Moisture transport in heated concrete, as studied by NMR, and its consequences for fire spalling

During the past 30 years concrete has developed enormously in both strength and durability. A drawback of these improvements is the increased risk of explosive spalling in case of fire. The moisture inside the concrete plays an important role in the spalling mechanism. In order to study the moisture migration inside concrete during intense heating, a dedicated nuclear magnetic resonance (NMR) setup was built. This setup can be placed inside a 1.5-T MRI scanner.With this setup one-dimensional moisture profiles can be measured while the concrete sample is heated up to 250 °C. Besides concrete, measurements were performed on fired-clay brick and calcium-silicate brick.The results show that water inside the concrete sample is superheated to a temperature of 170 °C, which results in an increased pressure inside the concrete. A model was developed to predict the movement of the observed drying front.     

Thermal and mechanical properties of fiber reinforced high performance self-consolidating concrete at elevated temperatures

This paper presents the effect of temperature on thermal and mechanical properties of self-consolidating concrete (SCC) and fiber reinforced SCC (FRSCC). For thermal properties specific heat, thermal conductivity, and thermal expansion were measured, whereas for mechanical properties compressive strength, tensile strength and elastic modulus were measured in the temperature range of 20–800 °C. Four SCC mixes, plain SCC, steel, polypropylene, and hybrid fiber reinforced SCC were considered in the test program. Data from mechanical property tests show that the presence of steel fibers enhances high temperature splitting tensile strength and elastic modulus of SCC. Also the thermal expansion of FRSCC is slightly higher than that of SCC in 20–1000 °C range. Data generated from these tests was utilized to develop simplified relations for expressing thermal and mechanical properties of SCC and FRSCC as a function of temperature.     

Assessment of statistical variations in impact resistance of high-strength concrete and high-strength steel fiber-reinforced concrete

The impact resistance variations of high-strength steel fiber-reinforced concrete (HSFRC), versus those of high-strength concrete (HSC), commanded this research. The impact resistance of the high-strength steel fiber-reinforced concrete improved satisfactorily over that of the high-strength concrete; the failure strength improved most, followed by first-crack strength and percentage increase in the number of post-first-crack blows. The two concretes resembled each other on the coefficient of variation values, respectively, on the two strengths, whereas the high-strength concrete was much higher in the value on the percentage increase. The Kolmogorov-Smirnov test indicates that the high-strength concrete was approximately normally distributed in first-crack and failure strengths, high-strength steel fiber-reinforced concrete was poorly normally distributed in the two strengths, and both concretes were hardly normally distributed in the percentage increase. Finally, for both concretes, failure strength regression models were developed, and then, the accompanying 95% prediction intervals for the strength were established.     

Predicting the pullout response of inclined hooked steel fibers

Steel fiber reinforced concrete (SFRC) is symptomatically an anisotropic material due to the random orientation of fibers within the cement matrix. Fibers under different inclination angles provide different strength contributions at a given crack width. Therefore the pullout response of inclined fibers is a paramount subject to understand and quantify SFRC behavior, particularly in the case of fibers with hooked ends, which are currently the most widely used. Several experimental results were considered to validate the approach and to assure its suitability on distinct material properties and boundary conditions. The good agreement on predicting the pullout behavior of these fibers encourages its use towards a new concept of design and optimization of SFRC.     

Orientation factor and number of fibers at failure plane in ring-type steel fiber reinforced concrete

Considering the probabilistic distributions of fibers in ring-type steel fiber reinforced concrete, the orientation factor and the number of ring-type steel fibers crossing the failure plane were theoretically derived as a function of fiber geometry, specimen dimensions, and fiber volume fraction. A total number of 24 specimens were tested incorporating different fiber types, specimen geometry, and fiber volume fractions of 0.2% and 0.4%: 5 beams and 5 panels containing straight steel fibers; and 6 beams and 8 panels containing ring-type steel fibers. Measurements were made to assess the number of fibers at fractured surfaces of steel fiber reinforced concrete. The developed theoretical expressions reasonably predicted the orientation factor and the number of ring-type steel fibers at failure plane: the average and the standard deviation for the ratios of the test to theory were 1.03 and 0.26, respectively. Theoretical investigations and comparisons were made for the values of orientation factor and the number of fibers at failure plane for straight steel fibers and ring-type steel fibers.     

