Luigi Coppola

Electrical Properties of Carbon Nanotubes Cement Composites for Monitoring Stress Conditions in Concrete Structures

Cement pastes reinforced with Multi-Walled carbon NanoTubes (MWNTs) are smart materials with piezoresistivity properties. Adding carbon nanotubes to the cement matrix, in fact, the electrical resistivity of cementitious composites changes with the stress conditions under static and dynamic loads. This particular behaviour can be used to evaluate the stress level in reinforced concrete structures, to monitor the traffic flow, to weigh vehicles. In this paper data on pressure-sensitive behaviour under compressive stress of cement pastes and mortars containing different percentages (from 0.0% to 1.0% vs. cement mass) of MWNTs are presented.In order to form a conductive network and enhancethe piezoresistive properties of cementitious mixtures, Carbon NanoTubes (CNTs) need to be efficiently dispersed in the cement matrix. Two different methods to disperse CNTsin the cement matrix were used. The first one uses a surfactant (Sodium Linear Alkyl Benzene Sulphonate - LAS): MWNTs were dispersed in a LAS aqueous solution,and thenmixed with cement and a defoamer (tributyl phosphate) to decrease the air bubble in MWNT filled cement-based composites. The second method consists in mixing CNTs with about 50% of the mixing water in a becker by means of a glass wand. Then, the solution is sonicated by an ultrasonic generator for 10 minutes. Finally, the sonicatedCNT-aqueous solution ismixed with cement (and sand for the mortars). The piezoresistivity properties of the cementitious mixtures manufactured with the two above mentioned CNTs dispersing methods will be compared.Experimental results show that the electrical resistance changes synchronously with the compressive stress levelsfor the specimens manufactured with both methods. Therefore, CNTs improve the pressure-sensitivity of cementitious composites. Moreover, the piezoresistive response is better for cementitious composites manufactured by using the surfactant agent to disperse CNTs. Data indicate that – thanks to the better dispersion of nanotubes promoted by the surfactant - the pressure-sensitivity properties of cement pastes can be achieved even by using a very low percentage of CNTS (0.1% vs. cement mass). These findings seem to indicate that self-sensing CNTs/cement composite can be produced. These smart materials have great potential and they could be used in the next future in concrete field for practical applications to monitor the stress level of reinforced concrete elements subjected to static, dynamic and impact loads. In particular, informations on actual stress existing under dynamic and impact loads could be improve design procedures in protective structures.

Sustainable Development and Durability of Self-Compacting Concretes

Self-compacting concretes (SCC) represent a move toward a sustainable material since they encourage the use of waste and recycled materials. The high volume of very fine powder necessary to achieve deformability and passing ability properties, in fact, permits SCC to consume large amount of fly-ash, very fine particles generated by the recycling of demolished concrete structures, and huge amount of calcareous filler available from the marble quarries. Moreover SCC turn out to be materials with an extended durability with respect to conventional concretes. Since fresh properties of self-compacting concretes (SCC) are significantly different from those of conventional concretes (CC) durability can be significantly improved when a SCC is used due to a modification of the microstructure of the interfacial transition zone between aggregates and cement matrix. This paper presents results of an experimental study carried out to evaluate changes in microstructure of interfacial transition zone (itz) and of bulk paste for both SCC and CC. Data on the influence of the calcareous filler, a fundamental ingredients to achieve self-compactability, on the hydration process of cement are also presented. Data indicate that the decrease in internal bleeding, when self-compacting concrete is used, seems to favour the formation of a stronger transition zone characterized by a less porous structure and with a limited amount of microcracking responsible for higher compressive strength values for SCC with respect CC. No differences were detected by EDAX analysis in the chemical nature of itz with respect the bulk matrix both for SCC and CC. Finally, observations of the cement hydration by analysis of the temperature profile vs time seem to indicate the calcareous grains promote formation of heterogeneous nucleation responsible for the increased crystallinity of ettringite, for a shorter normally dormant period and, hence, for higher strength values at early ages, when the calcareous filler is used.

