Faculty-of-Engineering-and-Technology (1).docx

Testing for density this test is used to determine

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Testing for Density This test is used to determine density (unit weight), yield and air content. After the density is determined, the yield and air content can then be calculated using given formulas. Density (unit weight) is a measure of weight per unit volume and is expressed in lbs/ft³ or kg/m³. The density of fresh concrete is similar to the density of hardened concrete. After the unit weight is determined, yield can be calculated. Yield is the ratio of total weight of mix material batched for a designed volume to the actual concrete density. It can also be expressed as the volume of concrete produced per batch. Yield will indicate if a mix proportioning problem exists. The density test is also a quick method for checking the air content of concrete. After the density and yield are determined, the air content can be calculated. This method should not replace volume or pressure testing methods for air content measurements. This test is also not appropriate for determining the air content of lightweight concrete.( Tech Notes, Concrete Sampling and Testing) . Testing for Compressive Strength One of the most important properties of concrete is its compressive strength. Testing for compressive strength helps determine whether a product meets specified strength requirements, when to strip forms, and when a product is ready for shipping and service. Strength results are also used for evaluating mix designs. Cylinders are usually tested in pairs to verify results and to
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rule out erroneous information based on unsatisfactory breaks. The objective of compressive- strength testing of concrete cylinders is to determine the amount of force it takes to break a cylinder of concrete. A test cylinder is placed in a compression-testing machine, and a constant load is applied to the cylinder until it breaks. The compressive strength is calculated based on the applied load and the size of the test cylinder. It is important to centre the cylinder between the loading plates and to apply the load at a constant rate. Loading rates, sample alignment and capping procedures can affect the results. Improper capping or surface irregularities of specimens will cause a non-uniform load, yielding improper results. The proper method for capping is described in ASTM C 617, “Practice for Capping Cylinders.” End grinders or rubber caps can also be used to prepare and test concrete test specimens. The type of break of a cylinder needs to be assessed and should be classified according to Figure 2 of ASTM C39. The break type that yields the most accurate reading is the hourglass. Other types may yield erroneous strength values. Compressive testing machines require proper calibration to ensure accurate results and should be recalibrated according to applicable standards (Magazine of Concrete Research ,20, July 1955, pp. 87-96). Figure1: Cylinder crashing f Figure1.1: Cylinder crashing Cube Testing The cubes are generally tested at 7 & 28 days unless specific early tests are required, for example to remove a concrete shutter safely prior to 7 days. Usually 1 cube will be tested at 7 days and 2 cubes at 28 days, however this may vary depending of the requirements, check the design first.
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