materials and methods

Materials and methods

Concrete1- Introduction:Concrete is mixture of cement or lime, sand, brick or stone ballast and water, which when placed in forms and allowed to cure, becomes hard like stone. The hardening is caused by the chemical reaction between cement and water. The cement and water form a paste which, upon hardening, binds the aggregate to a permanent mass. cement is called binding material , the stone or brick ballast called coarse aggregate as distinguished from the fine aggregate which is sand . The mortar used in concrete called (matrix) cement concrete when used by itself known as mass concrete. Concrete is stronger in compression than tension. Enable it to resist tensile stressed it is reinforced or strengthened with steel in the form of steel bars or wire netting etc. the concrete so obtained is called (reinforced concrete). A- fresh concrete B- hardened concrete A- Fresh concrete:- The first 48 hours are very important for the performance of the concrete structure. - Fresh concrete: from time of mixing to end of time concrete surface finished in its final location in the structure - Operations: batching, mixing, transporting, placing, compacting, surface finishing For fresh concrete to be acceptable, it should: 1. Be easily mixed and transported. 2. Be uniform throughout a given batch and between batches. 3. Be of a consistency so that it can fill completely the forms for which it was designed. 4. Have the ability to be compacted without excessive loss of energy. 5. Not segregate during placing and consolidation. 6. Have good finishing characteristics.

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2- properties of concrete:-

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Workability of Fresh Concrete: Fluidity & compatibility known as workability Higher workability concretes are easier to place and handle but obtaining higher workability by increasing water content decreases strength and durability Measurement of workability: 1- Slump test 2- Compaction factor test 3- Vebe consistometer test 4- Flow table test The slump test is the most well-known and widely used test method to characterize the workability of fresh concrete. The inexpensive test, which measures consistency, is used on job sites to determine rapidly whether a concrete batch should be accepted or rejected. The test method is widely standardized throughout the world, including in ASTM C143 in the United States and EN 12350-2 in Europe (In British standard BS 1881 Part 2).

a- Slump test :-

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Advantages:1- The slump test is the most widely used device worldwide. In fact, the test is so well known that often the terms workability and slump are used interchangeably, even though they have different meanings. 2- Specifications are typically written in terms of slump. 3- The slump test is simple, rugged, and inexpensive to perform. Results are obtained immediately. 4- The results of the slump test can be converted to yield stress in fundamental units based on various analytical treatments and experimental studies of the slump test. 5- Compared to other commonly used concrete tests, such as for air content and compressive strength, the slump test provides acceptable precision.

Disadvantages:1- The slump test does not give an indication of plastic viscosity. 2- The slump test is a static, not dynamic, test; therefore, results are influenced by concrete thixotropy (is the viscous behavior where the apparent viscosity decreases with shear stress). The test does not provide an indication of the ease with which concrete can be moved under dynamic placing conditions, such as vibration. 3- The slump test is less relevant for newer advanced concrete mixes than for more conventional mixes.

Limitations: The slump test is not considered applicable for concretes with a maximum coarse aggregate size greater than 37.5 mm (1.5 inches). For concrete with aggregate greater than 37.5 mm in size, such larger particles can be removed by wet sieving.

2-Compacting Factor Test:The compacting factor test (Powers 1968; Neville 1981; Bartos 1992; Bartos, Sonebi, and Tamimi 2002) measures the degree of compaction resulting from the application of a standard amount of work. The test was developed in Britain in the late 1940s and has been standardized as British Standard 1881-103.

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Compacting factor shape The apparatus, which is commercially available, consist of a rigid frame that supports two conical hoppers vertically aligned above each other and mounted above a cylinder, as shown in Figure. The top hopper is slightly larger than the bottom hopper, while the cylinder is smaller in volume than both hoppers The compacting factor is defined as the ratio of the weight of the concrete compacted in the compaction factor apparatus to the weight of the fully compacted concrete. The standard test apparatus, described above, is appropriate for maximum aggregate sizes of up to 20 mm. A larger apparatus is available for concretes with maximum aggregate sizes of up to 40 mm.

