Alessandra Buoso

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.

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 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%).