Accelerated and natural carbonation of concrete with high volumes of fly ash : chemical, mineralogical and microstructural effects

Today, a rather poor carbonation resistance is being reported for high-volume fly ash (HVFA) binder systems. This conclusion is usually drawn from accelerated carbonation experiments conducted at CO2 levels that highly exceed the natural atmospheric CO2 concentration of 0.03-0.04%. However, such accelerated test conditions may change the chemistry of the carbonation reaction (and the resulting amount of CH and C-S-H carbonation), the nature of the mineralogical phases formed (stable calcite versus metastable vaterite, aragonite) and the resulting porosity and pore size distribution of the microstructure after carbonation. In this paper, these phenomena were studied on HVFA and fly ash thorn silica fume (FA + SF) pastes after exposure to 0.03-0.04%, 1% and 10% CO2 using thermogravimetric analysis, quantitative X-ray diffraction and mercury intrusion porosimetry. It was found that none of these techniques unambiguously revealed the reason for significantly underestimating carbonation rates at 1% CO2 from colorimetric carbonation test results obtained after exposure to 10% CO2 that were implemented in a conversion formula that solely accounts for the differences in CO2 concentration. Possibly, excess water production due to carbonation at too high CO2 levels with a pore blocking effect and a diminished solubility for CO2 plays an important role in this

Estimated Service Life of Carbonation Exposed (Cracked) Concrete with Pozzolans or Self-healing Agents

Today, concrete with large portions of ordinary Portland cement (OPC) replaced by fly ash and self-healing concrete count as potential sustainable alternatives to traditional concrete. For the first concrete type, the increased sustainability lies in lowering the carbon footprint which is largely attributable to cement. For the second one that should be able to heal cracks autonomously upon occurrence, an extended service life is the key objective. In this paper, both concrete types were evaluated in terms of service life in carbonation exposed environments. When using the probabilistic model for carbonation-induced steel depassivation of fib Bulletin 34 with mix specific curing exponents, it was found that in uncracked High-Volume Fly Ash (HVFA) and Fly Ash + Silica Fume (FA+SF) concrete carbonation should not reach the reinforcing steel at a typical cover depth of 35 mm within the envisaged design service life of 100 years. This requires a longer optimal curing period though (HVFA: ≥ 20 days; FA+SF: ≥ 9.1 days versus OPC: ≥ 4.2 days). Evaluation of cracked concrete suggests that in presence of a 25 mm deep, 300 µm wide crack the depassivation period would take no longer than 5 years. In case of partial crack healing with the proposed encapsulated polymer, this could be extended to only 11 years regardless of the carrier concrete type (OPC, HVFA or FA+SF, all properly cured for a sufficiently long time). A 100 % crack healing efficiency implying a return to the uncracked state should therefore always be aimed for. The now considered polymer seems reasonably efficient since this was the case for 8 out of 9 samples. To account for the risk of insufficient curing in combination with possibly having an unhealed/partially healed crack anyway, OPC binder systems still have the preference as carrier concrete for incorporation of the healing agent in exposure class XC3

Life cycle assessment of a column supported isostatic beam in high-volume fly ash concrete (HVFA concrete)

Nowadays, a lot of research is being conducted on high-volume fly ash (HVFA) concrete. However, a precise quantification of the environmental benefit is almost never provided. To do this correctly, we adopted a life cycle (LCA) approach. By considering a simple structure and an environment for the material, differences between traditional and HVFA concrete regarding durability and strength were taken into account. This paper presents the LCA results for a column supported isostatic beam made of reinforced HVFA concrete located in a dry environment exposed to carbonation induced corrosion. With a binder content of 425 kg/m3 and a water-to-binder ratio of 0.375, the estimated carbonation depth after 50 years for a 50 % fly ash mixture does not exceed the nominal concrete cover of 20 mm. As a consequence, no additional concrete manufacturing for structure repair needs to be included in the study. Moreover, structure dimensions can be reduced significantly due to a higher strength compared to the reference concrete used in the same environment. In total, about 32 % of cement can be saved this way. The reduction in environmental impact equals 25.8 %, while this is only 11.4 % if the higher material strength is not considered

