 10% reduction in aggregate volume can double shrinkage High aggregate : cement ratio (A/C) desirable A/C mainly influenced by cement content Imparts unit weight to concrete Aggregate Classification Aggregates are classified as below: Based on • size:- F.A & C.A. • Specific Gravity:- Light Weight, Normal Weight and Heavy Weight Aggregates. • Availability:- Natural Gravel and Crushed Aggregates. • Shape:- Round, Cubical, Angular, Elongated and Flaky Aggregates. • Texture:- Smooth, Granular, Crystalline, honeycombed and Porous. 2 There are two types of Aggregates used in concrete making based on their size: • Coarse Aggregates. • Fine Aggregates. Fine Aggregate • Sand and/or crushed stone. • < 4.75 mm. • F.A. content usually 35% to 45% by mass or volume of total aggregate. Coarse Aggregate • Gravel and crushed stone. • ≥ 4.75 mm. • typically between 9.5 and 37.5 mm. Rock and Mineral Constituents in Aggregates 1.Minerals • Silica Quartz, Opal • Silicates Feldspar, Clay • Carbonate Calcite, DolomiteSulfate Sulfate Gypsum, Anhydrite Iron sulfide Pyrite, Marcasite Iron oxide Magnetite, Hematite 2.Igneous rocks • Granite • Syenite • Diorite • Gabbro • Peridotite • Pegmatite • Volcanic glass • Felsite • Basalt 3. Sedimentary rocks • Conglomerate • Sandstone • Claystone, siltstone, argillite, and shale • Carbonates • Chert 4. Metamorphic rocks • Marble • Metaquartzite • Slate 3 • • Phyllite Schist Normal-Weight Aggregate Most common aggregates • Sand • Gravel • Crushed stone Produce normal-weight concrete 2200 to 2400 kg/m3 Lightweight Aggregate Expanded • Shale • Clay • Slate • Slag Produce structural lightweight concrete 1350 to 1850 kg/m3  Pumice  Scoria  Perlite  Vermiculite  Diatomite Produce lightweight insulating concrete— 250 to 1450 kg/m3 Heavyweight Aggregate • Barite • Limonite • Magnetite • Ilmenite • Hematite • Iron • Steel punchings or shot Produce high-density concrete up to 6400 kg/m3 Used for Radiation Shielding 4 Aggregate Characteristics Grading of Aggregates Grading is the particle-size distribution of an aggregate as determined by a sieve analysis using wire mesh sieves with square openings. As per IS:2386(Part-1) Fine aggregate―6 standard sieves with openings from 150 µm to 4.75 mm. Coarse aggregate―5 sieves with openings from 4.75mm to 80 mm. Gradation (grain size analysis) Grain size distribution for concrete mixes that will provide a dense strong mixture. Ensure that the voids between the larger particles are filled with medium particles. The remaining voids are filled with still smaller particles until the smallest voids are filled with a small amount of fines. Ensure maximum density and strength using a maximum density curve Good Gradation Concrete with good gradation will have fewer voids to be filled with cement paste ( economical mix) Concrete with good gradation will have fewer voids for water to permeate ( durability) Particle size distribution affects: Workability Mix proportioning Freeze-thaw resistance ( durability) 5 Range of Particle Sizes 6 Fine-Aggregate Grading Limits IS -383 IS Sieve Designation Percentage passing by weight Grading Zone-I (Coarse Sand) 10mm 100 Zone-II Most Suitable/Desirable 100 Zone-III Zone-IV (Fine Sand) 100 100 4.75mm 90-100 90-100 90-100 95-100 2.36mm 60-95 75-100 85-100 95-100 1.18mm 30-70 55-90 75-100 90-100 600µm 15-34 35-59 60-79 80-100 300µm 5-20 8-30 12-40 15-50 150µm 0-10 0-10 0-10 0-15 Fineness Modulus 4.0-2.71 3.37-2.10 2.78-1.71 2.25-1.35 7 The percentage passing 600µm sieve will decide the zone of the sand. Zone-I Coarse Sand Zone-II Zone-III Zone-IV Fine Sand Grading Limits Can also be represented through a graph of sieve size on the x-axis and % passing on the Y-axis (Semi log sheet). Fineness Modulus (FM) The results of aggregate sieve analysis is expressed by a number called Fineness Modulus. Obtained by adding the sum of the cumulative percentages by mass of a sample aggregate retained on each of a specified series of sieves and dividing the sum by 100. The specified sieves are: 150 µm (No. 100), 300 µm (No. 50), 600 µm (No. 30), 1.18 mm (No. 16), 2.36 mm (No. 8), 4.75 mm (No. 4), 9.5 mm , 19.0 mm , 37.5 mm , 75 mm , and 150 mm. Results of Sieve Analysis and calculation of FM of Sand Percentage of individual fraction retained, by mass Sieve size Percentage passing, by mass Cumulative percentage retained, by mass 0 100 0 2.36 mm 2 13 98 85 2 15 1.18 mm 20 65 35 600 µm 20 45 55 300 µm 24 21 79 150 µm 18 3 97 3 100 0 — 283 10 mm 4.75 mm Pan Total Fineness modulus = 283 ÷ 100 = 2.83 • Index of fineness of an aggregate. • The fineness modulus of the fine aggregate is required for mix design since sand gradation has the largest effect on workability. A fine sand (low FM) has much higher effect paste requirements for good workability. • The FM of the coarse aggregate is not required for mix design purposes. 