Although, concretes are mainly containing CEM1 Portland Cement, concretes may contain a range of cementious materials such as fly ash, GGBS and silica fume. Select two of the cementious materials and describe the general features of use of the selected two cementious materials and their influence on the properties of concretes. Justify your answer by providing relevant graphs, illustrations and research.
WHAT IS SILICA FUME?
Silica fume is a very fine powder consisting mainly of globular particles; each particle is 100 times smaller than an average cement grain. Silica fume acts in concrete as a very efficient pozzolan, as it merges chemically with the calcium hydroxide formed by the hydration of the Portland cement to form calcium silicate hydrates (C-S-H) which combine the concrete together. It is very reactive due to the high quantity of non-crystalline SiO2 (figure 2), and a great surface area. Silica fume is dark grey or an off white colour (figure 1), and can be manufactured as a hardened powder or slurry depending on the purpose.
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Figure 1 - Silica fume grains
PRODUCTION OF SILICA FUME
Silica fume is a side product of the manufacture of silicon metal and ferro-silicon alloys. The process involves the reduction of high purity quartz (SiO2) in electric arc furnaces at temperatures over of 2000°C.
Figure 2 - Composition of Silica fume
USES AND BENEFITS OF SILICA FUME IN CONCRETE
Silica fume has specific benefits including: increased cohesiveness of the fresh concrete, which leads to improved handling characteristics so curing can start earlier as there is no need to wait for bleed water to dissipate. The dense microstructure of concrete including Silica fume leads to improvements in performance and resistance to chemicals such as oils, chlorides, acids, etc. Characteristics of hardened Silica fume concrete include: lower permeability and improved durability because of the fine particle size and reactivity of Silica fume, greater resistance to abrasion and impact than conventional concretes of similar strength grade. Silica fume can be used as an ingredient in high performance concretes containing micro-fibres to combat explosive spalling during exposure to fire. A properly designed Silica fume high performance concrete containing micro-fibres with a low water/cement ratio will outperform conventional concretes in terms of resistance to spalling during fire. Silica fume is perfectly suitable for the most severe applications, such as concrete slipways, dam spillways, etc, where chlorides, chemicals or abrasion resistance are necessary. Silica fume concretes perform well in these conditions as they are chemically stable and have very low permeability.
STANDARDS AND SPECIFICATION
The conformity of Silica Fume to relevant standards is significant so that its performance in concrete meets the necessary expectations. The standards covering SF in mainland Europe, the UK and the US are:
BS EN 13263 Silica fume for concrete
Part 1:2005 Definitions, requirements and conformity criteria
Part 2:2005 Conformity evaluation
ASTM C1240-97b Standard specifications for silica fume to use as a additional mineral mixture in hydraulic cement concrete, mortar, and grout.
These require Silica fume to be tested for compliance at the plant of origin. Sources of SF that do not meet the requirements of the standards may cause problems in concrete relating to rheology, admixture dosages, strength performance and permeability. Performance data for proprietary Silica fume concretes (source Tarmac
Silica Fume concrete has to be cured in agreement with good quality site practice in spite that bleed water does not appear on its surface. Curing can begin straight away as there is no need to wait for bleed water to dissipate.
WHAT IS GGBS?
Figure 3 - Ground GGBS
Ground granulated blast-furnace slag (GGBS) is a side product from the blast-furnaces used to make iron. These function at about 1,500°C and are fed with a mixture of iron-ore, coke and limestone. The iron ore is converted to iron and the left over materials form a slag that sits on top of the iron.
Figure 4 - 50% GGBS Bridge
The slag is then closed off as a molten liquid, if it is then to be used for the manufacture of GGBS it has to be quickly submitted in large volumes of water. The water gives it the cementitious properties and produces granules; this granulated slag is dried and ground to a fine powder (figure 3). GGBS is off-white in colour and substantially lighter than Portland cement. This whiter colour is also seen in concrete made with GGBS, especially at addition rates of 50%. The more aesthetically pleasing appearance of GGBS concrete can help soften the visual impact of large structures such as bridges and retaining walls (figure 4).
USES AND BENEFITS OF GGBS
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The major use of GGBS is in ready-mixed concrete. There are many benefits to using GGBS compounds to concrete, these include: improved workability, making insertion and compaction easier, decrease early-age temperature rise, reduce the risk of thermal cracking in big pours, eliminating the risk of damaging interior reactions such as ASR, high resistance to chloride ingress, reducing the risk of reinforcement corrosion, high resistance to attack by sulphate and other chemicals.
In the manufacture of ready-mixed concrete, GGBS substitutes a large fraction of the normal Portland cement content, usually about 50%. The higher the amount, the better is the durability. There is a disadvantage of the elevated replacement level, which is that early-age strength development is relatively slower.
