Nuno Cristelo

Structural Performance of Alkali-Activated Soil Ash versus Soil Cement

AbstractAlkaline activation of fly ash (FA) was used to improve the mechanical performance of a silty sand, considering this new material as a replacement for soil-cement applications, namely, bases and subbases, for transportation infrastructures. For that purpose, specimens were molded from mixtures of soil, FA, and an alkaline activator made from sodium hydroxide and sodium silicate. Uniaxial compression tests showed that strength is highly increased by the addition of this new binder. The results described a high stiffness material, with an initial volume reduction followed by significant dilation. All specimens have clearly reached the respective yield surface during shearing, and peak-strength Mohr–Coulomb parameters were defined for each mixture. The evolution of the microstructure during curing, responsible for the mechanical behavior detected in the previous tests, was observed by scanning electron microscopy. These results were compared with soil-cement data obtained previously with the same soi...

Conservation and New Construction Solutions in Rammed Earth

The conservation and rehabilitation of several sites of cultural heritage and of the large housing stock built from rammed earth requires adopting intervention techniques that aim at their repair or strengthening. The present work discusses the main causes of the decay of rammed earth constructions. The intervention techniques used to repair cracks and lost volumes of material are also discussed. Regarding the strengthening of rammed earth walls, the discussion is focused on the techniques that improve the out-of-plane behaviour. Special attention is given to the injection of mud grouts for crack repair in rammed earth walls, including the presentation of the most recent developments on the topic, namely regarding their fresh-state rheology, hardened-state strength and adhesion. Finally, the use of the rammed earth is discussed as a modern building solution. In addition, several typical techniques for improving rammed earth constructions are discussed, aiming at adequate those to modern demands. In addition, the alkaline activation of fly ash is presented and discussed as a novel improvement technique.

Stiffness Behavior of Soil Stabilized with Alkali-Activated Fly Ash from Small to Large Strains

AbstractAlkaline activation of fly ash creates a geopolymeric cement that can replace ordinary portland cement in several applications such as soil improvement, with the advantage of much lower carbon dioxide emissions and reusing an industrial by-product otherwise landfilled, which averts several environmental problems. In this paper, the behavior of a silty sand improved by the alkaline activation of fly ash is analyzed from small to large strains by presenting uniaxial and drained triaxial compression test results and seismic wave velocities measured throughout the curing period. The dynamic, cyclic, and static tests showed a significant increase in stiffness with curing time, even beyond the 28-day curing period. On the basis of the nondestructive wave-propagation technique, the increase of the shear and compression wave velocities with time were drawn, giving the evolution of the elastic shear modulus and the Poisson ratio values. The dynamic Young modulus was compared to the correspondent secant You...

Increasing the reaction kinetics of alkali-activated fly ash binders for stabilisation of a silty sand pavement sub-base

The paper addresses several options to improve the reaction kinetics of alkali-activated low-calcium fly ash binders for soil stabilisation in road platforms. For that purpose, an experimental programme was established to assess the strength evolution, with time, of different binders, based on ash, lime, sodium chloride and alkali solutions, applied in the stabilisation of a silty sand. The tests included unconfined compression strength tests, triaxial tests and seismic wave measurements performed at different curing periods. The results were compared with a binder made of Portland cement and a commercial additive specifically designed for soil stabilisation in road applications. The activated ash mixtures with lime were the most performing producing a significant increase in the reactions development and, consequently, in the strength gain rate. The sodium chloride significantly improved the lime and lime-ash mixtures, but provided only a slight improvement in the activated ash mixtures.

Improvement of a clayey soil with alkali activated low-calcium fly ash for transport infrastructures applications

The improvement of geotechnical properties is often achieved by the addition of traditional binders, such as cement or lime. However, the use of such binders implies a considerable financial and environmental cost that needs to be mitigated. An unconventional solution, similar to cement in terms of performance but more environmentally friendly, consists in the use of binders made from alkaline activated industrial residues. The technique consists on the activation of raw materials (such as fly ash or blast furnace slag) rich in Si, Al, or even Ca, with high pH alkaline solutions. The present work was developed aiming the possible stabilisation, using different fly ash contents, of a clayey soil with sand. The activator solution was composed of sodium hydroxide and sodium silicate. The extended experimental campaign included unconfined compressive strength (UCS), California Bearing Ratio (CBR), pulse velocity tests and triaxial tests to assess the geomechanical improvement induced by the new binder. As a mean of comparison, the experimental campaign included also the stabilisation of the same soil with either cement or lime. The obtained data indicates that the use of alkaline activation as a soil stabilisation technique provides competitive geomechanical results, when compared with those obtained with traditional binders.

Geotechnical characterization of recycled C&D wastes for use as trenches backfilling

DTU Orbit (03/11/2019) Recycling of MSWI fly ash in clay bricks-effect of washing and electrodialytic treatment Fly ash generated from municipal solid waste incineration (MSWI) is a hazardous waste due to presence and leachability of heavy metals and organic pollutants (e.g. dioxins and polycyclic aromatic hydrocarbons). In 2000, approximately 25 Mt/year of fly ash was generated in USA, Japan and EU (Reijnders 2005). Electrodialytic remediation (EDR) is one technique for MSWI fly ash treatment (Ferreira et al. 2005), where an electric DC field is applied to an ash-water suspension to extract and separate heavy metal by migration towards anode or cathode through ion exchange membranes. Ferreira et al. (2008) observed that in MSWI ash treated by water washing and EDR, metals were mainly in the strongly bonded and residual phases, indicating a reduction in the ash’s environmental risk. Belmonte et al. (2016) made Greenlandic bricks (∼2 g discs) containing 20% and 40% of EDR treated MSWI fly ash, and found that bricks had a low durability and high leaching of As and Cr. In the present study, fired fly ash-clay bricks with a larger size and with lower EDR-treated ash (water-washed before EDR) contents (5%, 10% and 20%) were made and characterized. These bricks were compared with 100% clay bricks and with bricks made from original MSWI fly ash at 20% substitution rate. The feasibility of incorporation of MSWI fly ash treated by combined washing and EDR in production of sintered clay bricks was investigated.

Geotechnical Properties of Sediments by In Situ Tests

River sediments are mainly composed by intermediate materials, between sand and clay, for which partial drainage conditions apply. In these cases, the interpretation of CPTU tests may be wrong since existing correlations are based on fully drained or fully undrained conditions. This paper presents results from CPTU tests performed in a river area to evaluate whether partial drainage conditions were observed. The results, presented in terms of the normalized velocity, show that great part of the analyzed profiles are in this condition. For this reason, the angle of shearing resistance was presented as a conservative estimate of the soil strength in these areas.