(i) Design a suitable concrete mix for the above structure by initially considering TWO different concrete mixes. Mix Design of both mixes should be given. Special attention need to be paid when placing the concrete with heavy reinforcement arrangement involved in an off-shore structure.
(ii) A design life of 30 years is expected of the structure. In relation to durability comment on the type of cement used and aggregates selected. (Typical cover to reinforcement and tendons need to be specified). Suggest a method to estimate the design life of the structure for the cover chosen (based on reinforcement corrosion).
Concrete Mix Design
Due to the population increase in the urban cities there is rise in the use of the fuel in these areas thereby leading to the construction of the concrete platform that will contain the oil to serve the eight million of the people by 2020.it is therefore a belief that by the year 2020 the concrete oil platform would be completed in terms of the construction. Concrete with the high performance is one of the important parts of the building that are tall. The construction reinforcement by the concrete enables the progress of the construction by completing about 2 to 3 floor per week; this is due to the existence of the strength and stiffness. During this construction, a number of factors are required to be considered. This includes the required design of the mixes and considerations of the construction thus a case study should be done on the self-compartment and other factors like the pumping ability.
The approximation of the volume of self-compact high-performance concrete was estimated to be 70,000m3. The variation of the strength was between 50MPa to 80MPa.the field displacement at the taw was estimated to be 600000 tones and the pre-stressing cable were 400 tones (Vecchio, 2010).
What is required and what should be considered.
The strength that is compressive
The colossal statue to which present-day tall structures are built instinctively brings about high loads which must be conveyed by vertical load-bearing basic individuals, for example, segments, center dividers and heaps. Keeping in mind the end goal to convey these heaps, without having segments and heaps of too much expansive cross-areas, high-quality cement (HSC) must be utilized alongside vigorously fortified steel confines. This is apparent from the high qualities required for the off-shore concrete oil platform.
For HSC concrete, the expanded pressing thickness accomplished because of a lower w/b proportion and the utilization of super plasterers combined with high fineness extenders (FA and CSF) brings about the lessened porosity of the bond glue and interfacial progress zone (ITZ). This microstructure refinement gives the higher compressive quality yet renders the total as the powerless connection regarding compressive quality with the disappointment plane in pressure more often than not through the totals. Since the total turns into the basic segment, high caliber and high-quality totals ought to be utilized (Baker, 2015).
Furthermore, to accomplish an adequately low w/b proportion a total with a low water necessity ought to be utilized. The appropriateness of the total for pumping ought to likewise be considered while picking a total to guarantee conservative pumping. The utilization of smashed nearby dolomitic limestone for the off-shore concrete oil platform gave adequate quality, had a low water prerequisite and turned out to be prudent for pumping (Chen, 2013).
Without the capacity of cement to be pumped, it would not be a suitable development material as extensive amounts of material should be lifted by cranes bringing about profoundly wasteful development due to hindered throwing rates. Pumping concrete, particularly in tall building, can bring about directing blockages because of total attachment, sweltering climate, throwing delays and hazardous cement blends which can cause deferrals and end up being exorbitant. Therefore, watchful blend plan, adequate trial blends and full-scale directing tests preceding development and efficient on location checking of rheology and pipe weights is critical to limit the potential for blockages (Orchard, 2016)
Directing cement for the off-shore concrete oil platform ended up being the most troublesome solid outline issue. Cement to be pumped ought to have an adequately high fine total substance, similar to the case for HSC. This builds the cohesiveness and lessens isolation of the blend which diminishes the potential for total connect whereby the blended water to the solid is constrained through in front of the blend. At the point when this happens, the course total particles interlock, the inner rubbing increments, and the solid quits moving. It ought to anyway be noticed that a fine total substance that is too high, making the blend be too much durable, will require expanded drawing weights because of expanded contact between the solid and the inward covering of the pipeline. The utilization of CSF at 10% substitution and fine total level of the half of the solid blend for the off-shore concrete oil platform anticipated total attachment and in the meantime caused the blend to be excessively durable (Neville, 2010).
Blockages because of the early setting of the solid in the pipeline can likewise be an issue because of the period of time (can associate with 30 min) required to draw the solid to high elevations. On account of the off-shore concrete oil platform, this was exacerbated by the sweltering summer climate experienced in the Dubai where temperatures can every so often achieve 50°C thus a particularly composed super putting a mixture which gave satisfactory impediment was important to guarantee adequate time for pumping of the solid before getting started. The measurements of the a mixture must be changed for the distinctive periods of the year as Dubai encounters moderately cool winters. The decision of course total for the solid blend can affect the cost of pumping. Two critical properties of course totals that must be considered while choosing a course total are the abrasiveness and greatest size. A grating total can cause unnecessary wear on the coating of the pipeline, particularly at high pumping weights required for tall structures, to such an extent that the life expectancy of the pipeline can be diminished to around 10 000 m3. For the off-shore concrete oil platform, a pipeline life expectancy of around 40 000m3 was accomplished by utilizing a less rough dolomitic limestone. The greatest total size influences the span of the pipeline. By and large, the pipe width ought to be no less than four times the greatest total size to decrease the potential for blockages. Be that as it may, a bigger pipe width requires a more prominent weight head. A 150 mm distance across pipeline was utilized for the off-shore concrete oil platform for the 20 mm total up to around a stature of 350 m. At elevations higher than this, the total size was lessened, and in addition, the pipe distance across diminished pumping weights (Ninet, 2013).
