Measurement Of Concrete Workability - Essay Example

Measurement Of Concrete Workability Concrete workability depends on a number of interacting factors: water content, aggregate type and grading, aggregate/cement ratio, presence of admixtures, and fineness of cement. The main factor is the water content of the mix since by simply adding water the inter-particle lubrication is increased. 1. Measure out each of the constituents using the balance provided. Note: Water is more accurately measured by volume (1kg of water is equal to 1 litre). Mix the dry ingredients first and gradually add the water while mixing. Water (kg) 4.97* Cement (kg) 13.2 Fine Aggregate (kg) 18.82 Coarse Aggregate (kg) 25.07 Percentage water content (%) 8.71 % Water/cement ratio 0.3765 Curing condition 1.803 * Water content may be reduced dependant on moisture conditions of fine aggregate Table 1: Concrete mix details (24Litres) Required formulae: 2. Workability Testing: Carry out the following workability tests: Slump test – BS EN 12350-2 • Cone of height = 305mm and base diameter = Slump: true, shear & collapse 203mm with smaller opening (102mm diameter) at the top is placed on a smooth surface. The inside of the mould and the base should be moistened at the beginning of the test. The mould must be firmly held against the base during the entire filling process - this is facilitated by handles or foot-rests brazed to the mould. The mould is filled with concrete in 3 approximately equal depth layers. Each layer is tamped 25 times with a standard 16mm diameter steel rod – when compacting the second and third layer, the rod should just penetrate the underlying layer. The top surface is struck off by means of screeding and rolling motion of the tamping rod. Prior to lifting the mould, the area immediately around the base of the cone should be cleared of concrete, which may have dropped accidentally. The cone is slowly lifted clear of the concrete in 2 – 5 seconds (vertically upwards and without twisting) and the unsupported concrete will slump. The difference in height between the upturned slump cone and the highest point of the concrete is called the slump (noted to the nearest 5 mm). Please clean the apparatus by rinsing with water at the end of the test. Measured slump value (mm) =160 mm 4 Place the reinforcement into the mould and pour the concrete, taking good care to achieve good compaction 5 Consider the following discussion points based on your results observations and further research: a What is the effect of the water content on the workability of concrete? Presence of water improves the workability of the concrete. The volume of water affects the density of the concrete. The more the water content, the greater the workability of the concrete. High contents of water results in higher fluidity and hence better workability. b How does the water/cement ratio and method of curing affect the compressive strength of concrete? An increase in the water/cement ratio decreases the compressive strength of the concrete. Large water/cement ratios increase the fluidity of the mixture and this can even cause the concrete to collapse. The method of curing used also affects the strength of the compressive strength of the mixture. All methods except the air curing method increase the compressive strength. c Why is the characteristic strength rather than mean strength of concrete used to specify structural concrete? Characteristic strength is used since it is normally based on statistical concepts as it is the strength below which no more than 5 % of all the cubes that have been tested from the concrete mixture will fall. It is expected that 95 % of all the cube samples will have strength in excess of the design characteristic strength. 6 Comment on the practicalities of placing concrete into a tightly spaced mould containing reinforcement. How did you achieve good compaction of the concrete placed for your beam? The practicality of placing concrete into a tightly spaced mould containing reinforcement is limited by the process of casting the concrete and the cost of performing this operation. In order to achieve a good compaction, vibration is done on the concrete mix using a screed board vibrator. This removes the air spaces present in the mixture. 7 Sketch the beam, showing the reinforcement and describe what each element within the cage is provided for Section A-A of the beam a Regularly spaced ribs They are not less than half of the clear distance between the ribs nor less than 5 cm. it is designed as continuous beam supported by ribs. The modulus of fracture f the ribs can be computed as: b Topping slab This is not less than 10 cm in width and the depth is not less than the minimum width. The spacing is not more than 15 cm. it is normally characterized by loads and spacing. 1) What is the effect of the water content on the workability of concrete? How can you use your Results to draw conclusions about the mix? The amount of water available in the mix improves the workability of the concrete. The volume of water has a direct effect on the density of the concrete. The greater the content of water, the greater the workability of the concrete. High contents of water usually results in higher fluidity and hence better workability. With a computed water content of 8.71 %, the concrete mix is fairly workable. 2) How does the water/cement ratio and method of curing affect the compressive strength of concrete? An increase in the water/cement ratio decreases the compressive strength of the concrete. Large water/cement ratios increase the fluidity of the mixture and this can even cause the concrete to collapse. The method of curing used also affects the strength of the compressive strength of the mixture. All methods except the air curing method increase the compressive strength 3) Why is the characteristic strength rather than the mean strength of concrete used to specify structural concrete? Characteristic strength is used since it is normally based on statistical concepts as it is the strength below which no more than 5 % of all the cubes that have been tested from the concrete mixture will fall. It is expected that 95 % of all the cube samples will have strength in excess of the design characteristic strength. 4) Comment on the practicalities of placing concrete into a tightly spaced mould containing reinforcements. How did you achieve good compaction of the concrete? The practicality of placing concrete into a tightly spaced mould containing reinforcement is limited by the process of casting the concrete and the cost of performing this operation. In order to achieve a good compaction, vibration is done on the concrete mix using a screed board vibrator. This removes the air spaces present in the mixture. 5) How does the tensile steel reinforcement type affect the steel /concrete bond strength? Reinforcement with steel increases the tensile strength of the bond. Bond stress acts parallel to the bar along the interface and hence the axial force emanating from the reinforcing steel bar generates tangential force which is transferred to the concrete. The bond ensures that there is very minimal slip of the steel bars. 6) Sketch the beam, showing the cracks and identify the cracks types according to the failure mode The type of crack is flexural type. The cracks vary between 0.5 and 1.5 units and it is dependent on the steel area and coverage. The average crack spacing usually decreases with time as a result of shrinkage. The width of the crack is dependent on the steel stress, the diameter of the steel bar and the adjacent spacing of the crack. Concrete Break 1. Sketch of the test apparatus, showing the forces acting on the beam. 2. Sketch showing the nature of failure, together with annotations describing why it failed in this way. The beam failed as a result of an inclined crack propagating in a rapid manner as a result of inadequate shear reinforcement on the beam. 3. Two other modes of failure in an RC beam and under what circumstances that they might occur. a. Shear compression failure: This type of failure may result from crushing of the concrete near the flange of compression of the beam and above the tip of the beam that is under compression. b. Shear tension failure: This type of failure occurs as a result of the longitudinal bars lacking adequate anchorage and diagonal cracks start propagating horizontally. 4. Record the final failure load of the beam and convert this into Kg. If the mass of an average small car is around 1500Kg, comment on how many cars, stacked on top of one another would be required to break the beam. 5. Use your knowledge of elastic and plastic behaviour of materials to describe how the beam behaved when loaded and unloaded in a cyclic fashion. What did you notice about the readings on the deflection dial gauge? When the beam is loaded with and unloaded in a cyclic manner, the deflection recorded on the gauge shifts from minimum to maximum and vice versa. When the beam is under loading, the amount of deflection increases due to the exerted load. The kind of deformation experienced by the beam is elastic in nature. If the load is increased up to a certain limit beyond which the deflection remains permanent, it is referred to as plastic deformation References G.D. TAYLOR , Materials in Construction, 2nd edition, Prentice Hall, 1997. Peter Domone and John Illston, Construction Materials their nature and behaviour, 4th edition, 2010. G.D. Taylor, Materials in Construction principles, practice and performance, Longman, 20 Read More