Concrete

1. Concrete

2. One Definition of Portland Cement Concrete… • Portland cement concrete (PCC) is a heterogeneous system of solid, discrete, gradiently sized, inorganic mineral aggregates, usually plutonic or sedimentary-calcareous in origin, embedded in a matrix compounded of synthesized polybasic alkaline and alkaloidal silicates held in aqueous solution and co-precipitate dispersion with other amphoteric oxides, this matrix being originally capable of progressive dissolution, hydration, re-precipitation, gelation and solidification through a continuous and co-existent series of crystalline, amorphous, colloidal and cryptocrystalline states and ultimately subject to thermo-allotriomorphic alteration, the system when first conjoined being plastic during which stage it is impressed to a predetermined form into which it finally consolidates, thus providing a structure relatively impermeable and with useful capacity to transmit tensile, compressive, and shear stresses. • (source unknown)

3. A Real Definition of PCC… • A mixture of: • Portland Cement • Fine Aggregate • Coarse Aggregate • Water • Air • Cement and water combine, changing from a moist, plastic consistency to a strong, durable rock-like construction material by means of a chemical reaction called “hydration”

4. Further Defined… • Concrete exists in three states • Plastic • Curing • Hardened

5. Mix Design • Combination of materials to provide the most economical mixture to meet the performance characteristics suitable for the application • Developed in laboratory - produced in a batch plant • Mix proportions will typically vary over a range for a given job • Required strength and exposure conditions • Mix consistency must be ensured to guarantee concrete performance

6. Mixture Design Concepts • Cement content • Sacks/yd3 or lbs/yd3 • To a point, increasing cement content increases strength and durability • Too much cement is uneconomical and potentially detrimental • Amount of water • Proper selection of aggregate and grading • Admixtures?

8. Water-to-Cement Ratio • The ratio of water-to-cement, or w/c, is the single most important parameter with regards to concrete quality • Theoretically, about 0.22 to 0.25 is required for complete hydration • Practically, the useful limit is around 0.33 • Reducing the water for a given amount of cement will move the cement particles closer together, which in turn densifies the hydrated cement paste • This increases strength and reduces permeability • It also makes the concrete more difficult to work • In combination, the w/c and degree of hydration control many of the properties of the hardened concrete

10. Voids in Hydrated Cement • Concrete strength, durability, and volume stability is greatly influenced by voids in the hydrated cement paste • Two types of voids are formed in hydrated cement paste • Gel pores • Capillary pores • Concrete also commonly contains entrained air and entrapped air

11. Voids in Hydrated Cement Paste • Gel Pores • Space between layers in C-S-H with thickness between 0.5 and 2.5 nm • Includes interlayer spaces, micropores, and small isolated capillary pores • Can contribute 28% of paste porosity • Little impact on strength and permeability • Can influence shrinkage and creep

12. Voids in Hydrated Cement Paste • Capillary Voids • Depend on initial separation of cement particles, which is controlled by the w/c • It is estimated that 1 cm3 of anhydrous portland cement requires 2 cm3 of space to accommodate the hydration products • Space not taken up by cement or hydration products is capillary porosity • On the order of 10 to 50 nm, although larger for higher w/c (3 to 5 mm) • Larger voids affect strength and permeability, whereas smaller voids impact shrinkage

13. w/c = 0.5 Source: Mindess, Young, and Darwin, 2004

14. Source: Mindess, Young, and Darwin, 2004

15. Source: Mindess, Young, and Darwin, 2004

17. High Permeability(Capillary Pores Interconnected) Capillary Pores C-S-H Framework Neville

18. Low-Permeability Capillary Pores Segmented and Only Partially Connected Capillary Pores C-S-H Framework

21. Dimensional Range of Solids and Voids in Hydrated Cement Paste Source: Mehta and Monteiro, 1993

22. Source: Mindess, Young, and Darwin, 2004

23. Source: Mindess, Young, and Darwin, 2004

24. Source: Mindess, Young, and Darwin, 2004

25. Interfacial Transition Zone • Zone between the aggregate and bulk paste • Has a major impact on the strength and permeability of the concrete • The interfacial zone is 10 to 50 mm in thickness • Generally weaker than either the paste or aggregate due to locally high w/c and the “wall effect” (packing problems) – in some cases predominately large crystals of calcium hydroxide and ettringite are oriented perpendicular to aggregate surface • Greater porosity and few unhydrated cement grains • Microcracking commonly exists in transition zone • Results in shear-bond failure and interconnected macroporosity, which influences permeability • Modification of transition zone is key to improving concrete

28. Entrained Air • Provides the path for water to migrate from larger voids to smaller voids • Water in smallest capillary/gel pores won’t freeze • For adequate protection • 6-8% air by volume • Entrained air spacing factor = 0.2mm

29. Entrained Air Measurement • Proper air entrainment is one of the most critical aspects of producing durable concrete • Air entrainment affects • Strength • Freeze-Thaw durability • Permeability • Scaling Resistance • Workability • Air content must be measured accurately at the job site

30. Air-Void System ASTM C 231 and C 173 Stereo Microscope ASTM C 457

31. Curing Concrete • Extremely important • Concrete will not achieve its potential strength unless it is properly cured • Concrete will crack if not properly cured • Curing should be started immediately after final set • Curing includes providing both moisture and temperature

33. Curing • Concrete must not dry out, especially at a young age • Preferably water is applied after the concrete has set • Steam curing applies both heat and moisture, accelerating hydration • Often, waterproof barriers are used to hold mix water in…not as good as wet curing

39. Durability • Concrete is inherently durable, having a history of exceptional long-term performance • In some instances, the structure’s service life has been adversely affected by the concrete’s inability to maintain its integrity in the environment in which it was placed • These distress manifestations are categorized as materials-related distress (MRD)

40. What is Materials-Related Distress? • MRD is commonly associated with the “durability” of the concrete • Durability is not an intrinsic material property • “Durability” cannot be measured • Concrete that is durable in one application may rapidly deteriorate if placed in another application • It is not related to loading, although loading can exacerbate the distress

41. Common Types of MRD • Physical Mechanisms • Freeze-thaw Deterioration of Hardened Cement Paste • Deicer Scaling/Deterioration • Freeze-Thaw Deterioration of Aggregate • Chemical Mechanisms • Alkali-Aggregate Reactivity • Alkali-Silica and Alkali-Carbonate Reactivity • Sulfate Attack • External and Internal Sulfate Attack • Corrosion of Embedded Steel

45. Important Considerations • The concrete constituents, proportions, and construction all influence MRD • Water is needed for deleterious expansion to occur • Severe environments (e.g. freezing and thawing, deicer applications, high sulfate soils, etc.) make it worse • Strength does not equal durability

46. Summary • Concrete is an immensely complex material that will perform to its potential only if treated properly during the entire construction phase • Mix design and proportioning • Transporting • Placing and consolidating • Finishing and curing • As billions are spent annually on concrete construction, the most sophisticated testing is used to ensure quality

47. ASTM C 143-00 Standard Test Method for Slump of Hydraulic Cement Concrete