For efficient asset management of reinforced concrete infrastructure, owners want to know when to... more For efficient asset management of reinforced concrete infrastructure, owners want to know when to expect corrosion in structures. At present, no widely accepted procedures for testing of existing structures for the remaining time to corrosion initiation are available. This paper describes such a procedure, based on the authors’ long-time experience. From about 20 years age, the concrete contains its mature response to actual environmental loads, e.g. chloride ingress. By measuring the mean and standard deviation of the actual cover depth, taking chloride profiles, assuming a few parameters and a simple, pragmatic model, the expected time to corrosion initiation for a particular test area can be predicted. Required numbers of cores and samples per core are given. Uncertainties are taken into account by subtracting a safety margin from the measured concrete cover depth. Results of six cores per test area are classified and interpretations are given. Because of large variability, the r...
Asset managers would benefit from knowing when to expect corrosion initiation in a particular rei... more Asset managers would benefit from knowing when to expect corrosion initiation in a particular reinforced concrete structure. However, accepted approaches to test existing structures for the remaining time to corrosion initiation are lacking. This paper proposes such an approach, based on experience in the field and additional considerations. From say 20 years age, existing structures embody the concrete’s response to actual environmental loads, e.g. in chloride profiles. Based on measuring the actual cover depth, taking (at least six) chloride profiles, some assumed parameters and a simple model, the expected time to corrosion initiation for a particular test area can be predicted. Sampling frequencies are given. Uncertainties can be taken into account by applying a safety margin to the cover depth. Results of at least six tests are classified and suggestions for interpretation are given. Because the accuracy is limited, the results are classified in three broad ranges: 5 years or l...
ABSTRACTIn this paper results of efforts to prepare stable aqueous suspensions of high surface ar... more ABSTRACTIn this paper results of efforts to prepare stable aqueous suspensions of high surface area (> 100 m2 /gr) yttria stabilized tetragonal zirconia powders are presented using a low molecular weight ammonium polyacrylic acid deflocculant. Zetapotentials, viscosities, agglomerate sizes and sedimentation volumes have been measured to find the optimum pH and deflocculant concentration. It has been found that the optimum pH-value coincides with the pH at which the polyacrylic acid is fully dissociated. Using deflocculant concentrations below the optimum value leads to a highly unstable system, while concentrations above the optimum value influence the stability in a much weaker way.
ABSTRACTA non-isothermal analysis of grain growth in tetragonal zirconia ceramics doped with vari... more ABSTRACTA non-isothermal analysis of grain growth in tetragonal zirconia ceramics doped with various amounts of yttria and/or ceria in the temperature range 700–1150°C is presented. In these nanocrystalline ceramics, prepared by the gel precipitation technique, two grain growth regimes are recognized. At temperatures up to 900–1000°C grain growth proceeds slowly at high values of porosity (55%→35%); probably by means of a surface diffusion mechanism. At higher temperatures grain growth is much faster and occurs in locally dense regions by a normal grain growth or an impurity drag mechanism. Apparent activation energies are given for five compositions in both regimes.The isothermal sintering behavior of 3 mol% Y2O3-containing TZP at 1050°C is analysed. A dense ceramic with a crystallite size in the nanometer regime (< 100nm) is obtained.
IABSE Congress, Seoul 2012: Innovative Infrastructures – Towards Human Urbanism, 2012
In this paper the risk on structural failure of some heavily deteriorated bridges constructed wit... more In this paper the risk on structural failure of some heavily deteriorated bridges constructed with post-tensioned prefabricated T-girders is described. The bridges have undergone three detailed inspections with an interval of approximately five years. All visual deterioration was recorded and classified. To check the actual state of corrosion, at four viaducts with severe deterioration, the risk of corrosion of the prestressing bars has been thoroughly assessed. With high pressure water-jetting the anchorages of the prestressing bars where carefully partly uncovered. Parallel to this for two of these viaducts thorough structural assessments has been carried out. For the other viaducts the risk on structural failure is determined by a specifically developed quick-scan-model. With this model the bridges are ranked with respect to the risk on structural failure. Depending on this risk, a solution for conserving these structures was proposed and a Design and Construct contractor was cho...
