Surface corrosion of steel fibre reinforced concrete

Corrosion of SFRC (Steel Fibre Reinforced Concrete) in an adverse environment, less harmful as compared to corrosion of steel reinforced concrete, is often considered to be of minor importance, however it exists. It can affect the fibres bridging the cracks and then decrease the strength of the concerned structures. As well, it results in the appearance of corrosion spots at exposed surfaces. Then the damaging effect is no longer expressed in terms of resistance, it is only aesthetic. It is especially undesirable in prefabricated structures.The work developed in this paper focuses on this second point: surface corrosion. Fibre reinforced concrete prisms have been subjected to cycles of salt fog (1 week) and drying (1 week). The results obtained demonstrate that with high W/C ratio (0.78), all the fibres which are embedded in concrete less than 1 mm are susceptible to give corrosion spots at the surface. When W/C is reduced to about 0.5, the minimum necessary cover to prevent surface corrosion drops to 1/10 mm or 2/10 mm and further decrease of W/C does not bring extra significant benefit. This result is in agreement with the analysis, by mercury intrusion, of the skin concrete porosity. It confirms a sharp change in the pore diameters when W/C is decreased from 0.78 to 0.48 and a quasi stability when it is varied from 0.48 to 0.36.     

Statistical analysis of the carbonation rate of concrete

The carbonation rate of concrete has implications for the lifecycle carbon emissions of concrete. This paper describes the reported effect of several variables on the rate of concrete carbonation and collates a data set of measurements published in the literature. Many studies producing predictive models for the carbonation rate constant, K, use only small data sets. 1999 measurements of carbonation depth as a function of time and other variables were collected for analysis. Models in the form ln (K) = a + bI1 + cI2 + … have been produced by which the rate of carbonation can be predicted. Hierarchical Models were used to combine different authors' data and introduces a new explanatory variable called ‘origin’, which indicates whether the concrete was taken from a working structure or cast specifically for experiments. Two models of the carbonation rate using concrete properties have been produced, allowing prediction of K over a range of conditions and compositions.     

Efficiency investigations of electrochemical realkalisation treatment applied to carbonated reinforced concrete — Part 1: Sacrificial anode process

The aim of this study was to assess the efficiency of a realkalisation treatment using sacrificial anodes applied to reinforced concrete degraded by carbonation. Analytical determinations (acid/base indicators, quantitative pH, alkaline profiles, SEM and micro-Raman) together with electrochemical characterizations (rest potential, impedance, linear polarisation resistance and corrosion current densities) were performed on artificially carbonated slabs, before and after treatment (mainly 15 days, 11 weeks, 6 and 12 months). The treatment efficiency was demonstrated by an increase of pH and by an alkaline ion penetration in the concrete cover. Rest potential and corrosion current densities indicated a slight decrease of the rebar corrosion activity. Complementary Raman spectroscopy showed a change in the oxide species and SEM observations indicated that the cement matrix remained almost unchanged.     

Self-sealing of cracks in concrete using superabsorbent polymers

Cracks in concrete can self-heal when exposed to prolonged wetting, but this is limited to narrow cracks. In practice, cracks > 0.2 mm cause leakage and impair performance of structures. The potential of superabsorbent polymers (SAPs) to self-seal such cracks was investigated via transport experiments, microscopy and modelling. Forty samples containing SAP and through-thickness cracks were subjected to 0.12 wt.% NaCl at 4 m/m pressure gradient to simulate groundwater seepage. Results show that SAP can re-swell and seal cracks, for example in the case of 0.3 mm cracks reducing peak flow rate and total flow by 85% and 98% respectively. Increasing SAP dosage accelerates sealing, but imparts a strength penalty and this limits practical applications. Modelling suggests that the effectiveness of SAP can be enhanced by increasing its re-swelling ratio and particle size, and depressing its initial swelling. These variables increase the SAP exposed in a crack and the gel volume available to seal it.     

Chloride-induced corrosion of steel in cracked concrete – Part I: Experimental studies under accelerated and natural marine environments

Parallel corrosion experiments were carried out for 2¼ years by exposing one half of 210 beam specimens (120 × 130 × 375 mm long) to accelerated laboratory corrosion (cyclic wetting and drying) while the other half underwent natural corrosion in a marine tidal zone. Experimental variables were crack width w_cr (0, incipient crack, 0.4, 0.7 mm), cover c (20, 40 mm), binder type (PC, PC/GGBS, PC/FA) and w/b ratio (0.40, 0.55). Results show that corrosion rate (i_corr) was affected by the experimental variables in the following manner: i_corr increased with increase in crack width, and decreased with increase in concrete quality and cover depth. The results also show that the corrosion performance of concretes in the field under natural corrosion cannot be inferred from its performance in the laboratory under accelerated corrosion. Other factors such as corrosion process should be taken into account.     

