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An Najah National University Faculty of Engineering

An Najah National University Faculty of Engineering. Construction Materials Course. Eng. Mohammed Abu Neamah. Portland Cement Concrete. Portland Cement Concrete.

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An Najah National University Faculty of Engineering

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  1. An Najah National UniversityFaculty of Engineering Construction Materials Course Eng. Mohammed Abu Neamah

  2. Portland Cement Concrete

  3. Portland Cement Concrete • Portland cement concrete is a “concrete”, or artificial rock composed of aggregates, water, and a cementing agent known as Portland cement. • Portland cement is made from(Limestone “Lime CaO”,Silica “SiO2”, Alumina “Al2O3”and Iron oxide “Fe2O3”) which aremixed, Burned in a kiln, and Ground (grind) to fine powder which will harden when mixed with water. • Main component is Lime (60%-75%)

  4. Manufacturing of Portland Cement • Raw materials are ground up. • Mix raw material to produce desired proportion of minerals. • Burning mixture in large kiln where the temperature in the kiln reach about 1500oC (2700oF) to produce new chemical composition called clinker. • The clinker is ground with about 5% gypsum (to control rate of hardening) and is ready for use as Portland cement.

  5. Portland Cement • The cement compounds produced are: • 3CaO.SiO2 , abbreviated C3S • 2CaO.SiO2 , abbreviated C2S • 3CaO.Al2O3 , abbreviated C3A • 4CaO.Al2O3.Fe2O3 abbreviated C4AF • Percentage of these four chemicals in the final product depends on the required properties such as: • Rate of hardening • Amount of heat given off • Resistance to chemical attack

  6. Portland Cement • Portland cement is a binding material with adhesive and cohesive. Use in building and civil engineering construction. These cements will set and hardens when mixed with water, therefore it is called hydraulic cements. • Particles of cement take up water, forming a gel which cements the individual particles together, and this chemical process called hydration which continues for months or years as long as water is present.

  7. Portland Cement • Chemical reaction for one cement compounds C3S is: • 2C3S + 6H2O 3CaO.2SiO2.3H2O + 3Ca(OH)2 • The main product is 3CaO.2SiO2.3H2O is calcium silicate hydrate and give concrete its strength • The total amount of water required to complete hydration of the cement is about 25% of the mass of the cement.

  8. Characteristics of Portland Cement Compounds • C3S: hardness rapidly, responsible for the initial set & early strength. • C2S: hydrates slowly, and is the main source of increased strength after the first week of hardening. low heat of hydration • C3S + C2S: are responsible for the strength of concrete • C3A: reacts very quickly and adds small amount of strength

  9. Characteristics of Portland Cement Compounds • C4AF: react slowly, main purpose toreduce temperature required during burning in the kiln • C3A & C3S: rate of release of heat is greatest, about 50% of total heat of hydration is released in the first three days. • Sulphates may combine with C3A to produce an expanding compound which could result in disintegration of the concrete.

  10. Characteristics of Portland Cement Compounds

  11. Types of Portland Cement • Type I : Normal • Type II: Moderate, moderate resistance to sulfate • Type III: High early strength, in case where early high strength is required. This type is ground finer to allow the water to reach the interior of the cement particles more quickly. • Type IV: Low heat of hydration, is used where a lower rate of heat increase is required as in dams • Type V: Sulfate resisting

  12. Properties of Portland Cement • The most important properties and tests used to check the quality of Portland cement are: • Fineness: govern rate of hydration • Setting: measure the setting time. • Compressive strength • Tensile strength = 10% of compressive strength • Relative density usually 3.15 for Portland cement.

  13. Properties of Portland Cement Concrete • Portland cement concrete is made up of: • Portland cement • Water Paste • Air • Fine aggregate • Coarse aggregate Aggregate • Strength is the main requirement, and concrete design is usually based on this property

  14. Properties of Portland Cement Concrete • Two factors are extremely important for the quality of concrete: • Water / Cement ratio:W/C ratio (w/c = 0.4 - 0.7) • Water is required in the mixture for two purposes: • Hydration • Workability • Whether or not the concrete is air entrained.

