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The Effect of Autogenous Shrinkage on Flexural Cracking Behavior of Reinforced HSC Beams and Improvement by Using Low-s

1. The Effect of Autogenous Shrinkage on Flexural Cracking Behavior of Reinforced HSC Beams and Improvement by Using Low-shrinkage HSC. Masahiro SUZUKI P.S. Mitsubishi Construction Co., Ltd., Japan Makoto TANIMURA Taiheiyo Cement Corporation, Japan Ryoichi SATO Hiroshima University, Japan.

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The Effect of Autogenous Shrinkage on Flexural Cracking Behavior of Reinforced HSC Beams and Improvement by Using Low-s

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  1. 1 The Effect of Autogenous Shrinkage on Flexural Cracking Behavior of Reinforced HSC Beams and Improvement by Using Low-shrinkage HSC Masahiro SUZUKI P.S. Mitsubishi Construction Co., Ltd., Japan Makoto TANIMURA Taiheiyo Cement Corporation, Japan Ryoichi SATO Hiroshima University, Japan Fourth International Seminar on Self-desiccation and Its Importance in Concrete Technology 20/June/05, Gaithersburg, Maryland, USA

  2. 2 Background • Autogenous shrinkage (AS) properties of HSC using various materials • Comprehensive approach for reducing AS ----- well studied!! However, few investigations on the effect of AS on structural performance. Only a few reports on evaluation method for cracking behavior considering AS Objectives • Evaluation of AS effect on cracking performance of RC flexural beams, effectiveness of low shrinkage-HSC on the improvement of cracking performance • Calculation method for crack width of RC beam considering shrinkage/expansion effect before loading

  3. 3 (Unit; mm) • Dimension; 200x250x2700 mm • Two-point loading; span:2100mm, pure flexural zone:800mm RC beam specimen • Sealed curing period;30-50 days • Crack width • Steel strain • Evaluations • Before loading Steel strainRestrained stress on bottom fiber • Under short-term loading - Crack width; Contact-type strain gauge

  4. 4 Mixture investigated • Reference HSC • Low-shrinakge HSCs • W/(C+EX); 0.3 • EX content; 40 kg/m3 • SRA content; 6 kg/m3 • Targeted concrete strength; 70 N/mm2 at 28 days

  5. 5 Equation for calculating restrained stress on extreme bottom fiber • Equilibrium of the force between concrete and rebar • Navier’s assumption

  6. Restrained-shrinkage/expansion stress 6 1.5 N/mm2 Tension Compression －1.5 N/mm2 • Low-shrinkage HSCs are obviously effective in reducing AS-restrained stress.

  7. 7 Flexural cracking moment • AS-restrained stress affects cracking load significantly. • LS-HSC markedly improve cracking load.

  8. 8 Maximum crack width/crack spacing • LS-HSCs improve cracking performance significantly. --Decrease in crack width, while increase in crack spacing!!

  9. 9 General evaluation method for crack width considering shrinkage/expansion effect before loading • Diferrenec in elastic strain component between steel and concrete; affect crack width This concept incorporated into JSCE CODE-2002

  10. 10 JSCE Code Equation for maximum crack width

  11. 11 Verification of proposed concept for evaluating maximum crack width (1) JSCE CODE-1996 JSCE CODE-2002 • Improved accuracy Reinforcement stress (N/mm2) Reinforcement stress change(N/mm2) Significant effect of AS on crack width; Reference HSC has 250x10-6 of steel stress at zero stress state; this means the influence of 25% for the steel stress of 200MPa(=1000x10-6)

  12. 12 Verification of proposed concept for evaluating maximum crack width (2) Range of conventional theory Range of Proposed concept Conventional theory Proposed concept Measured • Conventional RC theory cannot evaluate the tendency of crack width of RC beams with AS and expansion. • Proposed concept has acceptable accuracy for evaluating crack width.

  13. Conclusions 13 • AS decreases significantly the cracking performances of Reinforced HSC flexural beams. • LS-HSCs markedly improve the flexural cracking performances; the combined use of Belite-rich low heat Portland cement and expansive additive is particularly effective. • It is verified that a new concept can evaluate crack widths of the RC beams with wider range of early age deformation with acceptable accuracy. This concept was adopted into JSCE Code equation. Time-dependent structural performance of LS-HSC beams will be presented by TANIMURA, at 7th HS/HPC symposium.

  14. Thank you !!

  15. Unit; mm Specimen for autogenous shrinkage measurement

  16. Autogenous shrinkage/expansion strain

  17. Additional performance Necessity of low-shrinkage HSC • Significant autogenous shrinkage • Tensile restrained-stress before loading • Deterioration of serviceability performance of RC members Low shrinkage Low-shrinkage HSC-High cracking resistance-Durable RC structure High-strength High-flowability generalization Assignment High-mechanical performance and high-durability

  18. Approach for low-shrinkage HSC Low-shrinkage cement Special admixtures Belite-rich Portland cement Expansive additive Low-heat Portland cement Shrinkage reducing agent EX+SRA Autogenous strain Age Autogenous strain LPC EX SRA Conventional HSC Combination Control of autogenous shrinkage Expansion rather than shrinkage

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