The deformation characteristics of dense, reaction bonded, zirconia dispersed alumina (RBAO) have... more The deformation characteristics of dense, reaction bonded, zirconia dispersed alumina (RBAO) have been investigated in uniaxial compression and biaxial tension at temperatures of 1350–1500 °C. Microstructural observations demonstrate that grain boundary sliding is the dominant deformation mechanism. In the lower temperature range the stress exponent has a value near 1 and increases slightly at higher temperatures. Diffusional accommodation is most likely the rate limiting step during grain switching. The superplastic formability was assessed by deep drawing tests and it was found that high temperatures (1500°C) and low punch rates are needed to obtain crack-free specimens. Sinter forging tests were performed to analyze the potential of this pressure-assisted sintering technique to produce fine-grained, high-strength RBAO. Strengths of more than 1 GPa have been obtained by this process. The sinter forging process has been analysed in depth at 1320°C.
Y-TZP powders were prepared either by calcination in air or crystallization under hydrothermal co... more Y-TZP powders were prepared either by calcination in air or crystallization under hydrothermal conditions of a hydrous gel, obtained by coprecipitation. Differences in powder properties, green compact structure and sinterability were examined. Crystallization under hydrothermal conditions occurs at temperatures as low as 190°C in the presence of ammonia. The hydrothermally treated powders are composed of soft agglomerates, that collapse under very low pressures, resulting in green bodies with high densities and small pore radii. The sinterability is greatly improved by the hydrothermal treatment and allowed the production of dense, nanostructured Y-TZP by free sintering at 1050°C.
Superplastic forming under biaxial tension of tetragonal zirconia (Y-TZP) is investigated by push... more Superplastic forming under biaxial tension of tetragonal zirconia (Y-TZP) is investigated by pushing a hemispherical punch (radius 6 mm) on Y-TZP which was placed on a ring with an inner diameter of 16. 7 mm. Dense Y-TZP samples with a grain diameter of 125 nm could be elongated to a dome height of at least 8 mm at a temperature as low as 1160 C. Such elongations could not be achieved at this very low temperature when the ceramic had a grain diameter of 250 nm. At a grain size of 250 nm dierences in deformation behaviour were observed for dierent types of Y-TZP powders. This is explained by the fact that at higher cavity concentration in the sintered compact the sample deforms at lower forces but fractures at lower elongations.
The effects of compressive deformation on the grain boundary characteristics of fine-grained Y-TZ... more The effects of compressive deformation on the grain boundary characteristics of fine-grained Y-TZP have been investigated using surface spectroscopy, impedance analysis, and transmission electron microscopy. After sintering at low temperature (11SO"C), the grain boundaries are covered by an ultrathin (1 nm) yttrium-rich amorphous film. After deformation at 1200"-1300°C under low stress, some grain boundaries are no longer covered by the amorphous film. Yttrium segregation seems to occur only at wetted grain boundaries. Evidence has been found that the extent of dewetting increases with increasing applied stress.
The fracture toughness and ageing resistance of yttria, ceria-stabilized tetragonal zirconia poly... more The fracture toughness and ageing resistance of yttria, ceria-stabilized tetragonal zirconia polycrystals (Y, Ce-TZP) were evaluated as a function of grain size and ceria content. Very fine grained, fully dense materials could be produced by sinter forging at relatively low temperatures (11 50-1200 ~ The ageing resistance in hot water (185 ~ of 2 mol % Y203stabilized TZP is strongly enhanced by alloying with ceria. The ceria content necessary to avoid degradation completely, decreases with grain size. The toughness of fully dense Y, Ce-TZP is 7-9 MPa m 1/2 for grain sizes down to 0.2 ~tm. No or very little transformation took place during fracturing and no clear variation with grain size was observed for the toughness at grain sizes up to 0.8 pm. Reversible transformation and crack deflection may explain the observed toughness values.
The effect of additives (Bi203, Fe203) on densification and creep rates of tetragonal ZrO2-Y203 h... more The effect of additives (Bi203, Fe203) on densification and creep rates of tetragonal ZrO2-Y203 has been investigated. In 8i203-doped Y-TZP, a reactive liquid forms at temperatures above 800-900 ~ which leads to a strong enhancement of densification for concentrations of 1-2 mol % Bi203. However, during cooling from the processing temperature a strong, undesirable transformation of the tetragonal to the monoclinic phase occurs. The addition of 0.6-1.2 mol % Fe03/2 promotes densification without destabilizing the tetragonal phase. A concentration of 1.2 mol %, however, induces discontinuous grain growth, while this
F forms of natural garnets are (112), (110), (123), (001), (120) and {332). For Mg 3AI2(Si04)3, t... more F forms of natural garnets are (112), (110), (123), (001), (120) and {332). For Mg 3AI2(Si04)3, the surface configurations of the elementary growth layers d220 and d240 are bounded by Al ions. For d220, a different boundary configuration exists as well, that of Si and Mg ions, just as the configuration of d0~. For d112 two boundary configurations are possible, one with Mg and Al, and another with Al and Si. The attachment energies of the F faces have been calculated in different electrostatic point charge models, in which the point charges of the ions forming the Si-O tetrahedral bonds vary from the completely ionic model with formal charges, [Si 4~Or], to the covalent one with reduced charges, [Si°O~J. In all models the formal point charges of Al and Mg have been used. Because these energies are supposed to be directly proportional to their growth rates for F faces, theoretical growth forms have been constructed. For models with q 0 =-2 I e , the growth form consists only of (112). However, for the more covalent like models with lower q0, {100) and (110) appear, while at the same time, (211) disappears slowly. If q0 =-I e I, the growth form is merely bounded by {110). Ordering of the boundary ions changes fundamentally the growth form only at q0-1.751e1 producing a growth form with equally important (211}, (110) and {100). If the mineral growth of garnets proceeds with isolated silicate tetrahedra as building units, the theoretical growth form shows only {112}.
