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Concrete Admixtures. Concrete is composed mainly of cement (commonly Portland cement), aggregate, water, and chemical admixtures. Portland Cement. Coarse Aggregate. Chemical Admixtures. Fine Aggregate.
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Concrete is composed mainly of cement (commonly Portland cement), aggregate, water, and chemical admixtures. Portland Cement Coarse Aggregate Chemical Admixtures Fine Aggregate
Concrete solidifies and hardens after mixing with water and placement due to a chemical process known as hydration. The water reacts with the cement, which bonds the other components together, eventually creating a stone-like material.
Concrete is used more than any other man-made material in the world. The word concrete comes from the Latin word "concretus" (meaning compact or condensed). The first major concrete users were the Egyptians in around 2,500 BC and the Romans from 300 BC. Opus caementicium laying bare on a tomb near Rome. In contrast to modern concrete structures, the concrete walls of Roman buildings were covered, usually with brick or stone. Outer view of the Roman Pantheon, still the largest unreinforced solid concrete dome to this day.
Concrete has many applications and is used to make pavements, pipe, structures, foundations, roads, bridges/overpasses, walls and footings for gates.
Properties: • Concrete has relatively high compressive strength, but significantly lower tensile strength, and as such is usually reinforced with materials that are strong in tension (often steel). • The elasticity of concrete is relatively constant at low stress levels but starts decreasing at higher stress levels as matrix cracking develops. • Concrete has a very low coefficient of thermal expansion, and as it matures concrete shrinks. • All concrete structures will crack to some extent, due to shrinkage and tension. • Concrete can be damaged by fire, aggregate expansion, sea water effects, bacterial corrosion, leaching, physical damage and chemical damage (from carbonation, chlorides, sulfates and distillate water).
Types of Concrete: • There are various types of concrete for different applications that are created by changing the proportions of the main ingredients. • The mix design depends on the type of structure being built, how the concrete will be mixed and delivered, and how it will be placed to form the structure. • Examples include: • Regular concrete • Pre-Mixed concrete • High-strength concrete • Stamped concrete • High-Performance concrete • UHPC (Ultra-High Performance Concrete) • Self-consolidating concretes • Vacuum concretes • Shotcrete • Cellular concrete • Roller-compacted concrete • Glass concrete • Asphalt concrete • Rapid strength concrete • Rubberized concrete • Polymer concrete • Geopolymer or Green concrete • Limecrete • Gypsum concrete • Light-Transmitting Concrete
Basic Composition for Main Concretes • Regular Concrete • Cement, Aggregate, and water • Geopolymer (Green concrete) • Fly Ash and Regular • Concrete • High Strength Concrete ~<0.35% • Silica Fume • Strong Aggregates • Ultra High Performance Concrete (UHPC) • Cement • Coarse/Fine Aggregate • Air • Silica Fume • Polypropylene Fibers
Our Samples: • Sample 1: Portland cement + coarse aggregate + fine aggregate + water • Sample 2: Portland cement + coarse aggregate + fine aggregate + water + fly ash + water reducer • Sample 3: Portland cement + coarse aggregate + fine aggregate + water + fly ash + water reducer + silica fume • Sample 4: Portland cement + coarse aggregate + fine aggregate + water + fly ash + water reducer + silica fume + polypropylene fibers
Background There are two types of fly ash used in concrete which is classified as Class-C and Class-F. Class-F is more widely used because it is made from the burning of older anthracite (i.e. black coal , black diamond, etc.) which is in abundance, with an opposing amount of uses. Fly ash that is not used in concrete is poured in landfills with it’s micro dust particles to flutter in the atmosphere. As far as human health is concern, fly ash in itself contains traces of heavy metals which pertains to arsenic, selenium, lead, and more. Fly Ash • Background • Being a pozzolan, fly ash has the ability to act cementitious with the presence of cement and water. This process is able to happen because of fly having silica and alumina. • Fly ash on a micro level takes the form of a sphere which allows the particle to fit easily within the pores of the concrete. This circular form of the fly ash also allows the concrete to be more fluid and workable. When it comes to setting the concrete, it’s a benefit for workers by it having this feature making it easier to place. • Benefits • For every ton of Portland cement one ton of carbon dioxide is released into the atmosphere. Decreasing the amount of Portland cement would lower the carbon emissions. Replacing this portion with fly ash would help with decreasing the amount of Portland cement needed as well as making use for the ash that would otherwise be put in landfills or the factories. • Decreasing the amount of water is always a benefit when it comes to cement. Fly ash lowers the amount of water needed because it’s smoother and spherical shape on a micro level allows the concrete to have more consistency without plasticizing with more water. • Fly ash lowers the amount of voids (compared to regular cement) because of the particles’ small size.
Silica Fume Properties When silica is in combination of alkali which is found in the concentration of concrete a destructive reaction occurs. When alkali is in the presence of silica hydroxyl ions expansion occurs causeing crakes, which is why a low-alkali cement is used in the presence of silica fume. Silica fume, like fly ash, is a pozzolan and has cement properties. Silica fume as the ability to act as if it were cement (with the presence of water and cement of course) because its’ extremely small particles (at the size of about 1/100th to a cement particle) , having a considerable amount of silicon dioxide, and large surface area makes the admixture an active pozzolan. When concrete has silica fume and low water the outcome of the concrete becomes highly resistant which causes penetration by chloride ions. • Benefits • When concrete has silica fume the strength is greatly increased, having an average compressive strength of 15,000psi. • With silica fume being very resistant to corrosion, concrete with silica fume is now being used in bridges and for rebuilding older structures. • Silica fume molecules have the ability to combine with calcium hydroxide (which is exhaled from the cement during the hydration process) which increases the cement’s overall durability. • Since silica fume’s particle is extremely small which makes it able to fit into the voids made from the spacing between the cements’ particle, it reduces permeability. Being a microfiller helps protect the reinforcing steel from the concrete.
