P. Garcés

Multifunctional Cement Composites Strain and Damage Sensors Applied on Reinforced Concrete (RC) Structural Elements

In this research, strain-sensing and damage-sensing functional properties of cement composites have been studied on a conventional reinforced concrete (RC) beam. Carbon nanofiber (CNFCC) and fiber (CFCC) cement composites were used as sensors on a 4 m long RC beam. Different casting conditions (in situ or attached), service location (under tension or compression) and electrical contacts (embedded or superficial) were compared. Both CNFCC and CFCC were suitable as strain sensors in reversible (elastic) sensing condition testing. CNFCC showed higher sensitivities (gage factor up to 191.8), while CFCC only reached gage factors values of 178.9 (tension) or 49.5 (compression). Furthermore, damage-sensing tests were run, increasing the applied load progressively up to the RC beam failure. In these conditions, CNFCC sensors were also strain sensitive, but no damage sensing mechanism was detected for the strain levels achieved during the tests. Hence, these cement composites could act as strain sensors, even for severe damaged structures near to their collapse.

Effect of curing temperature in some hydration characteristics of Calcium Aluminate Cement compared with those of Portland Cement

Abstract In the present paper the hydration of 1:3 Calcium Aluminate Cement (CAC) and Portland Cement (PC) mortars at temperatures of 5 °C, 20 °C, and 60 °C have been studied. An evaluation of the compressive and flexural strengths obtained from different curing conditions is made. The evolution of phases presented was also studied from XRD spectra. The relationship between the values of ultrasonic propagation velocity, UPV, (Y1-axis) and compressive strengths (Y2-axis) versus w c ratios for CAC mortars, cured at 5 °C, 20 °C and 60 °C respectively, have also been obtained, at a curing age of 28 days. The great similarity of curves enables us to establish the hypothesis that it is possible to use a non destructive test as an empirical determination of compressive strength for CAC mortars with the same proportions of aggregates/ cements/water as those used in the test. Abrams rule was applied in order to check the reality of this model in the CAC mortars studied. The percentage of variability of the variable response (Rc), obtained in our regression models is highly satisfactory.

Triple Percolation in Concrete Reinforced with Carbon Fiber

This article discusses how the electrical resistivity of carbon fiber (CF) reinforced concrete with electrical continuity within the cement paste and sand-cement ratio (s/c) 0.75 depends on the CF content and gravel-sand ratio (g/s). For resistivity ≤500 Ω.cm, the mortar must exceed 62 vol.% and the cement paste must exceed 44 vol.%. The minimum resistivities provided by double percolation (continuity provided by the cement paste in the mortar but no continuity of either paste or mortar), pseudo triple percolation (continuity provided by the cement paste of the mortar), and true triple percolation (continuity provided by the mortar) are 355, 36, and 7.6 Ω.cm, respectively. The true triple percolation threshold is 0.75% CF (by mass of cement) if g/s ≤1.50 and 1.5% CF if g/s ≤2.00. The pseudo triple percolation threshold is 1.5% CF if g/s = 2.50 and 1.75% CF if g/s = 2.50 to 3.00. The double percolation threshold is 0.75% CF if g/s = 2.00 to 3.00.

Corrosion Behavior of Steel Reinforcement in Concrete with Recycled Aggregates, Fly Ash and Spent Cracking Catalyst

The main strategy to reduce the environmental impact of the concrete industry is to reuse the waste materials. This research has considered the combination of cement replacement by industrial by-products, and natural coarse aggregate substitution by recycled aggregate. The aim is to evaluate the behavior of concretes with a reduced impact on the environment by replacing a 50% of cement by industrial by-products (15% of spent fluid catalytic cracking catalyst and 35% of fly ash) and a 100% of natural coarse aggregate by recycled aggregate. The concretes prepared according to these considerations have been tested in terms of mechanical strengths and the protection offered against steel reinforcement corrosion under carbonation attack and chloride-contaminated environments. The proposed concrete combinations reduced the mechanical performance of concretes in terms of elastic modulus, compressive strength, and flexural strength. In addition, an increase in open porosity due to the presence of recycled aggregate was observed, which is coherent with the changes observed in mechanical tests. Regarding corrosion tests, no significant differences were observed in the case of the resistance of these types of concretes under a natural chloride attack. In the case of carbonation attack, although all concretes did not stand the highly aggressive conditions, those concretes with cement replacement behaved worse than Portland cement concretes.

Highly Conductive Carbon Fiber Reinforced Concrete for Icing Prevention and Curing

This paper aims to study the feasibility of highly conductive carbon fiber reinforced concrete (CFRC) as a self-heating material for ice formation prevention and curing in pavements. Tests were carried out in lab ambient conditions at different fixed voltages and then introduced in a freezer at −15 °C. The specimens inside the freezer were exposed to different fixed voltages when reaching +5 °C for prevention of icing and when reaching the temperature inside the freezer, i.e., −15 °C, for curing of icing. Results show that this concrete could act as a heating element in pavements with risk of ice formation, consuming a reasonable amount of energy for both anti-icing (prevention) and deicing (curing), which could turn into an environmentally friendly and cost-effective deicing method.

