EVALUATION ON APPARENT INSTANTANEOUS STIFFNESS DECREASE CONSIDERING - PDF document

EVALUATION ON APPARENT INSTANTANEOUS STIFFNESS DECREASE CONSIDERING THE EFFECT OF CREEP OF MASS CONCRETE BASED ON MEASUREMENT RESULTS OF ACTUAL STRUCTURES R.Ashizawa, K.Yokozeki and S.Fujioka Kajima Technical Research Institute T.Mizobuchi Hosei University JCI-RILEM International Workshop, CONCRACK5, April 24-26, 2017, Japan 1 １ INTRODUCTION Thermal cracks affect durability, water tightness, and appearance of concrete structures. Thermal cracks must be evaluated as accurately as possible. 3-D FEM analysis can improve accuracy by using data based on actual construction  Thermal analysis ： Relatively high accuracy  Stress analysis ： Not well accuracy JCI-RILEM International Workshop, CONCRACK5, April 24-26, 2017, Japan 2 1

１ INTRODUCTION Examples of stress analysis results 4.0 Reflect measured values of 3.0 Deviation temperature history, strength Stress （ N/ ㎜ 2 ） properties and thermal expansion 2.0 coefficient. 1.0 0.0 Measured value -1.0 Analytic value (Mass Concrete Guidline) -2.0 0 10 20 30 Material age （ day ） Of the various factors, focus on the influence of the apparent instantaneous stiffness decrease with the effect of creep. JCI-RILEM International Workshop, CONCRACK5, April 24-26, 2017, Japan 3 １ INTRODUCTION Purpose of the study  Calculate the reduction coefficient of Young's modulus from measurement result of several actual structures.  Evaluate influence of the reduction coefficient of Young's modulus on thermal stress analysis. JCI-RILEM International Workshop, CONCRACK5, April 24-26, 2017, Japan 4 2

２ MEASUREMENT IN ACTUAL STRUCTURES Outline of each structures Structure-A Structure-B Structure-C (Wall Structure) (Wall Structure) (Columnar Structure) 20m length 10m length  Crack control joints was arranged at intervals 5.0m ： Measurement positions JCI-RILEM International Workshop, CONCRACK5, April 24-26, 2017, Japan 5 ２ MEASUREMENT IN ACTUAL STRUCTURES  Temperature, total strain, non-stress strain and effective stress were measured. Thermocouple Embedded strain meter Embedded strain meter and Embedded concrete non-stress container effective stress meter JCI-RILEM International Workshop, CONCRACK5, April 24-26, 2017, Japan 6 3

２ MEASUREMENT IN ACTUAL STRUCTURES Structure-A (Wall Structure)  W/C 54.4% ・ C(BB) 315kg/m 3 ・ Slump 12cm  Concrete was placed in the end of May(ct:20 ℃ )  Cracking occurred only at crack control joints 50 1.5 Ambient temperature Effective stress （ N/ ㎜ 2 ） Temperature （ ℃ ） Concrete temperature 1.0 40 0.5 30 0.0 20 -0.5 10 -1.0 0 -1.5 0 5 10 15 20 25 0 5 10 15 20 25 Material age （ day ） Material age （ day ） JCI-RILEM International Workshop, CONCRACK5, April 24-26, 2017, Japan 7 ２ MEASUREMENT IN ACTUAL STRUCTURES Structure-B (Wall Structure)  W/C 50.0% ・ C(BB) 300kg/m 3 ・ Slump 8cm  Concrete was placed in the beginning of May(ct:20 ℃ )  Cracking occurred only at crack control joints 50 1.5 Ambient temperature Effective stress （ N/ ㎜ 2 ） Temperature （ ℃ ） Concrete temperature 1.0 40 0.5 30 0.0 20 -0.5 10 -1.0 0 -1.5 0 5 10 15 20 25 0 5 10 15 20 25 Material age （ day ） Material age （ day ） JCI-RILEM International Workshop, CONCRACK5, April 24-26, 2017, Japan 8 4

２ MEASUREMENT IN ACTUAL STRUCTURES Structure-C (Columnar Structure)  W/C 47.2% ・ C(L) 339kg/m 3 ・ Slump 12cm  Concrete was placed in the end of December(ct:11 ℃ )  Whether cracking has occurred is unknown 50 1.5 Ambient temperature Effective stress （ N/ ㎜ 2 ） Concrete temperature Temperature （ ℃ ） 40 1.0 30 0.5 20 0.0 10 -0.5 0 -1.0 -10 -1.5 0 10 20 30 40 50 60 0 10 20 30 40 50 60 Material age （ day ） Material age （ day ） JCI-RILEM International Workshop, CONCRACK5, April 24-26, 2017, Japan 9 ３ REDUCTION COEFFICIENTS OF YOUNG’S MODULUS Calculation method Relationship between effective strain and effective stress ※ Effective strain ＝ total strain － non-stress strain Effective Young’s modulus is obtained by dividing effective stress by effective strain ※ Because plot-by-plot calculation has considerable variation, the slope was calculated for each section that obtain the straight line. 1.0 Relationship between Young’s modulus and temperature-adjusted age of Process of temperature Effective stress （ N/ ㎜ 2 ） drop concrete 0.5 0.0 Calculate as static Young's modulus from compressive strength of -0.5 specimen and temperature measurement result of actual structure ※ Use Equation shown in JCI guideline -1.0 Process of temperature rise -1.5 Reduction coefficient is calculated dividing effective Young’s modulus by -400 -300 -200 -100 0 static Young’s modulus Effective strain （ μ ） JCI-RILEM International Workshop, CONCRACK5, April 24-26, 2017, Japan 10 5