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Degree of workability Very low Low

Slump mm 0 - 25 25 - 50

Compaction factor Small Large apparatus apparatus 0.78 0.85 0.80 0.87

Application Vibrated concrete in road or other Large sections Mass concrete foundation without vibration , simple reinforced section with vibration simple reinforced work without vibration Section with congested reinforcement without vibration

medium High

50 - 100 100 - 180

0.92 0.95

0.935 0.96

3-Vebe consistometer test:1- The Vebe test measures the remolding ability of concrete under vibration. The test results reflect the amount of energy required to remold a quantity of concrete under given vibration conditions. 2- The Vebe test is applicable to concrete with slumps less than 2 inches.

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Vebe consistometer test1- The apparatus, shown above , consists of a metal cylindrical container mounted on a vibrating table, which produces a sinusoidal vibration. 2- In the version of the test standardized in Europe as EN 12350-3, a slump cone is placed in the center of the cylinder and filled in the same manner as in the standard slump test. 3- After the slump cone is removed, a clear plastic disk is set atop the fresh concrete. 4- The Vebe table is started and the time for the concrete to remold from the slump cone shape to the shape of the outer cylindrical container is recorded as a measure of consistency. 5- The sliding clear plastic disk facilitates the determination of the end of the test.

B- Hardened concrete :12345Principal Properties of harden concrete are: Compressive Strength Tensile strength Porosity Permeability Fatigue and Fracture, etc.

- The properties are determined by: 1Laboratory Testing 2In-Situ Testing Concrete Testing Approaches 1- Destructive Testing - In this approach separately cast concrete samples and/or cores/cut samples are tested to destruct. 2- Non-Destructive Testing - In this approach tests are conducted without destructing the concrete. This approach is preferred when assessment the quality of existing concrete structures is required - Destructive Tests:- The standard method of evaluating the quality of concrete in buildings or structures is to test specimens cast simultaneously for compressive, flexural and tensile strengths. - The main disadvantages are that results are not obtained immediately; that concrete in specimens may differ from that in the actual structure as a result ofIndian standard Page 6


different curing and compaction conditions; and that strength properties of a concrete specimen depend on its size and shape.

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This category of testing covers the following tests:Engineering & Mechanical properties Compressive strength Flexural strength Tensile strength Modulus of elasticity Durability Permeability Chloride migration The most common test preformed on concrete is for compressive strength. There several reasons for this: It is assumed that the most important properties of concrete as directly related to compressive strength; Concrete has little tensile strength and is used primarily in compression; Structural design codes are based on compressive strength; The test is relatively simple and inexpensive to perform.

Compressive Strength: ASTM C39 Cylinder Test The normal compressive specimen in North America is a cylinder with length to diameter ratio of 2:1. 1- Molds may be reusable, made of heavy-gauge metal or single-use, made from sheet metal or waxed cardboard. 2- Cardboard molds have been found to yield slightly lower strength (+/-3%) than other types.