STRENGTH GAINS IN GBBS CONCRETE
With the identical content of cementitious material (the whole weight of Portland cement added to GGBS), similar 28 day strengths to Portland cement will be achieved when using up to 50% GGBS. At greater GGBS percentages there should be more cementitious content to achieve equal 28 day strength. GGBS concrete gains strength more progressively than concrete made with Portland cement. For the same 28 day strength, a GGBS concrete will have lower strength at beginning however its long-term strength will be higher. The reduced early-strength will be most obvious at high GGBS levels and low temperatures. Normally Portland cement concrete will reach about 75% of its 28 day strength at seven days, with a little increase of 5% to 10% 28 and 90 days. By contrast, a 50% GGBS concrete will normally reach about 45% to 55% of its 28 day strength at seven days, with an increase of between 10% and 20% from 28 to 90 days. When in normal conditions, the striking times for concretes containing up to 50% GGBS, do not increase sufficiently to significantly affect the construction programme. Although concrete that has a higher level of GGBS will not always achieve sufficient strength after one day to allow removal of vertical formwork, particularly at lower temperatures, lower cementitious contents and in thinner sections.
Describe and recommend good concrete practice, from production to finishing.
Initially the cement (frequently Portland cement) is prepared. After that the aggregates, admixtures (chemical additives), any fibres, and water are mixed together with the cement to form concrete. The concrete is then transported to the work site and positioned, compacted, and cured.
Stage 1 - Preparing Portland cement
Limestone, silica, and alumina that formulate Portland cement are dry ground into a fine powder, combined together in determined proportions, preheated, and heated to a high temperature in a large rotary kiln at 1400°C. The substances partially fuse into a matter called a clinker. The clinker is chilled and crushed into a fine powder in a tube or ball mill. A ball mill is a rotating drum filled with steel balls of different sizes that mash and pulverize the clinker. Gypsum is also put into the mixture during the grinding process.
Stage 2 - Placing and compacting
When at the site, the concrete must be placed and compacted. These two processes are carried out almost at once. Placing should be done so that separation of the different ingredients is avoided and that completed compaction without any air bubbles can be accomplished. Positioning is very important in achieving these goals. The rates of placing and of compaction should be identical, at the end it is usually achieved using interior or exterior vibrators. An interior vibrator uses a poker housing a motor-driven shaft. When the poker is put into the concrete, controlled vibration occur to compress and compact the concrete. Exterior vibrators are used for precast or in-situ pieces as they have a shape or thickness unsuitable for interior vibrators. These vibrators are clamped to the formwork, which rests on an elastic support. Both the form and the concrete are vibrated. As vibrating tables are also used, they create vertical vibrations by using two shafts rotating in opposite directions.
Stage 3 - Curing
Once the concrete is placed and compacted, it must be cured before it is finished to make sure that it does not dry too rapidly. Concrete's strength is determined by its moisture content during the hardening process because as the cement solidifies, the concrete shrinks. If site constraints stop the concrete from contracting, tensile stresses will increase, and weaken the concrete. To reduce this difficulty, concrete must be kept damp throughout the few days it is required to set.
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There are three ways to cure concrete, either add water to the surface to replace the water that is evaporating or seal the concrete to prevent the water from evaporating. Adding water to the surface will not work it into the concrete mix; it would increase the water-cement ratio of the surface concrete and weaken it. The simplest approach is to use evaporation retardant. This retardant should be sprayed on to form a thin layer on the surface that prevents the water from evaporating. It then completely dissipates during finishing operations without residue.
Design a concrete mix and state clearly the stages of a mix design procedures.
MIX DESIGN STAGES
The mix design is carried out in the following five stages.
Stage 1 Determine Free Water/Cement Ratio Required for Strength
A specified margin or calculated margin can be used for a given proportion of defectives and statistical standard deviation. Take the target mean strength by adding the margin to the compulsory characteristic strength.
Accept a specified free water/cement ratio or obtain the maximum free water/cement ratio which will offer the target mean strength for concrete made from the given coarse aggregate type and from cement with the known properties.
Stage 2 Determine Free Water Content Required for Workability
A specified free water content can be used, or obtain the minimum free water content, which will provide the required workability for concrete made with the given fine aggregate type, coarse aggregate type and maximum size of coarse aggregate.
If the free water content is known for workability, regulate the required free water content if air entrainment is specified, and adjust further if a water-reducing admixture is specified.
Stage 3 Determine Required Cement Content
Acquire the minimum cement content, which is necessary for strength, by dividing the free water content acquired in Stage 2 by the free water/cement ratio acquired in Stage 1.
Ensure the min cement content, which is required for strength, against the max cement content, and give a warning if the former exceeds the latter.
Ensure the min cement content, which is required for strength, against the min cement content, which is allowable for durability, and adopt whichever is greater to be the cement content in the mix.
Then the free water content is divided by the cement content used in the mix to acquire a modified free water/cement ratio.
Stage 4 Determine Total Aggregate Content
Work out a value for the overall aggregate density.
Work out the partial volume of the aggregate by subtracting the proportional volumes of the free water and the cement from a unit volume.
Work out the whole aggregate content by dividing the volume of the aggregate by the aggregate density.
Stage 5 Determine Fine Aggregate Content
Use a particular value of the percentage of fine aggregate, or acquire the percentage of fine aggregate, which will offer the desired workability for concrete made with the given grading of fine aggregate, maximum size of coarse aggregate and the free water/cement ratio acquired in Stage 3.
Work out the fine and coarse aggregate contents from the total aggregate content acquired in Stage 4 and the percentage of fine aggregate.