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The project was set out to compare the reinforcement of concrete using carbon fibre rod to existing materials being used to reinforce concrete e.g. ribbed high tensile steel and mild steel. The aim is to show that reinforcing concrete with carbon fibre will improve the tensile strength, flexural strength and the mechanical property better than the materials it is being compared with.
Reinforced samples with carbon fire were tested to determine how much tensile loading they could withstand before they fail and then they were compared against different reinforcement materials in order to make the results reliable and trustworthy. The other reinforced materials that were used in a series of tests were generally used as reinforcement s in the engineering industry. The reinforcement materials used to compare against carbon fibre were:
A concrete mix ratio of 4:2:1 was used because it provided enough strength required for the test samples and there was not much point in wasting materials and time when the samples do not need to be any stronger than what it is needed to be. In industry the mixing ratio would be determined by the project or the specification of the strength required for the job/structure. A 4:2:1 mix would not be suitable for setting the foundation of a house; regarded as a weak mix and non conformance to the job specification.
10mm coarse aggregate was used mix the cement because of the size of the test samples. A bigger aggregate size would have insufficient bond with the cement; thus making the concrete weak. Using the right sized aggregate is important for the strength of the concrete, bonding to the cement, strength of the concrete and importantly the flexural strength. The rough surface aggregate results in a better bond. Although it is rather difficult to determine the quality of bond of aggregate, generally, when a bond is good, a crushed concrete specimen should contain some aggregate particles broken right through. An excess of broken particles would suggest that the aggregate is weak; The bond strength also depends on the strength of the paste being used. The bond strength increases with the age of the concrete.
There are different ways of checking the consistency of the concrete. One of the methods used in site work is a slump test. The slump test is used in detecting variations in the uniformity of the mix. The slump test is a cone like shape with three different layers filled with concrete. The cone is placed on a smooth surface. Once the cone is full, it is slowly lifted, and the unsupported concrete will now slump. The decrease in the centre of the concrete is measured to the nearest 5mm. Example a slump measuring 50-100mm would be classed as having medium degree of workability and would be suitable for reinforced concrete.
28 days is classed as the standard technical curing time for concrete as that is when it reaches its maximum strength; which is about 340kj/kg. It is not always possible to leave the concrete to cure for 28 days, this might be due to time constraints and Strength of the concrete required for the job, what sort of work that is being carried out. The curing should be in a suitable environment during the early days of hardening The aim of curing is to keep the concrete saturated or as saturated as possible until the water filled in the fresh cement paste has been filled to the desired extent by the products of hydration of cement. If the concrete were left in site conditions, the active curing would stop long before the maximum possible hydration would have taken place. The necessity for curing arises because the hydration of cement can only take place in water filled capillaries. It is important that the loss of water by evaporation from the capillaries is prevented. Ingress of water into the concrete must be made possible.
The three unreinforced concrete prisms were also made on the 15/04/2011 from the same concrete used for the unreinforced cubes. Using the same concrete reduces variation in the concrete. The unreinforced prism cubes were left to cure immersed in water for 28 days before they were tested on an Avery Denison tension loading tester. The unreinforced concrete prisms were made to give an indication of how much tensile loading they can withstand without being reinforced.
It is important that the mixing is done properly. The aim of mixing is coat the surface of all the aggregate particles with cement paste and bland all the ingredients of concrete into a uniform mass. There are different methods of mixing concrete; it can be mixed by hand depending on the quantity of concrete. The bigger the quantity the longer it takes to mix and becomes more difficult to get the right texture. If the concrete is a dry mix, mixing by hand would be difficult because the workability of the concrete would be stiff and it would start drying before it has being poured. This leads to waste, increase in cost, labour and time. For example a pumping concrete machine would be used to lay the foundation of a house. It pumps the concrete at a fast rate, and fills up a large surface area in a short amount of time. This will reduce cost of labour and save on time. Using a pump; the water to cement ratio can be quite low which in turn saves on the amount of cement needed for the job.
The unreinforced concrete cubes were set in water to cure from the 04/05/2011 to the 13/05/2011. Which works out at 9 days; 19 days short of the 28 day required curing time. It is not always possible to leave the concrete to cure for 28days. In this case time constraints meant the unreinforced cubes had to be tested after 9 days. The unreinforced concrete cubes were presumed to be at 70% strength.