|
Abstract
The research was conducted to study the effect of vibration on the strength of concrete using recycled aggregates. The specimens were subjected to a series of pre-determined vibration effects (25 gal, 70 gal, 190 gal, 718gal) at selected time periods (15 minutes, 30 minutes, 1 hr, 10 hr, 20hr) after the concrete was mixed. In general, the results of the study exhibit that reduction of the 28-day strength of the concrete are 21-30%,23-32%, 24-34%, and 28-37% for the vibration equivalent to minor, moderate, strong, and devastating earthquake, respectively.
The results of the study also revealed that vibration introduces much more significant influence on the reduction of the concrete strength when the vibration was applied within a “critical time period” after the concrete was poured, and after which the influence becomes much less. The results indicated that the “critical time period” is about 70, 90, 120, and 140 minutes for the same series of vibration effects mentioned above. A preliminary conclusion was reached based on the test results that, the upper limit of the critical time is about 150 minutes (2-1/2 hrs), within which all levels of vibration effect introduce severe reduction of strength up to 30% compared with that subjected to no vibration.
The study results also revealed that vibration introduces very little effect on the strength of concrete beyond a certain time period after the concrete was mixed which is defined as the “minimal effect time period”. According to the test results, for specimens subjected to vibration effect equivalent to minor, moderate, strong, and devastating earthquake, the “minimal effect time period” was revealed to be 450, 520, 590, and 660 minutes respectively. It is preliminarily concluded that the upper limit of the “minimal effect time period” is about 720 minutes (12 hours) after which all levels of vibration effects introduce very little or no effect on the strength of the concrete with a reduction ratio less than 4%.
|