Concrete Properties What is Concrete Components Aggregates (Sand and Gravel) Water Cement Hydration What are the benefits of Concrete? Strong Durable Waterproof Cast into formed shape Relatively inexpensive Typical Concrete Structures Buildings Bridges Sidewalks Roadways Tunnels Statues Famous Concrete Structures Famous Concrete Structures Design of Concrete Buildings Concrete Components o What are concrete’s major components? Aggregate (60%-75% volume) o o o Cement Paste (23%-33% volume) o o o o Coarse Aggregate (gravel) Fine Aggregate (sand) Cement (Portland Cement) Water (potable water) Admixtures Air (2%-8% volume) Decreasing quantities by weight Concrete Components Aggregates- Where do they come from? Natural – Dredged from a pit, river, lake or seabed Crushed – Quarry Recycled Concrete Concrete Components Aggregates Selection of Aggregates Grading Durability Particle shape and surface texture Abrasion and skid resistance Unit weights and voids Absorption and surface moisture Concrete Components Aggregates - coarse Rounded and smooth Equidimensional Elongated Flaky Angular and rough Rounded and smooth Concrete Components Aggregates - grading Concrete Components Aggregate Distribution Dense Graded Open Graded Concrete Components Aggregate Distribution Dense Graded • • • • • Grain-to-grain contact Low void content High density Easy to compact Used in conventional concrete Open Graded • • • • • Grain-to-grain contact High void content Low, variable density Difficult to compact Used in pervious concrete Concrete Components Portland Cement Types Type I normal Type II general-purpose moderate sulfate resistance Type III High early strength Type IV Low heat of hydration drainage structures Type V Used where soils and groundwater have high sulfate content Severe sulfate resistance quick form removal accelerated curing in cold weather Minimizes rate and amount of heat generated through hydration For massive structures Concrete Components Admixtures Water reducers – workability and strength Plasticizers (low range) – 5%-10% water reduction Superplasticizers (high range) – 20%-30% reduction Air entraining – freeze-thaw resistance Accelerators – accelerates setting Cold weather Quick form removal Retarders – slows setting Hot weather Difficult placement Concrete Components Air Entrapped air macroscopic undesirable vibration for proper consolidation <1% desirable Concrete Components Air Entrained air microscopic desirable freeze-thaw resistance (durability) 2%-8% Freeze-Thaw Damage Press-UR-Meter Concrete Components Pozzolans and Supplementary Cementitious Materials Other cementitious materials that be substituted for Portland cement? Flyash (P) Silica Fume (SCM) Ground Blast Furnace Slag (P) Concrete Components Cost How much does concrete cost? approx. $100 /cu.yd. delivered Depends on supplier, admixtures, delivery time What is the most expensive component? Cement How can aggregate grading minimize cost? Maximize size of coarse aggregate Well graded (dense) mixture of aggregate sizes Concrete Components Typical Concrete Batch Mix (1 yd3) Ingredient* Conventional Pervious 400 – 500 lbs/yd3 500 – 600 lbs/yd3 Up to 50% 100-150 lbs/yd3 Coarse Aggregate 1,800-2,100 lbs/yd3 2,400-2,700 lbs/yd3 Fine Aggregate 1,200-1,400 lbs/yd3 0 – 250 lbs/yd3 Water (w/cm) 30-40 gal. (0.4-0.6) 18-20 gal. (0.27-0.34) Portland Cement SCM Admixtures • • • • • water reducer accelerator retarder air entraining others • • • • • water reducer viscosity modifier hydration stabilizer air entraining internal curing Voids minimal 15% - 35% Fibers optional 1.5 – 5 lbs/yd3 * NOTE: Any mix should always be tested by supplier and installer. Concrete Strength Setting What is the hardening process called? Setting – the transition from plastic to a solid state How long does setting take? Depends on w/c ratio, temperature, etc. Can setting time be controlled? cement type Admixtures When has concrete set? Continuous process Bleed water evaporated Supports worker with < ¼” depression. Concrete Strength Hydration What is hydration? The chemical bonding of Portland cement and water forming a cement paste or