Pore pressure development in hybrid fibre-reinforced high strength concrete at elevated temperatures

The present experimental work investigates the build-up of pore pressure at different depths of High Strength Concrete (HSC) and Hybrid-Fibre-Reinforced High Strength Concrete (HFRHSC) when exposed to different heating rates. First, the effect of the measurement technique on maximum pore pressures measured was evaluated. The pressure measurement technique which utilised a sintered metal and silicon oil was found to be the most effective technique for pore pressure measurement. Pore pressure measurements carried out showed that addition of polypropylene fibres is very effective in mitigation of spalling and build-up of pore pressure inside heated HSC. Addition of steel fibres plays some role in pore pressure reduction at relatively higher pressures in deeper regions of concrete during fast heating. Pore pressure development is highly influenced by the rate of heating with fast heating leading to higher pore pressures in the deeper regions of concrete compared to slow heating.     

Influence of the nature of aggregates on the behaviour of concrete subjected to elevated temperature

An experimental study is carried out on concretes composed of three different types of aggregates: semi crushed silico-calcareous, crushed calcareous and rolled siliceous. For each aggregate type, two water/cement ratios (W/C), 0.6 and 0.3 are studied. Aggregates and concrete specimens were subjected to 300, 600 and 750 °C heating–cooling cycles. We analyse the evolution of thermal, physical and mechanical properties of concrete in terms of behaviour and physical characteristic evolutions of aggregates with temperature. The study of thermal behaviour of aggregates showed the importance of initial moisture state for the flints. The crystallisation and microstructure of quartz play an important role in the thermal stability of siliceous aggregates. The residual mechanical behaviour of concrete varies depending on the aggregate and the influence of aggregates is also dependent on paste composition. This study allowed to better understand the influence of chemical and mineralogical characteristics of aggregates on the thermomechanical behaviour of concrete.     

How do polypropylene fibers improve the spalling behavior of in-situ concrete?

The results of a recently finished research project have shown a great influence of the amount of polypropylene (PP) fibers on the spalling behavior of concrete under fire loading. Starting from the identification of the permeability as the parameter with the greatest influence on spalling, results of permeability tests on normal-strength in-situ concrete without and with PP-fibers (1.5 kg/m³) are presented in this paper. The values for the permeability, which are obtained for concrete pre-heated to different temperature levels, are related to the pore structure, accessible by mercury-intrusion-porosimetry (MIP) tests. The considered concrete was prepared under on-site conditions, accounting for the workability and densification when casting at the construction site. In order to illustrate the effect of the permeability of concrete with and without PP-fibers on spalling, which was experienced during the aforementioned research project, a finite-element analysis, taking the coupling between heat and mass transport into account, is performed. The so-obtained results provide insight into the risk of spalling of concrete with varying amount of PP-fibers.     

Simulation of crack propagation in fiber-reinforced concrete by fracture mechanics

Mode I crack propagation in fiber-reinforced concrete (FRC) is simulated by a fracture mechanics approach. A superposition method is applied to calculate the crack tip stress intensity factor. The model relies on the fracture toughness of hardened cement paste (K_IC) and the crack bridging law, so-called stress-crack width (σ-δ) relationship of the material, as the fundamental material parameters for model input. As two examples, experimental data from steel FRC beams under three-point bending load are analyzed with the present fracture mechanics model. A good agreement has been found between model predictions and experimental results in terms of flexural stress-crack mouth opening displacement (CMOD) diagrams. These analyses and comparisons confirm that the structural performance of concrete and FRC elements, such as beams in bending, can be predicted by the simple fracture mechanics model as long as the related material properties, K_IC and (σ-δ) relationship, are known.     

Nonlinear ultrasonic evaluation of load effects on discontinuities in concrete

The presence of discontinuity surfaces in concrete structures, i.e. two or more layers in contact, may be an existing situation with evident relapses on damage formation and progression. Differences occur depending on the type of discontinuity, which could be a thin weaker layer or a pre-existing crack. The behavior of pre-existing interfaces is here studied by means of the Scaling Subtraction Method, a Nonlinear Ultrasonic Non-Destructive Technique, that revealed to be effective in describing the mechanical evolution of concrete samples with discontinuity surfaces under the effects of compressive loads.     

