SHRINKAGE COMPENSATING CONCRETE IN POST-TENSIONED STRUCTUERS
Ahmed Mohammed Ezzat Mahmoud;
Abstract
This research aimed to produce shrinkage compensating concrete and to investigate the effect of adding supplementary admixtures on their mechanical properties as well as measuring length change for the produced concrete. In addition, an application of the behavior of post-tensioned beams prepared from shrinkage compensating concrete mixes was accomplished in order to outline the most suitable mix for use in such vigorous concrete elements.
An extensive review of the previous literature and studies was presented at chapter 2 while an extensive overview of the experimental program - done at the Laboratory of Properties and Testing of Materials and Quality Control at Ain Shams University in Cairo - was introduced at chapter 3. Test results of the preceded experimental work are discussed in details at Chapter 4.
5.2 Conclusions
Based on the experimental results of this work, the following conclusions could be drawn –within the limitations of the investigated parameters-:
The effect of adding CaO expansive agent on the examined concrete mixes shows an accumulated expansion increase as a consequence of dose increase. The initial expansion reached, almost duplicates for each CaO supplementary dose (4, 8& 12% respectively). However, the difference between the maximum expansion reached and the residual expansion recorded for concrete at the end of the monitoring period is approximately constant regardless to the amount of CaO used.
According to ASTM specified limits for the acceptable expansion, it is believed that; the optimum acceptable expansive dose is 8% CaO.
Chapter 5 Conclusions and Recommendations
106
The compressive strength is slightly reduced as CaO dose increased at both 7 and 28 days age.
Incorporating SRA on the investigated shrinkage compensating concrete mixes has an adverse effect on the resulting expansion potential according to the recorded results, regardless to the amount of CaO used. Unfortunately, these results do not support the synergistic effect acknowledged in literature for the effect of combined use of SRA with CaO based expansive agents. This may be probably attributed to the base or type of the investigated SRA.
The presence of SRA decreases the difference (approximately 20%) between the maximum expansion reached and the residual expansion of concrete regardless to the amount of CaO used. This is the same percentage of reduction in shrinkage values that took place (obtained) when the mixes with and without SRA are compared for shrinkage potential showing that the effectiveness of the SRA is neither affected by the CaO dose.
The use of SRA alone reduces the compressive strength at both ages and as the dose of incorporated CaO increases the rate of loss in compressive strength increases.
The effect of incorporating silica fume upon the investigated shrinkage compensating concrete mixes also has an adverse effect on the expansion potential but in a rather different way than that exhibited by the addition of SRA. The increased shrinkage potential exhibited due to the incorporation of silica fume showed that the 4% dose of CaO used should not be used in the presence of silica fume as this mix exhibited shrinkage after 20 days of casting. The 8% and the 12% doses also should so not be used as they achieved very high levels of expansion above the specified limits.
The presence of silica fume does not change the difference between the maximum expansion reached and the residual expansion recorded for concrete regardless to the amount of CaO used showing that the rates of gain and loss of expansion are constant and not effected by the presence of silica fume.
Chapter 5 Conclusions and Recommendations
107
As commonly known, the enhancement of silica fume addition –only- for the compressive strength at both ages was recorded. While the increase of expansive agent dose resulted in a decrease in the strength gain potential incorporated from silica fume addition.
The load carrying capacity of the investigated post-tensioned beams was significantly improved by about 20% when cast with a shrinkage compensating concrete incorporating CaO expansive agent only. It was observed the improvement of beam stiffness for CaO mix compared with other investigated concrete mixtures as registered from load-deflection curves.
The addition of the SRA alone yielded a reduction of the load carrying capacity for the investigated post-tensioned beam. It is resulting an increment of the first crack load by about 15%
An extensive review of the previous literature and studies was presented at chapter 2 while an extensive overview of the experimental program - done at the Laboratory of Properties and Testing of Materials and Quality Control at Ain Shams University in Cairo - was introduced at chapter 3. Test results of the preceded experimental work are discussed in details at Chapter 4.
5.2 Conclusions
Based on the experimental results of this work, the following conclusions could be drawn –within the limitations of the investigated parameters-:
The effect of adding CaO expansive agent on the examined concrete mixes shows an accumulated expansion increase as a consequence of dose increase. The initial expansion reached, almost duplicates for each CaO supplementary dose (4, 8& 12% respectively). However, the difference between the maximum expansion reached and the residual expansion recorded for concrete at the end of the monitoring period is approximately constant regardless to the amount of CaO used.
According to ASTM specified limits for the acceptable expansion, it is believed that; the optimum acceptable expansive dose is 8% CaO.
Chapter 5 Conclusions and Recommendations
106
The compressive strength is slightly reduced as CaO dose increased at both 7 and 28 days age.
Incorporating SRA on the investigated shrinkage compensating concrete mixes has an adverse effect on the resulting expansion potential according to the recorded results, regardless to the amount of CaO used. Unfortunately, these results do not support the synergistic effect acknowledged in literature for the effect of combined use of SRA with CaO based expansive agents. This may be probably attributed to the base or type of the investigated SRA.
The presence of SRA decreases the difference (approximately 20%) between the maximum expansion reached and the residual expansion of concrete regardless to the amount of CaO used. This is the same percentage of reduction in shrinkage values that took place (obtained) when the mixes with and without SRA are compared for shrinkage potential showing that the effectiveness of the SRA is neither affected by the CaO dose.
The use of SRA alone reduces the compressive strength at both ages and as the dose of incorporated CaO increases the rate of loss in compressive strength increases.
The effect of incorporating silica fume upon the investigated shrinkage compensating concrete mixes also has an adverse effect on the expansion potential but in a rather different way than that exhibited by the addition of SRA. The increased shrinkage potential exhibited due to the incorporation of silica fume showed that the 4% dose of CaO used should not be used in the presence of silica fume as this mix exhibited shrinkage after 20 days of casting. The 8% and the 12% doses also should so not be used as they achieved very high levels of expansion above the specified limits.
The presence of silica fume does not change the difference between the maximum expansion reached and the residual expansion recorded for concrete regardless to the amount of CaO used showing that the rates of gain and loss of expansion are constant and not effected by the presence of silica fume.
Chapter 5 Conclusions and Recommendations
107
As commonly known, the enhancement of silica fume addition –only- for the compressive strength at both ages was recorded. While the increase of expansive agent dose resulted in a decrease in the strength gain potential incorporated from silica fume addition.
The load carrying capacity of the investigated post-tensioned beams was significantly improved by about 20% when cast with a shrinkage compensating concrete incorporating CaO expansive agent only. It was observed the improvement of beam stiffness for CaO mix compared with other investigated concrete mixtures as registered from load-deflection curves.
The addition of the SRA alone yielded a reduction of the load carrying capacity for the investigated post-tensioned beam. It is resulting an increment of the first crack load by about 15%
Other data
| Title | SHRINKAGE COMPENSATING CONCRETE IN POST-TENSIONED STRUCTUERS | Other Titles | استخدام الخرسانه محدوده الانكماش فى المنشآت لاحقه الاجهاد | Authors | Ahmed Mohammed Ezzat Mahmoud | Issue Date | 2015 |
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