ENB273 - Concrete Report.docx

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Queensland University of Technology
Mike Fordyce

th 15 of April, 2013 Dr. Client Department of Science and Engineering Queensland University of Technology Brisbane, Queensland Dear Client, I would like to present to you this report, containing the analysis of three concrete samples where all of which were designed to meet the compressive strength standards of a dam wall. The raw data was obtained through compression testing, which was then manipulated through calculations to determine their density, compressive strength, standard deviation and characteristic strength. The purpose of this report was to give an insight into the properties of concrete, and to investigate the effects that defects have on these properties. Furthermore, this report discusses the applications of different types of concrete in construction. I hope this report satisfies your requirements. Yours sincerely , (Civil Engineering Student) 0 | P a g e ENB273 – Civil Materials Concrete Practical Report QUT Engineering Student 30/4/2013 1 | P a g e Table of Contents Description Page no. 1.0 Executive Summary 3 2.0 Introduction 4 2.1 Slump Selection, Reasons and Measurement 5 2.2 Mix Design 5 2.3 Material Requirements 5 3.0 Method 8 4.0 Results 9 5.0 Discussion 12 6.0 Conclusion 14 7.0 References 15 8.0 Appendix 16 2 | P a g e 1.0 Executive summary This report was conducted to give an insight into the mechanical properties of concrete, and to investigate how variables in the mix design affect these properties. Compression testing took place to determine the maximum compressive strength of each sample, and the dimensions and mass of each sample were collected. This data was used in calculations to determine the density, compressive strength, standard deviation and characteristic strength (all calculations are found in the appendix). It was found that decreasing the moisture content in the sample decreased its workability and therefore decreased its compressive strength (since voids were created). This report also investigates different types of cement and their applications, as well as discussing the influencing factors and benefits of different types of aggregate. Due to the lack of time taken to mix the concrete and the lack of efficiency in doing so, the samples tested may not have been an accurate depiction of the true strength of the original mix design. This was a limiting factor in this experiment. 3 | P a g e 2.0 Introduction Concrete is a widely used material in civil engineering, across all aspects of the discipline. In order to design a concrete structure to perform a specific task, the engineer must have an understanding of how the mix design influences the concrete’s properties. Furthermore, to ensure that a concrete mix design is suitable for the task it is delegated, sample testing must be performed. In this practical, compression and hardness testing was performed on three concrete cylinders which were designed according to the provided dam wall specifications. The main factors influencing the compressive strength of concrete are the material ratios, and the compaction of these materials. It is expected that as the water to concrete ratio increases, the concrete’s compressive strength decreases, and the workability increases. A concrete mix was designed by analysing the aggregate size range and grading, to determine the water content to aggregate and water content to cement ratios. After the concrete samples were prepared and their dimensions and mass were recorded, testing was performed through the use of a Schmidt Hammer and compression machine. The sample’s mechanical compression and hardness values were manipulated to determine properties such as compressive strength (fc), density (p), standard deviation (s) and characteristic strength (f’c). The compressive strength was determined using the equation; fc= Ultimate Load (P)/Cross Sectional Area (A) Where ultimate load is in Newtons, cross sectional area is in mm , and therefore compressive strength is in Mega Pascals. The density values of each sample were determined using the equation; p = Mass (M)/Volume (V) Where mass is in kilograms, volume is in m , and therefore density is in kg/m (the densities were 3 rounded to the nearest 20kg/m ). The standard deviation was determined using the equation; 2 s = sqrt(∑(f – fc) /ncm 1) Where f cs compressive strength in MPa, f icmthe mean strength of all test cylinders in MPa, and n is the number of test cylinders. The characteristic strength was determined using the equation; f’ c f cm – (k)(s) Where f’ cs characteristic strength in MPa, cmis the mean strength of all test cylinders in MPa, k is a constant of 1.64, and s is the standard deviation in MPa. The mechanical property values for each cylinder were analysed to determine their influencing factors. 4 | P a g e 2.1 Slump Selection, Reasons and Measurement Slump is the measure of consistency of a fresh concrete mix, and may be determined by filling a slump cone. This is done by filling the cone in three layers with the design mix being tested, compacting each layer using a tamping rod with 25 strokes. The slump cone is then removed from the concrete and placed inverted adjacent to the sample being tested. The vertical distance from the top of the inverted slump cone to the top of the concrete mix is then measured using a ruler, and recorded as the slump value. The slump boundaries for the dam wall concrete mix design were 75mm to 25mm. In general as the percentage of water in a concrete mix increases, the workability increases, however the quality of the concrete decreases (is this case the compressive strength decreases, as that is the property being tested).This is due to excess water in the cured concrete which creates voids, which are a source of weakness. In this scenario, a slump value which is greater than what is designated will most likely be weaker than desired, just as a slump value which is smaller than designated will most likely be less workable than desired. 