Study of glassy polymers fractional accessible volume (FAV) by extended method of hydrostatic weighing: Effect of porous structure on liquid transport

In this study, 11 different hydrophobic materials made of high permeability glassy polymers such as PTMSP, PBTMST, PTMST, PTMGP, PMP, PIM-1, PVTMS, as well as polymeric blends based on PTMSP/PVTMS with varied fractional free volume, were studied by the extended method of hydrostatic weighing. Results clearly indicate the presence of interconnected pre-existing free volume elements (microcavities or micropores), which are accessible for the liquid molecules without polymer swelling. Depending on the polymer, the contribution of pre-existing microcavities to the free volume of polymer varies from 35% for PIM-1 to 85% for PTMSP/PVTMS (90/10). Using the proposed method, it was possible to estimate total fractional accessible volume FAV_t: PTMSP (30%) > PTMSP/PVTMS (90/10) (27%) > PTMSP/PVTMS (80/20) (25%) > PBTMST (24%) > PTMST (23%) > PTMGP (22%) > PTMSP/PVTMS (70/30) (21%) > PIM-1 (17%) > PMP (16%) > PTMSP/PVTMS (40/60) (11%) > PVTMS (4%). The applicability of the extended method of hydrostatic weighing for evaluation of the porous structure of the polymeric materials was confirmed by good agreement with the literature data on fractional free volume FFV estimated by PALS. FAV can be considered as a uniform parameter to describe the solvent transport regardless of the difference in the nature of high permeability glassy polymers. It was found that there is a threshold value of FAV_t, estimated as 12%, which is required for establishment of liquid permeability. FAV_t values obtained for PVTMS (4%) and PTMSP/PVTMS (40/60) (11%) were not high enough to provide the formation of liquid percolation clusters, and, hence, liquid transport across the membrane at 20 bar. Investigation of water–ethanol transport through PTMSP, PTMGP, PMP and PIM-1 showed that all polymers had two regions: (i) at lower concentration of ethanol, hydrophobic glassy polymers showed absence or hardly detectable liquid transport, (ii) the increase of ethanol concentration led to the establishment and further increase of liquid transport through the dense membranes. PTMSP, PTMGP, PMP and PIM-1 kept their barrier properties till the threshold value of FAV was equal to 26%, 17%, 15% and 12%, respectively. Such behavior was explained in terms of boundary conditions for formation of a percolation cluster within a specific glassy polymer with respect to its properties and chemical nature. Once such clusters were formed in the bulk material, no further noticeable increase in FAV_t was required to enhance the liquid transport through the membrane. The different behavior of PIM-1 was attributed to the presence of a noticeable fraction of isolated holes.     

Chloride-induced corrosion of steel in cracked concrete—Part II: Corrosion rate prediction models

Chloride-induced corrosion rate (i_corr) prediction models for RC structures in the marine tidal zone that incorporate the influence of crack width (w_cr), cover (c) and concrete quality are proposed. Parallel corrosion experiments were carried out for 2¼ years by exposing one half of 210 beam specimens (120 × 130 × 375 mm long) to accelerated laboratory corrosion (cyclic wetting and drying) while the other half underwent natural corrosion in the tidal zone. Experimental variables were w_cr (0, incipient crack, 0.4, 0.7 mm), c (20, 40 mm), binder type (PC, PC/GGBS, PC/FA) and w/b ratio (0.40, 0.55). The two proposed models (one each for accelerated and natural i_corr) can aid not only in quantifying the propagation phase, but also provide a novel way to select c, w_cr and concrete quality.     

