The concept and use of Ultra-High-Performance Concrete, UHPC, can bring several advantages in terms of strength and durability aspects over the conventional concrete materials in the concrete construction industry, mainly regarding infrastructure applications. As a matter of fact, the use of conventional materials requires cost and labor efforts for assembling the reinforcing bars, and maintenance and repair actions are demanded throughout the structure service life as a result of a somewhat unavoidable, though slow, deterioration of the structural performance. These problems could be successfully tackled by adopting high-performance concrete materials in infrastructure applications, particularly those that are exposed to aggressive environmental exposure and structural service scenarios. Nonetheless, the peculiarities brought by advanced materials and, in detail, the enhancement inborn in UHPC, for instance, the impressive residual tensile capacity, high compressive strength, and significant durability improvement in uncracked, and cracked states, trigger the need for formulating and validating new design procedures and methodologies. As expectable, the lack of understanding and dealing with these materials has delayed so far, their extensive implementation in a wide ray of infrastructure applications. Therefore, this study, through extensive experimental and theoretical investigations, aims to bring a contribution to consistently foster the use of UHPC materials, especially to structures exposed to extremely aggressive conditions. To obtain a rational comprehension of the UHPC materials, multi-scale level tests have been carried out with a focus on “structural durability” aspects, which meant the capacity of the material to maintain the required level of structural performance over time under the intended service conditions, including environmental exposure and expected cracking state. Experimental tests at different scales have been used to fulfill the objectives of the thesis: micro-scale to test the micromechanical performance of steel fiber-UHPC matrix interface, mesoscale tests to study the damage-healing mechanism in small thin beams, large-scale tests to study the flexural performance of UHPC panels and model the structural durability performance, and finally full-scale structural level tests to monitor the serviceability requirements of a pilot UHPC structure. The microscale level consists of single steel fiber pullout tests. Performed on 20mm long steel fiber embedded inside UHPC matrix with different nano-additions mix-designs. The samples were cured in tap water and 3.5% NaCl aqueous solution to understand the influence of the curing conditions on the micromechanical behavior. Upon one month curing period, some samples were tested with a direct pullout test to represent the reference case, whereas for other samples, fibers were pulled out partially to induce some damage on the fiber-matrix interface and then immersed either in tap water or 3.5% NaCl aqueous solution for another one month to investigate the healing and deterioration mechanisms respectively. Scanning Electron Microscopy, SEM, and Energy-Dispersive Spectroscopy, EDS was conducted on the steel fiber surface and attached healing/corrosion particles to understand the nature of the occurred processes, before being tested to complete pull-out failure to assess the effects of the same phenomena on the fiber-matrix bond. The mesoscale level involves an extensive experimental campaign on UHPC thin beams that have cross-sectional dimensions of 100mm × 30mm and length of 500mm. Durability and strength tests have been carried out on these beams via nondestructive and destructive measurements. In order to depict the real case condition, the beams have been placed under sustained loading conditions, where a suitable loading setup was introduced and applied to the beams. Beams from different UHPC mixes, incorporating different nano- and micro-functionalized constituents were first nondestructively surveyed to quantify their steel fiber contents, and further still nondestructively tested to evaluate their stiffness via resonant vibration test and ultra-sonic pulse velocity (UPV) tests. The beams were then cracked to 200µm crack opening displacement (COD) on 150mm tensile range length and thereafter subjected to sustained flexural stress to the same level of pre-cracking load and immersed in water to simulate three different scenarios: tap water, 3.5%NaCl aqueous solution and geothermal water, obtained from a geothermal power plant located in Tuscany. The specimens stayed in the exposure baths for up to 12 months. Every three months, specimens from different materials and exposure types were tested, firstly via the same nondestructive tests as