Testing strength of hardened concrete and quality control

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Andrew Wheeler
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1 CEEN 3144 Construction Materials Testing strength of hardened concrete and quality control Francisco Aguíñiga Assistant Professor Civil Engineering Program Texas A&M University Kingsville Page 1

2 Factors affecting compressive strength Water/cement ratio Time Maturity concept Cement Aggregate Admixtures Page 2

3 Water/cement ratio Strength of concrete depends on capillary porosity not easily measurable Capillary porosity is proportional to w/c Page 3

4 Water/cement ratio Factors not considered by Abrams law Degree of hydration Air content Aggregate effects Abrams law Page 4

5 Water/cement ratio Page 5

6 Time Compressive strength depends on time and w/c fc at 28 days = f c = days Page 6

7 Maturity concept Maturity = f (t x T) Concretes of same maturity have similar strengths regardless of combination of T and t leading to the maturity Nurse-Saul expression: Maturity ( C x days) = Σ a t (T + 10) Where: a t = time of curing days T = temperature in C 10 C = datum T below which hydration stops Page 7

8 Maturity concept Page 8

9 Maturity concept Limitations Does not take onto account curing moisture Cannot be applied to mass concrete only ambient temperature is considered Not useful at low maturities Not useful if large T variations during curing Strength affected by cement composition and fineness, and by w/c ratio Page 9

10 Cement Chemical composition and fineness Cement variability leads to variability in concrete strength Page 10

11 Aggregate Important parameters: Shape, texture, and maximum size Surface texture affects σ-ε curve but not ultimate strength Page 11

12 Cement and air contents Page 12

13 Admixtures Little effect on strength per se However admixtures alter w/c ratio and porosity Strength gain modified by accelerating or retarding cement hydration Page 13

14 Temperature Page 14

15 Length and diameter of specimen (l/d) Page 15

16 Cylinder size Page 16

17 Concrete age Page 17

18 Strain rate during loading Page 18

19 Concrete state of stress Page 19

20 State of stress Tensile strength Shear strength Impact strength Page 20

21 Shrinkage strain reduces tensile strength Page 21

22 Creep strains Page 22

23 Creep strains Page 23

24 In-place strength of concrete Nondestructive methods Surface hardness Rebound hardness Penetration resistance Pull-out tests Ultrasonic pulse velocity Tests on core samples Structural load test Page 24

25 Nondestructive methods Surface hardness Impact concrete with a standard mass Measure size of indentation Accuracy between 20 and 30% Page 25

26 Nondestructive methods Rebound hardness Schmidt hammer Results affected by Surface finish Moisture content Temperature Member rigidity Surface carbonation Direction of impact Page 26

27 Nondestructive methods rebound hardness Page 27

28 Nondestructive methods Penetration resistance - Windsor probe Harder aggregates yield higher penetration resistance Page 28

29 Nondestructive methods Pull-out tests Measure of concrete shear strength Correlated with compressive strength Page 29

30 Nondestructive methods Pull-out tests Page 30

31 Nondestructive methods Ultrasonic pulse velocity V = (E/ρ)^0.5 Page 31

32 Nondestructive methods Ultrasonic pulse velocity Page 32

33 Nondestructive methods Ultrasonic pulse velocity Pulse velocity increases with moisture Affected by surface smoothness Pulse velocity depends on path length Pulse velocity constant for 5-30 C Steel bars increase pulse velocity For same pulse velocity compressive strength is higher for older specimens Page 33

34 Quality control Factors that contribute to concrete variability Materials Production Testing Page 34

35 Measurement of variability Concrete distribution can be approximated with a normal or Gaussian distribution y 1 exp 2π = 2 s ( ) 2 x µ 2s s = Σ ( x µ ) n 1 2 µ = Σx n V = s µ Page 35

36 Measurement of variability Page 36

37 The normal curve Page 37

38 The normal curve Page 38

39 The normal curve Page 39

40 Area under the normal curve Page 40

41 Quality control If concrete compressive strength has a normal distribution Cannot use µ for design Cannot require all strengths be above design strength Must arbitrarily define acceptable % below design strength Cylinder tests are only estimates of strength Page 41

42 Quality control Concrete variability effects are reduced because Batches are intermixed when placed Tests are based on 28-days, strength with time Steel reinforcement redistributes stresses Page 42

43 ACI 214 approach to variability Required average strength f cr ' f c = 1 tv f = f ' + cr c ts Page 43

44 ACI 214 approach to variability Page 44

45 ACI 318 approach to variability Two requirements The probable frequency of tests more than 500 psi below f c should not exceed 1 in 100 ' f cr = f c s Probable frequency of the average of 3 consecutive tests below f c will not exceed 1 in 100 ' 2.326s ' f cr = f c + = f c s 3 Page 45

46 Choice of design strength ACI - When a strength record exists, increase f c by 400 psi if s < 300 psi 550 psi if 300 psi < s < 400 psi 700 psi if 400 psi < s < 500 psi 900 psi if 500 psi < s < 600 psi 1200 psi if 600 psi < s; or no records available Use previous equations only after sufficient data is available Page 46

47 Number of samples ACI sampling requirements Samples taken at least once a day At least once for every 110 m 3 (150 yd 3 ) At least once for every 450 m 2 (5000 ft 2 ) Apply procedure for each concrete type At least 5 samples for each concrete type Waive sampling if volume < 40 m 3 (50 yd 3 ) Page 47

48 Specifications compliance Page 48

Overview of Topics. Stress-Strain Behavior in Concrete. Elastic Behavior. Non-Linear Inelastic Behavior. Stress Distribution.

Overview of Topics. Stress-Strain Behavior in Concrete. Elastic Behavior. Non-Linear Inelastic Behavior. Stress Distribution. Stress-Strain Behavior in Concrete Overview of Topics EARLY AGE CONCRETE Plastic shrinkage shrinkage strain associated with early moisture loss Thermal shrinkage shrinkage strain associated with cooling