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3- Specimen should be cast on a firm level surface, free from vibration. If the slump is more than 3 inches, concrete is consolidated by rodding; if the slump is less than 1 inch, the concrete is consolidated by vibration. 4- Poorly compacted cylinders will have lower strength. 5- If the specimen is to be rodded, it should be filled in three equal layers, each rodded 25 times with 5/8 inch diameter steel rod with a rounded end. Compressive Strength: BS 1881-116/BS EN 12390-3 Cube Test 1- Cube test, standard in Great Britain and Germany, uses a 150mm cubic mold, which is filled in three layers, rodded 35 times with a 25 mm square rod or compacted with a vibrator. 2- The cube is tested at right angles to the position casted and therefore required no capping or grinding. The loading rate is 0.03 kg/mm2/s, for 150mm standard cube it will be about 5kN/s. Destructive Tests: Tensile strength There is as yet no standard test for directly determining tensile strength. However, there are two common methods for estimating tensile strength through indirect tensile tests. Split Cylinder Test Flexural Strength Tensile strength: Split Cylinder Test BS 12390-6 BS EN 12390-6 supersedes BS 1881-117, Method for the determination of tensile splitting strength. BS EN 12390-6 specifies a single method for determining the tensile splitting strength of concrete cylinders as the reference method. Splitting test carried out on a standard cylinder specimen by applying a line load along the vertical diameter. BS 1881-117 placed no essential dimensional requirements on the tests specimens. For cylinders, BS EN 12390-6 requires compliance with the dimensional requirements of Clause 4.3 of BS EN 12390-1. It is not practical to apply the true line load to the cylinder because the side is not smooth enough and because it would induce high compressive stresses at the surface. Therefore, a narrow loading strip made of soft material is used. Tensile Strength: Flexural Strength Test BS EN 12390-5 BS EN 12390-5 supersedes BS 1881-118, Method for the determination of flexural strength. BS EN 12390-5 specifies two methods for the determination of flexural strength of concrete by either centre or two-point loading of cast or sawn prisms.

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3- The two-point loading test is essentially the same as the test specified in BS 1881118, except in detail, whilst the centre-point loading test was not included in BS1881. 4- A specimen beam 150 x 150 x 500 mm is molds in two equal layers each rodded 60 times. 5- The beam may be vibrated and should be cured in the standard way. 6- This test tends to overestimate the true tensile strength by about 50%. 7- This can be explained by the fact that the simple flexural formula used is based on a linear stress-strain distribution whereas concrete has a nonlinear distribution. 8- This is an important test because it model how a concrete beam is normally loaded.

Non-Destructive Testing:These tests are useful to: Quality control; Determination of the time for form removal; and Help assess the soundness of existing concrete structures. NDT Categories: Rebound Test: The rebound hammer is a surface hardness tester for which an empirical correlation has been established between strength and rebound number. The only known instrument to make use of the rebound principle for concrete testing is the Schmidt hammer, which weighs about 4 lb (1.8 kg) and is suitable for both laboratory and field work. It consists of a spring-controlled hammer mass that slides on a plunger within a tubular housing. 1- The hammer is forced against the surface of the concrete by the spring and the distance of rebound is measured on a scale. 2- The test surface can be horizontal, vertical or at any angle but the instrument must be calibrated in this position.

REBOUND HAMMER TEST

NDT Categories: Dynamic Test

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At present the ultrasonic pulse velocity (UPV) method is the only one of this type that shows potential for testing concrete strength in situ. 1- It measures the time of travel of an ultrasonic pulse passing through the concrete. 2- The fundamental design features of all commercially available units are very similar, consisting of: A - a pulse generator, and B- a pulse receiver. 3- Pulses are generated by shock-exciting piezo-electric crystals, with similar crystals used in the receiver. 4- The time taken for the pulse to pass through the concrete is measured by electronic measuring circuits.

- NDT Categories: Penetration Test:1- The Windsor probe is generally considered to be the best means of testing penetration. 2- Equipment consists of a powder-actuated gun or driver, hardened alloy probes, loaded cartridges, a depth gauge for measuring penetration of probes and other related equipment. 3- A probe, diameter 0.25 in. (6.5 mm) and length 3.125 in. (8.0 cm), is driven into the concrete by means of a precision powder charge. 4- Depth of penetration provides an indication of the compressive strength of the concrete. 5- Although calibration charts are provided by the manufacturer, the instrument should be calibrated for type of concrete and type and size of aggregate used.

- NDT Categories: Radioactive Methods 1- Radioactive methods of testing concrete can be used to detect the location of reinforcement, measure density and perhaps establish whether honeycombing has occurred in structural concrete units. 2- Gamma radiography is increasingly accepted in England and Europe. 3- The equipment is quite simple and running costs are small, although the initial price can be high. 4- Concrete up to 18 in. (45 cm) thick can be examined without difficulty.

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Radioactive Methods figure

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