above, visually acquiring crack width imaged, UPV, and RF tests; then half of the specimens were tested in 4pbt, while the other half was tested in direct tension. From these tests, the evolution of the tensile constitutive laws was evaluated for up to 12 months of exposure and then extrapolated to be used in design approaches that can integrate and upgrade the durability performance from materials to the structural level. The Macro scale level included tests on UHPC panels of different sizes, representative of different structural schemes, that were also subjected to pre-cracking, exposure for up to 6 months to different scenarios, and final failure tests. A yield line approach was used to predict the performance of the UHPC slabs, also employing the constitutive tensile relationships identified from the previously described tests. These UHPC panels were meant to replicate at a lab scale a real structural application, assumed as a benchmark reference in this study: a basin to collect the geothermal water located in a geothermal power plant that is in Tuscany, Italy. The basin consists of three compartments, each one having a different material or structural scheme. The first compartment is 7.0m wide, 7.5m long, and 1.5m high and is made of a 100 mm thick ordinary reinforced concrete wall. The second compartment has the same plan and height dimensions but is made of 60 mm thick UHPC walls. The last compartment consists of 30 mm thick UHPC panels (1.4m × 1.5m) stiffened by 200mm×200mm UHPC columns. The service height level of the geothermal water is assumed equal to 1.3m. Upon the completion of the basin and entering its service condition, several monitoring and validation tests have been carried out including steel fiber survey and loading/unloading tests with measurements of wall displacements, strains in concrete and steel bars, and crack width monitoring. These measurements were aimed to verify the design concept and evaluate the durability criteria at specific time periods. The same monitoring and validation tests were conducted on the lab scales specimens to mimic the behavior of the second and the third compartments. The whole experimental campaign serves as a solid background for the formulation and validation of a durability performance-based design approach for UHPC structures, which constitutes the second part of this study. The proposed methodology encompasses the conventional structural design procedures, integrated with the material performance derived from the experiments as a function of exposure condition and time. Moreover, such approach allows to rationally predict the service life of UHPC structures, since the serviceability and strength limit states are checked and compared to the performance and resistance capacity respectively.
L’impiego di calcestruzzi ad altissime prestazioni (UHPC) può portare molteplici vantaggi in termini di resistenza e durabilità rispetto ai materiali tradizionali dell’industria delle costruzioni in calcestruzzo e, in particolare, per le applicazioni su infrastrutture. Infatti, l’utilizzo di materiali tradizionali richiede costi e manodopera per la posa delle barre di armatura, a cui si aggiungono i costi di manutenzione e riparazione che risultano necessari nel corso della vita utile a causa del progressivo e inevitabile deterioramento della struttura stessa. Le suddette problematiche possono essere efficacemente affrontate adottando calcestruzzi ad alte prestazioni per le applicazioni infrastrutturali, specialmente per le strutture soggette ad ambienti e condizioni di carico aggressivi. Inoltre, l’impiego di materiali tecnologicamente avanzati e le loro caratteristiche peculiari – per l’UHPC, resistenza a trazione, elevata resistenza a compressione, durabilità in condizioni integre e in presenza di fessure – impongono lo sviluppo di nuovi approcci alla progettazione, che devono poi essere validati. Attualmente, la limitata conoscenza di questi materiali ne ha impedito la diffusione, soprattutto in ambito infrastrutturale. Di conseguenza, il presente lavoro si propone di contribuire, mediante analisi teoriche e ampie campagne sperimentali, alla promozione dell’impiego degli UHPC, soprattutto per strutture esposte ad ambienti estremamente aggressivi. Per comprendere a fondo le caratteristiche degli UHPC, sono state eseguite numerose sperimentazioni su diversa scala, ponendo particolare attenzione agli aspetti legati alla durabilità delle strutture, intesa come la capacità del materiale di preservare la prestazione meccanica richiesta nel tempo quando soggetto alle condizioni di servizio (i.e. esposizione ambientale e quadro fessurativo atteso). A tale scopo sono state eseguite diverse prove sperimentali: prove a scala microscopica per indagare la prestazione