Post-fire residual mechanical properties of concrete made with recycled concrete coarse aggregates

This paper presents results of an experimental study on the residual mechanical performance of concrete produced with recycled coarse aggregates, after being subjected to high temperatures. Four different concrete compositions were prepared: a reference concrete made with natural coarse aggregates and three concrete mixes with replacement rates of 20%, 50% and 100% of natural coarse aggregates by recycled concrete coarse aggregates. Specimens were exposed for a period of 1 h to temperatures of 400 °C, 600 °C and 800 °C, after being heated in accordance with ISO 834 time–temperature curve. After cooling down to ambient temperature, the following basic mechanical properties were then evaluated and compared with reference values obtained prior to thermal exposure: (i) compressive strength; (ii) tensile splitting strength; and (iii) elasticity modulus. Results obtained show that there are no significant differences in the thermal response and post-fire mechanical behaviour of concrete made with recycled coarse aggregates, when compared to conventional concrete.     

Monitoring evolution of compressive damage in concrete with linear and nonlinear ultrasonic methods

An experimental study was conducted on different concrete cylinders damaged in compression. The evolution of damage was followed by means of linear and nonlinear ultrasonic methods, with the purpose to provide a better understanding of mechanical degradation processes in concrete and highlight the potentialities and limitations of the Non-Destructive Techniques used.     

Development of waste tire modified concrete

In this study, waste tire modified concrete was investigated experimentally. Two types of waste tire configurations were evaluated. One was in the form of chips, or particles, and the other was in the form of fibers. For the waste tire chip modified concrete, surface treatment by saturated NaOH solution and physical anchorage by drilling a hole at the center of the chips were also investigated. For the waste tire fiber modified concrete, fibers with various aspect ratios were utilized. A hybrid fiber reinforcement using waste tire fiber and polypropylene (PP) fiber was also investigated. The effect of waste tire resources (car tires or truck tires) on the strength and stiffness was evaluated. A total of 10 batches of concrete, which yielded sixty φ152.4×304.8 mm cylinders, were prepared. Compressive strength, compressive modulus of elasticity, Poisson's ratio, and split tensile strength tests were conducted on the prepared samples. Ways to further recover the lost strength and stiffness of waste tire modified concrete were discussed based on the test results.     

Derivation of a crack opening deflection relationship for fibre reinforced concrete panels using a stochastic model: Application for predicting the flexural behaviour of round panels using stress crack opening diagrams

This study is aimed at proposing a simple analytical model to investigate the post-cracking behaviour of FRC panels, using an arbitrary tension softening, stress crack opening diagram, as the input. A new relationship that links the crack opening to the panel deflection is proposed. Due to the stochastic nature of material properties, the random fibre distribution, and other uncertainties that are involved in concrete mix, this relationship is developed from the analysis of beams having the same thickness using the Monte Carlo simulation (MCS) technique. The softening diagrams obtained from direct tensile tests are used as the input for the calculation, in a deterministic way, of the mean load displacement response of round panels. A good agreement is found between the model predictions and the experimental results.     

Explosive spalling and residual mechanical properties of fiber-toughened high-performance concrete subjected to high temperatures

In this paper, an experimental investigation was conducted to explore the relationship between explosive spalling occurrence and residual mechanical properties of fiber-toughened high-performance concrete exposed to high temperatures. The residual mechanical properties measured include compressive strength, tensile splitting strength, and fracture energy. A series of concretes were prepared using OPC (ordinary Portland cement) and crushed limestone. Steel fiber, polypropylene fiber, and hybrid fiber (polypropylene fiber and steel fiber) were added to enhance fracture energy of the concretes. After exposure to high temperatures ranged from 200 to 800 °C, the residual mechanical properties of fiber-toughened high-performance concrete were investigated. For fiber concrete, although residual strength was decreased by exposure to high temperatures over 400 °C, residual fracture energy was significantly higher than that before heating. Incorporating hybrid fiber seems to be a promising way to enhance resistance of concrete to explosive spalling.     

