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L01- Aggregates in Concrete.pdf

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McMaster University
Civil Engineering
Ghani Razaqpur

CONCRETE AND ITS CONSTITUENTS Concrete is a mixture of aggregates + cement + water and air. The proportions are approximately as follows: Aggregates : 60-80 % by volume Cement : 7-15 % by volume (203-400 kg/m )3 Water : 14-21 % by volume (125-190 kg/m ) 3 Air : 3 % (non-air entrained), 8 % (air entrained) In order to produce high quality concrete, its ingredients must be carefully chosen and proportioned. Since the aggregates constitute such a large part, we begin our study with the aggregates and their properties, but we will return to concrete and study it in detail in subsequent sections. Aggregates Aggregates are particles of natural stones, produced either by natural processes such as erosion and weathering or by mechanically crushing rocks. Other types of aggregates are made from slag, which is produced during the making of steel. Although aggregates have a number of applications in construction works, the focus of this course is mainly on their use in concrete mixtures. Aggregates can be classified based on a number of criteria as described below. Coarse aggregates : gravel, crushed rock, blast furnace slag Fine aggregates : sand Aggregates should be sound, well-graded, with maximum achievable density. Although use of larger size aggregates reduces the cement requirements, practically the maximum size is limited by a number of considerations. Aggregates classification based on size: Coarse aggregate: > 5 mm in size. Strictly speaking > 4.75 mm Fine aggregate: > 150 μm , but less than 4.75 mm (nominally 5 mm) Aggregates classification based on density Coarse aggregates can be normal, light-weight or heavy-weight. Light- weight 3 aggregates such as vermiculite, which is a type of mica, has a density of 64-192 kg/ m ; expanded blast furnace slag, silicates and aluminates of calcium (lime), expanded shale, clay or slate are other examples of light-weight aggregates. Limestone and quartzite are examples of normal weight aggregate. For atomic shielding purposes, we use heavy-weight aggregates such as steel punching, barite (BaSO )4 magnetite (FeFe O ) 2nd4hematite (Fe O ) to s2ie3d against x-rays, gamma rays. These are heavy-weight aggregates. The bulk density (mass of aggregate plus air between particles per unit volume) of aggregates typically vary as follows: (a) Light-weight fine aggregate < 1120 kg/m 3 (b) Light-weight coarse aggregates < 880 kg/m 3 3 (c) Normal-weight aggregates: 1520-1683 kg/m (d) Heavy-weight: (2400-2880 kg/m ) On the other hand, structural low density concrete has a density of 1430-1900 kg/m . The3 densi3y of concrete made with normal weight aggregates can be between 2000 and 2300 kg/m while of that made with heavy-weight aggregates can be between 3400-4800 kg/m . Volume-Weight Relationship of Aggregates A certain volume of aggregates normally consists of solid particles of various sizes and of the spaces or voids between the particles. In addition, each solid particle may appear to be totally solid, but in fact it contains tiny holes or pores which can be penetrated by air or water. In addition, the surface of exposed aggregates may be weight due to rain or contact with other sources of high humidity. In certain applications, it is important to know the amount of voids as well as the porosity of aggregate particles. For instance, in making concrete mixes, the amount of cement past required to produce a certain volume of concrete will depend on the amount of voids while the amount of water needed to make the required volume of concrete will depend on the amount of water within the pores and on the surface of the aggregate. To measure these quantities, engineers perform tests and use certain definitions to precisely indicate the amount of solid versus pores and/or voids in a given volume of aggregate. Traditionally, the density of a material gives the relationship between its weight, or more precisely its mass, and its volume. Since the weight of aggregate may comprise the weight of the solid particle plus the weight of the surface and pore water, and the volume may include the volume of the solid, the voids and the pores, precise definitions are needed to differentiate among the different kinds of densities which would result from the different combinations of weights and volumes. Basic Definitions Bulk Volume (V ): volume of (solid + pores +voids) B Saturated Surface Dry (SSD) Volume (V SSD): volume of (solid + pores) Solid Volume (V ):Svolume of solid material only Wet Weight (W ): weight of (solid + pore water + surface water) SSD Weight (W ): weight of (solid material + pore water) SSD Oven Dry Weight (WOD): weight of solid only weight of material Specific Gravity = volume of material  density of water Basic Properties of Aggregates W SSD  W OD (a) Absorption Capacity = 100 W OD W  W (b) Moisture Content =W OD 100 W OD volume of pores (c) Porosity = total volume of particles (d) Unit Weight or Bulk Density= B = V B W (e) Bulk Specific Gravity = OD W SSD  WSUBMERGED W OD (f) Apparent Specific Gravity = W OD  W SUBMERGED (g) Void Content =G of Agg . DW  Bulk Density of Agg . 100 SG of Agg . DW Note: DW = Density of Water Agg. = Aggregate The average specific gravity of blended aggregates, i.e. mixture of aggregates with different specific gravities, can be calculated using 1 G  P1  P2  Pn 100G1 100G2 100G n where G = Average specific gravity of the blended aggregate G1,G2, …Gn = Specific gravity of each size or type in the blend P1,P2, …n = Weight percentage of each size or type in the blend Similarly, the average percent absorption (A) of the blend is: P A P A P A A  ( 1 1100 )  ( 2 2 100 )  ...(n n100 ) where A ,A , …A = Absorption percentage of each size or typw in the blend 1 2 n Note: Bulk density is also called dry-rodded weight, specific weight or dry loose weight. Specific gravity is also referred to as relative density. Other Important Properties of Aggregates Bulking: Percent increase in the volume of aggregate over its dry-rodded volume due to surface moisture. See the figure on the following page for increase in volume of aggregates due to bulking. Bulking is caused by the pulling of the particles from each other by the surface tension of the water Bulking of coarse aggregate: At 5% MC, bulking is approximately 22% At 6% MC, bulking is a maximum of 25% At higher moisture content than 6%, bulking decreases until it reaches 3% at approximately 12% MC. Bulking of fine aggregate: At 5-8% MC, it reaches a maximum of 30-35% At greater than 8% MC, bulking decreases until it reaches 15% at 20% MC Rule of Thumb: Bulking is 15% in coarse sand and 35% in fine sand. The figure on the following page shows typical bulking trends for aggregates of different size. Porosit
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