The deformation strain rate of nanocrystalline Y-TZP shows an increase by a factor 4 if the grain... more The deformation strain rate of nanocrystalline Y-TZP shows an increase by a factor 4 if the grain size decreases from 200 to 100 nm. Real superplastic deformation (strain rate > low4 s-t) is observed in these materials at relative low temperature (1 lOO-1200°C). Grain-boundary analysis indicates (partial) removal of an ultra-thin (1 nm), yttrium-rich grain boundary layer after deformation. Uniaxial pressure-assisted sintering techniques (= sinter-forging) provide the opportunity of large shear strains during densification. Sinter-forging experiments on zirconia-toughened alumina (15 wt% Zr02/85 wt% AlsOs) resulted in a dense composite within 15 min at 1400°C and 40 MPa, with effective shear strains up to 100%. Sinter-forging of Y-TZP and ZTA gives an increase in strength, reliability and fracture toughness. These improvements are caused by the large shear strains that result from the removal of processing flaws. Also, the number of microcraks at the grain boundaries and the interatomic spacing between the grains are reduced by the forging techniques, resulting in a strengthening of the grain boundaries if compared with pressureless sintering. Klc values of 10 MPa,/m are obtained for Y-TZP, while no classical stressinduced phase transformation toughening is observed. Sinter-forged ZTA samples showed a better wear resistance than free sintered ones.
For efficient asset management of reinforced concrete infrastructure, owners want to know when to... more For efficient asset management of reinforced concrete infrastructure, owners want to know when to expect corrosion in structures. At present, no widely accepted procedures for testing of existing structures for the remaining time to corrosion initiation are available. This paper describes such a procedure, based on the authors’ long-time experience. From about 20 years age, the concrete contains its mature response to actual environmental loads, e.g. chloride ingress. By measuring the mean and standard deviation of the actual cover depth, taking chloride profiles, assuming a few parameters and a simple, pragmatic model, the expected time to corrosion initiation for a particular test area can be predicted. Required numbers of cores and samples per core are given. Uncertainties are taken into account by subtracting a safety margin from the measured concrete cover depth. Results of six cores per test area are classified and interpretations are given. Because of large variability, the r...
Asset managers would benefit from knowing when to expect corrosion initiation in a particular rei... more Asset managers would benefit from knowing when to expect corrosion initiation in a particular reinforced concrete structure. However, accepted approaches to test existing structures for the remaining time to corrosion initiation are lacking. This paper proposes such an approach, based on experience in the field and additional considerations. From say 20 years age, existing structures embody the concrete’s response to actual environmental loads, e.g. in chloride profiles. Based on measuring the actual cover depth, taking (at least six) chloride profiles, some assumed parameters and a simple model, the expected time to corrosion initiation for a particular test area can be predicted. Sampling frequencies are given. Uncertainties can be taken into account by applying a safety margin to the cover depth. Results of at least six tests are classified and suggestions for interpretation are given. Because the accuracy is limited, the results are classified in three broad ranges: 5 years or l...
ABSTRACTIn this paper results of efforts to prepare stable aqueous suspensions of high surface ar... more ABSTRACTIn this paper results of efforts to prepare stable aqueous suspensions of high surface area (> 100 m2 /gr) yttria stabilized tetragonal zirconia powders are presented using a low molecular weight ammonium polyacrylic acid deflocculant. Zetapotentials, viscosities, agglomerate sizes and sedimentation volumes have been measured to find the optimum pH and deflocculant concentration. It has been found that the optimum pH-value coincides with the pH at which the polyacrylic acid is fully dissociated. Using deflocculant concentrations below the optimum value leads to a highly unstable system, while concentrations above the optimum value influence the stability in a much weaker way.