8 • It is computed by adding the cumulative percentages of aggregate retained on each of the specified series of sieves, and dividing the sum by 100 [smallest size sieve: No. 100 (150 µm)]. For concrete sand, FM range is 2.3 to 3.1 • Note: The higher the FM, the coarser the aggregate. • It is important to note that the fineness modulus is just one number which only characterizes the average size of the aggregate, and different grading may have the same fineness modulus. Fine Aggregate effect on concrete • • • • Oversanded (More than required sand) – Over cohesive mix. – Water reducers may be less effective. – Air entrainment may be more effective. Undersanded (deficit of sand) – Prone to bleed and segregation. – May get high levels of water reduction. – Air entrainers may be less effective. Sand grading – gap graded or single sized may enhance bleed and segregation. Air entrainment may help fill the gaps. Coarse aggregate – Poor grading may give a harsh mix at low workabilities and segregation at high workabilities. – Effect on admixtures is small. – Elongated or flaky aggregates may cause workability difficulties . 9 Reduction of Voids If uniform size aggregates are there there will be more voids as can be seen from the first two figures.If properly graded aggregates are used which contain suitable percentage of all size then the voids will be minimum which is explained in the figure. Maximum Size vs. Nominal Maximum Size of Aggregate Maximum size ― is the smallest sieve that all of a particular aggregate must pass through. Nominal maximum size ― is the standard sieve opening immediately smaller than the smallest through which all of the aggregate must pass. The nominal maximum-size sieve may retain 5% to 15% Nominal Maximum Size of Aggregate Size should not exceed ―  1/5 of the narrowest dimension between sides of forms.  3/4 clear spacing between rebars and between rebars and the form.  1/3 depth of slabs. Higher maximum aggregate size lowers paste requirements, increases strength and reduces w/c ratio. Excessively large aggregates reduce strength due to reduced surface 10 area for bonding. It affects the paste requirements, optimum grading depends on MSA and nominal max. size. The higher MSA, the lower the paste requirements for the mix. Aggregate size affects the following concrete properties: water demand, cement content, microcracking (strength). Effect of aggregate size on the surface area size # of particles volume surface area 1" 1 1 cubic inch 6 square inches .5" 8 1 cubic inch 12 square inches 0.25 64 1 cubic inch 24 square inches 0.125 512 1 cubic inch 48 square inches Larger particles, less surface area, thicker coating, easy sliding of particles. Smaller particles, more surface area, thinner coating, interlocking of particles Maximum Aggregate Size and Water Requirement Effect on water demand Max size of Aggregate 10 mm 20 mm 40 mm Slump 30 - 60 mm 230 kg/m3 210 kg/m3 190 kg/m3 Slump 60 - 180 mm 250 kg/m3 225 kg/m3 205 kg/m Effect on cement content at constant w/c of 0.60 Max size of aggregate 10 mm 20 mm 40 mm Water content 230 kg/m3 210 kg/m3 190 kg/m3 Cement content 380 kg/m3 350 kg/m3 315 kg/m3 A:C ratio 4.7 5.3 6.0 11 In general the grading and maximum size of aggregate affect the following: o Relative aggregate proportions (i.e. FA/CA and FA/TA ratios) o Cement and water requirements o Workability and pumpability of fresh concrete: very coarse sands and coarse aggregate can produce harsh, unworkable mixes o Uniformity of concrete from batch to batch o Porosity, shrinkage, and durability of hardened concrete o Economy in concrete production: very fine sands are often uneconomical Moisture In Aggregates Aggregates have two types of moisture: 1. Absorbed moisture – retained in pores 2. Surface moisture – water attached to surface Aggregates have four moisture states: Oven dry: all moisture removed Air dry: internal pores partially full & surface dry Saturated-surface dry: pores full & surface moisture removed Wet: pores full and surface film SSD aggregate does not add or subtract water Not easily obtained in the field Moisture Absorption We must determine how much water dry aggregate will consume into its voids This takes water away from the mix and reduces workability & W/C ratio We adjust mix proportions for absorption We want to: provide aggregates water for absorption maintain workability of the mix Shape and surface texture of aggregates The shape of aggregate is an important characteristic since it affects the workability of concrete.It is difficult to measure the shape of irregular shaped aggregates. Not only the type of parent rock but also the type of crusher used also affects the shape of the aggregate produced. Good Granite rocks found near Bangalore will yield cuboidal aggregates. Many rocks contain planes of jointing which is characteristics of its formation and hence tend to yield more flaky aggregates. The shape of the aggregates produced is also