TESTING of AGGREGATES for CONCRETE The properties of the aggregates affect both the fresh and hardened properties of concrete. It is crucial to know the properties of the aggregates to be used in the making of concrete in order to obtain the desired quality in a concrete. Therefore, the tests performed on aggregates are very important. Definitions (EN 12320) aggregate -granular material used in construction. Aggregate may be natural, manufactured or recycled. natural aggregate - aggregate from mineral sources which has been subjected to nothing more than mechanical processing. manufactured aggregate - aggregate of mineral origin resulting from an industrial process involving thermal or other modification. recycled aggregate -aggregate resulting from the processing of inorganic material previously used in construction. aggregate size - designation of aggregate in terms of lower (d) and upper (D) sieve sizes expressed as d/d fine aggregate - designation given to the smaller aggregate sizes with D less than or equal to 4 mm coarse aggregate - designation given to the larger aggregate sizes with D greater than or equal to 4 mm and d greater than or equal to 2 mm. all-in aggregate - aggregate consisting of a mixture of fine and coarse aggregates. fines - particle size fraction of an aggregate which passes the 0,063 mm sieve. grading - particle size distribution expressed as the percentages by mass passing a specified set of sieves. SAMPLING Before conducting any test on aggregates, the first thing to do is sampling. A sample is a representative small portion from a larger whole or group of materials. There are two methods of reducing the size of an aggregate sample: Splitting and quartering. Splitting Quartering 1. Determination of Particle Size Distribution Sieving Method (EN 933-1) In a sieve analysis, a sample of dry aggregate of known weight is separated through a series of sieves with progressively smaller openings. Once separated, the weight of particles retained on each sieve is measured and compared to the total sample weight. Particle size distribution is then expressed as a percent retained by weight on each sieve size. Results are usually expressed in tabular or graphical format. The test consists of dividing up and separating, by means of series of sieves, a material into several particle size classification of decreasing sizes. The aperture sizes and the number of sieves are selected in accordance with the nature of the sample and the accuracy required. The mass of the particles retained on the various sieves is related to the initial mass of the material. The cumulative percentages passing each sieve are reported in numerical form or in graphical form. 1
Test sieves set of sieves with given aperture sizes and shape. Individual retained the mass or percentage retained on one sieve after test. Cumulative retained sum of the mass or percentages retained on the sieve and on all coarser sieves. Cumulative passing sum of the mass or percentage passing the sieve (e.g. sum of the retained on all finer sieves and pan). The basic series of sieves (according EN 933-2): 0,063 mm; 0,125 mm; 0,250 mm; 0,500 mm; 1 mm; 2 mm; 4 mm; 8 mm; 16 mm; 31,5 mm; 63 mm; 125 mm Sieves with aperture size of 4 mm and above are perforated plate with square holes and sieves below 4 mm are from woven wire. Test portions - depends on maximum aggregate size and is specified in Table 1. Table 1. Required test portion mass for sieve analysis of aggregates for concrete. Sieving Procedure Pour the washed and dried material (or directly the dry sample) into sieving column. The column comprises a number of sieves fitted together and arranged, from top to bottom, in order of decreasing aperture sizes with pan and lid. Shake the column, manually or mechanically, then remove the sieves one by one, and shake each sieve manually ensuring no material is lost. Weight the retained material on each sieve with the largest aperture and record the retained masses in gram as shown in the example below (the first column in the example). Calculate the mass retained on each sieve as percentage of the original dry mass M (the second column in example) Calculate the cumulative percentage of the original dry mass passing each sieve down (the third column). Plot the sieve aperture size (in log scale) vs. cumulative passing (%) chart. Example: 2
Cumulative passing (%) CIV 204 MATERIALS of CONSTRUCTION Sieve aperture size (mm) Retained (g) Retained (%) Cumulative retained (%) Cumulative passing (%) 16 8 4 2 1 0.5 0.125 PAN TOTAL 100 90 80 70 60 50 40 30 20 10 0 0.125 0.25 0.5 1 2 4 8 16 Sieve aperture size (mm) 3
2. Cleanliness and Deleterious Materials. Aggregates must be relatively clean. Vegetation, soft particles, clay lumps, excess dust and vegetable matter may affect performance by quickly degrading, which causes a loss of structural support and/or prevents binderaggregate bonding. Clay Determination of clay, silt, and dust in fine and coarse aggregate can be tested by sedimentation method. The aggregate is carefully mixed with water in volumetric cylinder and then let to settle. The clay particles will form layer with different color and structure on the surface of aggregate. Organic Impurities Decaying vegetation may result in aggregates being contaminated with organic matter. This material may have a retarding effect on the setting of cementitious material and may result in lower strengths of the hardened material at all ages. Organic impurities can be tested by colorimetric test. Tested aggregate is mixed with sodium hydroxide (NaOH) or potassium hydroxide (KOH) to prepare colored solution. The color of solution is compared with color of standard solution after 24 hours, prepared according the standard. If the color of the test solution is darker than the standard solution, then the aggregate should be rejected. 