Microbiologically induced concrete corrosion: A case study from a combined sewer network

In this study, a strongly deteriorated concrete-based sewer system was investigated by using a multi proxy approach based on gaseous, hydro-geochemical, microbiological, mineralogical and mechanical analyses. Therefore, gas, liquid, and solid samples were taken throughout the entire sewer system. Long term measurements of gaseous hydrogen sulfide (H₂S) within the sewer atmosphere yielded concentrations up to 367 ppm. Interstitial fluids, extracted from deteriorated concrete by squeezing, contained sulfate (SO₄^2 −) concentrations of up to 104 g l^− 1 at strong acidic conditions (0.7 > pH > 3.1) and are close to the saturation state of gypsum. This sulfuric acid attack is indicative for a well-established biofilm containing sulfide oxidizing bacteria (SOB), which was analyzed to consist mainly of Acidithiobacillus thiooxidans. The micro-structure of the attacked concrete displays a progressing alteration zone, which is caused by microbially induced concrete corrosion (MICC), with a suggested pH gradient from about 13 to < 1, from the intact inner concrete zone to the outermost heavily deteriorated concrete. Calcium sulfate minerals such as gypsum (CaSO₄ · 2H₂O), bassanite (CaSO₄ · 1/2H₂O) and anhydrite (CaSO₄) are abundant in the altered concrete, which were formed from the dissolution of the cement phases and Ca-bearing aggregates. Remarkably high corrosion rates of different precast concrete manholes were quantified to reach values greater than 1 cm yr^− 1, despite the fact that C₃A-free cement, fly ash and a w/c of ~ 0.35 was used.     

Mechanical properties of geopolymer concrete exposed to dynamic compression under elevated temperatures

Through adoption of a self-designed high temperature SHPB apparatus herein, an experimental study is made on the mechanical properties of geopolymer concrete (GC) exposed to dynamic compression under elevated temperatures. As the results have turned out, the weight loss is remarkable within temperature ranges from room temperature to 200 °C as well as from 600 °C to 800 °C. The dynamic compressive strength of GC grows higher at 200 °C than at room temperature, but suffers a dramatic drop at 800 °C. The critical strain is higher at elevated temperature than that at room temperature. At 200 °C and 600 °C, respectively, its energy absorption property is superior to that at room temperature. However, at 400 °C and 800 °C, respectively, it is inferior to that at room temperature. The strain rate effect of the dynamic increase factor (DIF) obtained from test data can reflect the inherent nature of GC. The DIF assumes a linear relationship with the logarithm of strain rate.     

Hardened properties of high-performance printing concrete

This paper presents the hardened properties of a high-performance fibre-reinforced fine-aggregate concrete extruded through a 9 mm diameter nozzle to build layer-by-layer structural components in a printing process. The printing process is a digitally controlled additive method capable of manufacturing architectural and structural components without formwork, unlike conventional concrete construction methods. The effects of the layering process on density, compressive strength, flexural strength, tensile bond strength and drying shrinkage are presented together with the implication for mix proportions. A control concrete (mould-cast specimens) had a density of approximately 2250 kg/m³, high strength (107 MPa in compression, 11 MPa in flexure) and 3 MPa in direct tension, together with a relatively low drying shrinkage of 175 μm (cured in water) and 855 μm (cured in a chamber at 20 °C and 60% relative humidity) at 184 days. In contrast well printed concrete had a density of 2350 kg/m³, compressive strength of 75–102 MPa, flexural strength of 6–17 MPa depending on testing direction, and tensile bond strength between layers varying from 2.3 to 0.7 MPa, reducing as the printing time gap between layers increased. The well printed concrete had significantly fewer voids greater than 0.2 mm diameter (1.0%) when compared with the mould-cast control (3.8%), whilst samples of poorly printed material had more voids (4.8%) mainly formed in the interstices between filaments. The additive extrusion process was thus shown to retain the intrinsic high performance of the material.     

Fire spalling of concrete,as studied by NMR

The moisture transport in concrete subjected to fire is one of the most important processes with respect to fire spalling. The research on fire spalling of concrete is currently lacking experimental information of the moisture transport processes.We present combined moisture content and temperature profiles of one-sided heated concrete samples measured with our dedicated NMR setup. The concrete samples were equilibrated at different moisture contents ranging from 97 to 50% RH. The moisture content can be measured quantitatively and non-destructively while heating up the sample one-sided to 500 °C.We present the first experimental proof for the build up of a moisture peak in concrete, and the formation of a saturated layer. The temperatures measured at the boiling front indicate a vapour pressure in the order of 1.8 MPa. A simple vapour transport model was successfully used to describe the speed of the boiling front.     