３ REDUCTION COEFFICIENTS OF YOUNG’S MODULUS Effective Young’s modulus Structure-A (Wall Structure) Structure-C (Columnar Structure) 40 40 Young's modulus （ kN/ ㎜ 2 ） Effective Young's modulus Young's modulus （ kN/ ㎜ 2 ) Effective Young's modulus (Mass Concrete Guideline) (Mass Concrete Guideline) 30 30 Effective Young's modulus Effective Young's modulus 20 20 10 10 0 0 0 10 20 30 40 0 20 40 60 80 100 Temperature adjusted age of Temperature adjusted age of concrete （ day ） concrete （ day ） This age is just matched the time when the measured effective stress changed from compressive to tensile. JCI-RILEM International Workshop, CONCRACK5, April 24-26, 2017, Japan 11 ３ REDUCTION COEFFICIENTS OF YOUNG’S MODULUS Reduction coefficient for Young’s modulus Structure-A (Wall Structure) Structure-C (Columnar Structure) 2.0 2.0 Reduction coefficient Increase in Decrease in Increase in compressive Reduction coefficient compressive compresive stress 1.5 1.5 stress stress Decrease in compresive stress Increase in 1.0 Increase in tensile stress 1.0 tensile stress 0.5 0.5 0.0 0.0 0 10 20 30 40 0 20 40 60 80 Temperature adjusted age of Temperature adjusted age of concrete （ day ） concrete （ day ）  It is considered that difference of effects of creep depended on the stress state and material age. Evaluation by average value each of stress state (Increasing compressive stress, decreasing compressive stress, and increasing tensile stress) JCI-RILEM International Workshop, CONCRACK5, April 24-26, 2017, Japan 12 6

３ REDUCTION COEFFICIENTS OF YOUNG’S MODULUS Average of reduction coefficient of Young’s modulus for each stress state Mass Structure Previous Stress state Concrete literature A B C average Guideline Increase in 0.56 0.59 0.48 0.54 0.42 0.63 compressive stress Decrease in 0.67 1.31 0.61 0.86 1.02 compressive stress 0.65 Increase in tensile 0.47 0.28 0.34 0.36 0.61 stress  It is smaller than the values of JCI guideline and previous  Near the center value of JCI guidelines and previous  Although the values of the structure A and C are smaller literature than the previous literature, they tend to be larger than literature other stress states JCI-RILEM International Workshop, CONCRACK5, April 24-26, 2017, Japan 13 ４ EFFECT OF THERMAL STRESS ANALYSIS Analysis model  W/C 53.0% ・ C(N) 330kg/m 3 ・ Slump 12cm  Thermal analysis is reflected measured values of temperature history.  Stress analysis is reflected strength properties and thermal expansion coefficient, in addition to average value of reduction coefficient obtained in this study. 60 Concrete temperature （ ℃ ） Measured value 50 Analytic value 40 30 20 10 0 0 5 10 15 20 Material age （ day ） ※ the crack control joints of length 5m JCI-RILEM International Workshop, CONCRACK5, April 24-26, 2017, Japan 14 7

４ EFFECT OF THERMAL STRESS ANALYSIS Comparison of stress analysis 4.0 Stress （ N/ ㎜ 2 ） 2.0 0.0 Measured value Analytic value(Mass Concrete Guidline) Analytic value(average of structure A ～ C) -2.0 0 5 10 15 20 Material age （ day ） Analysis value(using average of structure A-C) is,  For compressive stress, larger than actually measured value.  For the tensile stress, as same as actually measured value. JCI-RILEM International Workshop, CONCRACK5, April 24-26, 2017, Japan 15 ４ EFFECT OF THERMAL STRESS ANALYSIS Comparison of minimum thermal cracking index 2.0 1.77 Minimum thermal 1.65 cracking index 1.5 0.97 1.0 0.5 0.0 Measured Mass average of value Concrete structure A ～ C Guidline ※ Dividing estimated tensile strength by measured value of the effective stress  Analysis value(using average of structure A-C) is as same as measured value.  Analysis value (JCI guideline) is more safe side evaluation. JCI-RILEM International Workshop, CONCRACK5, April 24-26, 2017, Japan 16 8

５ CONCLUSION  The reduction coefficient of Young's modulus with the effect of creep that obtained in this study may be smaller than the JCI guideline.  It was confirmed that the reduction coefficient of Young's modulus differs depending on the stress state.  Because the result was based on a limited amount of structures data, It is necessary to further accumulate data in the future. JCI-RILEM International Workshop, CONCRACK5, April 24-26, 2017, Japan 17 9