Frost Resistance of Concrete Containing Brick Aggregate - Dissertation Example

& No Frost Resistance of Concrete 10 February Introduction The human population continues to grow andthis growth is putting pressure on the Earth's finite resources. Critical resources are close to depletion; one example is oil which has been in decline since peak oil has been reached a few years ago. Crucial metals like iron, aluminium, gold, silver, copper, lead and tin are in short supply these days. Fossil fuels are non-renewable while metals can be recycled but demand is so great from the fast-growing economies like India and China. Man's technical abilities allow him to produce new materials out of various elements. This amazing ability also allowed him to predict, characterize and exploit the chemical and physical qualities of minerals, metals and other elements found in nature. It is this specific knowledge that makes it possible to develop new applications in the industries such as construction, shipbuilding, manufacturing, food production, etc. Early structures built by early civilizations of Egypt, China, India and other nations are still in existence today, although most of them are in ruins already. Man will always build new structures as habitat and places to work in, sometimes building skyscrapers to maximize the use of limited land. However, a growing realization that Earth's resources are not infinite has lead to the Green Movement in the construction industry these days. A big component of today's modern structures is concrete which makes up more than 60% of the total volume in construction materials. This paper discusses the exciting possibilities of using concrete with brick aggregates in them to improve their frost resistance and make them long-lasting. This paper examines the use of brick aggregates instead of the other usual aggregates in cement. Some studies showed brick aggregates concrete to have better qualities (Singh 27). I. Aim of the Project It is the aspiration of every architect and engineer to build structures which can last for long periods of time. The idea is to build for posterity if possible and this can be done with the right types of construction materials. Concrete has always been a favourite material due to its availability, strength and ease of use. The word concrete comes from the Latin concretus which means compact or condensed; this makes concrete desirable as a material of choice. In concrete, builders have found the ideal material to construct grand buildings of all shapes and sizes. Concrete is a composite construction material derived by mixing cement with fly ash, slags, aggregates (made up of gravel, crushed rocks, limestone, granite and a little bit of sand) water and several chemical admixtures. Concrete solidifies into place after mixing it with water in a process known as hydration. The water reacts chemically with the cement to form and bond all its other components together into a hard rock-like material. Concrete is the most widely-used man-made construction material in the world today. A lot of factors determine whether the concrete will stand the test of time. Moreover, engineering structural designs need to take into consideration the actual load weights imposed on the concrete. A concrete structure is exposed to various elements of nature such as wind, rain, earthquakes, fires, moisture and snow, to name just a few. In other words, the durability of a structure is to a large extent determined by its exposure to the elements. An important consideration that allows a concrete mixture to withstand the extreme elements of nature is the design and composition of the aggregates mixture forming the concrete. It is in this regard that I am examining how brick aggregates can improve the frost resistance of the concrete. A re-examination of its desired frost-resistant qualities is vital because it will improve concrete structures by making them more durable. It will help in minimising use of scarce construction materials by reducing a need to build new structures to replace old ones. The problem of frost resistance had been recognized as early as the Roman times. The Roman builders tried several materials to make their concrete more durable and these in turn partly explains why so many Roman structures still exist today. For example, they had added horse hair to make the concrete less liable to crack during the hardening process. A very odd interesting additive which the early Romans made to concrete was animal blood to make their mixture a bit more resistant to frost conditions. Concrete strength can be varied depending on the proportions of the main ingredients used in the mixture. The mix design of the concrete is the main factor that determines its compressive strength, density and thermal resistance. Frost resistance is a serious technical issue that has to be dealt with satisfactorily from an engineering or construction point of view. If the right solution can be found, then it will be possible to build structures which can withstand the repeated cycles of winter and spring. If a technical problem like frost resistance is solved, then it will be possible to build more durable structures even in the harshest wintry conditions. One of these exciting possibilities is the use of brick aggregates in concrete. Brick aggregate concrete has a modulus of elasticity which is 30% lower but an 11% higher tensile strength compared to stone aggregate concrete (ibid.). My project is to try solving this technical