# Presentation: 478876

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

***Fourth International Seminar on Self-desiccation and Its Importance in Concrete Technology ***
***20/June/05, Gaithersburg, Maryland, USA***

***Masahiro SUZUKI***
***  ******P.S. Mitsubishi Construction Co., Ltd., Japan***
***Makoto TANIMURA***
***  ******Taiheiyo Cement Corporation, Japan***
***Ryoichi SATO***
***  ******Hiroshima University, Japan***

**Notes:**
Thank you Mr. Chairman, ladies and gentlemen.
I’m very happy to be able to make my presentation in a title, “The Effect of Autogenous Shrinkage on Flexural Cracking Behavior of Reinforced HSC Beams and improvement by Using low-shrinkage HSC”. 

##  Autogenous shrinkage (AS) properties of HSC using various materials

*** ******Comprehensive approach for reducing AS***
***----- well studied!! ***

***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***

***However, few investigations on the effect of AS on structural performance. Only a few reports on evaluation method for cracking behavior considering AS  ***

_***Objectives***_

_***Background***_

**Notes:**
Firstly, let me explain briefly background.
High strength concrete has been increasingly applied in order to enhance durability and structural performance. However,  autogenous strain of HSC occurs remarkably at early ages, sometimes resulting in cracking. Control of cracking is important to make structures durable. Many researchers  has carried out to reduce autogenous shrinkage, which showed that combined use of expansive additive, shrinkage reducing agent and low heat Portland cement were the most effective in reducing shrinkage.
However, few investigation has been carried out on the effect of AS on structural performance including cracking behavior. 
Then in the presentation I would like to introduce the effect of AS on cracking behavior and propose a new concept for calculation of crack width of   RC beams considering shrinkage and expansion effect before loading.

## RC beam specimen

***Dimension; 200x250x2700 mm***
***Two-point loading; span:2100mm, pure flexural zone:800mm***

*** ******Evaluations***
***Before loading***
***    ******Steel strain******  ******Restrained stress on bottom fiber***
***Under short-term loading***
*** ******- Crack width; Contact-type strain gauge***

_**Crack width**_

_***Sealed curing period;30-50 days***_

_**Steel strain**_

**Notes:**
I would like to talk about outline of RC beams specimens.
The specimen dimension is 200 by 250 by 2700mm. Two reinforcing bars with nominal diameter 19mm were arranged at the depth of 210mm from upper fiber. A two-point loading was applied to the RC beams. The span and pure moment zone of the beams is 2100 mm and 800mm long.
Tension reinforcement strain at span center was measured until  beams were loaded. 
Under loading, crack widths at the depth of tension reinforcement were measured by a contact-type strain gauge.
Autogenous shrinkage strain was also measured by gauge embedded into concrete with the same cross section as that of the RC beam.  

## Mixture investigated

*** ******W/(C+EX); 0.3***
*** ******EX content; 40 kg/m******3***
*** ******SRA content; 6 kg/m******3***
***Targeted concrete strength; 70 N/mm******2****** at 28 days***

| Con | NC | NE | NS | NES | LC | LE | LS | LES |
| --- | --- | --- | --- | --- | --- | --- | --- | --- |
| Cem | Ordinary Portland |  |  |  | Belite-rich low heat Portland |  |  |  |
| EX | - | Add. | - | Add. | - | Add. | - | Add. |
| SRA | - | - | Add. | Add. | - | - | Add. | Add. |

_**Low-shrinakge HSCs**_

_**Reference HSC**_

**Notes:**
I would like to explain mixtures. 
We prepared  8 types of HSC in order to investigate the shrinkage/expansion effects on cracking behavior of RC beam.
Water-to-binder ratios of all types of concrete were fixed to 0.3.
As is shown in this table, ordinary Portland cement(designated as N) and belite-rich low heat Portland cement(designated as L) were used. Symbols of C, E, S and ES denote containing no admixtures, expansive additive, shrinkage reducing agent and both expansive additive and shrinkage reducing agent , respectively. 

## Equation for calculating restrained stress on extreme bottom fiber 

_***Equation for calculating restrained stress on extreme bottom fiber ***_

*** ******Equilibrium of the force between concrete and rebar***
*** ******Navier’s assumption***

**Notes:**
Let me enter result and discussion.
First of all I would like to explain an equation for calculating restrained stress on bottom fiber.
 This equation is derived from Navier’s assumption and the equilibrium requirement for the forces between concrete and reinforcement, in which it is defined that the stress is positive in tension and negative in comp. 

## Restrained-shrinkage/expansion stress

_***Restrained-shrinkage/expansion stress***_

- Tension

- Compression

- 1.5 N/mm2

- -1.5 N/mm2

*** ******Low-shrinkage HSCs are obviously effective in reducing AS-restrained stress.***

**Notes:**
This figures demonstrates the reduction performance for AS induced stress attained by EX and/or SRA and/or type of cement. 
As you can see from the figure, low heat PC is effective in reducing autogenous shrinkage stress and, furthermore, that combined with expansive additive such LE and LES produces compression stress in concrete, reaching about 1.5 MPa.

