This can lead to visible yellow viscous liquid exudations amber liquid droplets at the surface of affected concrete. Its viscosity increases due to gelation process and its mobility fluidity strongly decreases when C-S-H phases start to precipitate after reaction with calcium hydroxide portlandite. At this moment, the calcified gel becomes hard, hindering therefore the alkali gel transport in concrete. When the relatively fluid alkali gel continue to exude below the hardened superficial gel layer, it pushes the efflorescences out of the crack surface making them to appear in relief.
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This can lead to visible yellow viscous liquid exudations amber liquid droplets at the surface of affected concrete. Its viscosity increases due to gelation process and its mobility fluidity strongly decreases when C-S-H phases start to precipitate after reaction with calcium hydroxide portlandite. At this moment, the calcified gel becomes hard, hindering therefore the alkali gel transport in concrete.
When the relatively fluid alkali gel continue to exude below the hardened superficial gel layer, it pushes the efflorescences out of the crack surface making them to appear in relief. Because the gel drying and carbonation reactions rates are faster than the gel exudation velocity liquid gel expulsion rate through open cracks , in most of the cases, fresh liquid alkali exudates are not frequently encounterered at the surface of civil engineering concrete structures.
Decompressed concrete cores can sometimes let observe fresh yellow liquid alkali exudations viscous amber droplets just after their drilling. Calcium ions then react with the soluble sodium silicate gel to convert it into solid calcium silicate hydrates C-S-H. The C-S-H forms a continuous poorly permeable coating at the external surface of the aggregate. The resulting expansive pressure increases in the core of the aggregate. The accumulated pressure cracks the aggregate and the surrounding cement paste when the pressure exceeds the tolerance of the aggregate.
Compressive strength : The effect of ASR on compressive strength can be minor for low expansion levels, to relatively higher degrees at larger expansion. Swamy R. The modulus of elasticity is shown to be more sensitive to ASR than pulse velocity. Shear strength : ASR enhances the shear capacity of reinforced concrete with and without shear reinforcement Ahmed T. Mitigation[ edit ] ASR can be mitigated in new concrete by several complementary approaches: Limit the alkali metal content of the cement.
Many standards impose limits on the "Equivalent Na2O" content of cement. Limit the reactive silica content of the aggregate. Certain volcanic rocks are particularly susceptible to ASR because they contain volcanic glass obsidian and should not be used as aggregate. The use of calcium carbonate aggregates is sometimes envisaged as an ultimate solution to avoid any problem. However, while it may be considered as a necessary condition, it is not a sufficient one.
In principle, limestone CaCO3 is not expected to contain a high level of silica, but it actually depends on its purity. Indeed, some siliceous limestones a.
So, the use of limestone as aggregate is not a guarantee against ASR in itself. Add very fine siliceous materials to neutralize the excessive alkalinity of cement with silicic acid by deliberately provoking a controlled pozzolanic reaction at the early stage of the cement setting. Convenient pozzolanic materials to add to the mix may be, e.
Another method to reduce the ASR is to limit the external alkalis that come in contact with the system. In other words, as it is sometimes possible to fight fire with fire , it is also feasible to combat the ASR reaction by itself. A prompt reaction initiated at the early stage of concrete hardening on very fine silica particles will help to suppress a slow and delayed reaction with larger siliceous aggregates on the long term. Following the same principle, the fabrication of low-pH cement also implies the addition of finely divided pozzolanic materials rich in silicic acid to the concrete mix to decrease its alkalinity.
Beside initially lowering the pH value of the concrete pore water, the main working mechanism of silica fume addition is to consume portlandite the reservoir of hydroxyde OH— in the solid phase and to decrease the porosity of the hardened cement paste by the formation of calcium silicate hydrates C-S-H.
However, silica fume has to be very finely dispersed in the concrete mix, because agglomerated flakes of compacted silica fume can themselves also induce ASR if the dispersion process is insufficient.
This can be the case in laboratory studies made on cement pastes alone in the absence of aggregates. However, most often, in large concrete batches, silica fume is sufficiently dispersed during mixing operations of fresh concrete by the presence of coarse and fine aggregates. As part of a study conducted by the Federal Highway Administration , a variety of methods have been applied to field structures suffering from ASR-affected expansion and cracking.
Some methods, such as the application of silanes , have shown significant promise, especially when applied to elements such as small columns and highway barriers, whereas other methods, such as the topical application of lithium compounds, have shown little or no promise in reducing ASR-induced expansion and cracking.
Repair in damaged sections is possible, but the reaction will continue. In some cases, when a sufficient drying of thin components walls, slabs of a structure is possible, and is followed by the installation of a watertight membrane , the evolution of the reaction can be slow down, and sometimes stopped, because the lack of water to continue to fuel the reaction. Indeed, water plays a triple role in the alkali-silica reaction: solvent for the reaction taking place, transport medium for the dissolved species reacting, and finally also reagent consumed by the reaction itself.
However, concrete at the center of thick concrete components or structures can never dry because water transport in saturated or unsaturated conditions is always limited by diffusion in the concrete pores water present under the liquid form, or under the vapor state.
The water diffusion time is thus proportional to the square of its transport distance. Massive structures such as dams pose particular problems: they cannot be easily replaced, and the swelling can block spillway gates or turbine operations. Cutting slots across the structure can relieve some pressure, and help restore geometry and function. Example of standard for concrete in Belgium: NBN EN and its national supplement NBN B   ; by limiting the contact of underground or meteoritic water infiltrations with the concrete structure water tight membrane, roofing, sufficient water drainage, This last precaution is always advisable when possible and the only one also sometimes applicable for existing ASR-affected concrete structures.
A large pool of OH— anions is thus available to diffuse inside the mortar bar to dissolve silica present in aggregates. Consequently, this test is very severe and may exclude valuable aggregates. In case of non-decisive results, the long-term ASTM C test method has to be used for a final screening.
The main advantage of the ASTM C test is that it allows to quickly identify extreme cases: very insensitive or very reactive aggregates. The concrete prisms are not directly immersed in an alkaline solution, but wrapped with moist tissues and tightly packed inside a water-tight plastic foils. The Dungan method with superimposed additional thermal cycles. The concrete microbar test was proposed by Grattan-Bellew et al. Alkali reaction in concrete. Residual expansion tests on hardened concrete.
Known affected structures[ edit ] Surface of a concrete pillar of the building of the National Gallery of Canada at Ottawa presenting the typical crack pattern of the alkali-silica reaction ASR.
The 4 C — 03 method is particularly applicable to rapid control analysis of crystalloidal silica below 10 ppm. However, potentially deleterious aggregates aetm by points lying above the dashed line in Fig. The alkalisilica reaction asr, more commonly known as concrete cancer, is a swelling reaction that occurs over time in concrete between the highly alkaline cement paste and the reactive noncrystalline silica found in many common aggregates, given sufficient moisture. Reserve this diluted solution for the determination of the dissolved SiO2 and the reduction in alkalinity.