Factors causing defects in concrete placed in structures
There are many causes for defects in concrete structures, below are listed as per category;
Design and construction-related causes
• Difficult placement due to design of a member
• Improper design, construction and maintenance of forms
• Improper selection of concrete mixture proportions
• Failure to adjust concrete mixture proportions to suit
• Placement condition
• Improper placement practices
• Improper vibration and consolidation practices
• Improper steel detailing
Equipment-related causes
• Improper equipment
• Improper equipment maintenance
• Equipment failure (crane, pump, concrete plant)
• Interruption of utility service
Material-related causes
• Improper selection of release agent
• Cement characteristics
• Variation in mixture components
• Inappropriate use of admixtures
• Inappropriate use of release agents
Environmental causes
• Extreme weather conditions
Minimizing surface defects
To minimize the size and number of bugholes and other defects, arising out of the consolidation the following practices should be followed:
• Vibration period should be of sufficient duration
• Vibrator insertions should be properly spaced and overlapped and the vibrator removed slowly
• Each concrete layer should be consolidated from the bottom upward
• Vibration periods should be increased when using impermeable forms
• Inward sloping forms and other complex design details should be avoided
• Depth of placement layers should be limited
• Vibrator should penetrate into the previous layer
• Tightening devices and gaskets to prevent leakage at form joints should be provided as necessary
• Placing ports should be designed into the forms as necessary
Bugholes can be minimized by the use, where practical, of a 64 mm diameter vibrator of high frequency with medium to low amplitude. The vibrator should be immersed in the concrete around the perimeter of the form without damaging the form wall. Where reinforcement is placed near the form wall, the vibrator must be inserted inside the reinforcement. Care should be taken to insure that the vibrator has a sufficient radius of action to liquefy the concrete at the form wall. Form vibration may be used to supplement the internal vibration. An alternate procedure is to use a high frequency, low amplitude form vibrator. Vibration procedures should be evaluated at the beginning of a project to determine the vibration time for each type of vibrator for a given mixture.
In areas where voids are most prevalent, revibration may be used to reduce bugholes. Revibration is more effective if it is done at the latest possible time at which the vibrator head will penetrate the concrete under its own weight. Greater benefits are obtained with wetter concrete mixtures, especially in the top few feet of a placement where air and water voids are most prevalent.
Other measures, such as altering mix proportions, using superplasticizers and using smaller maximum aggregate size to improve workability should also be considered as methods of minimizing surface defects. These measures have often been successful, particularly when trying to consolidate concrete in congested areas.
There are many causes for defects in concrete structures, below are listed as per category;
Design and construction-related causes
• Difficult placement due to design of a member
• Improper design, construction and maintenance of forms
• Improper selection of concrete mixture proportions
• Failure to adjust concrete mixture proportions to suit
• Placement condition
• Improper placement practices
• Improper vibration and consolidation practices
• Improper steel detailing
Equipment-related causes
• Improper equipment
• Improper equipment maintenance
• Equipment failure (crane, pump, concrete plant)
• Interruption of utility service
Material-related causes
• Improper selection of release agent
• Cement characteristics
• Variation in mixture components
• Inappropriate use of admixtures
• Inappropriate use of release agents
Environmental causes
• Extreme weather conditions
Minimizing surface defects
To minimize the size and number of bugholes and other defects, arising out of the consolidation the following practices should be followed:
• Vibration period should be of sufficient duration
• Vibrator insertions should be properly spaced and overlapped and the vibrator removed slowly
• Each concrete layer should be consolidated from the bottom upward
• Vibration periods should be increased when using impermeable forms
• Inward sloping forms and other complex design details should be avoided
• Depth of placement layers should be limited
• Vibrator should penetrate into the previous layer
• Tightening devices and gaskets to prevent leakage at form joints should be provided as necessary
• Placing ports should be designed into the forms as necessary
Bugholes can be minimized by the use, where practical, of a 64 mm diameter vibrator of high frequency with medium to low amplitude. The vibrator should be immersed in the concrete around the perimeter of the form without damaging the form wall. Where reinforcement is placed near the form wall, the vibrator must be inserted inside the reinforcement. Care should be taken to insure that the vibrator has a sufficient radius of action to liquefy the concrete at the form wall. Form vibration may be used to supplement the internal vibration. An alternate procedure is to use a high frequency, low amplitude form vibrator. Vibration procedures should be evaluated at the beginning of a project to determine the vibration time for each type of vibrator for a given mixture.
In areas where voids are most prevalent, revibration may be used to reduce bugholes. Revibration is more effective if it is done at the latest possible time at which the vibrator head will penetrate the concrete under its own weight. Greater benefits are obtained with wetter concrete mixtures, especially in the top few feet of a placement where air and water voids are most prevalent.
Other measures, such as altering mix proportions, using superplasticizers and using smaller maximum aggregate size to improve workability should also be considered as methods of minimizing surface defects. These measures have often been successful, particularly when trying to consolidate concrete in congested areas.
