Presentation on theme: "MASSACHUSETTS SEMINAR SERIES 2010 ENGINEERS WITHOUT BORDERS – NORTHEASTERN UNIVERSITY CHAPTER TIM MCGRATH, PH.D., P.E. Turning Cement into Concrete."— Presentation transcript:
MASSACHUSETTS SEMINAR SERIES 2010 ENGINEERS WITHOUT BORDERS – NORTHEASTERN UNIVERSITY CHAPTER TIM MCGRATH, PH.D., P.E. Turning Cement into Concrete
Outline Speaker and Audience What is Concrete? Components Reinforced Concrete Mixing and Placing Field Mixing
Who am I? Senior Principal at Simpson Gumpertz & Heger, Waltham Degrees in Civil (NEU), Structures (MIT) and Geotechnical (UMass, Amherst) Primary work area is buried structures (pipes, culverts, and the soil around them) Mentor for EWB-NEU Uganda team since July ‘09
Who are you? Undergrads? Professionals? Engineers (Civil, mechanical, etc)? Architects? Others?
What is Concrete? Cement, sand, aggregate, water, additives Strong in compression Weak in tension Readily formed into many shapes
The History of Cement Ancient Egyptians used calcinated gypsum Greeks and Romans used lime mortars Early “Portland Cement” patented in 1824 True hydraulic portland cement manufactured in 1845
Cement - Process Fired at 2,550°F in rotary kiln Cooled and ground into an extremely fine powder with addition of gypsum
Cement – Finished Product Extremely fine powder (500,000,000,000 particles per pound) Huge surface area (1700-1850 ft 2 /lb) Bulk density of approximately 94 lb/ft 3 “Frozen” solution of many materials
Cement – Chemical Components C 3 S – Tricalcium silicate – 3CaOSiO 2 C 2 S – Dicalcium silicate – 2CaOSiO 2 C 3 A– Tricalcium aluminate – 3CaOAl 2 O 3 C 4 AF – Tetracalcium aluminoferrite – 4CaOAl 2 O 3 Fe 2 O 3 Gypsum – CaSO 4 2H 2 O
5 Types of Cement I – Standard II – Moderate Sulfate Resistance, Moderate Heat of Hydration III – High Early Strength IV – Low Heat of Hydration V – High Sulfate Resistance
Cement – Effect on Heat From Design and Control of Concrete Mixtures, PCA
Cement – Effect on Strength From Design and Control of Concrete Mixtures, PCA
Batch Design fine and coarse aggregate cement content water/cement ratio flow characteristics
Concrete Additives Air (freeze-thaw protection) Water reducers (strength, flow) Retarders (delay set) Fly ash (cost, flow, strength) ….
WATER Required for chemical hydration reaction Required for workability
Water – Effect of Poor Quality Setting time Strength Efflorescence Durability Staining Corrosion
Water – Permitted Sources Potable is generally OK Non-Potable can be OK if: pH is 6-8 Not saline or brackish No humic acid or algae
Water – Controlling Limits Chloride: < 500 ppm is “harmless” Alkali: < 600 ppm Turbidity: < 2000 ppm clay, fines Sulfates: < 1000 ppm “harmless”, 3000 limit Organics: no odor, no color
Reinforcement in Concrete Provides tensile strength for flexure Increases total compression strength for columns As transverse steel, increases shear capacity Controls cracking due to shrinkage and temperature
Concrete Design Strength f ’ c = strength used in calculations Typically taken as 28 day strength Often used as the basis for estimating: modulus of elasticity tensile strength shear strength
Concrete vs Steel ParameterSteelConcrete Compression strength60,0005,000 Tensile strength60,000500 Modulus of elasticity29,000,0004,500,000 All values in psi
Concrete Stress Strain Curves at Various Ages Concrete Strength and Stiffness
Reinforced Concrete Typical Reinforced Concrete Beam Span d
Mixing – Transit Mixed Used for longer haul times. Mixing efficiency, duration, and consistency depend on equipment and operator.
Transit-Mixing 70-100 revolutions at mixing speed. All subsequent at agitating speed. Overmixing can cause slump loss, irregular setting, slump loss, overheating
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