engineering problem. I had used a group of samples made up of 3 different concrete mixtures. The first sample group is made of 100% gravel, the second sample group is 100% crushed bricks and the third and final sample group is composed of 50% bricks and 50% gravel. A total of 11 concrete cubes were made for each group, or a total of 33 concrete cubes in all for this particular project. Varying the mixture of the components of concrete may help solve the problem of frost resistance. Other mixtures of concrete using clay brick aggregates had shown promising results such as those used in road construction (Vazquez, Hendriks & Janssen 64) with good performance under increased load factors. It is well know that bricks resist frost well due to water absorption (Appelbaum 335). II. Nature of the Project This project will entail subjecting the concrete cube samples to repeated cycles of the frosting and thawing process similar to the actual conditions experienced during winter and in the subsequent spring of each year. This is a simulation of the real-world conditions to which all concrete structures are subjected to each year, undergoing frost and thawing in the location where these structures were built. The research focus of this project is on frost resistance of a varied mixture of concrete because the durability of concrete is determined by how well it can hold up in the outer air with negative temperatures (Setzer, Auberg & Keck 296). Basic factor in determining frost-resistance quality of concrete is the mixture of water to the cement or its w-c ratio; however, other factors also contribute to frost resistance such as quality of materials, conditions of hardening, density of mixture and frost resistance of the aggregates. The research project will require subjecting concrete cubes to various temperatures in a laboratory. There will also be a control group for the samples. For each concrete mixture, 3 of the cubes will not be frozen at all (meaning, it will be observed at normal temperatures), a set of 3 cubes will undergo 10 freeze-thaw cycles and the last 3 cube samples will be subject to a maximum of 20 freeze-thaw cycles. The remaining 2 cubes will be subjected to the oven temperatures to determine their porosity. This set-up of testing procedures will be repeated for all 3 types of cubes I had made (the 100% gravel, 100% brick and a 50%-50% mix of gravel and crushed bricks). Each concrete cube will then be tested for its compressive strength (this is the ability of concrete to withstand or resist compression) as well as for its tensile strength (the quality of concrete to resist stretching, bending or twisting pressures). All results will be carefully recorded as test results will help determine which concrete mixture has the best quality in terms of durability (ability of concrete to resist deterioration) due to exposure from service conditions and to the outside elements of nature such as frost. III. Literature Review Concrete has been a builder’s choice because it is mostly readily available, relatively cheap compared to other building materials, easy to prepare, provides solidity and also bonds well with other construction materials (Construction Aggregate 6). As a synthetic construction material, concrete must have these 4 elements together: cement, fine aggregates (mostly sand) coarse aggregates (usually gravel, crushed rocks or stones) and water. The water and cement are considered as the active ingredients while both the course and fine aggregates are the inert elements of concrete (ibid.) and when water is added to the mixture, hydration occurs. It will create a chemical reaction that will cause the mixture to harden into concrete that is as hard as stone by binding all the components together. Contrary to popular opinion, the mixture should have adequate water during the initial hydration process or else the mixture is defective. This is a unique quality of cement in which the hardening process works equally well while underwater as well as in dry air or dry land (Newman & Choo 11). Concrete is inherently malleable when newly mixed and gets stronger over time. It is a fire-resistant building material which makes it ideal for homes, offices and all other buildings designed to withstand fire and extreme heat (Binggeli 38). Moreover, one of the chief benefits of using concrete is its ability to accept a wide variety of textures for surface finishes. It is an advantage as concrete can be used indoors as well as outdoors for aesthetic purposes. It can be used to beautify a building outside as well as enhance its interiors to great effect. It was mentioned that concrete is made of cement plus water and various aggregates. The ideal mixture should have 60% to 80% composed of aggregates which makes the cement strong, hard and fire resistant. Cement should be 10% to 15% while water should be around 15% to 20% of the entire mix. High quality concrete keeps the water to the cement ratio as low as possible without compromising the initial hydration process. III.A Frost Resistance of Concrete It has been determined from previous studies (Pigeon & Pleau 52) that frost resistance is greatly enhanced if the water that causes voids in the concrete can be minimized. It is the purpose of this paper to use another approach of eliminating excess water by a combination of gravel, crushed bricks and also a combination of 50% of gravel and 50% of crushed bricks. Using crushed bricks is a form of recycling used building materials (Sherwood 