CE 308 Structural Materials PDF

CE 308: Structural Materials 2. Properties of Cement and Tests Dr. Raid Alrashidi Civil Engineering Department, Jubail Industrial College Semester 461 Learning Objectives 1. To explain the physical properties of cement 2. To explain the commonly used tes...

CE 308: Structural Materials 2. Properties of Cement and Tests Dr. Raid Alrashidi Civil Engineering Department, Jubail Industrial College Semester 461 Learning Objectives 1. To explain the physical properties of cement 2. To explain the commonly used tests on cements 2 Comparison between the main compounds in cement (Review) Portland cement. 3 Comparison between the main compounds in cement (Review) 4 Portland Cements Definition and Uses Portland cement is a fine powder that when mixed with water become the glue that hold aggregate together in concrete. The main use of portland cement is to make portland cement concrete, but it can be used for other purposes, such as stabilizing soils and aggregate bases for highway construction. 5 Physical Requirements of Portland Cement The quality of a Portland cement is assessed in terms of the following physical properties determined through the lab tests on the cement samples collected in accordance with ASTM C183: 1. Bulk density of cement 2. Density and relative density (specific gravity) 3. Fineness of cement 4. Soundness of cement 5. Time of setting 6. Consistency of cement mortar or paste 7. Heat of hydration 8. Strength characterization 9. Loss on Ignition There are commonly used methods to test these properties. 6 1. Bulk Density (Unit weight) The bulk density of cement is defined as the mass of cement particles including the voids (air) between particles per unit volume The bulk density of cement does not remain constant, as it depends on how cement is handled and stored. As can be observed from the following figure that the same mass of cement has different volume before and after the compaction, therefore, has different bulk density before and after the compaction. For this reason, for batching concrete, the quantity of cement should be measured by mass (weight) instead of volume. The bulk density of cement may vary in the range of 830 to 1650 kg/m3. Before and after compaction (20% 7 difference) 2. Density and Relative Density (Specific Gravity) The density of cement is defined as the mass of a unit volume of the solids or particles, excluding air between particles Relative density or specific gravity is the density of cement per unit density of water The specific gravity of cement ranges from 3.10 to 3.25, averaging 3.15 It is needed for mixture-proportioning calculations The density of cement can be determined using a Le Chatelier flask and kerosene or by using a helium pycnometer according to ASTM C188 Particle Size of cement: finer than No. 200 sieve (75 μm) A commercial bag of cement in Saudi Arabia weighs around 50 kg Le chatelier flask Helium Pycnometer 8 3. Fineness of Cement Fineness of cement is the extent to which the cement is ground Fineness of cement particles is an important property that must be carefully controlled. Fineness of cement has considerable effect on the behavior of cement during hydration, as follows: The rate of hydration increases with increasing fineness. This leads to both a higher rate of strength development and a higher rate of evolution of heat, principally during the first seven days At high fineness, the amount of water required for workability is increased, which results in increased drying shrinkage A high cement fineness reduces the durability of concrete subjected to freeze-thaw cycles Fineness of cement is expressed in terms of “specific surface area” (i.e., surface area of particles per unit mass), usually in units of m2/kg. 9 3. Fineness of Cement Two commonly used methods to measure the fineness of cement: 1. Wagner Turbidimeter test and (ASTM C115) 2. Blaine Air Permeability test (ASTM C204) (mostly used) 3. Fineness can also be measured by determining the percent passing the 0.045 mm sieve (No. 325) (ASTM C430). (This is an old method, approx. 95% of materials passes) Wagner Turbidimeter test Blaine test 10 4. Soundness of Cement Soundness of cement is the ability of a hardened paste to retain its volume after setting without excessive expansion. A cement is said to be unsound if it is subjected to excessive volume change after setting, leading towards cracking Unsoundness of cement is caused by: Slow hydration of free lime or magnesia Reactions of gypsum with C3A Expansive reactions causing unsoundness take place very slowly, and so unsoundness will only appear after many months or even years 11 4. Soundness of Cement The most commonly used test is ASTM C150 standard test for Autoclave Expansion of Portland Cement. Capable of detecting unsoundness due to both excess free CaO and excess MgO ASTM C150 limits autoclave expansion to 0.8% 12 5. Time of Setting The term “setting” is used to describe the stiffening of the cement paste or the change from a plastic state to a solid state. The characterization of the time of setting of a cement is to determine the quality of a cement with regards to its rate of setting. “Initial set” indicates that the paste is beginning to stiffen considerably and can no longer be molded “Final set” indicates that the cement has hardened to the point at which it can sustain some load 13 5. Time of Setting The two commonly used tests are: 1. The Gillmore needle test (ASTM C266) 2. The Vicat test (ASTM C191) Both tests determine the initial setting time and final setting time of a cement. ASTM C150 Specification on Time of Setting: