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

Cement

Concrete

Concrete Truck

Cement Truck

Concrete Basic Ingredients (by volume)

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 ( 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

AGGREGATE

Aggregate –General Requirements  Clean  Hard  Strong  Durable

Aggregate – Size Effect  Larger stone provides  Less shrinkage  Lower water demand  Less expense  More difficult to place and consolidate

Aggregate – Shape Effect  Rounded  Lower bond  Lower strength  Lower water demand  Angular  Higher strength  Better Bond  Elongated  Higher water demand  Reduced workability

Aggregate – Gradation Curve From Design and Control of Concrete Mixtures, PCA

Reinforced Concrete

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, 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 revolutions at mixing speed. All subsequent at agitating speed. Overmixing can cause slump loss, irregular setting, slump loss, overheating

Placement Wheelbarrow Buggy Conveyor Bucket Pump …

Pumping

Finishing Spreading Consolidating Strikeoff Floating Troweling

Consolidating

Strikeoff

Floating

Troweling

Field Mixing

It’s Concrete!

Have a Good Time!