all’interfaccia tra le fibre d’acciaio e la matrice cementizia degli UHPC, prove a scala intermedia per studiare i processi di riparazione del danno su travi sottili, prove su larga scala per determinare la prestazione strutturale a flessione di pannelli in UHPC e modellare la durabilità a livello strutturale e, infine, prove in scala reale per monitorare una struttura pilota in UHPC soggetta ai carichi di servizio. In dettaglio, a scala microscopica sono state effettuate prove di estrazione di una singola fibra metallica, lunga 20mm, annegata all’interno di una matrice cementizia di alcuni mix di UHPC contenenti diversi nano-additivi. I campioni sono stati immerse all’interno di acqua e di una soluzione di cloruri (3.5% NaCl) per determinare l’effetto delle condizioni di maturazione sul comportamento a livello microscopico. Dopo un mese di esposizione, sono state eseguite prove di estrazione su alcuni campioni, adottati come riferimento; per gli altri campioni la fibra è stata solo parzialmente estratta, allo scopo di indurre un danno all’interfaccia tra fibra e matrice, per poi riporli nuovamente in acqua o nella soluzione salina per un ulteriore mese, andando poi ad indagare i meccanismi di degrado e di riparazione. Sulla superficie della fibra metallica sono state eseguite rilevazioni con il microscopio elettronico a scansione (SEM) e spettroscopia EDX, osservando le particelle formatesi in seguito alla riparazione o alla corrosione, a seconda del processo verificatosi. Infine, le prove di estrazione sono state eseguite fino a rottura, per determinare gli effetti dei suddetti processi sul legame tra la fibra e la matrice. Per le prove su scala intermedia sono state testate travi sottili realizzate in UHPC, con una sezione di 100mm × 30mm e una lunghezza di 500mm. Sono state eseguite prove di resistenza e di durabilità, di natura sia non-distruttiva che distruttiva. Sulle travi è stato applicato un carico permanente allo scopo di rappresentare le condizioni reali di servizio, sviluppando un apposito setup di carico. Le travi, realizzate con diversi mix di UHPC, contenenti ciascuno specifici micro- e nano-additivi, sono state inizialmente analizzate mediante prove non distruttive, definendo il contenuto di fibre di acciaio e determinandone la rigidezza iniziale mediante prove con frequenza di risonanza (RF) e velocità di volo degli ultrasuoni (UPV). Successivamente, le travi sono state fessurate fino a un’apertura di fessura (COD) di 200µm, sviluppata sulla porzione centrale, pari a 150mm, ed è stato poi applicato il carico permanente a flessione, pari al valore raggiunto per la pre-fessurazione. Una volta inserite nel setup, le travi sono state immerse in tre diverse esposizioni: acqua, soluzione con cloruri (3.5% NaCl) e acqua geotermica, ricavata da un impianto di produzione di energia geotermica situato in Toscana. I provini sono stati mantenuti all’interno delle vasche di esposizione fino a un massimo di 12 mesi; ogni tre mesi è stata estratta una coppia di provini per ciascun materiale ed esposizione, per un totale di 27 coppie per periodo. Sono state inizialmente ripetute le misurazioni non distruttive, determinando l’apertura di fessura mediante microscopio digitale e registrando UPV e RF. Di ciascuna coppia, una trave è stata testata mediante flessione a quattro punti, mentre l’altra mediante trazione diretta. È stata dunque determinate l’evoluzione del legame costitutivo fino a 12 mesi di esposizione, successivamente estrapolata per l’impego in fase di progettazione, implementando la durabilità tra i parametri prestazionali anche a livello strutturale. Per le prove su larga scala sono stati testati pannelli in UHPC di dimensioni variabili, rappresentativi di diversi schemi strutturali. Anche i pannelli sono stati pre-fessurati, immersi all’interno di diverse esposizioni e infine testati fino a rottura. Per prevedere la prestazione dei pannelli in UHPC è stato impiegato l’approccio con le linee di rottura, introducendo le leggi costitutive determinate mediante le prove su travi sottili. I pannelli sono stati realizzati allo scopo di riprodurre in laboratorio una struttura realmente esistente, adottata come riferimento nel presente lavoro. La struttura individuata è un bacino di raccolta di acque geotermiche, situato in un impianto di produzione di energia geotermica in Toscana (Italia). Il bacino è composto da tre compartimenti, di dimensioni in pianta pari a 7.0m × 7.5m e aventi un’altezza di 1.5m, distinti in funzione del materiale e della soluzione strutturale adottata. Il primo è stato realizzato con pareti in calcestruzzo ordinario, rinforzato con barre di armatura in acciaio, dallo spessore di 100mm. Nel secondo caso, lo spessore della parete è di 60mm ed è realizzato con UHPC. Il terzo