Corrosion inhibitors in reinforced concrete structures Part 1: Preventative technique

Abstract The corrosion of reinforcing bars (rebars) is the main cause of premature failure of reinforced concrete structures. The ingress of chlorides and the carbonation process that leads to the neutralisation of the alkaline pore solution are among the key phenomena promoting such corrosion. This paper deals with the effectiveness of commercial corrosion inhibitors to prevent the corrosion of reinforcement in concrete. Three organic inhibitors and, for comparison, a calcium nitrite based product, were added to fresh concrete in the dosages suggested by producers. Experimental tests were carried out both in chloride contaminated concrete (produced by adding chlorides directly to the mixture or by penetrating hardened concrete using ponding cycles) and in carbonated concrete. In order to study the effectiveness of the inhibitors on the initiation of corrosion, measurements were made over two years of two electrochemical parameters, the free corrosion potential and the polarisation resistance. These are discussed and compared with results obtained for concrete specimens without inhibitors. One of the organic corrosion inhibitors and the nitrite based product both delayed the initiation of corrosion.

Mechanical Characterization of Cement Composites Reinforced with Fiberglass, Carbon Nanotubes or Glass Reinforced Plastic (GRP) at High Strain Rates

Advanced researches on concrete are directed toward investigating the behavior of reinforced concrete structures in severe conditions such as those promoted by impact loads. Some particular structures (protective shelters, nuclear reactor containment, offshore structures, military structures, chemical or Energy production plant) may be subjected to loading at very high rate of stress or strain caused by impact of missiles or flying objects, also by vehicle collisions or impulses due to explosions and earthquakes. Resistance to impact loads is guaranteed by using cementitious materials having both high strength and ductility. In order to improve ductility cementitious mortars with Glass Reinforced Plastics (GRP) replacing partially the natural sand were manufactured. Moreover, glass fiber (GF) reinforced mortars were produced to enhance toughness. For this scope two types of glass fibers were used different in length and diameter. Since the use of GRP and GF don’t produce any increase in strength of the mortars Carbon Nanotubes were added in the cement matrix to enhance tensile strength of the cementitious composite. Flexural, compressive and Hopkinson bar tests were carried out to evaluate the role of the different materials used. Replacing partially the natural sand with Glass Reinforced Plastics (GRP), compressive and flexural strength decrease (about 20%) with respect those of the reference mortar both on static and dynamic condition as a consequence of an anomalous air entrapment. Adding glass fibers (GF), GRP or/and Carbon Nanotubes (CNTs) no substantial improvement in terms of mechanical properties under static condition was occurred. The Dynamic Increase Factor of the reference mortar was higher than that of the reinforced mixtures, but fracture energy was lower. In particular, combined addition of carbon nanotubes and GRP determines an increase in the energy fracture. The higher the carbon nanotubes content, the higher both fracture energy and tensile strength because nanoparticles oppose to wave and crack propagation, increasing the high strain rate strength. GRP and CNTs reinforced mortars need more fracture energy to failure at 150 s-1 strain rate.

The Rheological and Mechanical Performances of Concrete Manufactured with Blended Admixtures Based on Phosphonates

The paper deals with the effectiveness of blended phosphonate-based superplasticizers (PHN) for ready mixed concrete. Two phosphonates (PNH1 and PNH2) were added in different percentage to naphthalene sulphonate (NSF) or polycarboxylates (PCEs) based admixtures to improve both compatibility with different cements and workability retention of concrete. The performance of the obtained concrete mixtures was compared to concretes manufactured with the pure NSF or PCE based admixtures. Concretes with the same initial workability (flow table > 580mm) were produced at a temperature of 20 °C and 30 °C. Workability was measured at 0, 30 and 60 minutes to evaluate the flow retention performances of blended superplasticizers. Compressive tests were carried out to study the influence of the superplasticizer on concrete strength gain at the age of 1, 7 and 28 days. PNH1 in combination with NSF improved workability retention with respect to pure NSF, but caused a reduction in the early compressive strength when the dosage exceeded 0.10% (dry polymer vs. cement mass). Dosage of hybrid PCE-PNH superplasticizers to attain the targeted workability was lower with respect to hybrid PNH1/NSF admixtures. PNH1 was more effective than PNH2 in hybrid PCE admixtures in terms of workability retention. A threshold value for PNH dosage (about 0.18 - 0.20 %) exists also in hybrid PCE superplasticizers, but it is about two times higher than that of hybrid PNH1/NSF.