ABSTRACTA non-isothermal analysis of grain growth in tetragonal zirconia ceramics doped with vari... more ABSTRACTA non-isothermal analysis of grain growth in tetragonal zirconia ceramics doped with various amounts of yttria and/or ceria in the temperature range 700–1150°C is presented. In these nanocrystalline ceramics, prepared by the gel precipitation technique, two grain growth regimes are recognized. At temperatures up to 900–1000°C grain growth proceeds slowly at high values of porosity (55%→35%); probably by means of a surface diffusion mechanism. At higher temperatures grain growth is much faster and occurs in locally dense regions by a normal grain growth or an impurity drag mechanism. Apparent activation energies are given for five compositions in both regimes.The isothermal sintering behavior of 3 mol% Y2O3-containing TZP at 1050°C is analysed. A dense ceramic with a crystallite size in the nanometer regime (< 100nm) is obtained.
IABSE Congress, Seoul 2012: Innovative Infrastructures – Towards Human Urbanism, 2012
In this paper the risk on structural failure of some heavily deteriorated bridges constructed wit... more In this paper the risk on structural failure of some heavily deteriorated bridges constructed with post-tensioned prefabricated T-girders is described. The bridges have undergone three detailed inspections with an interval of approximately five years. All visual deterioration was recorded and classified. To check the actual state of corrosion, at four viaducts with severe deterioration, the risk of corrosion of the prestressing bars has been thoroughly assessed. With high pressure water-jetting the anchorages of the prestressing bars where carefully partly uncovered. Parallel to this for two of these viaducts thorough structural assessments has been carried out. For the other viaducts the risk on structural failure is determined by a specifically developed quick-scan-model. With this model the bridges are ranked with respect to the risk on structural failure. Depending on this risk, a solution for conserving these structures was proposed and a Design and Construct contractor was cho...
The deformation characteristics of dense, reaction bonded, zirconia dispersed alumina (RBAO) have... more The deformation characteristics of dense, reaction bonded, zirconia dispersed alumina (RBAO) have been investigated in uniaxial compression and biaxial tension at temperatures of 1350–1500 °C. Microstructural observations demonstrate that grain boundary sliding is the dominant deformation mechanism. In the lower temperature range the stress exponent has a value near 1 and increases slightly at higher temperatures. Diffusional accommodation is most likely the rate limiting step during grain switching. The superplastic formability was assessed by deep drawing tests and it was found that high temperatures (1500°C) and low punch rates are needed to obtain crack-free specimens. Sinter forging tests were performed to analyze the potential of this pressure-assisted sintering technique to produce fine-grained, high-strength RBAO. Strengths of more than 1 GPa have been obtained by this process. The sinter forging process has been analysed in depth at 1320°C.
Y-TZP powders were prepared either by calcination in air or crystallization under hydrothermal co... more Y-TZP powders were prepared either by calcination in air or crystallization under hydrothermal conditions of a hydrous gel, obtained by coprecipitation. Differences in powder properties, green compact structure and sinterability were examined. Crystallization under hydrothermal conditions occurs at temperatures as low as 190°C in the presence of ammonia. The hydrothermally treated powders are composed of soft agglomerates, that collapse under very low pressures, resulting in green bodies with high densities and small pore radii. The sinterability is greatly improved by the hydrothermal treatment and allowed the production of dense, nanostructured Y-TZP by free sintering at 1050°C.
Superplastic forming under biaxial tension of tetragonal zirconia (Y-TZP) is investigated by push... more Superplastic forming under biaxial tension of tetragonal zirconia (Y-TZP) is investigated by pushing a hemispherical punch (radius 6 mm) on Y-TZP which was placed on a ring with an inner diameter of 16. 7 mm. Dense Y-TZP samples with a grain diameter of 125 nm could be elongated to a dome height of at least 8 mm at a temperature as low as 1160 C. Such elongations could not be achieved at this very low temperature when the ceramic had a grain diameter of 250 nm. At a grain size of 250 nm dierences in deformation behaviour were observed for dierent types of Y-TZP powders. This is explained by the fact that at higher cavity concentration in the sintered compact the sample deforms at lower forces but fractures at lower elongations.
The effects of compressive deformation on the grain boundary characteristics of fine-grained Y-TZ... more The effects of compressive deformation on the grain boundary characteristics of fine-grained Y-TZP have been investigated using surface spectroscopy, impedance analysis, and transmission electron microscopy. After sintering at low temperature (11SO"C), the grain boundaries are covered by an ultrathin (1 nm) yttrium-rich amorphous film. After deformation at 1200"-1300°C under low stress, some grain boundaries are no longer covered by the amorphous film. Yttrium segregation seems to occur only at wetted grain boundaries. Evidence has been found that the extent of dewetting increases with increasing applied stress.
