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Introduction of concrete

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the introduction of concrete and a overview

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Introduction of concrete

  1. 1. Concrete and its mix
  2. 2. constituentsconstituents mixture of aggregate and paste paste 30 to 40% portland cement 7% to 15% by Vol. water 14% to 21% by Vol. Aggregates 60% to 70% coarse aggregates Fine aggregates Admixtures
  3. 3. Portland CementPortland Cement Dry powder of very fine particles forms a paste when mixed with water chemical reaction-Hydration paste coats all the aggregates together hardens and forms a solid mass
  4. 4. WaterWater needed for two purposes: chemical reaction with cement workability only 1/3 of the water is needed for chemical reaction extra water remains in pores and holes results in porosity Good for preventing plastic shrinkage cracking and workability Bad for permeability, strength, durability.
  5. 5. AggregatesAggregates cheap fillers hard material provide for volume stability reduce volume changes provide abrasion resistance
  6. 6. AdmixturesAdmixtures chemical set retarders set accelerators water reducing air entraining mineral fly ash silica fume slags
  7. 7. Properties of fresh concreteProperties of fresh concrete Workability ease of placement resistance to segregation homogeneous mass Consistency ability to flow
  8. 8. Slump TestSlump Test Inverted cone fill it up with three layers of equal volume rod each layer 25 times scrape off the surface 8” 4” 12”
  9. 9. Slump TestSlump Test slump cone rod concrete
  10. 10. Slump testSlump test Slump Ruler
  11. 11. Slump test resultsSlump test results stiff 0-2” massive sections, little reinforcement use vibration medium 2-5” columns, beams, retaining walls Fluid 5-7” heavily reinforced section, flowable concrete
  12. 12. Factors affecting slumpFactors affecting slump water cement ratio w/c = weight of water / weight of cement example: weight of water mixed at the plant 292 lbs. weight of cement 685 lbs./cu. yard w/c = 292/685 = 0.43
  13. 13. water cement ratiowater cement ratio if you add 10 gallons of water per cubic yard at job site, then: extra water 10 gallons/cubic yard * (3.8 liters/gallon) * (2.2 lbs./kg) *( 1kg/liter) = 83.77 lbs. total water 282 + 83.77 = 365.77 new w/c = 365.77 / 685 = 0.534 >> 0.43
  14. 14. Factors affecting slump-Factors affecting slump- paste contentpaste content constant water cement ratio increase paste content increase slump NO GOOD constant cement content increase water content increase slump NO GOOD
  15. 15. Factors Affecting Slump-Factors Affecting Slump- Water ContentWater Content Add water at the constant cement content, w/c increases, slump increases. Add water at a constant water cement ratio, have to increase cement as well, slump increases.
  16. 16. Factors affecting slump-pasteFactors affecting slump-paste contentcontent Low paste content Harsh mix High paste content Rich mix
  17. 17. ball bearing effect-startball bearing effect-start starting height
  18. 18. ball bearing effect-endball bearing effect-end slump
  19. 19. AdmixturesAdmixtures set retarding admixtures set accelerating admixtures water reducing admixtures superplasticizers air entraining admixtures
  20. 20. Factors affecting slumpFactors affecting slump Aggregates grading the larger the particle size, the higher the slump for a given paste content
  21. 21. effect of aggregate sizeeffect of aggregate size 1” 1” 1” Consider a single aggregate the size of 1”x1”x1”
  22. 22. Compute the surface area asCompute the surface area as you break up the particlesyou break up the particles volume = 1 cubic in surface area = 6 square inches volume = 1 cubic in surface area = 1.5*8= 12 square inches block surface area = 0.5*0.5*6=1.5 block surface area = 1*1*6= 6
  23. 23. Break it up furtherBreak it up further
  24. 24. Compute the surface areaCompute the surface area 0.5 in 0.25 in surface area = 0.25*0.25*6*8*8=24
  25. 25. Larger particles, less surface area,Larger particles, less surface area, thicker coating, easy sliding of particlesthicker coating, easy sliding of particles
  26. 26. Smaller particles, more surface area,Smaller particles, more surface area, thinner coating, interlocking of particlesthinner coating, interlocking of particles
  27. 27. Effect of aggregate sizeEffect of aggregate size size # of particles volume surface area 1" 1 1 cubic inch 6 square inches .5" 8 1 cubic inch 12 square inches 0.25 64 1 cubic inch 24 square inches 0.125 512 1 cubic inch 48 square inches
  28. 28. Angularity and surface textureAngularity and surface texture of aggregatesof aggregates angular and rough aggregate smooth aggregate river gravel
  29. 29. TemperatureTemperature fresh concrete aggregates paste
  30. 30. BleedingBleeding
  31. 31. Water accumulation on surfaceWater accumulation on surface Examine the concrete surface
  32. 32. Interaction between bleeding and evaporationInteraction between bleeding and evaporation surface water Evaporation Bleed water Bleed water = evaporation
  33. 33. Too much evaporation leads to surface crackingToo much evaporation leads to surface cracking no surface water Evaporation Bleed water < Evaporation drying
  34. 34. Side diagram of surface contractionSide diagram of surface contraction Wants to shrink Does not want to shrink
  35. 35. Free Shrinkage,Free Shrinkage, causes volume change, but no stressescauses volume change, but no stresses before shrinkage After Shrinkage
  36. 36. Restrained Shrinkage- createsRestrained Shrinkage- creates stresses, which may cause crackingstresses, which may cause cracking
  37. 37. Restrained shrinkage crackingRestrained shrinkage cracking Parallel cracking perpendicular to the direction of shrinkage
  38. 38. Bleeding and its controlBleeding and its control Creates problems: poor pumpability delays in finishing high w/c at the top poor bond between two layers causes lack of fines too much water content Remedies more fines adjust grading entrained air reduce water content
  39. 39. Causes of Plastic Shrinkage CrackingCauses of Plastic Shrinkage Cracking water evaporates faster than it can reach the top surface drying while plastic cracking
  40. 40. Plastic Shrinkage Cracking-Plastic Shrinkage Cracking- RemediesRemedies Control the wind velocity reduce the concrete’s temperature use ice as mixing water increase the humidity at the surface fogging cover w/polyethylene curing compound Fiber reinforcement
  41. 41. CuringCuring The time needed for the chemical reaction of portland cement with water. Glue is being made. concrete after 14 days of curing has completed only 40% of its potential. 70 % at 28 days.
  42. 42. Curing tipsCuring tips ample water do not let it dry dry concrete = dead concrete, all reactions stop can not revitalize concrete after it dries keep temperature at a moderate level concrete with flyash requires longer curing
  43. 43. Temperature effects on curingTemperature effects on curing The higher the temperature the faster the curing best temperature is room temperature strongest concrete is made at temperature around 40 F.(not practical) If concrete freezes during the first 24 hrs., it may never be able to attain its original properties.
  44. 44. Temperature effects on curingTemperature effects on curing real high temperatures above 120 F can cause serious damage since cement may set too fast. accelerated curing procedures produce strong concrete, but durability might suffer. autoclave curing.

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