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2. Concrete.pptx
- 1. In-Situ Concrete, Formwork &Reinforcement - II BE-1111 Building Design and Construction II Level-1, Semester-2 Dr. Thilini Jayawickrama
- 2. Slump test • Measurewetness or stiffness of concrete - workability • Indirect measurement of consistency Method • Slump cone • Fill properly and tamp • Remove the cone • The remaining concrete called “slump” • Shear or collapse slumps are undesirable • Advise to change the proportion
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- 4. Slump test contd.… • The slump can be controlled by a change in any one or all of the following: – Grade of aggregates, – Proportion of aggregates, or – Moisture content.
- 5. Cube test • Testfor compressive strength • Steel/cast iron molds in cubical shape • Tamping rod • Suitable identification number for each sample • Soak in water for curing • Test on a compression testing machine • For 3, 7, 28 days
- 6. Cube test contd.… Fill molds and tamping with rod Trowel Test for compression with machine
- 7. Classification of Concrete ByStrength • Grade (N/mm2) • In general at 28 days By composition • Normal weight concrete • Lightweight concrete Special types of concrete • Hot weather concrete • Self-compacted concrete • Pumped concrete
- 8. Lightweight concrete • Normalconcrete has a considerable self-weight (density 2200-2600 kg/m3) • Increase the size of building elements – uneconomical • Reduce the self-weight and increase the efficiency – Low-density concrete (non-structural purposes) (density 300-800 kg/m3) – Structural lightweight concrete (density 1350-1900 kg/m3) • The compressive strength is not as great as ordinary concrete • Methods of production – By replacing usual aggregate with lightweight aggregate – By Introducing gas or air bubbles into the mortar – By omitting the fine aggregate fraction - no-fines concrete
- 9. Lightweight concrete contd.… Advantages • Economical • Better fire resistant • Lower thermal conductivity - thermal comfort • Use of by-products/waste is environmentally beneficial Disadvantages • Higher moisture movement • Higher initial drying shrinkage (5-40%)
- 10. Lightweight aggregate Natural lightweightaggregate • Volcanic origins (Pumice, Scoria, Volcanic tuff, Foamed lava) • Rice husk • Sawdust • Diatomite Artificial lightweight aggregate • Sintered fly ash • Foamed slag • Bloated clay • Artificial cinders • Expanded clay, slate, shale • Coke breeze • Expanded perlite • Exfoliated vermiculite
- 11. Natural lightweight aggregate Sawdust Ricehusk Diatomite
- 12. Cellular concrete • Atype of lightweight concrete • This is produced by introducing gases or air bubbles in the concrete • Also known as Gas concrete, Aerated concrete, Foam concrete • The methods of manufacturing cellular concrete includes; – mixing foaming agents – adding expansion agent to the concrete mix to generate gas (chemical reaction ), and form a gas-bubble structure within the concrete • Cellular concrete has the following characteristics; – Very low density and strength – High fire resistance • This can be used for in-situ concrete as well as pre-cast concrete • Insulation, partition walls, prefabricated elements
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- 14. No-fines concrete • Atype of lightweight concrete • This is produced by omitting the fine aggregate fraction from the concrete • A mix consist of coarse aggregate particles, each surrounded by a coating of cement paste • It has the following characteristics; – Low shrinkage – High water absorption – Not easy to cut or nail – Very high skid resistance • This can be used for Block work, infill panels or partitions, Road work or road surfaces (to improve the water drainage)
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- 16. Cyclopean/plum concrete • Inclusionof large stone into a concrete mass • Method of placing – Placing a layer of normal concrete – Spreading large stones (plums) – Place another layer of concrete
- 17. Hot weather concrete •Recommended temperature for concrete mixing is 27+/- 20C • Not possible to hold construction during hot weather Problems with concrete in hot weather Fresh concrete • Quick settlement creates problems in pouring, compaction, and finishing Hardened concrete • Likelihood of shrinkage cracks due to difference in temperature between a concrete element and rest of the building or within the same section • Reduced durability, probability of corrosion of reinforcement bars due to cracks • Colour difference of the outer concrete surface due to different hydration rates
- 18. Hot weather concretecontd. … • Use cement types with slower rates of dehydration • Use cold water • Use low heat cement • Storing aggregate in a shaded area • Continuous spraying of water into aggregate
- 19. Self-compacted concrete • Thistype of concrete is independent of workers’ performance in compaction • Self-compaction by means of self-weight of concrete • Compact into every corner of formwork Benefits • Shorter construction period • Assuring compaction in confined parts where vibrating compaction is difficult • Eliminate noise during compaction
- 20. Pumped concrete • Pumpa Ready- mixed concrete • Pre-planning is required – Pump volume – Pressure capability – Pipeline diameter – Horizontal and vertical distance to be pumped • Quality is important to ensure uniformity • Precautions for hot/cold extreme weather conditions • Workability of concrete affects pumping (Slumps 50 – 150mm)
- 21. Pumped concrete contd.… Coarse aggregate • Maximum size should be considered (limited to 1/3 of the smallest inside diameter of the pipeline) • Content to be reduced Fine aggregate • Together with cement and water, fine aggregate provides the mortar (fluid) to convey coarse aggregate in suspension Water • Affects workability Admixtures • Those which are increasing workability will improve the ability to pump
- 22. Concrete properties Workability andConsistency • Consistency is the fluidity or wetness indicating workability • Fresh concrete should be easy to mix, transport, place, compact and finish • Workability is often considered to be a measure of the work needed to compact the wet concrete. • But it is also used to quantify the ease with which concrete can be placed, although this depends on other properties such as cohesiveness. • The workability of the mix is determined by the slump test. • It must retain free from segregation during these operations
- 23. Concrete properties contd.… Segregation • Segregation is the tendency for coarse aggregate to separate from the sand-cement mortar • This results in part of the batch having too little coarse aggregate (likely to shrink more) and the remainder having too much (poor resistance to abrasion) • The method and equipment used to transport and handle the concrete must not result in segregation of the concrete materials • When placing concrete, segregation can occur when mortar tends to flow ahead of coarse materials
- 24. Concrete properties Uniformity • Degreeof homogeneity or state of distribution within the mix. Testing uniformity • Two samples are taken and concrete properties are determined from each sample. – First sample of concrete - when approximately 15% of the total batch has been discharged – Second sample of concrete - when approximately 85% of the total batch has been discharged • The properties of two samples are then compared to each other and the property differences are determined.
