1 Class #26 Civil Engineering Materials – CIVE 2110 Concrete Material Concrete Compressive Strength, f’ c Cracking Aging, Maturity Fall 2010 Dr. Gupta Dr. Pickett

2 Cracking & Failure Mechanisms Concrete cracking process; - (MacGregor, 5 th ed., pp ) - In a Cylinder: (MacGregor, 5 th ed., pp ) - Compressive and Tensile strains are constant, - - In a Beam: - Compressive and Tensile strains vary with depth, - load is transferred to concrete having lower strain, - larger mass of concrete at lower strain, slows the growth of micro-cracks, - this reduces Unstable Crack Propagation.

3 Cracking & Failure Mechanisms Strength of concrete in a structure is than that of cylinder because of; Strength of concrete in a structure is lower than that of cylinder because of; - Different strain gradients, previous page, - Different placing, compaction, & curing procedures, - Size and shape effects, - Beams are deeper than cylinders, - Water rises to the top, - More voids at top, - Greater compaction at beam bottom - Drilled cores can be ≈ 0.85 f’ c, - Drilled cores can be ≈ 0.85 f’ c, because coring process relieves some stress.

4 Core Tests & Equivalent In-Place Strength Cores are drilled, capped, then tested in same manner as poured cylinders; (ASTM C42; ACI 318, section ); (ASTM C42; ACI 318, section ); - Water cooled drill-bit produces moisture gradient, - Wet outside surface, dry interior of core sample, - Moisture gradient causes stress gradient, - Reduces apparent test strength of core, - Must test cores between 48 hours and 7 days after drilling, - Moisture gradient dissipates after 48 hours, - Core size; - core diameter ≥ 3(max. size of aggregate) - - core length = (1.0  2.0)diameter of core - Concrete is structurally adequate if; ( ACI 318, Sect ) - Average of 3 cores ≥ (MacGregor advises taking 6 cores) - No single core <

5 Core Tests & Equivalent In-Place Strength In-place strength ≠ (core strength)/0.85 ; MacGregor proposes the following relationships: = equivalent specified strength, used in design calculations, = equivalent specified strength, used in design calculations, = equivalent in-place strength, = equivalent in-place strength, = mean equivalent in-place strength, = mean equivalent in-place strength, = standard deviation for equivalent in-place strength, = standard deviation for equivalent in-place strength, = core test strength, = core test strength,

6 Core Tests & Equivalent In-Place Strength Where: = equivalent in-place strength, = core test strength, = correction for length-to-diameter ratio, = correction for length-to-diameter ratio, = 0.87 for = 1.0 = 0.96 for = 1.50 = 0.87 for = 1.0 = 0.96 for = 1.50 = 0.93 for = 1.25 = 0.98 for = 1.75 = 0.93 for = 1.25 = 0.98 for = 1.75 = 1.0 for = 2.0 = 1.0 for = 2.0 = correction for diameter of core, = correction for diameter of core, = 1.06, for diameter = 2” = 1.00, for diameter = 4” = 1.06, for diameter = 2” = 1.00, for diameter = 4” = 0.98, for diameter = 6” = 0.98, for diameter = 6” = correction for presence of reinforcing bars, = correction for presence of reinforcing bars, = 1.00, for no bars = 1.08, for one bar = 1.00, for no bars = 1.08, for one bar = 1.13, for two bars = 1.13, for two bars

7 Core Tests & Equivalent In-Place Strength Where: = equivalent in-place strength, = core test strength, = accounts for effect of moisture of core at time of test, = accounts for effect of moisture of core at time of test, = 1.09, if core was soaked before test, = 1.09, if core was soaked before test, = 0.96 if core was air-dried at time of test, = 0.96 if core was air-dried at time of test, = accounts for damage to the core surface due to drilling, = accounts for damage to the core surface due to drilling, = 1.06 = 1.06 Factors in: - 1 st parentheses correct strength to that of standard core; ( diameter = 4”, length = 8”, with no rebars ) ( diameter = 4”, length = 8”, with no rebars ) - 2 nd parentheses account for differences between concrete in core vs. concrete in the structure. core vs. concrete in the structure.

8 Core Tests & Equivalent In-Place Strength Where: = equivalent specified strength = mean equivalent in-place strength, k 1 = factor dependent on number of core tests, k 1 = factor dependent on number of core tests, k 1 = 2.40, for 2 tests k 1 = 1.10, for 8 tests k 1 = 2.40, for 2 tests k 1 = 1.10, for 8 tests k 1 = 1.47, for 3 tests k 1 = 1.05, for 16 tests k 1 = 1.47, for 3 tests k 1 = 1.05, for 16 tests k 1 = 1.20, for 5 tests k 1 = 1.03, for 25 tests k 1 = 1.20, for 5 tests k 1 = 1.03, for 25 tests k 2 = factor dependent on number concrete batches in member or k 2 = factor dependent on number concrete batches in member or structure being evaluated, structure being evaluated, k 2 = 0.90, for cast-in-place member or structure, k 2 = 0.90, for cast-in-place member or structure, containing 1 or many batches containing 1 or many batches k 2 = 0.85, for a precast member or structure k 2 = 0.85, for a precast member or structure n = number of cores, n = number of cores,

