KONGRESSUNTERLAGEN | PROCEEDINGS - Bft-international.com

Kongressunterlagen ← PODIUM 3
spalling occurs on the finished surface, for example on concrete
steps in front of the building entrance, with a significant adverse
effect on appearance.
Such precast elements are subject to complaints although it is
undisputed that they do not exceed the scaling limit of 1,500 g/
m² specified in the standard. The consulted expert counted 150 of
these spots per square meter with a mean diameter of 8 mm and
a depth of 1 mm.
The fact that such a missing tiny concrete “cover” weighs only
about 0.2 g means that only a total of 30 g is reached. Assuming
further that the surface shows a scaling rate to an overall mean
depth of 0.3 mm, this adds another 700 g/m², resulting in a total
scaling of 730 g/m² (i.e. clearly below the threshold of 1,500 g/m²).
The concrete or installed precast element complies with the
agreed standard with respect to scaling, and thus meets the contractually
agreed conditions. The element is fully fit for its intended
purpose, and the scaling process comes to a halt. The surfaces
do not look appealing.
This issue is also dealt with in the 4/2011 DBV Code of Practice
for concrete bridge perimeter elements, where “usual quantities”
for which no repair is necessary are mentioned. A distinction is
made between three local imperfection categories:
»»
- pop-outs (swelling of isolated aggregate particles due to water
absorption, frost action or exposure to de-icing salts)
»»
- organic constituents (e.g. wood or coal)
»»
- spalling above intact (mostly smooth and level, near-surface)
aggregate particles
In this field, too, there is still disagreement among experts with
respect to the number of “permissible” imperfections.
Technical options to reduce their number include:
»»
preventing pop-outs/organic contamination: limiting the permissible
share in the aggregate in excess of the standard specifications
as far as such material is (locally) available
»»
measures to prevent spalling above flat, solid aggregates:
»»
avoiding smooth hard-rock aggregates
»»
reducing the maximum particle size
»»
preventing the accumulation of air or water below aggregate
particles in the production process and
»»
nsuring early, thorough finishing/surface treatment
→ 2 Massive tread
Blockstufe
»»
Popouts (Quellen einzelner Gesteinskörner infolge Wasseraufnahme,
Frost oder Frosttausalz)
»»
organische Bestandteile (z. B. Holz, Kohle o. ä.)
»»
Ablösung über gesunden (meist glatten und ebenen oberflächennahen)
Gesteinskörnern
Auch hier herrscht in der Fachwelt noch keine Einigkeit über diese
Anzahl an „zulässigen“ Fehlstellen.
Technisch zu reduzieren:
»»
Maßnahme gegen Popouts/organische Bestandteile: Begrenzung
der zulässigen Mengen im Gestein über Norm hinaus, soweit derartiges
Material (lokal) verfügbar
»»
Maßnahmen gegen Ablösungen über flachen festen Steinen:
»»
Vermeiden von glatten Hartgesteinen,
»»
Verminderung des Größtkorns,
»»
Vermeiden von Luft- oder Wasseransammlung unter Gesteinskörnern
bei der Produktion und
»»
frühzeitige und intensive Nachbehandlung
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