2.2 Mix Design In order to design a concrete mix first the design information must be gathered, such as the design strength (the characteristic strength multiplied by the control factor), the required slump (which is determined by the size of the aggregate and aggregate to cement ratio), and the sieve analysis data. From these values, the water to cement ratio can be determined through analysing Figure 4.0.In this experiment, the concrete cylinders were tested after 28 days. The fine to coarse aggregate mix ratio can be determined by plotting the percentage of coarse aggregate passing against the percentage of fine aggregate passing (refer to Figure 5.0).This chart is plotted by connecting the coarse aggregate passing against the fine aggregate passing, according to the sieve analysis table acquired in the design information stage. Through drawing a vertical line through the 40% coarse, 60% fine aggregate margin, the combined aggregate grading can be determined and compared to the grading number table, to determine the grading curve. After the slump value, water to cement ratio and grading curve has been determined, the corresponding aggregate to cement ratio can be determined through analysing Figure 6.0 (refer to Figure 6.0).After the aggregate to cement ratio is determined, calculation can be performed to determine the final mix design and therefore determine the masses of each material required (refer to Appendix). 2.3 Material Requirements Due to the high demand by the industry for cement to suit a variety of needs, cement is available with mechanical properties to suit a diverse range of applications. Five of the most common of which are; Gray Ordinary Portland Cement, White Portland Cement, Masonry or Mortar, Oil-Well Cement, and Blended Cement. Gray Ordinary Portland Cement is a cost effective material which is mainly composed of clinker (a rock like substance ranging between 3mm and 25mm, produced by clay and limestone). This cement is used for a wide range of applications from residential to industrial infrastructure. White Portland Cement is used for fine architectural works such as artificial granite and sculptural casts, due to its bright aesthetic properties. This cement is generally more expensive than Gray Ordinary Portland Cement, since it contains constituents such as low iron content kaolin clay. Masonry or Mortar Cement is derived from a Portland cement and mixed with limestone. Applications include 5 | P a g e templates, road surfaces and brick work. Oil-Well cement is a type of hydraulic cement, which is able to withstand high temperatures, pressures, and to some degree, chemical aggression. This cement is also produced using Gray Portland Clinker. Blended Cement is a mixture of Portland cement and a cocktail of supplementary cementitious materials such as fly ash, silica fume and hydrated limestone. According to Craig Blowns (2013), “The use of blended cements in ready-mix concrete reduces mixing water and bleeding, improves workability and finishing, inhibits sulfate attack and the alkali-aggregate reaction, and reduces the heat of hydration”. Water is generally an integral ingredient in concrete, and serves a range of purposes. Some of which includes wetting the surface of the aggregate to develop adhesion between the cement and the aggregate (since cement sticks better to a wet surface than a dry surface), and to hydrate the cement particles allowing them to harden during the curing process. Due to its significant contribution to the final concrete product and mechanical properties, the higher quality the water is in a mix design, the higher quality the concrete will be. For example, according to Dilip Saha (2009); “The presence of algae in mixing water causes air entrainments with a consequent loss of strength”. Furthermore, suspended soils and silts within the water can also decrease strength, as it covers the surface of the aggregate and cement particles, therefore decreasing the surface area in which the cement in able to bond. This illustrates that using water containing unnecessary constituents in a design mix may potentially decrease the strength of the final product. Aggregate is a key constituent in concrete, as it makes up 60% to 80% of the design mix. Aggregate variables such as the surface texture, grading size distribution, and moisture content all significantly affect the strength and workability of the concrete mix. In general, smooth concrete is more workable than rough angular or elongated aggregate, due to having less friction and a greater ability to slide between one another. However, more angular aggregate has a significantly higher surface area than smooth aggregate, and therefore can effectively hold more cement. In this case however, the workability of the mixture may be sacrificed. Grading curves can be classified into two main categories, these being well graded and poorly graded. Well graded aggregates have an even size distribution, and poorly graded aggregates have an uneven size distribution. Uniformly graded and gap graded aggregates can be considered as special cases of poorly graded aggregate, as uniformly graded refers to the aggregates all have similar sizes, and gap graded refers to having an abundance of very small and relatively large aggregates, with very little size variation between them. Having an aggregate which is poorly graded is less cost effective, since the mix may require more cement to fill the voids left between the a
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