Mix-Design and Control of Exposure Class of Durable Concrete

Durability requirements for concrete are determined by climatic, chemical and mechanical effects, depending on the climatic location and intended use of structural parts. The European standard EN 206-1 defines different types of exposure classes and specifies requirements for concrete, such as type and minimum quantity of cement, maximum w/c ratio, minimum compressive strength or minimum entrained air pore content. Additional characteristics required for a higher workability often lead to contradictory situations in practice and cannot be satisfactorily resolved by varying the mix design of the concrete. Based on a real life scenario where a C35/45 XD3 concrete had been used in the construction of a multi-storey car park, which presented serious cracking after only a short service time, this paper proposes a number of methods designed to assess the quality and durability of concrete. The proof of exposure classes at the hardened building concrete is not regulated in existing national standards. Therefore, in many court disputes it is difficult to prove the quality of the hardened concrete because of lacking regulations. However, it is possible to verify the w/c-ratio using the capillary porosity. The basic relationships between cement content, w/c-ratio and capillary porosity are derived and discussed. Higher proportions of cement, which are necessary to achieve a better workability or higher strength, result in a reduced modulus of elasticity and increased shrinkage as well as a higher risk of cracking. The associated increase of capillary pore volume and of permeability reduces the service life of concrete. The determination of performance-based concrete properties such as permeability, chloride migration or electrical conductivity further complements the assessment of concrete durability. These properties can be used for estimating the resistance of the capillary pore structure to specific damage mechanisms.