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- 1. Construction Materials CE1100 Chapter 6:Early age properties of concrete Meenakshi Department of Civil Engineering Meenakshi@ce.iith.ac.in Office no – 303, Block B 1 Students B. Tech 2nd Semester
- 2. 2 Concrete • Chapter 1:introduction to concrete • Chapter 2: cement • Chapter 3: aggregates • Chapter 4: admixtures or additives • Chapter 5: Concrete mix design • coming chapters - Properties of concrete • Fresh concrete • Hardened concrete
- 3. 3 Concrete • Chapter 1:introduction to concrete • Chapter 2: cement • Chapter 3: aggregates • Chapter 4: admixtures or additives • Chapter 5: Concrete mix design • coming chapters - Properties of concrete • Fresh concrete • Hardened concrete
- 4. 4 Concrete at earlyages • Requirements- strength and durability • Steps - selection of proper materials and mix proportions • Is it enough?
- 5. 5 Concrete at earlyages • Importance to operations at early age • Early age – insignificant amount of time – 2 to 3 days mostly • numerous operations are performed • Batching, mixing, transportation, placing, compaction, finishing and removal of form work • Structural adequacy • Characteristics of fresh concrete: workability, setting time, and maturity or rate of strength gain • Early-age operations
- 6. 6 Concrete • Workability • Segregation •Bleeding • Setting • Early age cracking
- 7. 7 Workability • ASTM C125 •Workability is property determining the effort required to manipulate a freshly mixed quantity of concrete with minimum loss of homogeneity. • Manipulate – placing, compacting, and finishing • effort required to manipulate • minimum loss of homogeneity • Workability is a composite property • Consistency - the ease of flow • Cohesiveness – stability or the tendency to bleed (opposite of water-holding capacity) or segregate (opposite of coarse-aggregate-holding capacity) https://civilquery.com
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- 9. 9 Workability • Is notabsolute, it depends on • Type of construction • Massive construction without reinforcement • Congested reinforced members • Method of placement • Pumps or chutes • Method of compaction • Vibrators or hand tamping • Method of finishing
- 10. 10 Concrete production process BatchingMixing Transport Placing Finishing Concrete Curing Compaction Formwork removal
- 11. 11 Concrete production process- workability Batching Mixing Transport Placing Finishing Concrete Curing Compaction Formwork removal
- 12. 12 Workability: measurements • Nosingle method for testing workability • the composite nature of the property • Consistency • Slump test • Vee bee test • Flow table test • Tests of compaction • Compaction factor test • Test for cohesiveness • Bleeding test • Manual inspection and experience is necessary Compaction factor test Bleeding test
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- 14. 14 Cohesiveness and consistency:Slump flow test
- 15. 15 Cohesiveness: Slump flowtest https://techconsults.in/
- 16. 16 Segregation • Segregation –as separation of the components of fresh concrete so that they are no longer uniformly distributed. • kinds of segregation • Larger and heavier particles tend to settle due to difference in densities • Water tends to rise – bleeding • Water and fine particles (cement, sand, etc.) tends to rise – laitance • Effects of segregation • Decrease in homogeneity • Honey combing in concrete • Reduction in strength • Deterioration and reduction in durability https://dailycivil.com
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- 18. 18 Causes of segregationin concrete • Excessive water content in the mix. • Use of high water-cement ratio • Use of poor graded aggregates • Low amount of fines in concrete • Improper design of the mix • Improper placement practices • Poor compaction of concrete • Over vibration of concrete
- 19. 19 Bleeding • Appearance ofwater on the surface after a concrete has been placed and compacted but before it has set • Solids in suspension tend to move downward under the force of gravity • The inability of the constituent materials to hold all the mixing water in a dispersed state as the relatively heavy solids settle • Effects of bleeding • Weak and non-uniform top surface • Channeled bleeding results in porous concrete • Laitance reduces cement content in concrete
- 20. 20 Bleeding • Control ofbleeding • Increase in fineness of cement and fine particles • Reducing water content • Increase in C3A, CaCl2 and alkali content • Use of pozzolana • Air entrainment
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- 22. 22 Factors affecting workability •Consistency & cohesiveness - oppositely influenced by changing a particular variable. • Water content • High water content – more consistency – may segregate and bleed • Cement content • Very low cement content leads to harsh mixes • very high proportion of cement or a very fine cement – excellent cohesiveness but sticky • Aggregate characteristics • Fine sand and angular aggregates reduce workability. Poor grading leads to segregation • Admixtures • Entrained air increases the paste volume and improves the consistency • Pozzolanic admixtures
- 23. 23 Slump loss • Lossof consistency in fresh concrete with elapsed time – normal phenomenon • Slump loss is negligible first 30 min • Unusually large loss of slump due to • Absorption in aggregates • Evaporation of water • Abnormal setting cements • Factors affecting slump loss • Temperature of the raw material • Time • Ambient temperature, sun, wind, etc. • Cement reactivity
- 24. 24 Setting • Reactions betweencement and water – primary cause of setting of concrete • Setting of cement doesn’t coincide with setting of concrete • Stiffening, setting, and hardening are physical manifestations of the progressive hydration reaction of cement • setting of concrete – the onset of solidification in a fresh concrete mixture • The initial set • the time at which fresh concrete can no longer be properly mixed, placed, and compacted • Point at which the stiffening starts • The final set • the time after which strength begins to develop at a significant rate • Point at which the structure is fully rigid
- 25. 25 Setting • Cement • Vicatapparatus • w/c - 0.85P (P – standard consistency) • Concrete • Concrete penetrometer • Mortar is extracted from concrete • force required to cause a needle to penetrate 25 mm into the mortar.