Challenges of Pumping Concrete for Tall Buildings
The diagram below shows the typical arrangement of the unit at the base of the oil platform.
Regularly, grouting is completed by driving funnels or drilling openings into the ground and after that pumping the grout arrangement at high weight through embedded tubes. The degree of grouting required for a specific zone is resolved through examination of ground conditions and the figuring of a penetrating example. This considers the size, dispersing and profundity of the openings required. The sort of grout and the specific ground conditions will impact the dispersing of the openings. Site conditions will impact the instruments utilized for the drilling procedure, yet pneumatic apparatuses, jewel bore or wash-drilling are the most widely recognized. Alluvial soils are inclined to crumple thus gaps are generally cased. The weight of the grout infusion is subject to soil conditions, and in-situ testing might be completed before the right weight is resolved. Weights for the most part extend from 1 N/mm2 for sands to 7 N/mm2 for shake.
Modulus that is elastic, ability to shrink and to creep
The utilization of littler vertical load-bearing part cross-areas in tall structures, influenced conceivable by the utilization of HSC, implies that more noteworthy burdens must be conveyed by cross-segments of decreased size. With a specific end goal to diminish the flexible shortening of these individuals, a higher versatile modulus is required. This is clear in the C80 concrete utilized for off-shore concrete oil platform which had a predetermined versatile modulus of roughly 44 GPa. Cement to be pumped is frequently outlined with a diminished coarse total substance to counteract blockages and lessen pipe wear which isn't helpful for acquiring a higher versatile modulus. Another choice is the utilization of harder totals, anyway, these totals are rougher thus increment pipe wear. Accomplishing an expanded flexible modulus thusly involves a tradeoff between an adequately high versatile modulus and adequate pump ability. A dolomitic limestone as coarse total for the off-shore concrete oil platform, which as a rule displays a high versatile modulus with a generally low abrasiveness, was observed to be ideal as was utilized. The lion's share of shrinkage of HSC is expected to autogenously shrinkage which happens because of self-drying up. Together with the high hydration temperatures of HSC and high surrounding temperatures of Dubai (up to 50°C), HSC can encounter extreme early-age splitting (Shinn, 2014).
To diminish the danger of splitting and guarantee the functionality and solidness necessities of the solid are kept up, extraordinary consideration ought to be paid to curing and proper curing methodology ought to be utilized. Broad curing tests on full-scale example were directed to decide ideal curing methodology. Showered on curing mixes were resolved to be the most effective and handy curing strategy for the vertical individual (Browner, 2013).
Ensuring Adequate Curing to Prevent Cracking
Differential shortening of the vertical load-bearing individuals in the tall building because of shrinkage and sneak can bring about expanded anxieties which ought to be represented in rebar of vertical individuals that abbreviate less. Furthermore, differential limitation ought to be limited to guarantee appropriate working and insignificant harm of lifts, pipelines, shade dividers, and so on. The expectation of vertical shortening because of versatile, shrinkage and crawl distortions ought to consequently be directed on the structure (Merrow, 2011).
The rate of warmth advancement of HSC is essentially more prominent than that of ordinary quality cement anyway the aggregate warmth of hydration is brought down. Subsequently, vast temperature slopes may create which can bring about early-age warm breaking, especially in substantial individuals, for example, mass establishments. Full-scale testing to screen the solid temperature was led for the off-shore concrete oil platform 3.7 m thick pontoon establishment when outlining the solid blend (Suderow, 2015). The solid for the pontoon establishment of the off-shore concrete oil platform had a trademark 3D square compressive quality of 50 MPa from a blend containing 40% FA and a w/b proportion of 0.34. The high substitution of FA decreased the warmth of hydration because of it’s to some degree idle pozzolanic response. No CSF was utilized as a part of the blend as it is exceedingly receptive because of its high fineness and subsequently would build the rate of warmth development.
Because of the outrageous surrounding temperatures experienced in Dubai, the danger of warm splitting is significantly higher. Keeping in mind the end goal to lessen concrete setting temperatures in the mid-year months, the larger part of throwing was finished amid the night, when the temperature was generally lower. Furthermore, fitting water curing and halfway supplanting of the blended water with chipped ice was utilized to bring down the warmth of solid temperature (Davies, 2014).
The figure below shows the finished view of the oil platform
It is obvious from the various execution prerequisites, blend plan and development contemplations for the solid utilized on the off-shore concrete oil platform that extraordinary consideration is required when making utilization of cement in this tall structures. To reduce the danger of splitting and put in place the functionality and solidness necessities of the solid are kept up, extraordinary consideration ought to be paid to curing and proper curing methodology ought to be utilized. Broad curing tests on full-scale example were directed to decide ideal curing methodology. Showered on curing mixes were resolved to be the most effective and handy curing strategy for the vertical individual. This approximation of the volume of self-compact high-performance concrete that is estimated to be 70,000m3 with the variation of the strength between 50MPa to 80MPa makes the structure stronger when combined with the field displacement at the taw that is estimated at 600000 tones with the pre-stressing cable of 400 tones.
