EVALUATION ON APPARENT INSTANTANEOUS STIFFNESS DECREASE CONSIDERING - - PDF document

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JCI-RILEM International Workshop, CONCRACK5, April 24-26, 2017, Japan

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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

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１ INTRODUCTION Thermal cracks affect durability, water tightness, and appearance of concrete structures.

Thermal cracks must be evaluated as accurately as possible.  Thermal analysis：Relatively high accuracy  Stress analysis ：Not well accuracy

3-D FEM analysis can improve accuracy by using data based on actual construction

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JCI-RILEM International Workshop, CONCRACK5, April 24-26, 2017, Japan

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１ INTRODUCTION

• 2.0
• 1.0

0.0 1.0 2.0 3.0 4.0 10 20 30 Stress（N/㎜2） Material age（day）

Measured value Analytic value (Mass Concrete Guidline)

Deviation

Examples of stress analysis results

Reflect measured values of temperature history, strength properties and thermal expansion coefficient.

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

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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.

１ INTRODUCTION Purpose of the study

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JCI-RILEM International Workshop, CONCRACK5, April 24-26, 2017, Japan

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２ MEASUREMENT IN ACTUAL STRUCTURES

Structure-A (Wall Structure) Structure-B (Wall Structure) Structure-C (Columnar Structure) ：Measurement positions

 Crack control joints was arranged at intervals 5.0m

Outline of each structures

20m length 10m length

JCI-RILEM International Workshop, CONCRACK5, April 24-26, 2017, Japan

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２ MEASUREMENT IN ACTUAL STRUCTURES  Temperature, total strain, non-stress strain and effective stress were measured.

Thermocouple Embedded strain meter Embedded strain meter and non-stress container Embedded concrete effective stress meter

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JCI-RILEM International Workshop, CONCRACK5, April 24-26, 2017, Japan

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２ MEASUREMENT IN ACTUAL STRUCTURES

Structure-A (Wall Structure)

10 20 30 40 50 5 10 15 20 25 Temperature（℃） Material age（day）

Ambient temperature Concrete temperature

• 1.5
• 1.0
• 0.5

0.0 0.5 1.0 1.5 5 10 15 20 25 Effective stress（N/㎜2） Material age（day）

 W/C 54.4% ・ C(BB) 315kg/m3 ・ Slump 12cm  Concrete was placed in the end of May(ct:20℃)  Cracking occurred only at crack control joints

JCI-RILEM International Workshop, CONCRACK5, April 24-26, 2017, Japan

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２ MEASUREMENT IN ACTUAL STRUCTURES

Structure-B (Wall Structure)

10 20 30 40 50 5 10 15 20 25 Temperature（℃） Material age（day）

Ambient temperature Concrete temperature

• 1.5
• 1.0
• 0.5

0.0 0.5 1.0 1.5 5 10 15 20 25 Effective stress（N/㎜2） Material age（day）

 W/C 50.0% ・ C(BB) 300kg/m3 ・ Slump 8cm  Concrete was placed in the beginning of May(ct:20℃)  Cracking occurred only at crack control joints

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JCI-RILEM International Workshop, CONCRACK5, April 24-26, 2017, Japan

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２ MEASUREMENT IN ACTUAL STRUCTURES

Structure-C (Columnar Structure)

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10 20 30 40 50 10 20 30 40 50 60 Temperature（℃） Material age（day）

Ambient temperature Concrete temperature

• 1.5
• 1.0
• 0.5

0.0 0.5 1.0 1.5 10 20 30 40 50 60 Effective stress（N/㎜2） Material age（day）

 W/C 47.2% ・ C(L) 339kg/m3 ・ Slump 12cm  Concrete was placed in the end of December(ct:11℃)  Whether cracking has occurred is unknown

JCI-RILEM International Workshop, CONCRACK5, April 24-26, 2017, Japan

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• 1.5
• 1.0
• 0.5

0.0 0.5 1.0

• 400
• 300
• 200
• 100

Effective stress（N/㎜2） Effective strain（μ）

Process of temperature rise Process of temperature drop

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.

