Abstract
The need to improve the durability and performance of concrete structures has led to higher research efforts on performance based concrete design. The ability of concrete to withstand aggressive chloride and deicing environments is a requirement in regions associated with high amount of snow fall. In other regions with high seismic activities, the requirements would be sufficient ductility for the concrete structure to undergo the inelastic deformation that usually occur during earthquakes. Therefore, the focus of this study is to evaluate, and develop new guidelines on sustainable and affordable concrete mixtures that could be used in aggressive chloride and deicing environments as well as in high seismic regions. Supplementary cementitious materials (SCMs) such as fly-ash, silica fume, and glass powder, are used in concrete structures to be exposed to aggressive chloride and deicing environment during their service life. However, SCMs are either expensive or unavailable due to government policy on burning of coal. Biomass-fly ash (BFA) obtained from residues of woods in fluidized-bed system, or pyrolysis of de-inking sludge can be used as an alternative supplementary cementitious materials (ASCMs).
Concrete samples in form of cylinders and prism bars were made from the concrete mixtures developed in this study. The concrete mixtures were replicated from the commonly used mixture design by state department of transportation (DOTs), including the Idaho department of Transportation (ITD). These samples were tested for short-term and long-term durability performance including deicing scaling, freezing and thawing cycles, continuous soaking, and mechanical properties. The mixtures without SCMs displayed heavy deterioration in form of spalling, cracking and deicing scaling.
Subsequently, alternative concrete mixtures were developed by altering the type of SCMs and percentage content of the SCMs up to 25% by weight of the cement. These alternative mixtures were also tested for durability and mechanical properties. The alternative mixtures performed well, with minor to no scaling.
The limitations to the implementation of the alternative include economics and availability of fly ash. As a result, an alternative SCMs, the nanoglass powder (NGP) was suggested. The NGP was used as a replacement for cement in concrete mixture. The nanoglass powder (NGP) was obtained from the crushing of soda glass into nano size particles. The NGP was used as a replacement for fly ash because of its high silica content. The concrete samples made from the mixtures containing NGP were tested for expansion due to alkali silica reaction, and mechanical properties. The cost of producing the NGP made it difficult to implement the concrete in commercial quantity.
To this end, the BFA being abundantly available as waste products from saw milling industries and steam plants, was collected, characterized and used instead of conventional fly ash as cement replacement up to 30% by weight.
As challenges of dredging of rivers for natural sand continue to increase, a shift to the use of manufactured sand (MS) as an alternate source of aggregate is necessary. MS is obtained by the mechanical crushing of virgin rock to a desire aggregate gradation. However, the characteristics of the MS particles completely differ from natural sand or river sand in terms of shape, surface texture and mechanical strength. The BFA was obtained from the University of Idaho steam plant and the Portland limestone cement (PLC) was obtained from a premix concrete factory. The BFA was analyzed for mineral and chemical composition. The natural sand (NS) and manufactured sand (MS) were also analyzed for surface texture and particle shape.
It was observed that the mineral composition of the BFA is like class C fly ash. At 15% of cement replacement with BFA, the paste exhibits better rheological properties: lower yield stress and lower viscosity up till 120 min after mixing, which is an important factor in ready mix concrete plant. However, a better pozzolanic behavior was observed at 20% cement replacement. From the results obtained, the properties of the paste containing BFA is very sensitive to water/binder ratio (w/b). Above 20% cement replacement, it is suggested to use viscosity modifying agent (VMA) to get a better rheology and pozzolanic behavior.
The addition of BFA to the concrete mixtures resulted in high strength concrete (HSC) with improved durability and mechanical properties, but lower ductility. The lower ductility is because of crack propagation through the aggregate rather than the hydrated paste. Six reinforced concrete beams were cast with the resulting high strength concrete and the beams were tested under different load regime to evaluate the influence of shear span to effective depth ratio on the structural behavior of the high strength concrete beams.
The experimental testing comprised high strength reinforced concrete beams, with and without shear reinforcement, tested under four-point bending with a/d ranging from 2.0 to 3.0. The experimental results were validated with empirical equations in various design codes. The a/d ratio has higher influence on the shear strength of reinforced HSC beams without shear reinforcement than beams with shear reinforcement. Most of the shear resistance prediction models underestimate the concrete shear strength of the beams but overpredict shear resistance of beams with shear reinforcement. However, the fib Model code 2010 accurately predicted the shear resistance for all the beams within an appropriate level of approximation.
The outcome of this study reveals the need for performance based concrete design guidelines because there is no one fit all design that can fulfil the requirements of durability and ductility, even though sustainable and affordable concrete construction can be produced from manufactured sand and biomass fly ash.