  15. Properties of Portland Cement Concrete • Consider mixes A and B with typical proportion of the paste fraction for 1 m3 of concrete : • The strength of concrete is determined to a large degree by the strength of the paste

  16. Properties of Portland Cement Concrete • Air entrained to protect concrete against disintegration resulting from freezing and thawing of water in pores in concrete. • Adding special material called air entraining agents to the mix will assist in forming small bubbles of air, approximately 0.025 mm – 0.075 mm in diameter. These bubbles relieves the pressure caused by freezing of water in pores. • All concrete that will be exposed to freezing and thawing conditions should be air entrained. • To be effective, the entrained air should be distributed uniformly throughout the volume between coarse aggregate particles.

  17. Properties of Portland Cement Concrete • Experience has shown that about 9% air in the mortar fraction (paste plus fine aggregate) provide adequate protection. • Air as a proportion of the whole mix increases as the size of the largest particles in the mix decreases.

  18. Properties of Portland Cement Concrete • The benefit of air entrained concrete: • Increase workability of the mix • Decrease segregation of the aggregate from the paste • Decrease bleeding of the paste to the surface. • But excess air entrained lead to decrease strength of concrete.

  19. Properties of Portland Cement Concrete

  20. Properties of Portland Cement Concrete • Some properties of hardened concrete: • Compressive strength • Tensile strength. • Durability • Permeability • Hardness or abrasion resistance

  21. Compressive Strength • Compressive strength of concrete expressed in Mpa, is usually measured by the load required to break a cylinder that has been cured for 28 days. • It is obtained by dividing total failure load by cross sectional area. • The use of 28 days as the age at which concrete is assumed to have reached its design strength is standard. • The strength at 3,7,and 14 days is about 40%,60%, and 75% respectively of 28 day strength. • See fig. 7-5

  22. Compressive Strength

  23. Tensile Strength • Tensile strength of concrete is very low = 10% of the compressive strength. • Tensile strength is not considered in structural concrete, for reinforcing steel is used to carry tensile stresses.

  24. Flexural Strength (Modulus of Rupture) • Flexure strength or modulus of rupture is often used to evaluate the strength of pavement concrete. • Flexural strength is usually =15% of compressive strength for ordinary concrete. or • Flexural strength = • where : • K= 0.7 (for compressive strength in MPA) • K= 8.4 (for compressive strength in Psi)

  25. Durability • Durability of concrete is its resistance to disintegration due to cycles of freezing and thawing. • The following figure illustrate the effect of air entrainment and W/C ratio on freeze - thaw resistance. • Deicing chemical such as calcium chloride frequently cause break up. These chemicals increase pressure in the concrete, and stronger concrete with a higher air content is recommended. • Concrete exposed to sulphates should be air entrained as well as having a lower W/C ratio. (Type II or Type V cement may be required)

  26. Durability

  27. Durability

  28. Permeability • Permeability or water tightness varies greatly with the W/C ratio. • Concrete with high W/C ratio will have more water and/or air voids will be more permeable

  29. Abrasion resistance • Abrasion resistance varies greatly with concrete strength

  30. Workability • Workability indicates the consistency or plasticity or ease of placing and molding the concrete without segregation. • Bleeding: movement of water to the surface • Segregation: separation of coarse aggregate from the mortar. • Excessive vibration during placement, too much water in the mix, or poor mix design cause bleeding, and/or segregation. • The concrete mix should be designed with the minimum water required for placing • Workability is measured by the slump test.

  31. Harshness • Harshness: indicates the finished quality of the concrete. • Harsh mix will have too much coarse aggregate and it will be hard or impossible to finish smoothly. • Over sanded mix, while it will finish well, it will be uneconomical.

  32. Volume change • Volume change due to: • Temperature • Shrinkage • Creep

  33. Volume change due to temperature • Temperature causes changes in volume of concrete, as it does for steel and other materials. • The amount of change varies with the type of aggregate used, but the average value for coefficient of expansion is 10 µm/m per o C • Change in length = change in temp. x coefficient of expansion x original length • Example: if the temperature increases by 25oC from winter to summer, find the change in length of 20 m slab? • Solution: change in length=20m x 10 µm/m/cx 35oC= 7µm. • = 7 mm

  34. Volume change due to shrinkage • Shrinkage: occurs as the concrete dries out during curing (unless it is kept continually under water). • The amount of this shrinkage varies approximately with the water content of the mixture. • For plain concrete value ranges from 400-800 µm/m when exposed to air at relative humidity of 50% • Reinforced concrete = 200-300 µm/m • About one third of this shrinkage occurs in the first month and about 90% in the first year.