The key necessities and contemplations seen from the off-shore concrete oil platform are as follows;
- High compressive qualities are required to decrease vertical load-bearing part cross-segments and along these lines, the course total utilized should be of a high-quality and excellent so it doesn't restrict the compressive quality. Furthermore, it ought to have a low water prerequisite and ought to be sparing for pumping
- Pump ability of the solid is basic to make RC development practical, however, can be troublesome when pumping to high heights. Adequately high fine total substances are basic to expand union and decrease isolation. In any case, a fine total substance that is too high will bring about an excessively strong blend which will be hard to pump. For temperate pumping, coarse totals ought not to be grating and too substantial. Pumping can essentially change the rheology of the solid which must be represented in the blended plan (Malhotra, 2012).
- Workability for sufficient course through congested rebar confines is to a great extent met by the necessities for pumping of the solid. The adjustment in rheology in the wake of pumping ought to anyway be represented. The utilization of basically self-compacting pump solid outcomes in parallel weight on formwork thus formwork ought to be adequately solid.
- The decreased cross-areas of vertical load-bearing individuals with higher burdens bring about the requirement for a higher flex modulus. Accomplishing a higher versatile modulus brings about an exchange off with practical pump ability prerequisites where lessened total substance and milder, less grating totals are utilized. High early-age shrinkage strains, which may cause breaking, can create if HSC concrete isn't properly cured. Differential shortening of vertical load-bearing individuals because of shrinkage and crawl ought to be anticipated to guarantee suitable working of vertical administrations, for example, lifts and pipelines (Gambhir, 2013).
- High rates of warmth advancement related with HSC can bring about huge temperature inclinations prompting early warm splitting, particularly in expansive individuals. The utilization of a high FA substitution can help in lessening the warmth of hydration. The utilization of CSF isn't suggested. On account of outrageous surrounding temperatures, throwing ought to be directed around evening time when the temperature is cooler. Also, water curing and an incomplete swap of chipped ice for blend water can decrease solid temperatures (French, 2016).
Davies, P., Bowden Drilling Services Ltd, 2014. Off shore drilling platform construction. U.S. Patent 3,727,414.
Merrow, E.W., 2011. Industrial megaprojects: concepts, strategies, and practices for success (Vol. 8). Hoboken, NJ: Wiley.
Suderow, G.E., De Long Corp, 2015. Off-shore drill rig. U.S. Patent 3,001,594.
Greenberg, J., 2007. SOLIDS.
Shinn, E.A., 2014. Oil structures as artificial reefs. L. Colunga and R. Stone, editors, pp.91-96.
Ninet, J.L. and Vaillant, R., Ateliers et Chantiers de Bretagne ACB SA, 2013. Device for positioning an off-shore platform on its support structure. U.S. Patent 4,436,454.
Mehta, P.K., 2013. Concrete. Structure, properties and materials.
Neville, A.M. and Brooks, J.J.,2010. Concrete technology.
Orchard, D.F., Curran, A. and Hearne, R., 2016. CONCRETE TECHNOLOGY-VOLUME 1-PROPERTIES OF MATERIALS (No. Monograph).
Malhotra, V.M., 2012. Introduction: sustainable development and concrete technology. Concrete International, 24(7).
Newman, J. and Choo, B.S. eds., 2013. Advanced concrete technology 3: processes. Butterworth-Heinemann.
Gambhir, M.L., 2013. Concrete Technology: Theory and Practice. Tata McGraw-Hill Education.
Mehta, P.K., 2010. Advancements in concrete technology. Concrete International, 21(6), pp.69-76.
Mehta, P.K., 2012. Concrete. Structure, properties and materials.
French, S.E., 2016. Reinforced concrete technology. Delmar Publishers.
Chen, P.W. and Chung, D.D., 2013. Carbon fiber reinforced concrete for smart structures capable of non-destructive flaw detection. Smart Materials and Structures, 2(1), p.22.
Baker, A.L.L., 2015. The ultimate load theory applied to the design of reinforced & prestressed concrete frames. Concrete Publ. Lmd.
Vecchio, F.J. and Collins, M.P., 2010. The modified compression-field theory for reinforced concrete elements subjected to shear. ACI J., 83(2), pp.219-231.
Grünewald, S., 2014. Performance-based design of self-compacting fibre reinforced concrete (Doctoral dissertation, TU Delft, Delft University of Technology).
Li, V.C., 2013. On engineered cementitious composites (ECC). Journal of advanced concrete technology, 1(3), pp.215-230.
Browner, R.D., 2013. Design prediction of the life for reinforced concrete in marine and other chloride environments. Durability of building materials, 1(2), pp.113-125.