Life cycle assessment of completely recyclable concrete

Since the construction sector uses 50% of the Earth. s raw materials and produces 50% of its waste, the development of more durable and sustainable building materials is crucial. Today, Construction and Demolition Waste (CDW) is mainly used in low level applications, namely as unbound material for foundations, e.g., in road construction. Mineral demolition waste can be recycled as crushed aggregates for concrete, but these reduce the compressive strength and affect the workability due to higher values of water absorption. To advance the use of concrete rubble, Completely Recyclable Concrete (CRC) is designed for reincarnation within the cement production, following the Cradle-to-Cradle (C2C) principle. By the design, CRC becomes a resource for cement production because the chemical composition of CRC will be similar to that of cement raw materials. If CRC is used on a regular basis, a closed concrete-cement-concrete material cycle will arise, which is completely different from the current life cycle of traditional concrete. Within the research towards this CRC it is important to quantify the benefit for the environment and Life Cycle Assessment (LCA) needs to be performed, of which the results are presented in a this paper. It was observed that CRC could significantly reduce the global warming potential of concrete

Resistance to chloride penetration of self-healing concrete with encapsulated polyuretyhane

Reinforcement corrosion induced by diffusion of chlorides is one of the most important damage mechanisms that leads to the deterioration of reinforced concrete structures. Cracking of reinforced concrete structures during their service life is almost inevitable. Cracks form preferential pathways for the ingress of chlorides and will accelerate the onset of corrosion and its propagation. In this paper, autonomous self-healing of cracks by encapsulated polyurethane is investigated as a possible method to heal cracks and reduce chloride ingress through cracks without human intervention. Cracks in concrete specimens were created in two ways: by means of thin metal plates to create standardized artificial cracks and by means of splitting tests to create realistic cracks. A crack width of 0.3 mm was chosen since most design codes limit the crack width to that value. The resistance to chloride penetration of autonomously healed concrete was evaluated by the diffusion test as described in NT Build 443. Uncracked, cracked and healed specimens were subjected to a 165 g/l NaCl solution for 7 weeks. After that period chloride profiles in the crack region and in an area further away from the crack were obtained by potentiometric titrations. From the resulting chloride profiles it was concluded that the polyurethane was very well able to seal both artificial and realistic cracks and reduce the chloride content in the crack zone significantly. At depths below the surface larger than 14 mm, healing was able to reduce the total chloride content in the crack zone by more than 70%

Sustainability effects of including concrete cracking and healing in service life prediction for marine environments

With today’s focus on sustainable design, it is necessary to adequately predict and prolong service life of concrete in marine environments. By introducing self-healing properties, service life extension can be achieved. However, in prediction models, the required concrete mix specific input is usually not available. Moreover, little attention goes to the unavoidable presence of cracks. Finally, autonomous crack healing has almost never been taken into account. In this paper, the relevant model input was estimated from experimental chloride profiles. It enabled an adequate prediction of the chloride-induced steel depassivation period for cracked and uncracked 15% fly ash concrete (8–104 years, respectively). Comparison with self-healing by means of encapsulated polyurethane indicated a 48–76% self-healing efficiency. It could extend the corrosion initiation period to 36–68 years. Being much less subject to time-dependent repair, PU based self-healing concrete has a 77–88% lower environmental impact than traditional (cracked) concrete

Towards an adequate deicing salt scaling resistance of high-volume fly ash (HVFA) concrete and concrete with superabsorbent polymers (SAPs)

The deicing salt scaling resistance has been investigated for two types of concrete, i.e., air entrained high-volume fly ash (HVFA) concrete with a 50% cement replacement and non-air entrained concrete containing superabsorbent polymers (SAPs). A full characterization of their air void systems from the moment of casting until the freeze/thaw test was also done. Due to the presence of the highly AEA adsorptive fly ash an increased AEA dosage (7.0 ml/kg binder) was needed to achieve an adequate air void system in terms of air content and spacing factor to keep salt scaling within acceptable limits. For the novel non-air entrained concrete type with SAPs, which are able to absorb up to 500 times their weight in fluids, the salt scaling resistance is surprisingly high. The microstructural analysis revealed the formation of macro-pores due to these SAPs, creating an air void system as can be found in air-entrained concrete. Another advantage is that the strength of concrete with SAPs is much higher than for a conventional air-entrained concrete. This substantiates the further use of these SAPs as admixture in precast concrete road elements