Multiple cracking and strain hardening in fiber-reinforced concrete under uniaxial tension

Fiber-reinforced concrete (FRC) showing strain hardening after cracking is commonly defined as High Performance Fiber-Reinforced Cementitious Composite (HPFRCC). In the post-cracking stage, several cracks develop before complete failure, which occurs when tensile strains localize in one of the formed cracks. As is well known, multiple cracking and strain hardening can be achieved in cement-based specimens subjected to uniaxial tension by increasing the volume fraction of steel fibers with hooked ends, or by using plastic fibers with and without steel fibers, or by means of high bond steel fibers (e.g., twisted fibers or cords). To better understand why, in such situations, high mechanical performances are obtained, an analytical model is herein proposed. It is based on a cohesive interface analysis, which has been largely adopted to investigate the mechanical response of FRC or the snubbing effects produced by inclined fibers, but not the condition of multiple cracking and strain hardening of HPFRCC. Through this approach, all the phenomena that affect the post-cracking response of FRC are evidenced, such as the nonlinear fracture mechanics of the matrix, the bond–slip behaviour between fibers and matrix, and the elastic response of both materials. The model, capable of predicting the average distance between cracks as measured in some experimental campaigns, leads to a new design criterion for HPFRCC and can eventually be used to enhance the performances of cement-based composites.     

Influence of steel fibers on the development of alkali-aggregate reaction

This work presents the results of an experimental research concerning the use of fibers in mortar specimens subjected to alkali-aggregate reaction (AAR). Two types of steel fibers (0.16 mm diameter and 6.0 mm length, and 0.20 mm diameter and 13.0 mm length) were used with fiber volume contents of 1% and 2%. Besides the expansion accelerated tests, compressive tests and flexural tests have also been carried out to display the main mechanical characteristics of the fiber-reinforced mortars after being subjected to AAR. Moreover, the microstructure of the specimens was analyzed by scanning electron microscopy and energy dispersive X-ray. The results shown that the addition of steel fibers reduced the expansion due to AAR for the experimental conditions studied in this paper. The most expressive benefit corresponded to the addition of 13.0 mm fibers in the mixture containing 2% fiber content. This fiber volume content also corresponded to the maximum increment in the mechanical properties compared to the reference mortar, mainly for the post-cracking strength and for the toughness in bending. It was observed that the fibers have a beneficial effect on the material, without compromising its main mechanical properties.     

On the mathematics of time-dependent apparent chloride diffusion coefficient in concrete

This paper provides an improved mathematical analysis of chloride penetration into concrete employing a time-dependent diffusion coefficient for the solution of Fick's second law of diffusion. In the paper the possible errors caused by the application of oversimplified mathematical expressions used in some models for the evaluation of service life of reinforced concrete structures are discussed. The results from this mathematical analysis demonstrate that some models based on the oversimplified error function complement (ERFC) solutions may easily overestimate the service life by orders of magnitude, especially when the age factor is high. Some chloride profiles after up to 10 years' field exposure were used to compare the oversimplified with the improved models. The results show that both the oversimplified and the improved models fairly well predict the 10 years' chloride ingress in Portland cement concrete, but the oversimplified ERFC model significantly underestimates the chloride ingress in concrete with fly ash.     

An experimental approach for the analysis of early-age behaviour of high-performance concrete structures under restrained shrinkage

This paper presents a systematic approach for the experimental testing and analysis of the early-age thermo-mechanical behaviour of large prismatic high-performance concrete specimens under restrained autogenous shrinkage and realistic temperature conditions. The loading system can apply a partial degree of restraint in order to enable the characterisation of high-performance concrete specimens without premature termination of the test in case of significant restrained shrinkage. The instrumentation system can measure the strains and other parameters from the setting time with high accuracy and reliability. The analysis method takes into account the temperature effects on the measured properties and provides equations to determine the time-evolution of shrinkage, thermal expansion, stiffness and creep of the concrete. Results from the testing of 200 × 200 × 1000 mm specimens made with a 0.34 water-cement ratio concrete are presented, analysed and discussed in the paper to demonstrate the application of the proposed approach.     

Cohesive fracture model for functionally graded fiber reinforced concrete

A simple, effective, and practical constitutive model for cohesive fracture of fiber reinforced concrete is proposed by differentiating the aggregate bridging zone and the fiber bridging zone. The aggregate bridging zone is related to the total fracture energy of plain concrete, while the fiber bridging zone is associated with the difference between the total fracture energy of fiber reinforced concrete and the total fracture energy of plain concrete. The cohesive fracture model is defined by experimental fracture parameters, which are obtained through three-point bending and split tensile tests. As expected, the model describes fracture behavior of plain concrete beams. In addition, it predicts the fracture behavior of either fiber reinforced concrete beams or a combination of plain and fiber reinforced concrete functionally layered in a single beam specimen. The validated model is also applied to investigate continuously, functionally graded fiber reinforced concrete composites.