Major infrastructure projects such as dams, roadways or airport runways require enormous quantities of aggregates, sought in closest proximity to construction sites. Some aggregates can exhibit an increased or high risk of ASR. Alkali-Silica-Reaction is a chemical reaction which occurs between the amorphous silica in the aggregate and the pore solution (alkalis) of the cement matrix.
The reaction results in an increase of concrete volume, causing cracking and spalling when the generated forces exceed the tensile strength of the concrete. Essential conditions for occurrence of ASR are moisture within the concrete, a high alkaline content in the pore solution and reactive aggregates. Selection of the correct concrete mix design is critical for avoidance of ASR. Choice of the right solutions can prevent damages resulting from ASR even if highly reactive aggregates are used.
Cement clinker contributes the greatest proportion of alkaline material. The higher the cement content is, the more alkaline the mix will be. Blended cements introduce a lower alkaline content. A low w/c-ratio is considered the central factor for achievement of dense, watertight concrete. Dense concrete slows the diffusion of free alkalines and the migration of water to aggregates. For ASR to occur it requires aggregates particularly sensitive to alkalines, such as siliceous limestone, sandy limestone, limestone, gneisses and strongly deformed quartzite.
Porous, cracked, weathered or crushed aggregates are more reactive than those with dense structure and round surfaces.
Aggregates constitute a major portion of concrete. Their influence on the fresh and hardened concrete is considerable. Sources of high quality aggregates are gradually dwindling in number, as a result of which the building and construction materials industry and builders of major infrastructure projects seek solutions for the use of aggregates with lower quality.
The Alkali-Silica-Reaction (ASR), which can occur with aggregates, presents a particular challenge and can affect the durability of concrete.
Pozzolanic additives such as fly ash, granulated blast furnace slag or silica fume react with and consume hydroxyl (alkaline) ions during hydration. This reaction lowers the pH value of the pore solution, suppressing the occurrence of ASR.
Pozzolanic additives differ in shape and reactivity depending on their source, but generally their effect is more homogeneous if added to the cement grinding process as opposed to the concrete mix. There remains however some dispute regarding the efficiency of additives in lowering the speed of ASR.
Admixtures such as traditional accelerators for shotcrete may introduce considerable quantities of alkalines and greatly increase the reactivity of the pore solution. In case of aggregates considered sensitive, alkali free accelerator should be used.
Experience has shown that inclusion of special admixtures can hem the ASR reaction, thus preventing expansion. A further possible solution is proposed with the addition of an air-entraining agent to create artificial expansion room (air voids) for the reaction products. If the possible occurrence of ASR represents a major concern, reaction trials are suggested to define the ASR potential.
The measures are:
- Partial replacement of the Portland cement by slag or other additions (Silica fume/fly ash) with low equivalent Na20.
- Analysis of the AAR/ASR potential of the aggregate and its classification (petrographic analyses/microbar test/performance testing etc.).
- Replacement or partial replacement of the critical aggregates (blending of available aggregates)„.Keep moisture access to the concrete low or prevent it (seal/divert).
- Reinforcement design for good crack distribution in the concrete (i.e. very fine cracks only)„.Impermeable concrete design to minimise the penetration of moisture.