Polypropylene Fibers Background Polypropylene is a recent additive to cement as of the 1960s, whereas other fibers are underway of being tested strength wise for concrete. • Properties • When regular concrete is under a great amount of compression it will spilt and deform on the spot into separate pieces once it reaches its greatest tensile load. Mixing sporadically polypropylene fibers into the cement will balance this effect by attaching to the other piece that wants to spilt away and maintain both sides for a longer duration. • Benefits • With the addition of polypropylene fiber in the mixture of concrete it enhances the toughness and tensile strength. When concrete is by itself it has the tendency to be very brittle especially in the area of a tensile test which is where the fibers come into play to build in where regular concrete lags, which can increase the compressive strength to a dramatic level. • In coastal areas there is a high concentration of chloride ions from the salty air, this creates corrosion with the steel product which produces rust as a result. This rust has the capacity to expand four to ten times larger than the iron causing a large expansion which makes crakes and voids. Polypropylene fibers now are underway in replacing the reinforcing steel in concrete, which has a much greater strength and can reach up to 20k psi.
Slump Test • The goal of the test is to measure the consistency of concrete through out the mix. • "Slump" is simply a term coined to describe how consistent a concrete sample is. • The test also further determines the workability of concrete, how easy is it to handle, compact, and cure concrete. • By adjusting the cement-water ratio or adding plasticizers to increase the slump of the concrete will give a desired mix.
Process • Fill one-third of the cone with the concrete mixture. Then tamp the layer 25 times using the steel rod in a circular motion, making sure not to stir. • Add more concrete mixture to the two-thirds mark. Repeat tamping for 25 times again. Tamp just barely into the previous layer(1") • Fill up the whole cone up to the top with some excess concrete coming out of top, then repeat tamping 25 times. (If there is not enough concrete from tamping compression, stop tamping, add more, then continue tamping at previous number) • Remove excess concrete from the opening of the slump cone by using tamping rod in a rolling motion until flat.
Slowly and carefully remove the cone by lifting it vertically (5 seconds +/- 2 seconds), making sure that the concrete sample does not move.
After the concrete stabilizes, measure the slump-height by turning the slump cone upside down next to the sample, placing the tamping rod on the slump cone and measuring the distance from the rod to the original displaced center.
A change in slump height would demonstrate an undesired change in the ratio of the concrete ingredients; the proportions of the ingredients are then adjusted to keep a concrete batch consistent. This homogeneity improves the quality and structural integrity of the cured concrete.
Data & Results What We Learned
Our Procedures Test first sample at 11 days, second sample at 18 days. Test third sample (comprised of two cylinders) at 25 days. First two samples and one of the third samples loaded wet side down. Last sample loaded dry side down. Photograph and record ultimate failure loads.
Our Failure ModesTest 1 Sample 1 Sample 2 Load:28000# Psi:990 Load:32000# Psi:1132 Sample 3 Sample 4 Load:38500# Psi:1362 Load:33000# Psi:1167
Our Failure Modes Test 2 Sample 2 Sample 1 Load:25500# Psi:902 Load:46000# Psi:1626 Sample 3 Sample 4 Load:62500# Psi:2210 Load:20000# Psi:707
Our Failure ModesTest 3 (Two Cylinders) Sample 1 Sample 2 (Averaged) Load: 33750# Psi:1194 (Averaged) Load:28250# Psi:999 Sample 4 Sample 3 (Averaged) Load:41500# Psi:1468 (Averaged) Load:39500# Psi:1397
Interpretation of Data General trend of all samples (except sample 2) were upward. Silica Fume and Silica Fume/Fiber mix did appear to increase overall compressive strength. Fly Ash data may be inconclusive when considering other sample’s upward trends. Appears that all samples may have been affected more by the constant water ratio (.45) than admixtures.
Potential Sources of Error(Based on Standardized Testing Methods) “A test result is the average of at least two standard-cured strength specimens made from the same concrete sample and tested at the same age. In most cases strength requirements for concrete are at an age of 28 days” “To provide for a uniform load distribution when testing, cylinders are capped generally with sulfur mortar” “The loading rate on a hydraulic machine should be maintained in a range of 20 to 50 psi/s”
Conclusion Failure mode observed was non standard, but appears potentially related to machined grooves in testing apparatus. Lack of over-all data points makes first two tests relatively inconclusive. Concrete never reached potential compressive strength (even with anomalous samples). Water Ratios appear to affect compressive strength more than admixtures.
References Images: http://img.ecplaza.com/my/metfabExportHouse/7.jpg http://www.fhwa.dot.gov/pavement/pccp/pubs/04150/images/fig138.gif http://www.buildinggreentv.com/files/u5/concrete.jpg http://www.luxuryhousingtrends.com/artistic-concrete-flooring.jpg http://www.engineeringfiber.com/images/products/polypropylene_fiber.jpg Text: http://en.wikipedia.org/wiki/Anthracite http://www.toolbase.org/Technology-Inventory/Foundations/fly-ash-concrete http://www.nytimes.com/2008/12/25/us/25sludge.html http://www.silicafume.org/general-silicafume.html http://www.concretenetwork.com/concrete/concrete_admixtures/silica_fume.htm http://www.uritc.uri.edu/media/finalreportspdf/536101.pdf www.nrmca.com/aboutconcrete/cips/35p.pdf
Very special thanks to Burt Lockwood and everyone at Superior Supplies Inc.! As a group we cannot express enough how much we appreciate the help, materials, time and knowledge given to us for this concrete compression project!
