Concrete structure presentation
- 1. ADAM MUSLIM HUSSAINI(阿达姆） PRESENTATION • Concrete Structure • Concrete is the second most used material for construction after water in the world. Concrete structure can take compressive stresses very effectively but it cannot take tensile stresses. So the reinforcement is given to concrete where the structure is under the tension load. Concrete is widely used in today construction industry today because of its durability and compatibility. Moreover concrete can be mould in any shape which make it a very useful.
- 2. • Type K cement: • It is also referred to as Klein cement. It comprises of C4A3S (C=CaO, A=Al2O3, and S=SO3), calcium sulphate (CaSO4) and lime CaO. • Type M cement: • It is constituted by
- 3. Plain Cement Concrete • Plain cement concrete has very low tensile strength. To improve the tensile strength of concrete some sort of requirement is needed which can take up the tensile stresses developed in the structure. The most common type of reinforcement is in the form of steel bars which are quite strong in tension. The reinforced concrete has innumerable uses in construction. For eg: in building, flyovers, water tanks, etc.
- 4. Prestress concrete • In ordinary reinforced cement concrete, compressive stresses are taken up by concrete and tensile stresses by steel alone. The concrete below the neutral axis is ignored since it is weak in tension. Although steel takes up the tensile stresses, the concrete in the tensile zone develops minute cracks. The load carrying capacity of such concrete sections can be increased if steel and concrete both are stressed before the application of external loads. This is the concept of prestress concrete. • The prestressed concrete is uses in the structures where tension develops or the structure is subjected to vibrations, impact and shock like girders, bridges, railway sleepers, electric poles, gravity dam, etc.
- 5. Advantages of concrete structure • Ingredients used in concrete such as, cement, aggregates and water are readily available and cheap. • Concrete assumes the shape of its mould and it can be poured and cast into any shape. • Concrete when used along with reinforcement, is capable of taking bending and tension forces. • The compressive strength of concrete is very high making it reliable to be used for structures and components under compressive loads. • The break through in prestressed concrete applications enables reduced section sizes and reduction in self-weight. • Due to massive nature, high unit weight and water tightness, concrete can be used for water retaining structures like Dams.
- 6. Disadvantages of concrete structure • Due to drying shrinkage and moisture expansion concrete may crack. Therefore construction joints are provided to avoid these types of cracks. • Concrete is weak in tension. • High Self weight of concrete is not always favorable for seismic prone structures. • Sustained loads develop creep in structures. • If salts are present in the concrete then it will results in the efflorescence in concrete structure
- 7. Concrete Creep Definition, Creep deformation Stages & Design Strategies • When load is applied on a concrete specimen, shows an instantaneous deformation followed by slow increase of deformation is called concrete creep. Creep is a time- dependent permanent (plastic) deformation under a certain applied load. Generally, creep occurs at high temperature (thermal creep) but can also happen at room temperature depending on the material (e.g. lead or glass), although this happens at a much slower rate.
- 8. • As a result, the material undergoes a time- dependent increase in length, which could become quite dangerous while in use. The rate of deformation is called the creep rate. It is the slope of the line in a creep strain vs. time curve (see below).
- 9. stages of creep deformation • Creep deformation has three stages; Primary creep starts rapidly and slows down with time. Secondary creep progresses at a relatively uniform rate. Tertiary creep has an accelerated rate of deformation which terminates when the material fails (breaks or ruptures). It is associated with both necking and the formation of grain boundary voids. There are several design strategies that can be adopted to avoid creeping in materials: • Reduce the effect of grain boundaries (use single crystal material with large grains). • Add solid solutions to fill the voids in the material. • Use materials with high melting temperatures. • Consult creep test data during materials selection.
- 10. Creep In Detail • When a load is applied on a concrete specimen. the specimen first shows an instantaneous deformation which is then followed by slow further increase of deformation. This slow increase of deformation, discovered in 1907 by Hatt, is called creep. There is always strain associated with applied stresses to any material. ASTM E 6-03 defines creep as “the time-dependent increase in strain in a solid resulting from force.” To define creep one must consider two identical specimens subjected to exactly the same environmental histories, one specimen being loaded and the other load-free (companion specimen). The difference of the deformation of these two specimens defines the instantaneous deformation plus creep.
- 11. Reinforcements in a Reinforced Concrete Beam • In RCC beams, elements are designed to resist the loads that cause bending moment, shear forces and sometimes in some cases, it also causes torsion along their length. It’s a known fact that concrete is strong in compression and very weak in tension. The steel reinforcement present in the concrete, is used to take up tensile stresses in reinforced concrete beams. • Mild steel bars of round section were utilized in RCC work. But when deformed and twisted bars were introduced, the usage of mild steel bars declined. Ribs are indented on the surface of the deformed or HYSD bars which results in increasing the bond strength by atleast 40% when compared to that of mild steel bar.