Gillmore Vicat Minimum Initial Setting Time: 60 minutes 45 minutes Maximum Final Setting Time: 600 minutes 375 minutes 14 5. Time of Setting- Vicat Test Vicat’s apparatus used for setting time is same as that for consistency except the size of needle, which is 1 mm diameter for setting time “Initial setting time” is the time from the instant at which water is added to the cement to the instant at which the Vicat’s initial set needle penetrates 25 mm into the cement paste ASTM C 150 prescribes a minimum initial setting time of 60 minutes for Portland cements “Final setting time” corresponds to the time at which the needle does not sink visibly into the cement paste ASTM C 150 prescribes a maximum final setting time of 10 hours for Portland cement 15 5. Time of Setting- Gillmore Test Gillmore apparatus is less commonly used than the Vicat apparatus and gives different results The cement is considered to have acquired initial set when the initial Gillmore needle (left needle) fails to penetrate the paste Final set is reached when the final Gillmore needle (Right needle) fails to penetrate the paste 16 Terminology related to Setting and Hardening of Cement Stiffening is the loss of consistency by the cement paste as the cement hydrates. Early stiffening is the early development of stiffness of cement paste, mortar or concrete. Early stiffening includes: False set Flash (quick) set False set False set is evidenced by a significant loss of plasticity without the evolution of much heat shortly after mixing False set cause no difficulty in placing and handling of concrete if the concrete is mixed for a longer time than usual or if it is remixed without additional water before it is transported or placed 17 Terminology related to Setting and Hardening of Cement Flash (quick) set Flash (quick) set is evidenced by a rapid and early loss of workability in paste, mortar or concrete It is usually accompanied by the evolution of considerable heat resulting primarily from the rapid reaction of aluminates It happens in the absence of gypsum Unlike the false set, flash set cannot be dispelled, nor can the plasticity be regained by further mixing without the addition of water 18 Flash (quick) Set vs False Set Gypsum in the cement regulates setting time. Setting time is also affected by: cement fineness, w/c ratio, and admixtures  What happens if gypsum content is too low?  Flash (quick) set  What happens if gypsum content is too high?  False set 19 6. Consistency of Cement Paste or Mortar Consistency of a cement paste or mortar refers to its ability to flow, or resistance to flow. “Normal or Standard Consistency” value of a cement sample is used for preparing the pastes for the determination of the setting time, unsoundness, compressive and tensile strength of the cement Two commonly used method to measure and control the consistency: 1. Vicat apparatus (ASTM C187) 2. Flow table (ASTM C230) 20 7. Heat of Hydration 21 Heat of Hydration Following are the factors affecting heat of hydration: Fineness of cement w/c ratio The temperature at which hydration occurs (i.e., curing temperature) The heat of hydration is determined in accordance with ASTM C 186 or by conduction calorimetr This test measures the heat of hydration at 7 days and at 28 days and reports the results in cal/g or kJ/kg. The heat of hydration can provide information for calculating temperature rises in mass concrete. 22 Stages of Hydration Stage 1: heat of wetting or initial hydrolysis (C3S and C3A hydration) Stage 2: dormant period related to initial set Stage 3: an accelerated reaction of the hydration products that determines rate of hardening and final set Stage 4: decelerates formation of hydration products and determines the rate of early strength gain Stage 5: slow, steady formation of hydration products establishing the rate of later strength gain 23 Stages of Hydration 25 8. Strength Characterization Compressive strength of cement is the most important property for assessing the quality of cement. However, strength of cement cannot be used to accurately relate the strength of CONCRETE because many other factors also affect concrete strength. Standard Test: Compressive Strength of Hydraulic Cement Mortars (ASTM C109). The purpose of this test is to ensure the quality of a cement with regards to strength development. Basic Procedures: Prepare a cement mortar (using one part of cement to 2.75 parts of graded standard sand, and a w/c of 0.485 for non-entraining cements) and mold 2-inch (50mm) cube specimens. Remove specimens from mold at 24 hours and immerse them in water saturated with lime (except those tested at 24 hours). Test the mortar cubes for compressive strength at 24 hours, 3 days, 7 and 28 days Required Compressive strengths for Type I cement: 13 MPa at 3 days & 20 MPa psi at 7 days. 26 8. Strength Characterization 27 9. Loss on Ignition Loss on Ignition (LOI) (ASTM C114 ) is an important factor to consider because it affects the strength and durability of the concrete that is made with the cement. Basic Procedure: A 1 g sample of cement is placed in a crucible and ignited in a muffled furnace at a temperature of 950 ± 50 °C until constant weight is reached. Then % weight loss is calculated and reported The weight loss is assumed to represent the total moisture and CO2 in the cement. A high LOI gives an indication of pre-hydration and carbonation, which may be caused by improper storage and handling of the cement. A high LOI can result in a weaker and less durable concrete, while a low LOI can result in a stronger and more durable concrete 28 9. Loss on Ignition (furnace) 29