Development, Durability Studies and Application of High Performance Green Hybrid Fiber-Reinforced Concrete (HP-G-HyFRC) for Sustainable Infrastructure and Energy Efficient Buildings
- Author(s): Hay, Rotana;
- Advisor(s): Ostertag, Claudia P.;
- et al.
Concrete-related construction industry consumes considerable amount of energy, resulting in large CO2 release into the atmosphere. Cement which is used as the main binder in concrete is energy intensive to produce and contributes about 7% to total global anthropogenic carbon emission. Infrastructure across the globe suffers from durability problems and requires frequent repair and maintenance. This brings about high direct cost for rehabilitation and unaccounted indirect cost resulted from loss of productive time, traffic congestion and diversion, and in the process more CO2 emission. In the meantime, buildings which are part of the overall civil infrastructure system require extensive amount of energy to keep the internal environment comfortable to users. The sector accounts for about 40% of global primary energy consumption. With increasing population and demand, actions from various building disciplines are needed to build a more sustainable industry. This research addresses these issues through the development of a new high performance fiber-reinforced concrete, its durability studies and its application to reduce operational energy in buildings. Durability is critical for infrastructure systems whose frequent maintenance and rehabilitation pose adverse impacts to the environment and add considerable costs to the economy. By accounting for sustainability aspects from materials conception to usage and disposal, this study encompasses the concept of sustainability through life cycle consideration. This represents a deviation from conventional sustainable approach where a focus is usually spent on reducing embodied energy of concrete composites.
The first area of focus was on the development of a new concrete composite called high performance green hybrid fiber-reinforced concrete (HP-G-HyFRC) reinforced with polyvinyl alcohol (PVA) micro- and hooked-end steel macrofibers. For easy construction and durability, the design criteria were defined to cover high workability, high strength and deflection hardening which is defined as an ability of the composite to carry increasing load after the first crack is formed. It was demonstrated that theoretical analysis could be used to limit the number of trials in determining the critical fiber volume fractions for the deflection hardening behavior in the composite. As compared to conventional self-consolidating concrete (SCC), fine aggregate over coarse aggregate ratio had to be increased in FRC for enhanced workability. Addition of supplementary cementitious materials (SCMs) in concrete especially fly ash helped to improve the composite’s workability. This is attributed to fly ash’s favorable fineness, size distribution and spherical shape which resulted in ball-bearing action provided to other concrete constituents. PVA microfibers controlled propagation of micro cracks inherent in concrete or formed during loading. They also provided toughening around steel fibers and ensured a gradual pullout of steel fibers. The synergy of PVA micro- and steel macrofibers led to a smooth deflection hardening behavior of the composite under flexure at a relatively low fiber volume fractions of 1.5% steel fibers and 0.15% PVA fibers.
A study on corrosion performance of HP-G-HyFRC with accelerated corrosion test with an impressed current was then conducted. It was found that wide cracks ranging from 1.1 to 2 mm were observed in high performance concrete (HPC) without fibers. The presence of hybrid fibers in HP-G-HyFRC, on the other hand, reduced corrosion rates by half, attributable to crack bridging of fibers and the resulting formation of distributed cracks of small sizes. Also, under no applied current, all embedded steel rebars in HP-G-HyFRC were in the inactive corrosion zone even with the presence of 4% NaCl in the mixing water. Microscopic observation at steel-concrete interface showed a densification of corrosion products, which is postulated to limit iron dissolution and subsequently to reduce corrosion rates of the embedded bars. HP-G-HyFRC corrosion samples were also able to retain most of its strength after the accelerated corrosion tests.
As corrosion resistance of HP-G-HyFRC was considered at a composite level, the effects of individual mix component such as slag and fibers on corrosion were yet unknown. The next area of focus was on the influence of high-volume slag as cement replacement, hybrid fibers and steel-concrete interface on corrosion of steel in concrete. The studies elaborated various phenomena observed in the corrosion study of HP-G-HyFRC and also provided a fundamental understanding of different concrete parameters on corrosion.
It was found that due to shrinkage-induced cracking and possibly poor quality passive film due to the presence of reducing agents in concrete pore solutions, samples with 60% slag replacement and with no fiber reinforcement showed an early corrosion initiation and higher mass loss induced by the impressed current. Microstructural imaging showed that the samples with slag, despite having a higher gas permeability, showed a denser matrix but more continuous distributed microcracking in the matrix. This led to its poor ability to accommodate corrosion products at the interface and as a result the concrete experienced an early onset of cracking. Under the same regime of applied current, samples made of slag concrete also experienced higher gravimetric mass losses. This is attributed to a less stable passive film and more intense acidification at the interface due to a reduction in calcium hydroxide (CH) in the matrix. Also, an inclusion of hybrid fibers in concrete slightly increased concrete permeability although this did not adversely affect corrosion initiation performance of concrete. However, under propagation stage achieved by an induced current, hybrid fibers in concrete significantly reduced corrosion rates through confinement and densification of corrosion products at steel-concrete interface. The influence of interface qualities on corrosion of steel in concrete showed conflicting performance in corrosion initiation and propagation stages. It was found that higher porosity at the steel-concrete interface initiated an early corrosion. However, the porous interface could accommodate more corrosion products. This led to a smaller pressure buildup from the corrosion products and less damage to the surrounding concrete. As a result, smaller corrosion rates were observed in the samples with more porous interfaces after impressed current regimes. The finding helps to explain the more extensive damage in high performance concrete (HPC) as compared to normal strength concrete. This warrants the inclusion of fibers in HPC to extend the service life of structures constructed with the composite.
The study ended with a proposed application of HP-G-HyFRC in an innovative double skin façade (DSF) system in place of a conventional solid façade system to enhance operational energy performance of buildings. It was found that although the DSF is more energy intensive and more costly to construct, it allowed for a full recovery of the additional embodied energy within the first year of operation and cost recovery within the first 6 years of operation. The overall study exemplifies a life-cycle consideration adopted for materials design, durability investigation and application to ensure more sustainable infrastructure and buildings for our society.