gel Develops in 3 stages ○ Setting ○ Hardening ○ Strength Gain How long does hydration take to complete? Most is within 28 days after pouring For life of concrete if sufficient moisture present ( >80% humidity) Decreasing rate Concrete Strength Hydration What are the effects of hydration on concrete? Improves all engineering properties ○ Increase strength, durability ○ Decrease permeability Are there any side effects? Heat – exothermic chemical reaction How can concrete temperature be controlled? Aggregate temperature Water temperature Cement type SCM’s Admixtures Concrete Strength How is concrete strength measured? Crushing stress Cylinder test 28 day compr. strength ( fc’) Casting Concrete Cylinders https://www.youtube.com/watch?v=FS5k uRHlQNw Concrete Strength Cylinder Test 12” 6” Concrete Strength Cylinder Test 140,000 lbs Testing Concrete Cylinders https://www.youtube.com/watch?v=lWVe YTOJBzA Concrete Strength Stress Stress = Force/Area Force Given: P = 140,000# d = 6” Find: Area Reaction Strength of Test Cylinder Concrete Strength Stress Stress = Force/Area Force Given: P = 140,000# d = 6” Find: Area Reaction Strength of Test Cylinder Solution: fc= P/πr2 fc = 140,000#/28.3 in2 fc = 4,950 psi Concrete Strength Strain Strain = Change/Length Length (L) Force Change (∆L) Given: At Cylinder Failure P = 140,000# L = 12” ∆L = 0.036” Find: Average Strain at Failure Concrete Strength Strain Strain = Change/Length Length (L) Force Change (∆L) Given: At Cylinder Failure P = 140,000# L = 12” ∆L = 0.036” Find: Average Strain at Failure Solution: ε= ∆L/L ε = .036”/12” ε = .003 Concrete Strength Strain Strain = Change/Length L=42’ concrete column average ε=.002”/” Force Length (L) Given: Find: Total deformation ∆L Change (∆L) Concrete Strength Strain Strain = Change/Length L=42’ concrete column average ε=.002”/” Force Length (L) Given: Find: Total deformation ∆L Change (∆L) Solution: ε = ∆L/L ∆L = ε x L ∆L = .002 x 42’ x 12”/’ = 1” Concrete Strength Testing What is the sampling frequency for concrete testing on a construction project? – Slump, Air, Cylinders Example Specification: 1 test per 50 yd3 or 1 test per day’s pour 4 (or more) cylinders per test 1 slump test 1 air entrainment test How often are cylinders tested? In project specifications Some combination of 3,7,14,21,28,56 days from casting One spare is kept from each pour Concrete Strength Tension How strong is concrete in tension? Approximately 1/10 compressive strength Given: fc’ = 4950 psi Find: Concrete Tensile Strength Concrete Strength Tension How strong is concrete in tension? Approximately 1/10 compressive strength Given: fc’ = 4950 psi Find: Concrete Tensile Strength Solution: ft= fc’ /10 ft = 495 psi Concrete Strength Elastic Modulus How stiff is concrete? ◦ About 1/10 as stiff as steel ◦ E = 57,000√f’c Given: fc’ = 4950 psi Find: Concrete Stiffness E Concrete Strength Elastic Modulus How stiff is concrete? ◦ About 1/10 as stiff as steel ◦ E = 57,000√f’c Given: fc’ = 4950 psi Find: Concrete Stiffness E Solution: E = 57,000√fc’ E = 57,000√4950 E = 4.01 x 106 psi Water in Concrete effects How is water measured? w/cm – ratio of water to cementitious material by weight in a concrete volume How does w/cm affect workability? greater w/cm ○ greater slump ○ More flowable concrete ○ Easier to place Water in Concrete effects How does increasing w/cm affect engineering properties? Decreases strength Decreases durability Increases permeability Increases bleeding Increases shrinkage w/cm is the single most important determinant of the quality of finished concrete Water in Concrete effects Can there be too little water? Complete hydration w/c = 0.25 Practical minimum w/c = 0.50 Given: 5 bag mixture (94 lbs/bag) per yd3 Find: Minimum water for complete hydration of 1 yd3 concrete Solution: w/c = 0.25 w = 0.25 x c w = 0.25 x 94 lbs/bag x 5 bags w = 117.5 lbs / (8.34 lbs/gal) w = 14.1 gal water Concrete Consistency Slump test Concrete Consistency Slump test slump Concrete Slump Test https://www.youtube.com/watch?v=zLBs pIRPKJI Concrete Consistency Slump test What is a “reasonable” slump? Around 4” slump Questions??