- 12. Beam and Slab Floor System • This system consists of beams framing into columns and supporting slabs spanning between the beams. It is a very traditional system. The relatively deep beams provide a stiff floor capable of long spans, and able to resist lateral loads. However, the complications of beam formwork, co- ordination of services, and overall depth of floor have led to a decrease in the popularity of this type of floor. • The traditional reinforced concrete beam-and-slab floor has an economical span ‘L’ of D x 15 for a single span and D x 20 for a multi-span, where D is the depth of the slab plus beam. The depth of slab between the beams can be initially sized using the span- to-depth ratios for a flat plate. Prestressing is not normally used with this system.
- 13. • Advantages: • Traditional effective solution • Long spans. • Disadvantages: • Penetrations through beams for large ducts difficult to handle • Depth of floor • Greater floor-to-floor height.
- 14. Formwork for Beams and Slabs • Forms are molds to receive concrete in its’ plastic form. • Formwork is temporary structure, as such, it is not normally shown on the drawings. • Formwork Materials • Wood • Either all-wood or some wood components • Plywood • Aluminum • Steel • Plastics • Lumber • Designated by Cross Sections, Nominal Dimensions (prior to finishing) • After cut length wise, finishing operations reduces actual dimensions • 2 x 4 Plank 1 1/2 x 3 1/2 ® 2” by 4” in S4S • Lengths are multiples of 2 ft (8, 10, 12, 14, 16,…) • Specified by type and grade • Type: pine, oak, fir • Grade: Selected (A, B, C, D) and Common (1, 2, 3, 4) • Selected (A best, D poor quality) • Cost ® Kind, grade, size, length, milling, quantity, freight
- 15. Shrinkage Cracking in Concrete Structures | Long Term Effects • Internal or external restraints in a concrete structural element may resist free shrinkage of concrete, over time this leads to Shrinkage Cracking. In concrete structures, the volumetric reduction that happen due to the loss of moisture by evaporation is known as the shrinkage of concrete. This deformation is time dependent and it reduces the volume without any impact of external forces. When cement paste is pure (cement+water), it may shrink upto 1%. Further, the aggregates added to it resists the deformation which reduces the magnitude of this volume decrease to 0.06%. Deydration of cement is one of the reasons for shrinkage in concrete. This dehydration is the loss of water due to evaporation. Thus, concrete continues to shrink during its life span at a reduced rate.
- 16. What is Shrinkage Compensating Concrete? • Shrinkage compensating concrete is the most efficient and extensively used material in the recent years used to eradicate or minimize cracking which is caused due to drying shrinkage in RCC structures. Drying shrinkage is basically decrease in volume caused by moisture loss in the process of hardening of concrete, which ultimately results in crack formation. • Shrinkage Compensating Concrete can be defined as an expansive concrete or a concrete consisting of an expansive cement or expansive admixture, which induces expansion whilst hardening and consequently offsets the contraction during drying shrinkage. When the expansive concrete is restrained accurately with appropriate reinforcement, it will expand to a value equal or slightly higher than the foreseen amount of drying shrinkage.
- 17. Mechanism of Shrinkage compensating concrete • Shrinkage compensating concrete is designed to work co-jointly with internal restraints, reinforcements. Compressive stresses will be induced in the concrete as well as tension in the steel restraints during time of expansion of the concrete. This results in off-setting of tensile stresses and negative strains induced in the concrete by drying shrinkage. Further, a residual compression will always remain in the concrete, eradicating the risk of shrinkage cracking. • Expansion is achieved by huge amount of ettringite formed, as soon as the type K cement hydrates. Ettringite is nothing but a mineral crystal that forms rapidly when Portland cement and water is mixed. The crystal nature of ettringite takes up a significantly huge amount of volume in concrete. This property of ettringite results in the primary expansion of concrete after setting.
- 18. Types of Expansive Materials • Expansive component is the key ingredient of shrinkage compensating mechanism. These are added to a dosage of 5.0 to upto 15% by cement weight. The mix proportioning is adjusted accordingly when these additive are used. Various types of expansive materials are as follows: • Type K cement: • It is also referred to as Klein cement. It comprises of C4A3S (C=CaO, A=Al2O3, and S=SO3), calcium sulphate (CaSO4) and lime CaO. • Type M cement: • It is constituted by blending mixes of Portland cement, calcium-aluminate cement (CA and C12A7), and calcium sulphate proportioned in accordance with the requirement. • Type S cement: • It is primarily Portland cement consisting of large amount of Tricalcium aluminate (C3A) and gypsum (Calcium Sulphate (CS)). • Admixtures • Expansive admixtures are made of clinker, alumstone, anhydrous gypsum.
- 19. Methods to repair Shrinkage Cracks • If shrinkage cracks have already occurred in the structure, following repair methods can be adopted. These repair methods will prevent further deterioration of the structure or reinforcement due to environmental effects. • Epoxy injection • Routing and sealing depending on width of crack • Drilling and plugging can be adopted where cracks run in straight lines and are accessible at one end • Gravity filling • Dry packing • Polymer impregnation