 
Below is the
proceedings of the International RILEM Conference in 1996 - technical report Production Methods and Workability of Concrete Proceedings of the
International RILEM Conference Paisley, Scotland June 3-5, 1996 EDITED BY P.J.M. Bartos and D.L.
Marrs
and D.J. Cleland
E & FN SPON An
imprint of Chapman & Hall London - Weinheim - New York - Tokyo -
Melbourne - Madras CONTINUOUS CONTROL OF FRESH CONCRETE USING THE FCT 101 TESTER T. STEINER
- Research and Consulting Institute of the Swiss Cement
Industry, Wildegg, Switzerland Abstract The new test method provides results combining those of the
traditional testing methods of fresh concrete. Advantages of the new method are
the speed of the test procedure, the possibility to test concrete in-situ
without removing any material, ability to carry out a number of workability
tests in a rapid succession on the same sample, to store the results and print
them out with a portable PC and provide complete practical test documentation. Keywords: Fresh concrete tests,
FCT-tester, workability, slump, water-cement ratio, continuous recording,
computer processing, concrete temperature. 1. Introduction Tests such as the slump, flow, compaction, Vebe and other test
methods are usually used to determine the variability of the workability of a
fresh concrete mix. Such tests are
described in appropriate standards and guidelines exist for use of these tests
in practical production quality control. The standard tests tend to provide
reproducible results and the basic tests are indispensable tools in modern
concrete construction practice. All the
standard workability tests are relatively labour intensive. A considerable time
usually elapses between the extraction of a test sample and the final results
being available. In practice this means that the control of the workability of
concrete is generally only made at intervals or at random. A systematic,
virtually gap-less, continuous control of workability dependent on the process
of stiffening of the cement paste and the water loss is practically impossible
with the traditional methods. Furthermore,
it has to be considered that in each test in which a sample of fresh concrete
is removed there is a probability of the sample not representing the in-situ
concrete on the building site Practical
concrete construction requires a method, which allows judging the workability
of a mix within max. 2 minutes if a fresh concrete is to be placed in a
shuttering or it is to be rejected because of its low workability. Not only the
consideration of easy pouring and compaction but also workability required for
all finishing work which has to be undertaken must be taken in order to
determine the correct workability. Concrete mixes of good workability are
particularly necessary in the case of air entrained concrete, concrete with
silica-fume or for concretes used for building conservation. 2. The FCT tester The operating principle of the test is based on a determination of
a torque of a rotating probe. The tester gives values for concrete with coarse
aggregate up to 32 mm and a slump between 35 and 180 mm with an accuracy of +
3%. The equipment is hand-held (2.4 kg) and battery operated. The tests can be
carried out in any location provided there is at least a volume of 50 litres of
fresh concrete available. The calibration of the tester is very simple. The
required FCT values can be determined at the same time as the fresh concrete is
tested in the usual conventional manner. Depending of the requirements and
problems which have to be solved several test-series may be necessary.
Selection of integrated working characteristics allows the required quality
parameters to be called upon in the shortest of time (slump, flow,
compaction...) under the condition that the mix design has not changed. Within
one minute the workability can be measured and with the established correlation
factors other parameters like water-cement-ratio can be determined. A significant
advantage is that the tests can be repeated using the same batch of fresh
concrete. It is therefore possible to observe the development of the stiffness
of the mix. The measured values are stored in the tester and can be recalled
and printed out via a PC. The quality control can be thus documented
continuously. 3. Application High
performance reinforced concrete construction elements require a continuous
quality control. In such cases a single non-conforming mix can cause the
failure of the whole construction. Bridge decks, waterproof concrete
containers, pipes, slim supporting piers and similar structures have to guarantee
the required safety demands even in their weakest parts. In such circumstances
it is necessary to test not just some of the concrete, selected batches only,
but much more of the concrete, virtually all the mix prior to being poured into
the formwork must be tested. The continuous testing should also eliminate
external effects such as long transportation times, weather etc. The continuous
and inexpensive monitoring proposed will always make it cheaper to reject
doubtful concrete rather than accept the potential cost of repair of the failed
part of a construction Requirements
and problems which have to be solved several test-series may be necessary.
Selection of integrated working characteristics allows the required quality
parameters to be called upon in the shortest of time (slump, flow,
compaction...) under the condition that the mix design has not changed. Within
one minute the workability can be measured and with the established correlation
factors other parameters like water-cement-ratio can be determined. A significant
advantage is that the tests can be repeated using the same batch of fresh
concrete. It is therefore possible to observe the development of the stiffness
of the mix. The measured values are stored in the tester and can be recalled
and printed out via a PC. The quality control can be thus documented
continuously. |