Mechanical Properties of High-Performance Concrete with Additives under Restraint

1. Mechanical Properties of HPC withExpansive Additive andShrinkage Reducing Admixture under Simulated Completely-Restrained Condition at Early Age Takafumi Noguchi The University of Tokyo, Japan Park Sun-Gyu Yonsei University, Korea Ippei Maruyama Hiroshima University, Japan

2. Background • High-performance Concrete (Low W/C) • Self-desiccation • Autogenous Shrinkage • Under Restraint Condition • Cracking • Influence on Durability & Aesthetic • Use of • Expansive Addition • Shrinkage Reducing Admixture

3. Objective of Study • Behaviour of Early Age HPC with and without • Expansive Addition • Shrinkage Reducing Admixture • Restraint Free Condition • Shrinkage Strain • Under Restraint Condition • Strain & Stress • Creep Behaviour • Control of Autogenous Shrinkage Cracking

4. Variable Restraint Testing Machine Fresh concrete is cast into the framework of the testing machine. The ends of specimen are fixed to the cross-head, which is fixed to the frame, by claws which hold the concrete specimen and are able to exert tensile or compressive force. Specimen size is 1500 mm in length and 100 mm x 100 mm in cross sectional area Experiment is commenced after the concrete setting. The longitudinal deformation of concrete specimen is monitored by four LVDTs with accuracy of 0.125 μm. The load through the specimen is monitored by a load cell with accuracy of 1 N.

5. Program Flow of Simulated Completely-Restrained Test There are two controlling triggers. Completely restrained condition is simulated by maintaining the total deformation of the specimen within a threshold, which is defined as the permissible change in the length of the specimen. While repeating this process in VRTM, a completely-restrained condition is achieved and the stress generated by shrinkage is measured. One is stress trigger. Another is strain trigger.

6. Mix Proportions of Concrete

7. Experiments • Compressive Strength • Tensile Strength • Modulus of Elasticity • Free Autogenous Shrinkage • Sealed with a polyester film at 20 ºC • Stress Development under Simulated Completely-Restraint • Sealed with a polyester film at 20 ºC • Trigger of stress and strain : 0.01 MPa and 2 x 10-6

8. Mechanical Properties of Concrete

9. Autogenous Shrinkage In EHC, after a few hours expansion is observed. Expansive addition and shrinkage reducing admixture can obviously reduce the autogenous shrinkage of HPC. Autogenous shrinkages of NHC and SHC occurres at a rapid rate in the first few hours and the rate decreased afterward.

10. Temperature Histories Almost constant temperature never causes significant expansion.

11. Strain under Simulated Completely-Restraint Tension Compression Deformation is well controlled within the range of the threshold value, 1m.

12. Invisible Crack Stress under Simulated Completely-Restraint In EHC and SHC, lower tensile stress and no cracking. Tensile Strength of NHC at 1 day = 2.2 MPa (x 0.7 = 1.54 MPa)

13. Elastic Strain measured from the recovery cycles of VRTM Strain Trigger Accumulation of Elastic Strain Creep Strain Free Autogenous Shrinkage εi,creep = εi,free - εi,elastic Schematic Diagram for Creep Estimation Age Strain

14. Creep Strain Creep strain shows the tendency to increase rapidly immediately after the setting up to 10 hours. Creep is quite significant in the deformation of HPC at early age, corresponding to 90 % of the free shrinkage strain. A considerable tensile stress in HPC can be relaxed under restraint at early age.

15. Creep Coefficient in Each Step Creep coefficient of NHC is lower than those of EHC and SHC in the beginning. Tensile stress in restrained EHC and SHC is lower than that in NHC at early age. εi,co-creep = εi,creep / εi,elastic

16. Concluding Remarks (1st) • The variable restraint testing machine can show how tensile stress and strain develop under restrained condition in HPC with and without expansive addition and shrinkage reducing admixture. • The tensile stress in HPC with expansive addition or shrinkage reducing admixture under completely restrained condition at early age was lower than that of normal HPC.

17. Concluding Remarks (2nd) • Normal HPC shows larger creep strain but smaller creep coefficient than concrete with expansive addition or shrinkage reducing admixture. Normal HPC is sensitive to autogenous shrinkage cracking. • Expansive addition and shrinkage reducing admixture make a crack prevention effect on HPC at early age.