- 26. 26 Factor affecting settingtime • Cement composition • quick setting, false setting, or flash setting • water/cement ratio • filling of void space • Temperature • Temperature influences rate of hydration • Admixtures • Accelerate or retard
- 27. 27 Volume changes inconcrete • Concrete changes slightly in volume • Volume change – an increase or decrease in volume due to temperature and moisture cycles, chemical factors, etc. • Volume changes may or may not lead to cracking • Volume changes • Early age and later ages • Cracking • Restraint leads to cracking • Cracking • Concrete is weak in tension • Strength is evolving • Cracking will occur at any moment of time • If shrinkage stress > strength
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- 29. 29 Early age crackingin concrete • Cement paste is not stable – it can shrink and expand and can lead to cracking • Early age – cracking is easily possible due to lower strength • Reasons • Rapid evaporation of bleed water before setting • Chemical shrinkage • High thermal gradients • Autogenous shrinkage • Drying of concrete • Early removal of formwork • Improper compaction
- 30. 30 Early age cracking •Reduction in volume – plastic shrinkage • Vertical and horizontal deformation • Plastic settlement cracks • Top surface of concrete – subsides after a few hours • immediately after casting when it is in its plastic phase until the initial setting time of the concrete • Over obstructions such as large aggregates and reinforcement bar
- 31. 31 Early age cracking •Plastic shrinkage cracks • Rate of evaporation > rate of bleed water • Water from the surface concrete evaporates • concrete surface is exposed to drying • a complex series of menisci are formed inside the capillary pores • Capillary stresses develops due to evaporation of water • tensile forces acting on the solid particles lead to a reduction in the volume of the concrete known as a plastic shrinkage • Early age, tensile strength is extremely low
- 32. 32 Prevention of plasticshrinkage • Minimize evaporation of water • Fogging • Wind breaks • Shading • Plastic sheet covers • Wet burlap • Spray on finishing aids • Moisten the subgrade and form • Moisten aggregates that are dry and absorptive • Keep the fresh concrete temperature low by cooling the aggregate and mixing water.
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- 35. 35 Chemical and autogenousshrinkage • Chemical shrinkage • Volume change during the hydration resulted by formation of hydration products with lower volume in comparison with the volume of the initial reactants (water and cement) during the hardening process • Driving force for autogenous shrinkage • autogenous shrinkage • Macroscopic volume reduction of cementitious materials when cement hydrates after initial setting. • It does not include volume change due to loss or ingress of substances, temperature variation, and application of an external force and restraint.
- 36. 36 Autogenous shrinkage • Concretewith low w/c ratio • Concrete with high binder content • Concrete with silica fume • Concrete with finely ground slag • High strength concrete is more susceptible to autogenous shrinkage
- 37. 37 Drying shrinkage • Concreteshrinks on the removal of water • Also happens in mature concrete • Drying shrinkage is due to loss of water from fine capillaries • causes an increase in tensile stress, which may lead to cracking
- 38. 38 Thermal shrinkage • solidsexpand on heating and contract on cooling. • The strain associated with change in temperature will depend on the • coefficient of thermal expansion and the magnitude of temperature drop or rise. • In massive structures • High heat is produced by cement hydration • Relatively poor heat dissipation conditions due to large structures • results in a large rise in concrete temperature within a few days after placement • Subsequently, cooling to the ambient temperature often causes the concrete to crack
- 39. 39 Summary • Workability • Segregation •Bleeding • Setting • Early age cracking