Heat transport enhancement of thermal energy storage material using graphene/ceramic composites

Graphene/ceramic composites are proposed by directly depositing graphene on the insulating Al₂O₃ particles by chemical vapor deposition without any metal catalysts. Carbothermic reduction occurring at the Al₂O₃ surface is vital during the initial stage of graphene nucleation and the graphene sheet can connect with neighboring sheets to completely cover Al₂O₃ particles. The quality and layer number of graphene on Al₂O₃ can be finely tailored by changing the growth temperature and gas ratio. Graphene coated Al₂O₃ (G-Al₂O₃) composites are used as effective fillers of stearic acid (SA) to increase the thermal transport property. By the optimization of the layer number of graphene, size of Al₂O₃ particles and ratio of G-Al₂O₃/SA in a quantitative, their thermal conductivities significantly increase up to 11 folds from 0.15 to 1.65 W m⁻¹ K⁻¹. The great improvement is attributed to the high thermal transfer performance of graphene and excellent wettability between graphene and SA. When the G-Al₂O₃/SA composites with the graphene coated porous Al₂O₃ foam, the thermal conductivity further reaches to 2.39 W m⁻¹ K⁻¹, and the corresponding latent heat is 38 J g⁻¹. It demonstrates the potential applications of graphene in thermal transport and thermal energy storage devices.     

Permeability measurement on high strength concrete without and with polypropylene fibers at elevated temperatures using a new test setup

A new test setup for permeability measurement at room and high temperature is presented. The experimental results obtained by employing the new setup are reported and validated. The experiments are performed on high performance concrete, without and with addition of polypropylene fibers under temperatures ranging from 20 °C to 300 °C as well as after cooling of previously heated specimens to the room temperature. The results show that plain concrete exhibits steady increase in permeability with increasing temperature, whereas concrete with fibers exhibit a sudden increase of permeability at temperatures between 80 °C and 130 °C. The results confirm the governing role of permeability on explosive spalling and suggest the existence of mechanisms of pressure relief other than just melting of fibers. The microstructure of concrete with fibers is investigated using SEM before and after exposure to high temperature. It is observed that the melted polypropylene flows only into the micro-cracks and does not penetrate into cement paste.     

Effect of fibre type and geometry on maximum pore pressures in fibre-reinforced high strength concrete at elevated temperatures

This paper presents results of an experimental and statistical study which investigates the effect of fibre type and geometry on the amount of maximum pore pressures measured at different depths in fibre-reinforced high strength concrete (HSC) exposed to elevated temperatures. Polypropylene, polyvinyl alcohol and steel fibres of varying lengths and diameters were used. Pore pressure measurements showed that addition of organic fibres regardless of the type significantly contributes to pore pressure reduction in heated HSC. Polypropylene fibres were more effective in mitigating maximum pore pressure development compared to polyvinyl alcohol fibres while steel fibres had a slightly low effect. Longer organic fibres of length 12 mm with smaller diameters of 18 μm showed better performance than shorter ones of length 6 mm with larger diameters of 28 and 40 μm. Based on experimental observations and using statistical analysis, a relationship to predict maximum pore pressures in heated concrete was developed.     

Cavitation resistance of refractory concrete: Influence of sintering temperature

The purpose of this study was to investigate possible application of refractory concrete in conditions of cavitation effect where the metal materials are usually applied. Also, influence of different sintering temperatures on cavitation resistance of refractory concrete was analyzed. As the refractory concrete, low cement castable was synthesized, cured, and then sintered at three different temperatures: 1100, 1300, and 1600 °C. Mass loss and surface degradation of investigated samples were monitored for 3 h during the exposure to the cavitation erosion. The results show that the samples sintered at 1100 °C are not suitable for application in conditions of cavitation since they were significantly destroyed only after 30 min of testing. On the other side, the samples sintered at 1300 and 1600 °C exhibited very good cavitation resistance, since they were stable during 360 min of cavitation testing with the damage level below 30% compared to the original surface.     

Simulation of supercritical water–hydrocarbon mixing in a cylindrical tee at intermediate Reynolds number: Formulation,numerical method and laminar mixing