9 Core Tests & Equivalent In-Place Strength Where: = equivalent specified strength = mean equivalent in-place strength, = coefficient of variation due to length/diameter correction, = coefficient of variation due to length/diameter correction, = 0.025, for = 1.0 = 0.006, for = 1.5 = 0.025, for = 1.0 = 0.006, for = 1.5 = 0, for = 2.0 = 0, for = 2.0 = coefficient of variation due to diameter correction, = coefficient of variation due to diameter correction, = 0.12, for diameter = 2” = 0, for diameter = 4” = 0.12, for diameter = 2” = 0, for diameter = 4” = 0.02, for diameter = 6” = 0.02, for diameter = 6” = coefficient of variation due to presence of reinforcing bars in the cores, = coefficient of variation due to presence of reinforcing bars in the cores, = 0, if none of the cores contain bars, = 0, if none of the cores contain bars, = 0.03, if > a third of the cores contain bars, = 0.03, if > a third of the cores contain bars,

10 Core Tests & Equivalent In-Place Strength Where: = equivalent specified strength = mean equivalent in-place strength, = coefficient of variation due to correction for moisture condition of cores = coefficient of variation due to correction for moisture condition of cores at time of testing, at time of testing, = 0.025, = 0.025, = coefficient of variation due to damage to cores during drilling, = coefficient of variation due to damage to cores during drilling, = 0.025, = 0.025, If a specific correction factor, Then the corresponding coefficient of variation,

11 Factors Affecting f’ c Factors Affecting : (1) Water-Cement ratio; (1) Water-Cement ratio; (2) Type of Cement; (2) Type of Cement; (3) Type of Aggregate; (3) Type of Aggregate; (4) Moisture conditions during Curing; (4) Moisture conditions during Curing; (5) Temperature during Curing; (5) Temperature during Curing; (6) Age of Concrete; (6) Age of Concrete; (7) Maturity of Concrete; (7) Maturity of Concrete; (8) Rate of Loading; (8) Rate of Loading; (1) Water-Cement ratio: A Water-Cement ratio produces; A low Water-Cement ratio produces; Smaller number of voids after water evaporates, Smaller number of voids after water evaporates, Larger number of interlocking solids (aggregate), Larger number of interlocking solids (aggregate), Increased strength. (MacGregor, 5 th ed., pp )

12 Factors Affecting f’ c (2) Type of Cement: Type I, Normal; Type I, Normal; used in ordinary construction. used in ordinary construction. Type II, Modified; Type II, Modified; used for moderate exposure to Sulfates, used for moderate exposure to Sulfates, used to slightly moderate heat of hydration. used to slightly moderate heat of hydration. Type III, High Early; Type III, High Early; compared to Type I; compared to Type I; gives higher strength, earlier, gives higher strength, earlier, gives higher heat of hydration. gives higher heat of hydration. Type IV, Low Heat; Type IV, Low Heat; heat of hydration is dissipated slowly, heat of hydration is dissipated slowly, used in massive structures; dams. used in massive structures; dams. Type V, Sulfate Resisting; Type V, Sulfate Resisting; used in foundations, sewers. used in foundations, sewers. (Fig. 3.5, MacGregor, 5 th ed.)

13 Factors Affecting f’ c (3) Type of Aggregate: Strength of aggregate; Strength of aggregate; strength aggregate gives. High strength aggregate gives high f’ c. If aggregate fails before cement-mortar paste, If aggregate fails before cement-mortar paste, failure occurs. Brittle failure occurs. of aggregate; Size of aggregate; size gives, Larger size gives lower f’ c, higher interface stress between aggregate and cement-mortar paste. higher interface stress between aggregate and cement-mortar paste. of aggregate; Texture of aggregate;, angular pieces give ; Rough, angular pieces give high f’ c ; More interlocking edges. More interlocking edges. of aggregate; Grading of aggregate;, gives less pores,, Well graded, gives less pores, high f’ c, M arbles of would easily roll over each other,. M arbles of equal size would easily roll over each other, low f’ c.

14 Factors Affecting f’ c (4) Moisture Conditions during Curing: ; Prolonged moist curing, gives high f’ c ; Fig. 3-6, MacGregor. Fig. 3-6, MacGregor. (5) Temperature during Curing: ; Colder than 73˚F curing, gives low early, high later, f’ c ; ; Higher than 73˚F curing, gives high early, low later, f’ c ; Fig. 3-7, MacGregor. Fig. 3-7, MacGregor. (Fig. 3.6, MacGregor, 5 th ed.) (Fig. 3.7, MacGregor, 5 th ed.)

15 Factors Affecting f’ c (6) Age of Concrete: ; Older, gives high f’ c ; For Type I cement: For Type I cement: (t = days of moist curing, 70˚F) For Type III cement: For Type III cement: (7) Maturity of Concrete: Strength is a function of ; Time at Temperature ; Use as a guide to determine ; when to remove forms ; (Fig. 3.8, MacGregor, 5 th ed.)

16 Factors Affecting f’ c (8) Rate of Loading: ; Standard rate, gives high f’ c ; ; Very much slower rate, gives 0.75 x (standard test strength) ; ; Earthquake rate, gives 1.15 x (standard test strength) ; 0.10  0.15 seconds load to failure 0.10  0.15 seconds load to failure (MacGregor, 5 th ed., p. 52)