Binders alternative to Portland cement and waste management for sustainable construction—part 1:

This review presents “a state of the art” report on sustainability in construction materials. The authors propose different solutions to make the concrete industry more environmentally friendly in order to reduce greenhouse gases emissions and consumption of non-renewable resources. Part 1—the present paper—focuses on the use of binders alternative to Portland cement, including sulfoaluminate cements, alkali-activated materials, and geopolymers. Part 2 will be dedicated to traditional Portland-free binders and waste management and recycling in mortar and concrete production.

Cement-Based Renders Manufactured with Phase-Change Materials: Applications and Feasibility

The paper focuses on the evaluation of the rheological and mechanical performances of cement-based renders manufactured with phase-change materials (PCM) in form of microencapsulated paraffin for innovative and ecofriendly residential buildings. Specifically, cement-based renders were manufactured by incorporating different amount of paraffin microcapsules—ranging from 5% to 20% by weight with respect to binder. Specific mass, entrained or entrapped air, and setting time were evaluated on fresh mortars. Compressive strength was measured over time to evaluate the effect of the PCM addition on the hydration kinetics of cement. Drying shrinkage was also evaluated. Experimental results confirmed that the compressive strength decreases as the amount of PCM increases. Furthermore, the higher the PCM content, the higher the drying shrinkage. The results confirm the possibility of manufacturing cement-based renders containing up to 20% by weight of PCM microcapsules with respect to binder.

Repair and conservation of reinforced concrete tent-church by Pino Pizzigoni at Longuelo – Bergamo (Italy)

ABSTRACT In 2016, maintenance and conservation of the reinforced concrete tent-church of Pino Pizzigoni, located in the district of Longuelo in Bergamo (Italy) was completed. The church represents one of the most impressive examples of very thin reinforced concrete vaults structures in Italy. However, the very thin cross section of these structures has required in the years a constant maintenance for the particular vulnerability of vaults to environmental aggressive agents. The intervention consisted in the reconstruction of the deteriorated sections of load bearing structures by means of a self-compacting shrinkage-compensating mortar poured in a wood formwork that faithfully reproduced the original texture of the reinforced concrete elements. The reconstruction of thin vaults occurred, moreover, with mortar applied by trowel. Repair was completed by the application, by spraying, on the surface of concrete elements of a thin mortar layer. Finally, an elastomeric acrylic coating completed the maintenance work.

The effect of sodium silicate on the behaviour of shotcretes for tunnel lining

Present case study investigates the rheological, mechanical and in-placing performances of fiber reinforced shotcrete manufactured with different fibers (steel, glass and polypropylene) and with sodium silicate based set-accelerating admixture for tunnel linings. The study compares the performances of concretes manufactured and fully compacted with those shotcretes which are manufactured directly on the job-site. The influence of sodium silicate accelerator on mechanical and rheological properties of fiber-reinforced shotcretes with respect to reference concrete were evaluated. It was observed that: The addition of fibers does not influence slump and workability retention with respect to reference concrete, independent of type and dosage of fibers; Spraying and set accelerator dosage determined a decrease about of 10-30% in compressive strength compared to that of concrete placed and vibrated without sodium silicate accelerator; The set accelerating admixture has negative effect on compressive strength of fiber-reinforced shotcrete (15%).