Chapter 3-Properties of Hardened Concrete PDF

Classwork 1. Define setting and hardening steps of concrete. 2. List 4 properties of hardened concrete. 3. Define and explain the test done to find the compressive strength of concrete. Setting and Hardening Steps Properties of Hardened Concrete Properties of Hardened Concrete 1. Strength Compressiv...

Classwork 1. Define setting and hardening steps of concrete. 2. List 4 properties of hardened concrete. 3. Define and explain the test done to find the compressive strength of concrete. Setting and Hardening Steps Properties of Hardened Concrete Properties of Hardened Concrete 1. Strength Compressive Strength Tensile Strength Flexural Strength Shear Strength Properties of Hardened Concrete 1. Strength Strength of concrete it is defined as the maximum stress it can resist or the maximum it can carry. The compressive strength of a concrete is defined as the ability of the concrete to withstand specific compressive forces. Compressive strength is one of the main structural design requirements to ensure that the structure will be able to carry the intended load. Cubes, Cylinders and prisms are the 3 type of compression Test Specimens. Compressive Strength takes as the maximum load these specimens can carry per unit are. Compressive Strenght Test of Concrete ØThe compressive strength test is the most commonly performed test on hardened concrete. Compressive strength is one of the main structural design requirements to ensure that the structure will be able to carry the intended load. ØCompressive Strength is determined by loading properly prepared and cured cubic, cylindrical or prismatic specimens under compression Cubic specimens without capping Cylindrical specimens with capping The compressive strength value depends on the shape and size of the specimen Compressive Strenght Test of Concrete Specimen Testing Using a testing machine, specimens are tested by applying axial compressive load with a specified rate of loading until failure. The compressive strength of the specimen is determined by dividing the maximum load carried by the specimen during the test by the average cross sectional area. !′ ! = Maximum Load (N) = (MPa) Cross Section Area (mm2 ) 80% of the samples taken should have a compressive strength equal or greater to the compressive strength required Gain Strength of concrete = !′! Grade of Concrete *100 Example Calculate the compressive strength of a cylindrical specimen having a diameter of 150 mm and height of 300 mm, if the maximum rupture load was 565 kN. Find the Gain Strength, if the grade of concrete is 45 MPa Example Calculate the compressive strength of a cubic specimen with side is equal to 150 mm, if the maximum rupture load was 600 kN. Grade of Concrete is M30 Find the Gain strength in percentage Example A 20Mpa Concrete strength is specified for a concrete mix Find the expected test result of the compressive strength of the concrete in Mpa when tested after 7 days of pouring at the time of testing Example A design engineer specifies a required compressive strength of concrete equal to 28 Mpa. The lab results of compressive strength test for different samples showed the following strength in Mpa: 32.6, 31.4, 28.5, 23.2, 33.3, 30.5, 22.5, 34.6, 27.7, 25.8, 28.3, 29.8, 32.9, 32.9, 39.6, 40.2, 29.5, 31.8 The concrete is judged to be adequate? 2-Degree of Compaction 2-Degree of Compaction Air voids are generally formed because of the evaporation of the water used in making concrete and by entrained air. In General the air voids depends on th water cement ratio. There is an increase in the air voids when there is an increase in the weight of water. When this situation happens this will affect the strength of the concrete. When the concrete is exposed to compaction this will affect the voids ratio which will affect the strength of concrete 3-Curing of Concrete Curing is used for promoting the hydration of cement and consists of a control of temperature and moisture movement from and into the concrete. The longer period of curing the greater its final strength. - Early – age: ages less than seven days - Later – age: ages exceeding 28 days 4-Admixtures The effect of particular Admixtures for example like accelerator, retarder or plasticizer depends on the precise nature of the admixtures themselves. Added to the batch of concrete before or during mixing. 2. Deformation Under Load It is a stress strain relationship under normal loading and under sustained loading. Under normal loading: the first effect of applying a load to concrete is to produce an elastic deformation i.e. as the load increases deformation increases. Under sustained loading: the continue application of stress causes a slow deformation known as creep. The increase of deformation is not proportional , as the time passes the deformation is lesser. 4. Permeability of Concrete Concrete has a tendency to be porous due to the presence of voids formed during or after placing. Penetration by substance may adversely affect durability e.g. Ca(OH)2 leaches out. The penetration of moisture into concrete causes corrosion of reinforcement and results in the volume expansion of steel bars, consequently causing cracks and spalling of concrete cover 4. Permeability of Concrete To produce concrete of low permeability, full compaction & proper curing is essential. Low permeability is important in increasing resistant to frost action and chemical attack and in protecting embedded steel against corrosion. 4. Permeability of Concrete Factors influencing permeability are: i. W/C Ratio ii. Curing iii. Method of compaction iv. Workability v. Soundness & porosity of the aggregate vi. Age (permeability decrease with age) vii. Grading of aggregate 5. Shrinkage of Concrete Factors influencing permeability are: Caused by the settlement of solids and the loss of free water from the plastic concrete (plastic shrinkage), by the chemical combination of cement with water and by the drying of concrete (drying shrinkage). The shrinkage is dependent on the amount of drying that can take place. Influenced by the humidity and temperature of the surrounding air, the rate of air flow over the surface and the proportion of the surface area to volume of concrete 5. Shrinkage of Concrete 2 types of shrinkage: a)Plastic Shrinkage Shrinkage which takes place before concrete has set. Occurs during the first few hours after fresh concrete is placed During this period, moisture may evaporate faster from the concrete surface than it is replaced by bleed water from lower layers of the concrete mass. 5. Shrinkage of Concrete Plastic cracking (Plastic Shrinkage Cracking) is cracking that occurs in the surface of the fresh concrete soon after it is placed and while it is still plastic. Factors Affecting Shrinkage of Concrete 1.Aggregate - Concrete with higher aggregate content exhibits smaller shrinkage. Concrete with aggregates of higher modulus of elasticity or of rougher surfaces is more resistant to the shrinkage process. 2.Water-cement ratio - The higher the W/C ratio is, the higher the shrinkage. As W/C increases, paste strength and stiffness decrease; and as water content increases, shrinkage potential increases. Factors Affecting Shrinkage of Concrete 3.Member size - Shrinkage decrease with an increase in the volume of the concrete member. However, the duration of shrinkage is longer for larger members since more time is needed for shrinkage effects to reach the interior regions. 4.Medium ambient conditions - The rate of shrinkage is lower at higher values of relative humidity. Shrinkage becomes stabilized at low temperatures. Factors Affecting Shrinkage of Concrete 5. Admixtures - effect varies from admixture to admixture. Any material which substantially changes the pore structure of the paste will affect the shrinkage characteristics of the concrete. In general, as pore refinement is enhanced, shrinkage is increased. Concrete in Different Environment