The pozzolanic properties of inorganic and organomodified nano-montmorillonite dispersions

The pozzolanic potentials of three non-thermally treated, nano-montmorillonite (NMt) dispersions were investigated by a new method involving the analysis of NMt/lime putty pastes via TGA/dTG and XRD crystallographic and semi-quantitative analysis. The criterion conceived was validated at eight days and eight months and was additionally verified via the Chapelle method. The inorganic NMt dispersion showed the most pronounced pozzolanic behaviour promoting Ca(OH) 2 consumption towards calcium silicate/aluminate hydrates formation and binding behaviour. The two organomodified NMt dispersions exhibited pozzolanicity increasing with time. The results can pave the way for advances in cement science and restoration mortars development for historical structures rehabilitation, where low CO 2-footprint, natural inorganic materials are a prerequisite. </p

Durability of Structural Lightweight Concrete Containing Different Types of Natural or Artificial Lightweight Aggregates

Different structural lightweight concrete mixtures of specific density and strength classes were produced by using various lightweight aggregates (LWAs) such as pumice, perlite, and rice husk ash. Their properties were evaluated in fresh and hardened states with regards to compressive strength and durability parameters such as water absorption (open porosity and capillary absorption), chloride’s penetration resistance, and carbonation depth. According to the results, most LWA concrete mixtures performed satisfactorily in terms of the designed strength and density and they could be used as structural LWA concrete mixtures. As far as the durability of LWA concrete was concerned, open porosity and resistance to the carbonation of LWA concrete were burdened with the porous nature of LWAs, while sorptivity in some mixtures and especially chlorides’ penetration resistance in all mixtures were reported to be significantly improved. The overall strength and durability performance of the designed LWA concrete mixtures could mitigate the concerns stemming from its vulnerability to extreme exposure conditions