compartimento, invece, è costituito da pannelli in UHPC (1.4m × 1.5m) dallo spessore di 30mm, irrigiditi mediante colonne in UHPC di sezione pari a 200mm × 200mm. Per definire la condizione di carico di servizio, è stata assunta un’altezza dell’acqua di 1.3m. Sono state condotte diverse indagini per monitorare e validare la struttura sin dal completamento della costruzione, in condizioni di servizio. Le prove eseguite comprendono sondaggi non distruttivi del contenuto di fibre metalliche, prove di carico e scarico con contestuale misurazione degli spostamenti delle pareti, delle deformazioni del calcestruzzo e delle barre di armatura e monitoraggio delle aperture di fessura. Le misurazioni sono state eseguite allo scopo di validare la concezione strutturale e valutarne la durabilità a diversi intervalli temporali. Le stesse indagini sono state eseguite su scala di laboratorio, simulando il comportamento della seconda e della terza vasca. L’ampia campagna sperimentale condotta costituisce un solido fondamento per la formulazione e la validazione di un approccio alla progettazione di strutture in UHPC basato sulla durabilità, che costituisce la seconda parte del presente lavoro. La metodologia proposta include le procedure di progettazione tradizionali, integrandole con la prestazione del materiale nel tempo e in funzione della condizione di esposizione, determinata mediante la sperimentazione. Inoltre, tale approccio consente di prevedere la vita utile delle strutture in UHPC, poiché gli stati limite ultimo e di servizio sono verificati e confrontati rispettivamente con la capacità portante e la prestazione in esercizio.
Durability-Based Design of Ultra-High-Performance Concrete (UHPC) Structures: from micro materials to structural scale
Al-Obaidi, Salam Maytham Jaber
2022/2023
Abstract
The concept and use of Ultra-High-Performance Concrete, UHPC, can bring several advantages in terms of strength and durability aspects over the conventional concrete materials in the concrete construction industry, mainly regarding infrastructure applications. As a matter of fact, the use of conventional materials requires cost and labor efforts for assembling the reinforcing bars, and maintenance and repair actions are demanded throughout the structure service life as a result of a somewhat unavoidable, though slow, deterioration of the structural performance. These problems could be successfully tackled by adopting high-performance concrete materials in infrastructure applications, particularly those that are exposed to aggressive environmental exposure and structural service scenarios. Nonetheless, the peculiarities brought by advanced materials and, in detail, the enhancement inborn in UHPC, for instance, the impressive residual tensile capacity, high compressive strength, and significant durability improvement in uncracked, and cracked states, trigger the need for formulating and validating new design procedures and methodologies. As expectable, the lack of understanding and dealing with these materials has delayed so far, their extensive implementation in a wide ray of infrastructure applications. Therefore, this study, through extensive experimental and theoretical investigations, aims to bring a contribution to consistently foster the use of UHPC materials, especially to structures exposed to extremely aggressive conditions. To obtain a rational comprehension of the UHPC materials, multi-scale level tests have been carried out with a focus on “structural durability” aspects, which meant the capacity of the material to maintain the required level of structural performance over time under the intended service conditions, including environmental exposure and expected cracking state. Experimental tests at different scales have been used to fulfill the objectives of the thesis: micro-scale to test the micromechanical performance of steel fiber-UHPC matrix interface, mesoscale tests to study the damage-healing mechanism in small thin beams, large-scale tests to study the flexural performance of UHPC panels and model the structural durability performance, and finally full-scale structural level tests to monitor the serviceability requirements of a pilot UHPC structure. The microscale level consists of single steel fiber pullout tests. Performed on 20mm long steel fiber embedded inside UHPC matrix with different nano-additions mix-designs. The samples were cured in tap water and 3.5% NaCl aqueous solution to understand the influence of the curing conditions on the micromechanical behavior. Upon one month curing period, some samples were tested with a direct pullout test to represent the reference case, whereas for other samples, fibers were pulled out partially to induce some damage on the fiber-matrix interface and then immersed either