Particularly in underground construction, concrete structures are exposed alongside loads and wear of decade-long use to influences emerging from the sub grade such as permanent mechanical stresses and aggressive water.
Concrete is nevertheless characterized by its outstanding durability. Solutions containing sulfates, such as in natural or polluted groundwater, represent a considerable deteriorating impact on concrete. This can eventually lead to loss of strength, expansion, spalling of surface layers and ultimately to disintegration.
The intended life cycle of a concrete structure is ensured by a suitable concrete mix design that is adapted to the expected exposition to various impacts. Sulfate contained in water reacts with the tricalcium aluminate (C3 A) in the cement to form ettringite (also thaumasite under certain conditions), which leads to increases in volume. This volume increase results in high internal pressure in the concrete structure which induces cracking and spalling. Such attack is classified among types of chemical attack under which standard concrete designed without dedicated measures can experience significant damages. Field experience demonstrates that loss of adhesion and strength are usually more severe than concrete damage resulting from expansion and cracking.
Sulfate resistance of concrete is determined by the sulfate resistance of the cement matrix as well as its ability to withstand diffusion of sulfate ions through the matrix. Concrete intended to be sulfate-resistant should therefore be characterized by high impermeability as well as higher compressive strength on the one hand. Furthermore, cements with low C3A and Al2 O3 content should be used. Doing so reduces the potential for any deteriorating reactions. In addition the inclusion of silica fume is favorable, since this contributes to higher density of the cement matrix in conjunction with enhanced bonding between the cement matrix and aggregates, and thus leading to higher compressive strength.
Sulfate attack is designated as exposure class chemical attack according to EN 206-1. Therefore, the exposure class is determined by the expected sulfate content in the water contacting the concrete. Depending on the exposure class, a minimum cement content in combination with a maximum w/c-ratio is required, as well as a mandatory utilization of cement with high sulfate resistance.
In tunneling, durability is of decisive importance and sulfate attack is a constantly occurring and challenging phenomenon. This is especially true in the case of production of precast tunnel lining segments for tunnel boring machine (TBM) and rock support applied by sprayed concrete. In excavations in which high sulfate attack is anticipated, it is difficult to fulfill all technical requirements unless appropriate measures regarding the concrete mix design are also taken.
For sprayed concrete the use of alkali free accelerators is mandatory to achieve adequate sulfate resistance. The industrialised, swift production of tunnel lining segments requires production cycles is very difficult with conventional sulfate resistant cements, due to the fact that these cements exhibit slow strength development.
A concrete mix containing silica fume and a superplasticizer fulfills both criteria, productivity and sulfate resistance, but this system is very sensitive to proper curing due to crack formation. With the application of a water-based epoxy emulsion immediately after formwork release of the segments, micro-crack free concrete can be produced.