77). These are bricks from demolished buildings, structures and houses which had bricks in them and these bricks are crushed to be used again as concrete aggregates (Halliday 231). The concrete mix with crushed brick aggregates is found to have the same compressive value, bending strength and waterproofness with smaller shrinkage strains but slightly greater creep strains. In general, the frost resistance of concrete is measured by how many freeze-thawing cycles it can withstand (Bertolini, Elsener, Pedeferri & Polder 52). The degeneration or the degradation of the concrete happens when there is loss of mass beyond a certain level. The degradation occurs when there is a marked decrease in the elastic modulus of the concrete. There is a certain critical level when this happens, primarily if there is saturation of the pores in the concrete structure. The critical value is reached when 80% to 90% of the total porous volume becomes filled with water during the spring season, did not dry sufficiently and when winter comes around, becomes frozen solid and expands, thereby weakening the concrete. Below this critical level, the concrete has a much greater chance of resisting frost over a longer period and can withstand a high number of freeze-thaw cycles. Above a critical level, even a few cycles can sufficiently damage the concrete and hence the entire structure. A very crucial parameter of any concrete mix is the water-to-cement ratio (abbreviated as w/c ratio). Generally speaking, the lower this w/c ratio, the more resistant is the concrete to the effects of frost. A lower ratio has another practical benefit, which is the reduction in potential number of pores (but not below a certain level either, since pores are necessary to lower the pressure). In good concrete mixes, there must be sufficient pores for the water movement and allow space for the growth of ice crystals to take place. In other words, the porosity must be just right enough. Concrete cured for prolonged periods has a better than frost resistance. III. B Types of Bricks Almost similar to concrete, there are likewise several types of bricks. These are used for a particular purpose and their designs are intended for whatever utility is intended. Bricks are made of clay and sand; these are formed into blocks and then fired in a kiln at a very high temperature. The colour of the clay used and the firing determines to a large extent the colour of the brick (Creative Homeowner 60). Brick textures (fineness or coarseness) can vary too. Furthermore, bricks are graded by their reaction to water or moisture. This is measured by the amount of water that is absorbed; very dry brick is not good because it tends to absorb water from the mortar which in turn weakens the bonding quality of the mortar. Specifically, bricks are classified based on their resistance to freeze-thaw damage. The first category is termed as MW or moderate weathering that means the bricks can be exposed to moisture, rain or snow but not freezing. The next category is SW or severe weathering in which the bricks are frozen when saturated. The last category is NW or no weathering which means the bricks are not to get wet in whatever circumstance. This last type is ideally used for indoor purposes only. The bricks are further classified according to use, and hence their shape and size. B.1 Building Brick – this is used mostly for foundations of walls where aesthetic or a facade is not very important. This is resorted to primarily for getting structural strength of the building rather than for architectural purposes. It is just a backing or filler material only. B. 2 Facing Brick – sometimes called as face bricks, these are used to enhance beauty of a building's exteriors. In other words, appearance is important and so face bricks are used in areas of a structure where they are usually seen by the public, such as walls. A big majority of bricks manufactured today belong to this category. B. 3 Hollow Brick – this is a brick with a hollow core in the centre. The hole in middle portion of the brick is usually made when the bricks are extruded through a die machine. The holes make the bricks lighter, allows for a stronger bond with the mortar and reinforced steel is passed through these holes to make a floor or the wall much stronger. These are sometimes called modular bricks because they are made using moulds instead of by hand. B. 4 Paving Brick – this brick type is intended for use on roads either for pedestrian or light vehicular traffic. These are unusually strong and thicker bricks designed to withstand all types of weather conditions. Paving brick is also very dense compared to other types of bricks and machine-pressed to remove as much water as possible. This will make the brick to absorb less water which is critical for it to withstand repeated cycles of freezing and thawing. Bricks of this type are cooked or fired for a much longer period of time than ordinary bricks. B. 5 Ceramic Glazed Brick – this is a brick with ceramic glazing attached to it. This is actually a type of facing brick designed to enhance the outer appearance of a building. These are used to add vibrancy to a structure because most glazed bricks are in bright colours. B. 6 Veneer Bricks – these are actually bricks which are thinner than the usual bricks. They are used mainly for appearance and installed similar to tiles. People install them for the illusion of a traditional look such as in walls around inset fireplaces at the home. IV. Entrapment of Air