Self-Sensing Properties of Alkali Activated Blast Furnace Slag (BFS) Composites Reinforced with Carbon Fibers

In recent years, several researchers have shown the good performance of alkali activated slag cement and concretes. Besides their good mechanical properties and durability, this type of cement is a good alternative to Portland cements if sustainability is considered. Moreover, multifunctional cement composites have been developed in the last decades for their functional applications (self-sensing, EMI shielding, self-heating, etc.). In this study, the strain and damage sensing possible application of carbon fiber reinforced alkali activated slag pastes has been evaluated. Cement pastes with 0, 0.29 and 0.58 vol % carbon fiber addition were prepared. Both carbon fiber dosages showed sensing properties. For strain sensing, function gage factors of up to 661 were calculated for compressive cycles. Furthermore, all composites with carbon fibers suffered a sudden increase in their resistivity when internal damages began, prior to any external signal of damage. Hence, this material may be suitable as strain or damage sensor.

Effect of Type of Anodic Arrangements on Efficiency of Electrochemical Chloride Removal from Concrete

Electrochemical techniques can be used to arrest or prevent reinforced concrete corrosion. This article reports on a study that evaluated the effect of the type of anodic arrangement on the efficiency of the technique of electrochemical chloride removal (ECR) applied to reinforced concrete structures. Two different arrangements were tested for maintaining the electrolytic medium in contact with the concrete surface: complete immersion of the system (structure and anode) in water during the electrochemical treatment; and the wrapping of the concrete structure with a sandwich-like anodic system, which consisted of two layers of polypropylene felt embedding the anodic material. Results demonstrate that the efficiency of an ECR treatment applied with a sandwich-like anodic system may be practically equal to that obtained with a liquid electrolyte (immersion) if a good contact between the concrete surface and the polymeric system is ensured. The use of a continuous graphite felt for the sandwich-like anodic system, instead of a discontinuous and more rigid metallic wire mesh anode, makes it easier to achieve the necessary electrical contact. The authors caution that the net influence of carbonation on the efficiency of ECR may vary for different experimental conditions.

Shape Effect of Electrochemical Chloride Extraction in Structural Reinforced Concrete Elements Using a New Cement-Based Anodic System

This article shows the research carried out by the authors focused on how the shape of structural reinforced concrete elements treated with electrochemical chloride extraction can affect the efficiency of this process. Assuming the current use of different anode systems, the present study considers the comparison of results between conventional anodes based on Ti-RuO2 wire mesh and a cement-based anodic system such as a paste of graphite-cement. Reinforced concrete elements of a meter length were molded to serve as laboratory specimens, to closely represent authentic structural supports, with circular and rectangular sections. Results confirm almost equal performances for both types of anode systems when electrochemical chloride extraction is applied to isotropic structural elements. In the case of anisotropic ones, such as rectangular sections with no uniformly distributed rebar, differences in electrical flow density were detected during the treatment. Those differences were more extreme for Ti-RuO2 mesh anode system. This particular shape effect is evidenced by obtaining the efficiencies of electrochemical chloride extraction in different points of specimens.

Carbon Nanofiber Cement Sensors to Detect Strain and Damage of Concrete Specimens Under Compression

Cement composites with nano-additions have been vastly studied for their functional applications, such as strain and damage sensing. The capacity of a carbon nanofiber (CNF) cement paste has already been tested. However, this study is focused on the use of CNF cement composites as sensors in regular concrete samples. Different measuring techniques and humidity conditions of CNF samples were tested to optimize the strain and damage sensing of this material. In the strain sensing tests (for compressive stresses up to 10 MPa), the response depends on the maximum stress applied. The material was more sensitive at higher loads. Furthermore, the actual load time history did not influence the electrical response, and similar curves were obtained for different test configurations. On the other hand, damage sensing tests proved the capability of CNF cement composites to measure the strain level of concrete samples, even for loads close to the material’s strength. Some problems were detected in the strain transmission between sensor and concrete specimens, which will require specific calibration of each sensor one attached to the structure.

Electronic and Electrolytic Conduction of Cement Pastes with Additions of Carbonaceous Materials

Additions to concrete may change some of its basic properties in several ways depending on the nature of these additions. The addition of fibers, in particular, has enabled new concrete characteristics, making standard concrete a very modern composite material. If the fibers are electrical conductors, the properties that change in addition to the mechanical ones, are the thermal and electrical conductivities. The results indicate that the arrangement of the electrodes and the electrode-material interface are relevant, because the use of sponges between electrode and concrete prevents the contact between the metallic electrode and the carbon material which ends in different values of electrical resistance with and without sponges. Moisture conditions, that critically influence the electrical resistance of concrete without additions, resulted, also very relevant when conductive substances are present in the matrix. If the proportion of the carbonaceous addition, that lowers significantly the resistivity, is to be quantified, the best procedure seems to measure in dry concrete (0 % relative humidity) with sponges or, alternatively, wet concrete (100 % relative humidity) with silver painted electrodes (without sponges).