Errors during casting 47
represent large relative errors. On the other hand, the effects can become
significant due to possible side effects of the admixtures. This will be illustrated by two real cases.
3.2.1 Wrongly proportioned accelerator
Until recently, the application of calcium chloride was regularly used as
an accelerator for concrete hardening in winter times. As a side effect,
it is clear that the chlorides within the accelerator increased the risk of
reinforcement corrosion. For this reason, nowadays chloride based admixtures are prohibited by the European Standard EN 206-1. In the 1990s,
one specific case clearly illustrated these risks. In winter time in Belgium,
the underground concrete basement walls of an office building were cast
with the implementation of calcium chloride as an accelerator. Two months
later, when the structure had climbed already a few levels, the basement
walls started colouring brownish. Investigation showed that the chloride
content in the concrete was significantly higher than what was generally
accepted as a critical value with respect to reinforcement corrosion. It was
shown that the applied dosage of the calcium chloride accelerator was
significantly higher than the maximum allowed dosage in an attempt to
keep up with the casting schedule in spite of the severe winter conditions.
As a consequence, the reinforcement started corroding heavily, leading to
the brownish discolouring of the walls. An attempt was made to extract the
chloride by means of electrochemical techniques, however, without success.
Finally, the partially completed concrete structure was torn down, and the
contractor could start all over again.
3.2.2 Wrongly proportioned plasticizer
In another remarkable example, a water tower was built, consisting of a vertical concrete shaft on which an inverted cone reservoir had to be positioned.
The shaft construction went on very smoothly, following a continuous
operation with day and night shifts. However, after time, some horizontal rings became visible showing a sequence of lighter and darker concrete
zones coinciding with the day and night shifts. Investigation showed that
the darker zones showed lower compressive strength of the concrete below
target values. Detailed investigation pointed to a wrong proportioning of a
plasticizer. According to the instructions, for each concrete mix (produced
on site) 30 cc of the plasticizer had to be added. The technician responsible
for adding the plasticizer wrongly considered that 30 cc was about the
volume of a beer glass leading to an over dosage of the plasticizer by almost
a factor of ten! As a consequence, the hydration process of the concrete was
totally disturbed, leading to insufficient strength. Also, in this case, only one
solution was found: tear down the water tower, and start all over.
represent large relative errors. On the other hand, the effects can become
significant due to possible side effects of the admixtures. This will be illustrated by two real cases.
3.2.1 Wrongly proportioned accelerator
Until recently, the application of calcium chloride was regularly used as
an accelerator for concrete hardening in winter times. As a side effect,
it is clear that the chlorides within the accelerator increased the risk of
reinforcement corrosion. For this reason, nowadays chloride based admixtures are prohibited by the European Standard EN 206-1. In the 1990s,
one specific case clearly illustrated these risks. In winter time in Belgium,
the underground concrete basement walls of an office building were cast
with the implementation of calcium chloride as an accelerator. Two months
later, when the structure had climbed already a few levels, the basement
walls started colouring brownish. Investigation showed that the chloride
content in the concrete was significantly higher than what was generally
accepted as a critical value with respect to reinforcement corrosion. It was
shown that the applied dosage of the calcium chloride accelerator was
significantly higher than the maximum allowed dosage in an attempt to
keep up with the casting schedule in spite of the severe winter conditions.
As a consequence, the reinforcement started corroding heavily, leading to
the brownish discolouring of the walls. An attempt was made to extract the
chloride by means of electrochemical techniques, however, without success.
Finally, the partially completed concrete structure was torn down, and the
contractor could start all over again.
3.2.2 Wrongly proportioned plasticizer
In another remarkable example, a water tower was built, consisting of a vertical concrete shaft on which an inverted cone reservoir had to be positioned.
The shaft construction went on very smoothly, following a continuous
operation with day and night shifts. However, after time, some horizontal rings became visible showing a sequence of lighter and darker concrete
zones coinciding with the day and night shifts. Investigation showed that
the darker zones showed lower compressive strength of the concrete below
target values. Detailed investigation pointed to a wrong proportioning of a
plasticizer. According to the instructions, for each concrete mix (produced
on site) 30 cc of the plasticizer had to be added. The technician responsible
for adding the plasticizer wrongly considered that 30 cc was about the
volume of a beer glass leading to an over dosage of the plasticizer by almost
a factor of ten! As a consequence, the hydration process of the concrete was
totally disturbed, leading to insufficient strength. Also, in this case, only one
solution was found: tear down the water tower, and start all over.