One factor that determines the resistance of concrete to winter freeze-thaw conditions is the amount of air trapped during the mixing of the concrete. This is a process known as air entraining (entrapment) and good concrete mixes usually contain 5% to 8%. This is a process intended to improve the frost resistance of concrete because all the voids (empty spaces of air) help to relieve hydraulic pressure when the water freezes during winter. Concrete technology has advanced sufficiently in recent years to allow for very small air bubbles to spread evenly throughout the cement mixture (Love 13). Air entraining agents are usually derivatives of the wood resins, animal fats, vegetable oils, alkali salts or water-soluble soaps (ibid.). Finally, it is equally important that aggregates used must be able to withstand the frost as well. Entrained air helps to improve frost resistance of concrete because the air voids serve to act as reservoirs for the excess water forced into the concrete whenever the water inside the concrete freezes during winters. The air voids allow the water to expand during freezing and thereby prevent damage due to ruptures by relieving the hydraulic pressure. Entrained air also helps to prevent damage from de-icing chemicals and improves the sulphate resistance of the concrete from seawaters or sulphate soil waters. V. Freeze-Thaw Mechanism Cycles Repeated freezing and thawing during winter and spring does the most damage to the concrete structures because water expands at about 8.3% in volume when frozen at 0°C under normal atmospheric pressure with a corresponding increase in pressure and density of water (Giordano 451). The micro-structure of concrete is porous and proper air entraining allows for smaller but more uniform air bubbles within the concrete rather than large air voids which are often not dispersed uniformly throughout the concrete mixture. The strength of the concrete is a function of how the air entraining prevents the formation of inter-connected channels where water can seep in that is frozen during winters and thawed during spring that cause damage. VI. Water Tightness Porosity is therefore a critical factor in determining the actual strength of both cement and bricks when these are mixed using crushed bricks as substitutes to the usual aggregates. It determines the strength of the cement when cured. The purpose of this research project is to determine how crushed bricks used as rough aggregates will compare with concrete mixtures that use gravel as aggregates, or how a 50% combination of bricks and gravel will compare to the usual cement mix. The alternate aggregate materials will be investigated with regards to their sinking properties during the hardening period. This is crucial because sinking causes the voids once water evaporates and 1% void in concrete causes loss of 5% in strength (Toit 245). VII. Characteristics of a Good Concrete Mix There are many types of cement and the quality of the resulting mixture depends on the purpose of the cement. My experiment is a variation of a usual mixture using the crushed bricks as aggregates instead of sand. The two other sample groups utilize gravel and half-half of crushed bricks and gravel. It must be borne in mind that quality of the concrete depends on using the right mixture of water-to-cement ratio, all other things being equal. This means mix must have just the right water-to-cement ratio to complete the hydration process; too much of water results in over-fluidity that can result in defects. The ideal should be all spaces are filled with cement so as not to leave air spaces and attain a high density (Lamond & Pielert 304). As a general rule, the denser the concrete, the more frost resistant it becomes. This is because there are less pores in it which allow water to seep in and cause saturation. This is the process by which concrete is degraded because the saturated concrete expands from all the ice that will be formed when the water solidifies during winter. The pores must just be at the right level for water to expand into (must also be near enough to be accessible to this water coming from the ice formation) and thereby lower the pressure by relieving it through its capillaries. It was mentioned earlier that frost resistance of concrete likewise depends to a large degree on its aggregate components which must be frost resistant as well such as bricks. It is along this line that this research experiment is undertaken in the search for a right aggregates mix that will hopefully help improve the frost resistance of concrete. Bricks are a good potential source of useful aggregates since these can be crushed and re-cycled into new concrete. If the experiment results produce or indicate good resistance to frost, then this study shall have helped in the conservation of scarce resources by replacing sand or gravel with the crushed bricks which would otherwise be thrown away in the dumps and cause pollution and harm to the environment. Only good bricks will be used to avoid spalling (Cobb 151). Works Cited Appelbaum, Barbara. Conservation Treatment Methodology. Burlington, MA, USA: Butterworth-Heinemann. Bertolini, Luca, Elsener, Bernhard, Pedeferri, Pietro and Rob B. Polder. Corrosion of Steel in Concrete: Prevention, Diagnosis, Repair. Weinheim, Germany: Wiley- VCH Verlag, 2004. Print. Binggeli, Corky. Materials for Interior Environments. 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Bagneux, France: RILEM Publications, 2004. Print. Read More