A short concrete theory
Architecture is rediscovering concrete which is left as it is after pouring. Representative buildings
the world over testify to this trend, e.g. the “Freilichtpavillon in Grafenegg” or the “Wirtschaftsuniversität Wien”.
This is not limited to architecture – the growing demand for superior finish has reached industrial and engineering applications. Not only the concrete finish, but the complexity of concrete structures has changed the face of concrete pours: flowable and self-compacting concrete types enable delicate geometries with a high reinforcement ratio. These structures go beyond the capabilities of standard concrete.
The challenge is how to handle the increased pressure that these types of concrete exert on formwork. Standards applied hitherto did not take the behaviour of these concrete mixtures into account. The new German standard DIN 18218, revised and extended through the efforts of MEVA formwork systems, does so.
Various scientific publications deal with the pressure behaviour of flowable and self-compacting concrete. The research, in part laboratory tests, assumed pour rates that are not compatible with on-site practice. The
predominant recommendation from these investigations is to assume full hydrostatic pressure on the formwork
This approach leads to formwork which is overdimensioned. Thus research correlated laboratory findings with theory and real-life, on-site tests. This is a more realistic basis for correctly calculating concrete pressure for flowable and self-compacting concrete types and, in consequence, to correctly and safely predicting the pressure capacity required of the formwork.
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Previously, the estimation of concrete pressure on vertical formwork was based on concrete consisting of three components, as was the case in the 1970´s, whose behaviour was outlined in the German standard DIN 1045 and
DIN 1048. Concrete mixtures in use throughout Europe not only register larger slump (flow diametre), but also
behave differently during setting. This makes it more difficult – if not impossible – to predict concrete pressure.
A new method of calculating concrete pressure, developed by MEVA, has officially been adopted into the new German standard (DIN 18218) which is now in force. Even beyond the standard´s realm of influence the method has proved successful on sites all over the world, especially for challenging pours with self-compacting or flowable concrete mixtures.
Normative Values & Pressure Factors
Norm for fresh concrete pressure
Previously, the normative equation for calculation for concrete pressure was:
This equation generates the concrete pressure diagram shown above (dependent on the respective concrete consistency) taking into account the conditions as defined.
The pressure impact on vertical formwork according to the German standard DIN 18218 is shown right.
According to this normative calculation, the factor „5“ in factor „5 vb“ implies that the fresh concrete will reach final setting after at most 5 hours. Other factors may be taken into account to account for the influence of setting delay agents on the final setting time. The impact exerted on setting time by different admixtures of fly ash cannot be predicted. This approach shows the impact of fresh concrete pressure at different pour heights (H) in terms of its pressure on the formwork.
The lightly coloured levels of concrete have begun their setting process, with the result that pressure no longer occurs as lateral strain. The pressure switches to silo pressure. Continuous setting process during the concrete pour until setting is concluded. The silo pressure curve stays constant as the pour goes on and the concrete setting process continues.
The switch from cumulative pressure to uniform silo pressure curve was already part of the DIN 18218, backed and proved by elasticity theory.
This calculation suggests that concrete which is in the process of setting exerts 40 % of the total pressure exerted by fresh concrete. During setting, water is drawn from the fresh concrete.
This leads to a reduction of total volume and to shrinkage in all three directions. Thus, the concrete does not exert pressure any more.