Minimum air void content of exposed aggregate concrete - Laboratory investigations on the freeze-thaw resistance of exposed aggregate concrete with specified minimum air content and on the effect of mixing intensity and cement type on air void development

Duration: 01.07.2009 - 31.12.2010

In the relevant codes of practice (TL Beton, Merkblatt "LP-Beton" /1/), a sufficient air content in the fresh concrete is required for concrete with high freeze-thaw resistance, which is determined according to the different mortar proportions depending on the maximum grain size of the aggregates. In today's standard construction method in exposed aggregate concrete, the maximum aggregate size is usually limited to 8 mm for the top concrete, which is only 5 to 8 cm thick. At the same time, this concrete is produced with a cement content of around 400 to 450 kg/m³ and a water/cement ratio of around 0.40, which practically always requires the addition of a concrete plasticizer/plasticizer to ensure adequate workability. According to the current regulations, this then results in an average minimum air content in the fresh concrete of 6.5% by volume. (Individual values may fall below these limits by a maximum of 0.5% by volume). A comparison of the above-mentioned requirements in Germany with foreign regulations shows that in Austria, for example, according to the local RVS 08.17.02 /4/, which regulates the production of concrete road pavements, an air content in the fresh concrete of between 4.0 and 6.0% by volume must be verified for road concrete including exposed aggregate concrete. The associated national application standard for concrete, Ö-Norm B 4710-1, requires that for a concrete with a maximum grain size of 8 mm in exposure class XF4, the air content in the fresh concrete (in % by volume) must be at least 13 % of the binder-size volume. With a cement content of 420 kg/m³ cement and a w/c ratio of approx. 0.40, a binder-size volume of approx. 300 liters/m³ results in a minimum air content in the fresh concrete of only 4.0 % by volume even for an exposed aggregate concrete with a maximum grain size of 8 mm. The aim of the investigations proposed here is to check the extent to which the minimum air content specified in the code of practice for the production and processing of air-entrained concrete for a concrete with 8 mm maximum aggregate size is actually necessary for adequate freeze-thaw resistance in exposed aggregate concrete. In addition, the extent to which mixing intensity and cement type have an effect on unscheduled air entrainment is to be determined.