dependent on type of crusher and the reduction ratio of the crusher. Quartzite which does not possess cleavage planes tend to produce cubical shape aggregates. From the standpoint of economy in cement requirement for a given water cement ratio rounded aggregates are preferable to angular aggregates. On the other hand, the additional cement required for angular aggregates is offset to some extent by the higher strengths and some times greater durability as a result of greater Interlocking 12 texture of the hardened concrete. Flat particles in concrete will have objectionable influence on the workability of concrete, cement requirement, strength and durability. In general excessively flaky aggregates make poor concrete. while discussing the shape of the aggregates, the texture of the aggregate also enters the discussion because of its close association with the shape. Generally round aggregates are smooth textured and angular aggregates are rough textured. Therefore some engineers argue against round aggregates from the point of bond strength between aggregates and cement. But the angular aggregates are superior to rounded aggregates from the following two points: Angular aggregates exhibit a better interlocking effect in concrete, which property makes it superior in concrete used for road and pavements.The total surface area of rough textured angular aggregate is more than smooth rounded aggregates for the given volume. By having greater surface area, the angular aggregates may show higher bond strength than rounded aggregates.The shape of the aggregates becomes all the more important in case of high strength and high performance concrete where very low water/cement ratio is required to be used . In such cases cubical aggregates are required for better workability. Surface texture is the property, the measure of which depends upon the relative degree to which particle surface are polished or dull, smooth or rough. Surface texture depends upon hardness, grain size, pore structure, structure of the rock and the degree to which the forces acting on it have smoothened the surface or roughened. Experience and laboratory experiments have shown that the adhesion between cement paste and the aggregate is influenced by several complex factors in addition to the physical and mechanical properties. As surface smoothness increases, contact area decreases, hence a highly polished particle will have less bonding area with the matrix than a rough particle of the same volume. A smooth particle , however, will require a thinner layer of paste to lubricate its movements with respect to another aggregate particle. It will therefore permit denser packing because of enhanced workability. Aggregate:Shape and Surface Texture Ideal aggregates: spherical or cubical round shape, fine porous surface reduced particle interaction (friction) results in good workability and good surface area for bonding natural sands are good examples of this Non Ideal aggregates: angular elongated flaky or rough high particle interaction requires more cement paste to achieve workability results in increased cost 13 Rounded: Good workability, low water demand, poor bond Irregular: Fair workability, low water demand Angular: Increased water demand, good bond Elongated : May lack cohesion and require increased fines Flaky: Aggregate stacks give workability problems Coarse Aggregate Texture Glassy. Smooth. Granular. Crystalline Honeycombed and porous. • • • • • • • • • • Depends on: rock hardness, grain size, porosity, previous exposure. Aggregate shape and texture affect the workability of fresh concrete through their influence on cement paste requirements. Sufficient paste is required to coat the aggregates and to provide lubrication to decrease interactions between aggregate particles during mixing. Ideal particle is one close to spherical in shape (well rounded and compact) with a relatively smooth surfaces (natural sands and gravels come close to this ideal). More angular shapes - rough surfaces – interfere with the movement of adjacent particles (less workable) –They also have a higher surface –to –volume ratio – more paste. Flat or elongated aggregates should be avoided. Rough surface requires more lubrication for movement (crushed stone). Shape can influence strength by increasing surface area available for bonding with the paste. Rough surfaces –improve mechanical bond. Irregular aggregates (angulars) –higher internal stress concentrations –easier bond failure. 14 Flaky Aggregates 15 16 Aggregate characteristics like Shape, Size and Textues Influence the following Fresh concrete • Mix proportions • Workability / water demand • Cohesion / pumpability • Air content / entrainment Hardened concrete • Strength • Density • Shrinkage • Skid & abrasion resistance • Elastic modulus • Durability • Color. For tests on aggregates please refer text books and concrete testing manuals. 17