# Presentation: 320730

## Mechanical Properties of HPC withExpansive Additive andShrinkage 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

## 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

## 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

## Variable Restraint Testing Machine 

- Fresh concrete is cast into the framework of the testing machine.

- Specimen size is 1500 mm in length and 100 mm x 100 mm in cross sectional area

- 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.

- The load through the specimen is monitored by a load cell with accuracy of 1 N.

- The longitudinal deformation of concrete specimen is monitored by four LVDTs with accuracy of 0.125 μm.

- Experiment is commenced after the concrete setting.

## Program Flow of Simulated Completely-Restrained Test

**Program Flow of Simulated Completely-Restrained Test**

**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**.

- There are two controlling triggers.

- One is **stress** trigger.

- Another is **strain** trigger.

- While repeating this process in VRTM,
- a completely-restrained condition is achieved and
- the stress generated by shrinkage is measured.

## Mix Proportions of Concrete

| Composition | NHC | EHC | SHC |
| --- | --- | --- | --- |
| Cement (kg/m3) | 550 | 530 | 550 |
| Expansive additive (kg/m3) | 0 | 20 | 0 |
| Shrinkage reducing admixture (kg/m3) | 0 | 0 | 6 |
| Water (kg/m3) | 165 | 165 | 165 |
| Fine aggregate (kg/m3) | 781 | 781 | 781 |
| Coarse aggregate (kg/m3) | 869 | 869 | 869 |
| High-range water-reducing admixture (cement weight %) | 0.7 | 0.7 | 0.7 |

## 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

## Mechanical Properties of Concrete

|  | Compressive Strength (MPa) |  |  | Tensile Strength (MPa) |  |  | Modulus of Elasticity
 (GPa) |  |  |
| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
|  | 1
day | 3
days | 5
days | 1
day | 3
days | 5
days | 1
day | 3
days | 5
days |
| NHC | 25.6 | 56.5 | 65.4 | 2.2 | 4.4 | 4.9 | 22.5 | 30.6 | 32.3 |
| EHC | 25.1 | 51.4 | 57.7 | 2.4 | 3.6 | 4.1 | 22.6 | 29.7 | 33.1 |
| SHC | 24.9 | 53.8 | 64.1 | 2.3 | 3.1 | 3.8 | 23.1 | 29.9 | 33.0 |

## Autogenous Shrinkage

- Autogenous shrinkages of NHC and SHC occurres at a rapid rate in the first few hours and the rate decreased afterward.

- In EHC, after a few hours expansion is observed.

**Expansive addition and shrinkage reducing admixture can obviously reduce the autogenous shrinkage of HPC.**

## Temperature Histories 

**Almost constant temperature never causes significant expansion.**

## Strain under Simulated Completely-Restraint

**Deformation is well controlled**
**within the range of the threshold value, 1********m.**

- Tension

- Compression

## Stress under Simulated Completely-Restraint

**Tensile Strength of NHC at 1 day = 2.2 MPa (x 0.7 = 1.54 MPa) **

**In EHC and SHC,**
**lower tensile stress and no cracking.**

## Schematic Diagram for Creep Estimation

- Age

- Strain

## Creep Strain

**Creep is quite significant in the deformation of HPC at early age, corresponding to 90 % of the free shrinkage strain.**

- Creep strain shows the tendency to increase rapidly immediately after the setting up to 10 hours.

**A considerable tensile stress in HPC can be relaxed under restraint at early age.**

## Creep Coefficient in Each Step

***ε******i,co-creep****** = ε******i,creep****** / ε******i,elastic***

- 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.**

## 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.

## 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.