The objective of this work is to study the flow dynamics and mixing of supercritical water and a model hydrocarbon (n-decane), under fully miscible conditions, in a small scale cylindrical tee mixer (pipe ID = 2.4 mm), at an intermediate inlet Reynolds number of 500 using 3-D CFD simulations. A Peng–Robinson EoS with standard van der Waals mixing rules is employed to model the near-critical thermodynamics with the mixture binary interaction parameter obtained from a Predictive Peng–Robinson EoS using group contribution theory (PPR78). The n-decane stream is introduced at the colder temperature of 700 K to ensure operation above the Upper Critical Solution Temperature (UCST, 632 K) of the water n-decane system while the water stream enters at a higher temperature of 800 K. Under these conditions, the flow in the tee mixer remains laminar and steady-state is reached. Mixing occurs predominantly due to the circulating action of a counter-rotating vortex pair (CVP) in the body of the hydrocarbon jet entering from the top. This CVP is formed due to the reorientation of the streamwise vorticity pre-existing within the hydrocarbon jet as it flows down the vertical pipe of the tee junction. The advective transport is further assisted by a secondary flow of water from the bottom stream, around the hydrocarbon jet, toward the space vacated near the top of the downstream pipe section by the downward motion of the HC jet. The CVP becomes progressively weaker due to vorticity diffusion as it is advected downstream and beyond 10–12 diameter lengths downstream of the mixing joint, transport is mainly controlled by molecular diffusion. It was found that the variations of density and transport properties with temperature do not have a significant impact on the flow and mixing dynamics for a ΔT = 100 K between the two streams. Local cooling of the fluid mixture was also observed in the mixing of water and n-decane streams entering at the same temperature (initially isothermal). This cooling effect is due to the diffusion of species along a gradient in their partial enthalpy in the mixture. Such gradients in species partial enthalpies are non-zero under near-critical conditions even for initially isothermal flows due to the non-ideality of the fluid mixture under these conditions. This local heating/cooling effect at near-critical conditions could give rise to unexpected formation of phases when operating close to critical points.     

Preparation of high-surface-area carbon nanoparticle/graphene composites

A method has been developed for synthesizing high-surface-area carbon nanoparticle/graphene composites. Functionalized carbon nanoparticles were anchored to the graphene planes and function as spacers to prevent the restacking of graphene sheets during drying. The composite has a layered structure in which functionalized carbon nanoparticles are sandwiched between graphene stacks. This layering leads to a porous structure with a specific surface area as high as 1256 m²/g. Such a structure provides easy access to both sides of the graphene for either gas or liquid species and allows their fast transfer. A specific capacitance as high as 324.6 F/g at a current density of 0.3 A/g was achieved using the composites in a supercapacitor.     

Modelling concrete deterioration in sewers using theory and field observations

Samples of new and 70 year old pre-corroded OPC concrete were exposed for up to 48 months in 6 sewers throughout Australia. Corrosion losses at each site followed the bi-linear trend originally proposed by Wells and Melchers [1]. During an initial phase (lasting < 2 years) negligible loss of material occurs however once the surface pH = 6 losses commence and accumulate linearly at a rate that is likely to remain constant over time. Corrosion rates were found to be sensitive to humidity but insensitive to concrete alkalinity. A first pass model which predicts the rate of concrete sewer pipe corrosion from a knowledge of local average sewer gas temperature, humidity and H₂S concentrations was also developed. The equation predictions were in good agreement with rates determined from field observation and historical data.     

Assessment of the protective effect of carbonation on portlandite crystals

The kinetics of many reactions important to cement hydration and use are not well understood: this is in part due to the great complexity of many supposedly “simple” processes. One such process, carbonation of portlandite, Ca(OH)₂, in moist air at ~ 23 °C has been investigated by microscopy and microchemical analysis. Single crystals of portlandite were grown, carbonated at relative humidities between ~ 25 and ~ 90%, and the transport properties of the self-generated calcite, CaCO₃, product film were determined.The calcite films thus grown within days or weeks varied in thickness but typically were polycrystalline and epitaxial: a variety of morphologies and surface features are recorded. Permeation was measured by determining the time taken for Ca^2 + ions, arising from the Ca(OH)₂ substrate, to diffuse through the calcite coat into initially pure water. The spontaneous formation of self-protecting films on concrete has long been envisaged: results demonstrate that passivation can actually be achieved.     

Open circuit voltage increase by substituted spacer and thieno[3,4-c]pyrrole-4,6-dione for polymer solar cells

We reported on two donor polymers containing thieno[3,4-c]pyrrole-4,6-dione(TPD) derivatives as electron withdrawing units for organic photovoltaics (OPVs). To control molecular weight and solubility of polymers, hexyl side chains are inserted to thiophene spacers. Due to the electron donating characteristic of hexyl side chains, highest occupied molecular orbital (HOMO) energy level of polymer is decreased as 0.18 eV, whereas the open circuit voltage is increased to 1.08 V. When bulk heterojunction devices were fabricated, the best PCE value of 0.360% (V_OC = 0.89 V, J_SC = 1.2 mA/cm², FF = 36.3%) under 100 mW/cm² irradiation.