3. Determination of Particle Shape of Coarse Aggregates. Particle shape and surface texture are important for proper compaction, deformation resistance and workability. Rounded particles create less particle-to-particle interlock than angular particles and thus provide better workability and easier compaction. Flat or elongated particles tend to impede compaction or break during compaction and thus, may decrease strength. Particle shape can be described by flakiness index or shape index (according EN 933-3, EN 933-4). Flakiness Index The flakiness index (FI) is calculated as the mass of particles that pass the bar sieves with parallel slots, expressed as a percentage of the mass of the test portion. Shape Index Shape index is determined only on the coarse aggregates. The principle of determination of shape index is to measure the thickness E and length L of each grain in a sample of several hundred stones and then to calculate the ratio L/E between the thickness and the length of each particle. If this ratio is higher than 3, than the particle is too long (non-cubic particle). EN 933-4 sets down the amount of measured grains to minimum 100 grains from each size of the coarse aggregate. Shape index SI is defined as a ratio between the weight of particles with L/E 3 and weight of all measured particles in percents. Shape index caliper 4
4. Abrasion Resistance (Los Angeles Test) The test methods described in ASTM C131 and EN 1097-2 are different each other. The general concept is provided here. This test method covers a procedure for testing coarse aggregate resistance to degradation (due to breaking the aggregates) using the Los Angeles testing machine. For this test, 5 kg of oven-dried aggregate sample and abrasive charges made of steel spheres are placed into the drum of the machine. Then, the machine is started to rotate at 30-33 rpm (revolution per minute). After 500 revolutions (100 revolutions can also be applied to aggregates which are going to be used in concrete elements facing light abrasion conditions), aggregate samples are removed from the drum and sieved on No.12 (1.6 mm) sieve. Los Angeles coefficient is determined by the LA equation given below (m is the retained fraction of aggregates on 1.6 mm sieve in gram). The aggregates are assumed to be resistant against abrasion when the weight loss is less than 10 % and 50 % at the end of 100 and 500 revolutions of the cylinder, respectively. Test samples and number of spheres should comply with one of the following limitations: GRADINGS of TEST SAMPLES (ASTM C131) Measured parameter Weight of the aggregate before test (g) Weight of the aggregate fraction retained on 1.6 mm sieve (g) Calculated s LA coefficient 5
5. Determination of Bulk Density (Unit Weight) This test method covers the determination of bulk density ( unit weight ) of aggregate in compacted or loose condition, and calculated voids between particles in fine, coarse, or mixed aggregates based on the same determination. To do this test, aggregate samples, which are oven dried at 110±5 o C to constant mass, are filled in a measure, size of which is determined by the maximum aggregate size. The measure is filled in 3 equal layers and each layer is rodded 25 times by tamping rod. When the measure is filled, the excess aggregates are struck off by using the side of tamping rod so that the surface of the measure is leveled. Then, knowing the volume of the measure and the weight of aggregates inside the measure, unit weight is calculated as weight over volume. The unit weight calculated by this procedure is called as dry rodded unit weight and if no rodding operation is applied, it is called as dry loose unit weight. Self-weight of the container (g) Weight of the container filled with water (g) Loose weight of concrete in container (g) Compacted (rodded) weight of aggregate in container (g) Volume of the container (m 3 ) Loose unit weight of the aggregate (kg/m 3 ) Compacted (rodded) unit weight of the aggregate (kg/m 3 ) 5.1 Determination of Specific Gravity, and Water Absorption Capacity of Coarse Aggregate The amount of sample should conform to the given limits: Procedure: Immerse the sample in water at room temperature for a period of 24 hours. Remove the sample from the water and roll it in a large absorbent cloth until all visible films of water are removed. Obtain the weight of the sample in saturated surface dry condition (SSD) (B). After determining B, immediately place the sample in a wire basket and determine its weight in water (C). Dry the sample to constant weight at a temperature of 105±5 o C, allow it to cool to room temperature and weigh (A). Calculations: Specific gravity - Dry (SG-dry) = A / (B C) Specific gravity SSD (SG-ssd) = B / (B C) Apparent specific gravity (SG-app) = A / (A C) Absorption Capacity (%) = (B A) / A 100 Dry weight of the sample (g) Weight of the sample in SSD condition (g) Weight of the sample in water (g) Specific gravity dry Specific gravity ssd Water absorption capacity (%) 6
5.2 Determination of Density, Relative Density (Specific Gravity), and Water Absorption Capacity of Fine Aggregate Apparatus: Balance, flask, small cone mold, tamping rod. Preparation of Sample: Place approximately 1 kg of fine aggregate in a pan and cover it with water and permit to stand 24 hr. Then, spread the sample on a flat surface exposed to a gently moving current of warm air and stir frequently to secure uniform drying. Continue drying until saturated surface dry condition is maintained. For the determination of this condition either cone method or eye inspection shall be employed. Procedure: Saturated surface dry sample shall be divided into two equal sections. One of which shall be weighed (B) and dried to constant weight at a temperature of 105±5 o C and after cooling weighted (A). The other section shall be introduced into the flask which is filled with some water. The flask then shall be rolled on a flat surface in order to eliminate all air bubbles. It shall be placed in a constant temperature water bath maintained at 23±2 o C. After 1 hr., the flask shall be filled with water up to 500 ml mark and weighed (C). D: Weight of flask filled with water up to 500 ml mark. Calculations: Specific gravity oven dry = A / (B + D C) Specific gravity - ssd = B / (B + D C) Apparent specific gravity = A / (A + D C) Absorption Capacity (%) = (B A) / A 100 Measured s Weight of the sample in SSD condition (g) Weight of the sample in oven-dry condition (g) Weight of the flask + sample + water (g) Weight of the flask + water (g) Specific gravity dry Specific gravity ssd Apparent specific gravity Water absorption capacity (%) 7