## Flexural cracking moment

_***Flexural cracking moment***_

*** ******AS-restrained stress affects cracking load significantly.***
*** ******LS-HSC markedly improve cracking load.***

**Notes:**
Next I would like to explain about HSC flexural member under short-term loading.
Flexural cracking moments observed in 8 types of beams are shown in figure, in which values are ratios for cracking moment of reference beam NC. Blue colored bars are measurements, and white bars with lattice are calculations using tensile strength and neglecting the restrained stress at the bottom fiber. As you can see in the quite difference of cracking moment of NC between experiment and calculation, the effect of autogenous shrinkage on cracking moment is remarkable. And it is found that cracking moment of expansive concrete specimens LE and LEX are more than 2 times larger than that of NC. 

## Maximum crack width/crack spacing

_***Maximum crack width/crack spacing***_

*** ******LS-HSCs improve cracking performance significantly.***
***  ******--Decrease in crack width, while increase in crack spacing!!***

**Notes:**
The maximum crack width and maximum crack spacing measured are shown in figure. The stress in tension reinforcement calculated by RC theory is 200 MPa. 
According to this figure, as expansion performance is enhanced the maximum crack spacing becomes wider and the maximum crack width becomes smaller. 
This result differs from the conventional concept; wider crack spacing results in a wider crack width.  
The reason for this could be explained by the stress difference between shrinkable and expansive concretes between adjacent cracks , and difference of stress change described next. 

## General evaluation method for crack width considering shrinkage/expansion effect before loading

_***General evaluation method for crack width considering shrinkage/expansion effect before loading***_

***This concept incorporated into JSCE CODE-2002***

**Diferrenec in elastic strain component between steel and concrete; affect crack width**

**Notes:**
The aforementioned difference of crack width depending on the type of concrete should be explained by strain change in RB from the state where the stress in concrete at the depth of the tension RB is zero to that under load. This figure shows the basic concept of the strain change in RB. The strain change in RB in expansive concrete beam is obviously smaller than that in shrinkable concrete beam. The former results in smaller crack width and the latter in larger one.    
This concept was incorporated into Design Code of JAPAN SOCIETY OF CIVIL ENGINEERS(JSCE). 

## JSCE Code Equation for maximum crack width

***JSCE Code Equation for maximum crack width***

**Notes:**
This equation is that of maximum crack width in JSCE Design Code. ‘σse denotes stress change in tension RB from the zero stress state in concrete at the depth tension RB to the stress state under load.

In other word, the great advance of low-shrinkage HSCs is more smaller crack width.  

## Verification of proposed concept for evaluating maximum crack width (1)

_***Verification of proposed concept for evaluating maximum crack width (1)***_

***JSCE CODE-1996***

**Improved accuracy**

***JSCE CODE-2002***

**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****)**

**Reinforcement stress (N/mm****2****)**

**Reinforcement stress change(N/mm****2****)**

**Notes:**
Left figure shows the relationship between maximum crack width and stress in tension RB calculated by neglecting shrinkage/expansion effects on crack width. According to the figure , it is found that difference of crack width among mixtures is marked. 
On the other hand, as is shown in the right figure, applying the concept of stress change, the difference among 8 beams is decreased, which shows that the present concept is valid for the evaluation of crack widths of concrete beams with early age deformation. 

## Verification of proposed concept for evaluating maximum crack width (2)

*** ******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.***

**Proposed**
**concept**

**Measured**

**Conventional theory**

**Range of conventional theory**

**Range of Proposed**
**concept**

**Notes:**
In order to verify the concept, left figure shows the relationship between RB strain at zero stress state and maximum crack width. Black line is measured, blue line is proposed , purple is conventionally calculated relationships, respectively. You can see that the relationship obtained by the present concept has the same tendency as that by measurement, while crack width of the former is larger than that of the latter. This should be due to neglecting the contribution of tension concrete at cracked section.
Right figure shows the comparison of maximum crack width between measurement and calculation, in which blue bars are that considering and white bars are that neglecting the effect of restrained stress.
Conventional RC theory cannot evaluate crack widths  of RC beams with early age deformation with satisfactory accuracy, which is improved by the present concept.

## Conclusions

*** ******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.***

_***Conclusions***_

***Time-dependent structural performance of LS-HSC beams will be presented by TANIMURA, at 7******th****** HS/HPC symposium. ***

**Notes:**
I would like to talk about conclusions.

○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.
Thank you very mach for your attentions.

## Thank you !!

***Thank you !!***

## Specimen for autogenous shrinkage measurement

## Autogenous shrinkage/expansion strain

_***Autogenous shrinkage/expansion strain***_

## Necessity of low-shrinkage HSC

***High-mechanical performance and high-durability***

_***Necessity of low-shrinkage HSC***_

***Significant autogenous***
***  ******shrinkage***
***Tensile restrained-stress   ***
***  ******before loading***
***Deterioration of serviceability performance of RC members***

**Low-shrinkage HSC****-High cracking resistance****-Durable RC structure**

***High-strength***

***Low shrinkage***

***High-flowability***

***Assignment***

***Additional performance***

***generalization***

## Approach for low-shrinkage HSC

***Belite-rich Portland cement***

***Low-heat Portland cement***

***Special admixtures***

***Low-shrinkage cement***

***Expansive additive***

***Shrinkage reducing agent***

***Autogenous strain***

***LPC***

***Conventional HSC***

***EX+SRA***

***EX***

***SRA***

***Combination***

***Expansion rather than shrinkage***

***Autogenous strain***

***Age***

***Control of autogenous shrinkage***