Exploitation of kaolins in concrete technology

The subject of this dissertation is the Exploitation of Kaolins in Concrete Technology. Kaolinite, the main mineral in kaolin, is transformed, after thermal treatment, in metakaolinite, a material with a remarkable pozzolanic reactivity. The target of this work is the exploitation of Hellenic poor kaolinitic deposits for the production of metakaolin and its use for the production of high performance concrete. The dissertation comprises three well defined stages. Thorough study of the kaolin thermal treatment and the reactivity of the produced metakaolin. This stage includes i) the selection and full characterization of Hellenic kaolins having different chemical and mineralogical composition, ii) the optimization of thermal treatment as far as time, temperature and fineness are concerned, iii) the investigation of the relation between the kaolin's structure and the reactivity of the produced metakaolin and iv) the study of the pozzolanic reaction of metakaolin. As it is concluded the crystallinity of kaolinite and especially the degree of its crystallinity, affects the reactivity of the produced metakaolin, while the chemical composition of kaolin affects the kind of the hydration products. The thermal treatment of kaolin at 650 °C for 3 hours is recommended for the Hellenic kaolins. Preparation of blended metakaolin cements and determination of characteristic mechanical and physical properties. In this stage the metakaolins were prepared, under the previously defined conditions, superfine metakaolins were produced, in a special grinding device, and blended cements, with the incorporation of 10% and 20% metakaolin, were prepared. After the measurement of their properties, it was concluded that the incorporation of 20% metakaolin improves the mechanical properties of cements. Use of metakaolin for concrete production. In this stage concrete, having a varying content of a Hellenic and a commercial metakaolin, was prepared and the properties of fresh and hardened concrete were measured. The quality of the concrete and its specification as high performance concrete were evaluated on the basis of special durability tests. As it was concluded the incorporation of metakaolin in the concrete improves the durability of the concrete and leads to the production of high performance concrete. Based on the above remarks, it is generally concluded that Hellenic kaolins of low purity can be transformed after proper thermal treatment, in highly reactive metakaolins. Their reactivity is fully competitive with the reactivity of commercial products of high purity. The concrete, incorporating Hellenic metakaolins, is high performance concrete and meets the contemporary demands for structural materials of high quality and specific requirements.Το θέμα της διδακτορικής διατριβής είναι η Αξιοποίηση Καολινών στην Τεχνολογία Σκυροδέματος. Ο καολινίτης, το κύριο ορυκτολογικό συστατικό του καολίνη, μετατρέπεται κατά τη διάρκεια θερμικής επεξεργασίας σε μετακαολινίτη, υλικό με αξιόλογη ποζολανική δραστικότητα. Στόχος της εργασίας είναι η αξιοποίηση Ελληνικών καολινιτικών κοιτασμάτων φτωχών σε καολινίτη για την παραγωγή μετακαολίνη και η χρήση του μετακαολίνη αυτού στην παραγωγή σκυροδέματος υψηλής απόδοσης (high performance concrete). Η εργασία περιλαμβάνει 3 διακριτά στάδια: Επιστημονική εμβάθυνση στη διεργασία της θερμικής μετατροπής του καολίνη και στη δραστικότητα του παραγόμενου μετακαολίνη. Το στάδιο αυτό περιλαμβάνει i) την επιλογή και τον πλήρη χαρακτηρισμό Ελληνικών καολινών με διαφορετική χημική και ορυκτολογική σύσταση, ii) την αριστοποίηση της θερμικής κατεργασίας του καολίνη μέσω της μελέτης της επίδρασης διαφόρων παραμέτρων (χρόνος, θερμοκρασία, κοκκομετρία), iii) τη διερεύνηση της σχέσης "δομής του καολινίτη" και "δραστικότητας" του αντίστοιχου μετακαολινίτη και iv) τη μελέτη της ποζολανικής αντίδρασης του μετακαολίνη. Όπως προέκυψε, η κρυσταλλικότητα του καολινίτη και ιδιαίτερα ο βαθμός κρυσταλλικότητας του, επηρεάζει τη δραστικότητα του παραγόμενου μετακαολινίτη, ενώ η χημική σύσταση του καολίνη επιδρά στο είδος των προϊόντων ενυδάτωσης. Η θερμική επεξεργασία των καολινών σε θερμοκρασία 650°C επί 3h θεωρείται η πιο ενδεδειγμένη για τους Ελληνικούς καολίνες. Παρασκευή συνθέτων τσιμέντων με συμμετοχή μετακαολίνη και μέτρηση χαρακτηριστικών μηχανικών και φυσικών ιδιοτήτων. Στο στάδιο αυτό παρασκευάσθηκαν οι αντίστοιχοι μετακαολίνες στις ήδη καθορισμένες βέλτιστες συνθήκες, πραγματοποιήθηκε η άλεση τους σε ειδική διάταξη, με στόχο την παραγωγή υπέρλεπτου μετακαολίνη και παρασκευάστηκαν σύνθετα τσιμέντα με περιεκτικότητα 10% και 20% σε μετακαολίνη, όπου μετρήθηκαν μηχανικές (αντοχές θλίψης διαφόρων ηλικιών) και χαρακτηριστικές φυσικές ιδιότητες (απαίτηση σε νερό, χρόνος πήξης). Όπως προέκυψε τα σύνθετα τσιμέντα με μετακαολίνη έως 20% παρουσιάζουν σαφέστατα βελτιωμένες μηχανικές ιδιότητες σε σχέση με το αμιγές τσιμέντο. Χρήση του μετακαολίνη στην παραγωγή σκυροδέματος. Στο στάδιο αυτό έγινε παρασκευή σκυροδέματος με τη συμμετοχή διαφόρων ποσοστών ενός Ελληνικού και ενός εμπορικού μετακαολίνη και μετρήθηκαν χαρακτηριστικές ιδιότητες του νωπού και του σκληρυμένου σκυροδέματος. Η ποιότητα του σκυροδέματος και ο χαρακτηρισμός του ως υψηλής απόδοσης αξιολογήθηκε με βάση ειδικές δοκιμές (διαπερατότητα CI-, διαπερατότητα σε αέρα, υδατοαπορροφητικότητα, πορώδες, κατανομή πόρων). Όπως προέκυψε η χρήση του μετακαολίνη οδηγεί στην παραγωγή σκυροδέματος υψηλής απόδοσης, αφού όλες οι ιδιότητες που διαμορφώνουν την ανθεκτικότητα του σκυροδέματος παρουσιάζουν σαφή βελτίωση. Με βάση τα παραπάνω προκύπτει ότι Ελληνικοί καολίνες χαμηλής περιεκτικότητας σε καολινίτη, μετατρέπονται μετά από κατάλληλη επεξεργασία, σε μετακαολίνες με δραστικότητα απόλυτα ανταγωνιστική της δραστικότητας εμπορικών μετακαολινών υψηλής καθαρότητας. Το σκυρόδεμα που περιέχει Ελληνικούς μετακαολίνες είναι υψηλής απόδοσης και ανταποκρίνεται πλήρως στις απαιτήσεις της σύγχρονης εποχής για δομικά υλικά συγκεκριμένων προδιαγραφών και υψηλής ποιότητας

Chloride Penetration in Lightweight Aggregate Mortars Incorporating Supplementary Cementing Materials

The current research is aiming to investigate the chlorides’ penetration resistance of 9 different pumice lightweight aggregate mortars (LWAMs). The composition of the mixture is altered by the addition of metakaolin, fly ash, silica fume, and ground granulated blast furnace slag, substituting 10% and 20% of cement by mass. Workability and compressive strength of the mortars are studied together with durability parameters like water absorption due to capillary absorption and porosity. Moreover, the mortar resistance to chloride penetration is evaluated through non-steady-state chloride migration coefficient, complying to NT BUILD 492 standard. Summarizing all the laboratory test results, while the addition of metakaolin in the range of 20% led to better workability, higher compressive strength, lower sorptivity, and lower volume of open porosity, it did not improve the resistance against chlorides’ penetration