  35. Volume change due to shrinkage • Example: using average value (600 µm/m, find the drying shrinkage for a 30 m concrete slab • Solution: • Shrinkage = 30 m x 600 µm/m = 18000 µm = 18 mm

  36. Volume change due to creep • Creep: change in volume caused by a continuously applied load.

  37. Materials • Concrete composed of: • Portland cement • Water • Fine aggregate • Coarse aggregate • Admixtures. • Supplementary cementing materials (SCM)

  38. Materials • Water • Drinkable, No definite taste or odor • Seawater is suitable for unreinforced concrete. Early strength would be higher with salt water, but lower at 28 days • In reinforce concrete could lead to corrosion of bars • Lower w/c & thick cover recommended when concrete exposed to sea water • Water with other chemicals, industrial wastes, dirt, oil may be acceptable if concentration is not too great. • Tests should be made, if the strength of test cubes made with water is 90% of that obtained with pure water, the water is generally satisfactory.

  39. Materials • Aggregates • Aggregate should be composed of clean, hard, strong and durable particles. Important characteristics: • Gradation • Hardness • Soundness • Chemical stability • Particle shape & texture • Relative density & absorption • Deleterious substances

  40. Materials • Gradation of aggregates is important for: • Workability • Strength • Economy • Minimum amount of fine aggregates passing 300 µm (No. 50) and 150 µm (No. 100), usually 10% & 2% respectively is required to insure smoothness (surface can be finished easily) • Maximum amounts of these fine sand sizes must be specified because cement paste has to be sufficient to cover each particle. • Excessive quantity of fines result in an uneconomical mix.

  41. Materials • Fineness Modulus is measured to ensure that the mix is not too harsh or unworkable, which often results if the gradation is very close to the fine side or the coarse side of the limits on all sieves. • Fineness Modulus is the sum. of accumulative % retained on the 9.5mm, 4.75mm, 2.36mm, 1.18mm,600 µm, 300 µm and 150 µm sieves divided by 100. • Coarse limits: harshness, unworkable mixture. • Useful in choosing proportions in trial mixes and in monitoring the consistency of production of sand from a plant over a period of time.

  42. Materials • Fineness Modulus = 291/100 = 2.91

  43. Materials • Most specifications limit the maximum size of coarse aggregate according to its use. • It is not exceed either 1/5 the width of the forms, ¾ of the space between reinforcing bars, or 1/3 the depth of a slab or pavement. • Various coarse aggregate gradation are allowed, depending on the maximum size of particles that can be used in the structure. • The larger size of aggregate, the more economical the concrete is. This is because there will be less voids space between aggregates particles to be filled with expensive paste concrete. • Max aggregate size must be limited to ensure that the concrete can be placed in the form without leaving voids or honeycombed areas.

  44. Materials • The larger of size aggregates the less need of water, so the more strength of concrete, or the less amount of needed cement.

  45. Admixtures • Admixtures are used to: • Improve the concrete properties (concrete strength) • Aid in construction procedures • Provide economy • Chemical admixtures are usually added in small quantities 1 to 2% of the mass of cement with the mixing water. Although some, such as certain types of air entraining agents, may be in the cement powder. • Mineral admixtures or SCM are added to the mix in significant quantities , 10 to 100% of the quantity of Portland cement, and are usually in the cement powder, or added separately.

  46. Admixtures • Types of admixtures: • Air-entrained agents • Accelerating admixtures • Retarding admixtures • Water reducing admixtures • Workability agents • Damp proofing and permeability reducing agents • Grouting agents • Super plasticizers

  47. Admixtures • Accelerating admixtures: • Used to accelerate the setting and strength development of the concrete. • Used in cold weather concreting. • Calcium chloride CaCl2 is often used as an accelerator. • Accelerators will not be effective if the concrete actually freezes before hydration. • Amount of Calcium chloride used should not exceed 2% of the cement mass because calcium chloride may cause corrosion of reinforcing bars in the concrete.

  48. Admixtures • Retarding admixtures: • Delay the time required for setting and hardening. • Very important for hot weather concreting and in situations where more time is required for finishing operations. • Without retarding, the rate of hydration would proceed very quickly, and workability would rapidly decrease.

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