in tap water or 3.5% NaCl aqueous solution for another one month to investigate the healing and deterioration mechanisms respectively. Scanning Electron Microscopy, SEM, and Energy-Dispersive Spectroscopy, EDS was conducted on the steel fiber surface and attached healing/corrosion particles to understand the nature of the occurred processes, before being tested to complete pull-out failure to assess the effects of the same phenomena on the fiber-matrix bond. The mesoscale level involves an extensive experimental campaign on UHPC thin beams that have cross-sectional dimensions of 100mm × 30mm and length of 500mm. Durability and strength tests have been carried out on these beams via nondestructive and destructive measurements. In order to depict the real case condition, the beams have been placed under sustained loading conditions, where a suitable loading setup was introduced and applied to the beams. Beams from different UHPC mixes, incorporating different nano- and micro-functionalized constituents were first nondestructively surveyed to quantify their steel fiber contents, and further still nondestructively tested to evaluate their stiffness via resonant vibration test and ultra-sonic pulse velocity (UPV) tests. The beams were then cracked to 200µm crack opening displacement (COD) on 150mm tensile range length and thereafter subjected to sustained flexural stress to the same level of pre-cracking load and immersed in water to simulate three different scenarios: tap water, 3.5%NaCl aqueous solution and geothermal water, obtained from a geothermal power plant located in Tuscany. The specimens stayed in the exposure baths for up to 12 months. Every three months, specimens from different materials and exposure types were tested, firstly via the same nondestructive tests as above, visually acquiring crack width imaged, UPV, and RF tests; then half of the specimens were tested in 4pbt, while the other half was tested in direct tension. From these tests, the evolution of the tensile constitutive laws was evaluated for up to 12 months of exposure and then extrapolated to be used in design approaches that can integrate and upgrade the durability performance from materials to the structural level. The Macro scale level included tests on UHPC panels of different sizes, representative of different structural schemes, that were also subjected to pre-cracking, exposure for up to 6 months to different scenarios, and final failure tests. A yield line approach was used to predict the performance of the UHPC slabs, also employing the constitutive tensile relationships identified from the previously described tests. These UHPC panels were meant to replicate at a lab scale a real structural application, assumed as a benchmark reference in this study: a basin to collect the geothermal water located in a geothermal power plant that is in Tuscany, Italy. The basin consists of three compartments, each one having a different material or structural scheme. The first compartment is 7.0m wide, 7.5m long, and 1.5m high and is made of a 100 mm thick ordinary reinforced concrete wall. The second compartment has the same plan and height dimensions but is made of 60 mm thick UHPC walls. The last compartment consists of 30 mm thick UHPC panels (1.4m × 1.5m) stiffened by 200mm×200mm UHPC columns. The service height level of the geothermal water is assumed equal to 1.3m. Upon the completion of the basin and entering its service condition, several monitoring and validation tests have been carried out including steel fiber survey and loading/unloading tests with measurements of wall displacements, strains in concrete and steel bars, and crack width monitoring. These measurements were aimed to verify the design concept and evaluate the durability criteria at specific time periods. The same monitoring and validation tests were conducted on the lab scales specimens to mimic the behavior of the second and the third compartments. The whole experimental campaign serves as a solid background for the formulation and validation of a durability performance-based design approach for UHPC structures, which constitutes the second part of this study. The proposed methodology encompasses the conventional structural design procedures, integrated with the material performance derived from the experiments as a function of exposure condition and time. Moreover, such approach allows to rationally predict the service life of UHPC structures, since the serviceability and strength limit states are checked and compared to the performance and resistance capacity respectively.| File | Dimensione | Formato | |
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Salam's thesis.pdf
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https://hdl.handle.net/10589/196155