Relationship between Young’s modulus and temperature-adjusted age of concrete Calculate as static Young's modulus from compressive strength of specimen and temperature measurement result of actual structure

※Use Equation shown in JCI guideline

Reduction coefficient is calculated dividing effective Young’s modulus by static Young’s modulus

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JCI-RILEM International Workshop, CONCRACK5, April 24-26, 2017, Japan

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３ REDUCTION COEFFICIENTS OF YOUNG’S MODULUS Effective Young’s modulus

Structure-A (Wall Structure) Structure-C (Columnar Structure)

10 20 30 40 10 20 30 40 Young's modulus（kN/㎜2） Temperature adjusted age of concrete （day）

Effective Young's modulus (Mass Concrete Guideline) Effective Young's modulus

10 20 30 40 20 40 60 80 100 Young's modulus（kN/㎜2) Temperature adjusted age of concrete （day）

Effective Young's modulus (Mass Concrete Guideline) Effective Young's modulus

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

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３ REDUCTION COEFFICIENTS OF YOUNG’S MODULUS Reduction coefficient for Young’s modulus

Structure-A (Wall Structure) Structure-C (Columnar Structure)

0.0 0.5 1.0 1.5 2.0 10 20 30 40 Reduction coefficient Temperature adjusted age of concrete （day）

Increase in compressive stress Decrease in compresive stress Increase in tensile stress

0.0 0.5 1.0 1.5 2.0 20 40 60 80 Reduction coefficient Temperature adjusted age of concrete （day）

Increase in compressive stress Decrease in compresive stress Increase in tensile stress

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)

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JCI-RILEM International Workshop, CONCRACK5, April 24-26, 2017, Japan

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３ REDUCTION COEFFICIENTS OF YOUNG’S MODULUS Average of reduction coefficient of Young’s modulus for each stress state

Stress state Structure Mass Concrete Guideline Previous literature A B C average Increase in compressive stress 0.56 0.59 0.48 0.54 0.42 0.63 Decrease in compressive stress 0.67 1.31 0.61 0.86 0.65 1.02 Increase in tensile stress 0.47 0.28 0.34 0.36 0.61

Near the center value of JCI guidelines and previous literature Although the values of the structure A and C are smaller than the previous literature, they tend to be larger than

• ther stress states

It is smaller than the values of JCI guideline and previous literature

JCI-RILEM International Workshop, CONCRACK5, April 24-26, 2017, Japan

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４ EFFECT OF THERMAL STRESS ANALYSIS Analysis model

※ the crack control joints of length 5m

10 20 30 40 50 60 5 10 15 20 Concrete temperature（℃） Material age（day）

Measured value Analytic value

 W/C 53.0% ・ C(N) 330kg/m3 ・ 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.

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JCI-RILEM International Workshop, CONCRACK5, April 24-26, 2017, Japan

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４ EFFECT OF THERMAL STRESS ANALYSIS Comparison of stress analysis

• 2.0

0.0 2.0 4.0 5 10 15 20 Stress（N/㎜2） Material age（day）

Measured value Analytic value(Mass Concrete Guidline) Analytic value(average of structure A～C)

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

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４ EFFECT OF THERMAL STRESS ANALYSIS

1.65 0.97 1.77 0.0 0.5 1.0 1.5 2.0

Measured value Mass Concrete Guidline average of structure A～C

Minimum thermal cracking index

Comparison of minimum thermal cracking index Analysis value(using average of structure A-C) is as same as measured value. Analysis value (JCI guideline) is more safe side evaluation.

※ Dividing estimated tensile strength by measured value of the effective stress

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JCI-RILEM International Workshop, CONCRACK5, April 24-26, 2017, Japan

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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.