The fracture toughness and ageing resistance of yttria, ceria-stabilized tetragonal zirconia poly... more The fracture toughness and ageing resistance of yttria, ceria-stabilized tetragonal zirconia polycrystals (Y, Ce-TZP) were evaluated as a function of grain size and ceria content. Very fine grained, fully dense materials could be produced by sinter forging at relatively low temperatures (11 50-1200 ~ The ageing resistance in hot water (185 ~ of 2 mol % Y203stabilized TZP is strongly enhanced by alloying with ceria. The ceria content necessary to avoid degradation completely, decreases with grain size. The toughness of fully dense Y, Ce-TZP is 7-9 MPa m 1/2 for grain sizes down to 0.2 ~tm. No or very little transformation took place during fracturing and no clear variation with grain size was observed for the toughness at grain sizes up to 0.8 pm. Reversible transformation and crack deflection may explain the observed toughness values.
The effect of additives (Bi203, Fe203) on densification and creep rates of tetragonal ZrO2-Y203 h... more The effect of additives (Bi203, Fe203) on densification and creep rates of tetragonal ZrO2-Y203 has been investigated. In 8i203-doped Y-TZP, a reactive liquid forms at temperatures above 800-900 ~ which leads to a strong enhancement of densification for concentrations of 1-2 mol % Bi203. However, during cooling from the processing temperature a strong, undesirable transformation of the tetragonal to the monoclinic phase occurs. The addition of 0.6-1.2 mol % Fe03/2 promotes densification without destabilizing the tetragonal phase. A concentration of 1.2 mol %, however, induces discontinuous grain growth, while this
F forms of natural garnets are (112), (110), (123), (001), (120) and {332). For Mg 3AI2(Si04)3, t... more F forms of natural garnets are (112), (110), (123), (001), (120) and {332). For Mg 3AI2(Si04)3, the surface configurations of the elementary growth layers d220 and d240 are bounded by Al ions. For d220, a different boundary configuration exists as well, that of Si and Mg ions, just as the configuration of d0~. For d112 two boundary configurations are possible, one with Mg and Al, and another with Al and Si. The attachment energies of the F faces have been calculated in different electrostatic point charge models, in which the point charges of the ions forming the Si-O tetrahedral bonds vary from the completely ionic model with formal charges, [Si 4~Or], to the covalent one with reduced charges, [Si°O~J. In all models the formal point charges of Al and Mg have been used. Because these energies are supposed to be directly proportional to their growth rates for F faces, theoretical growth forms have been constructed. For models with q 0 =-2 I e , the growth form consists only of (112). However, for the more covalent like models with lower q0, {100) and (110) appear, while at the same time, (211) disappears slowly. If q0 =-I e I, the growth form is merely bounded by {110). Ordering of the boundary ions changes fundamentally the growth form only at q0-1.751e1 producing a growth form with equally important (211}, (110) and {100). If the mineral growth of garnets proceeds with isolated silicate tetrahedra as building units, the theoretical growth form shows only {112}.
The deformation strain rate of nanocrystalline Y-TZP shows an increase by a factor 4 if the grain... more The deformation strain rate of nanocrystalline Y-TZP shows an increase by a factor 4 if the grain size decreases from 200 to 100 nm. Real superplastic deformation (strain rate > low4 s-t) is observed in these materials at relative low temperature (1 lOO-1200°C). Grain-boundary analysis indicates (partial) removal of an ultra-thin (1 nm), yttrium-rich grain boundary layer after deformation. Uniaxial pressure-assisted sintering techniques (= sinter-forging) provide the opportunity of large shear strains during densification. Sinter-forging experiments on zirconia-toughened alumina (15 wt% Zr02/85 wt% AlsOs) resulted in a dense composite within 15 min at 1400°C and 40 MPa, with effective shear strains up to 100%. Sinter-forging of Y-TZP and ZTA gives an increase in strength, reliability and fracture toughness. These improvements are caused by the large shear strains that result from the removal of processing flaws. Also, the number of microcraks at the grain boundaries and the interatomic spacing between the grains are reduced by the forging techniques, resulting in a strengthening of the grain boundaries if compared with pressureless sintering. Klc values of 10 MPa,/m are obtained for Y-TZP, while no classical stressinduced phase transformation toughening is observed. Sinter-forged ZTA samples showed a better wear resistance than free sintered ones.
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