Durability of Structural Lightweight Concrete Containing Different Types of Natural or Artificial Lightweight Aggregates

Different structural lightweight concrete mixtures of specific density and strength classes were produced by using various lightweight aggregates (LWAs) such as pumice, perlite, and rice husk ash. Their properties were evaluated in fresh and hardened states with regards to compressive strength and durability parameters such as water absorption (open porosity and capillary absorption), chloride’s penetration resistance, and carbonation depth. According to the results, most LWA concrete mixtures performed satisfactorily in terms of the designed strength and density and they could be used as structural LWA concrete mixtures. As far as the durability of LWA concrete was concerned, open porosity and resistance to the carbonation of LWA concrete were burdened with the porous nature of LWAs, while sorptivity in some mixtures and especially chlorides’ penetration resistance in all mixtures were reported to be significantly improved. The overall strength and durability performance of the designed LWA concrete mixtures could mitigate the concerns stemming from its vulnerability to extreme exposure conditions

Shaking table tests of a resilient bridge system with precast reinforced concrete columns equipped with springs

This paper presents the shake table test results of a novel system for the design of precast reinforced concrete bridges. The specimen comprises a slab and four precast columns. The connections are dry and the columns are connected to the slab by an ungrouted tendon. One of the tendon ends is anchored above the slab, in series with a stack of washer springs, while the other end is anchored at the bottom of the column. The addition of such a flexible restraining system increases the stability of the system, while keeping it relatively flexible allowing it to experience negative post-uplift stiffness. It is a form of seismic isolation. Anchoring the tendon within the column, caps the design moment of the foundation, and reduces its size. One hundred and eighty-one shake table tests were performed. The first 180 caused negligible damage to the specimen, mainly abrasion at the perimeter of the column top ends. Hence, the system proved resilient. The 181st excitation caused collapse, because the tendons unexpectedly failed at a load less than 50% of their capacity (provided by the manufacturer), due to the failure of their end socket. This highlights the importance of properly designing the tendons. The tests were used to statistically validate a rigid body model. The model performed reasonably well never underestimating the median displacement response of the center of mass of the slab by more than 30%. However, the model cannot predict the torsion rotation of the slab that was observed in the tests and is due to imperfections.ISSN:0098-8847ISSN:1096-984

Shaking table tests of a re-usable, sustainable, and resilient bridge system with rocking columns

This paper presents the results of shaking table tests of a system exhibiting negative stiffness. As the system uses dry connections, it is suitable for precast construction, and it can be easily disassembled and re-used. The specimen was composed of four restrained rocking columns and a slab. The reinforced concrete columns were protected by steel jackets at both ends and restrained by an ungrouted tendon in series with disc springs. The tendons were anchored at the bottom of the columns and above the slab. The intention of the design concept is to reduce the size of pile foundations, which, in conventional bridges, can comprise a large portion of the total reinforced concrete of the project and is often governed by seismic loading. Hence it saves material and is a contribution towards sustainable design. The system was simultaneously excited in the longitudinal, transversal, and vertical directions by 181 scaled real seismic records. During the tests, it could be observed that the columns not only rocked, but also twisted and slid. The only damage noticed during the first 180 excitations was abrasion on the edges of the steel jackets. The last excitation (#181), however, caused collapse. The tendons unexpectedly failed at a load less than 50% of their capacity (provided by the manufacturer), due to failure of their end socket. This failure highlights the importance of the tendons for the stability of the system at large displacements. The response of the system is discussed in detail presenting the main characteristics of the system

Statistical Validation of a Rocking Numerical Model

Validation of numerical models is a paramount task for the design of structures and introduction of new construction solutions in practice. Up to now, rocking structures have been considered unpredictable, hampering their use in practice. Using a recently proposed statistical validation method, this paper claims that rocking motion can be predicted. Herein, a simple 3D wobbling rocking model is statistically validated against multiple shake table tests. Both numerical and experimental seismic response are clustered in six bins, sorted according the excitation type and intensity, and their cumulative distribution functions and median response are compared. The paper shows that, even though the numerical model does not account for sliding and twisting of the columns, it still represents well the statistics of the horizontal displacement of the rocking system. Only in 1 out of the 6 cases the numerical model fails to predict well the statistics of the tested rocking system. Thus, as structures are usually designed for set ground of motions and not individual ground motions, the paper claims that the statistical validation is sufficient and appropriate for validation of rocking models