- 25. Concrete properties contd.… Bleeding • Bleeding is the tendency of water to rise to the surface of freshly placed concrete • Develop a layer of water at the top or surface • It is caused by the settlement of solid particles (cementing materials and aggregate) and simultaneous upward migration of water • Inability of solid particles of the mixture to hold the water amount as they settle down • Bleeding is normal and it should not diminish the quality of properly placed, finished, and cured concrete
- 26. Concrete properties contd.… Bleeding contd. … • Excessive bleeding increases the water-cement ratio near the top surface • Poor durability particularly if finishing operations take place while bleed water is present • Water rising to the top surface carries fine particles • a weak top layer with poor durability • not resistance to abrasion • A water pocket or void can develop under a prematurely finished concrete
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- 28. Types of Cement OrdinaryPortland Cement (OPC) • The most common type due to suitability for many types of works • It has medium type setting and hardening time Rapid Hardening Portland Cement • Sets faster than OPC • This type is used for quick construction where speed is very critical Sulphate Resisting Portland Cement • It has better resistance to Sulphate attack than OPC • This type is suitable for marine structures and foundations which built in soils having high risk of Sulphate attack Low Heat Portland Cement • Suitable for mass concrete • Fineness is higher than OPC and has lesser early strength
- 29. Types of Cementcontd. … High Alumina Cement • It has very good resistivity for Sulphate but weak in acids • The water requirement for hydration is higher (about twice) than in OPC Supersulphated Cement • This is suitable for mass concrete work due to its low heat generation during the hydration • This type has resistance to chemicals Portland Blast Furnace Cement • This also generates less heat than OPC and has high resistivity to Sulphate White Cement • Metal oxides, primarily iron and manganese • This is expensive than OPC but not strong as OPC
- 30. Admixtures • Admixtures maybe used to modify the properties of the concrete to make it better serve its intended use or for better economic reasons. Retarding admixtures • Slow down the hydration of cement, lengthening set time • Suitable for hot weather conditions • Most retarders also act as water reducers (frequently called water- reducing retarders) Accelerating admixtures • Shorten set time of concrete • Allowing cold-weather pour • Facilitate early removal of formwork, early surface finishes, and sometimes early load application • Drying shrinkages of concrete can be increase – care must be taken while selecting type and proportion of accelerators
- 31. Admixtures contd. … Superplasticizers • Also known as plasticizers • Includes water-reducing admixtures • These are “high range water reducers” • Allow large water reduction • Gives great flowability without substantially slowing set time or increasing air entrainment • They can maintain a specific consistency and workability at a greatly reduced amount of water Water reducing admixtures • Require less water to make a concrete of equal slump, or increase the slump of concrete at the same water content • They can have the side effect of changing initial set time • Mostly used in hot weather concrete placing and for pumping
- 32. Admixtures contd. … Air-entrainingadmixtures • Entrain small air bubbles in the concrete • While some loss of strength typically accompanies increase air in concrete, it generally can be overcome by reducing the water- cement ratio via improved workability (due to air-entraining agent itself) or by using other appropriate admixtures
- 33. Quality assurance • Concretewith high quality and to the required international standard and project specification • Calibrate laboratory devices periodically • Obtain samples correctly according to the standard specifications • Perform and review tests based on the quality system
- 34. Formwork • Molds tokeep concrete mix in place • Sufficiently water-tight to prevent leakage • Properly braced or tied to maintain position and shape • Design as not to damage previously place structures • Consider method of placing concrete • Consider loads • Uniform surfaces for smooth finish • Special design requirements (domes) • Reuse where possible
- 35. Types of Formwork Bymaterial • Wood • Steel • Aluminum • Fiberglass • Plastic By usage • Temporary • Left-in
- 36. Formwork – typesfor tall buildings Flying (table forms) • Typical span length • Continual re-use • Once the concrete gets strong enough, remove, clean, flown with a crane to the next level to re-use • Useful for high-rise buildings with typical floors
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- 38. Formwork – typesfor tall buildings Slip • Continuously re-employed • Using jacks, “slip” the section of formwork above the level • Use in wall sections to higher levels as the concrete cures