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The widespread use of carpets in residential and commercial buildings and their relatively short life span result in large volumes of carpet being landfilled. A potential solution to this problem is the use of post-consumer carpet fibers in concrete. To this end, this paper systematically identifies the common fiber types in a typical post-consumer carpet fiber bale and evaluates their durability under exposure to varying levels of alkalinity. The tensile strengths and toughness of the fibers belonging to the nylon and polyethylene terephthalate (PET) families (the dominant fibers in most post-consumer carpets) are reduced by up to 50% following exposure to extreme alkalinity, the reasons for which are determined using spectroscopic and microscopic evaluations. The chloride ion transport resistance of concretes (~40 MPa strength) containing 2.5% carpet fibers by volume (~25 kg of fibers per cubic meter of concrete) is comparable to that of the control mixture, while mortar mixtures containing the same volume fraction of carpet fibers demonstrate negligible enhancement in expansion and loss of strength when exposed to 1 N NaOH. This study shows that moderate-strength concretes (~40 MPa) for conventional building and infrastructure applications can be proportioned using the chosen volume of carpet fibers without an appreciable loss of performance. Consideration of low volume fractions of carpet fibers in low-to-moderate-strength concretes thus provides a sustainable avenue for the use of these otherwise landfilled materials in construction applications. [ABSTRACT FROM AUTHOR]

Structural applications of hybrid reinforced concrete (HRC) involve a composite system with a fiber-reinforced concrete (FRC) matrix in conjunction with conventional reinforcement. The system offers economic benefits by reducing the number of rebars, minimizing labor costs, and shortening the construction schedule. This paper presents the generalized approach to evaluating closed-form moment–curvature relationships from constitutive models with multilinear equations. The interaction of various moment–curvature solutions is by sequential switching from one mode to the other to form an envelope curve as the minimum moment among all available options. Results lead to explaining the interaction of material and geometrical parameters as well as the interaction of various failure modes. Furthermore, the model can be used for both the ultimate limit state (ULS) and serviceability limit state (SLS) since the strain history is the primary independent variable. These lead to parametric design charts for adjusting material parameters, reinforcement amount and location, and section geometry within the bounds of ρmin and ρbal is applicable for HRC, RC, and FRC sections. Analytical equations show that as the concrete flexural crack extends in a hybrid section, the bridging effect of fibers delays the stress accumulation into the rebar and its subsequent yielding. The efficiency of the volume of concrete that is under tensile loads is significantly improved, extending the serviceability range and increasing the stiffness of cracked concrete. The extension of the serviceability limit range is also associated with potential improvements in durability since the HRC model predicts smaller apparent strains and crack widths at a given load. The proposed solutions are validated by simulating the load-deflection responses of various full-scale flexural tests published in the literature. [ABSTRACT FROM AUTHOR]

Textile-reinforced concrete (TRC) is used widely in thin-walled structures due to its unique construction characteristics and superior mechanical properties. However, previous studies of the dynamic flexural properties of alkali resistant (AR)-glass TRC thin plates did not consider the influence of exposure to high temperature. In this study, the effect of impact velocity (range from 1.29 to 4.04 m/s) on AR-glass TRC after exposure to elevated temperatures up to 400°C was investigated experimentally using a drop-tower impact system. AR-glass TRC specimens were heated to target temperatures (100°C, 200°C, 300°C, and 400°C) in a furnace and then tested at room temperature. The effects of exposure to high temperature and the strain rate of loading (impact velocity) on the flexural mechanical properties were measured, and the microstructure and damage evolution of the composites was evaluated. Results show that the strain rate had a direct effect on the flexural properties of AR-glass TRC composite under impact after exposure to high temperatures. Furthermore, the weakening effect of high temperature in AR-glass TRC material could equal or even exceed the hardening effect due to the strain rate. The results were evaluated by comparing dynamic flexural stress and toughness features. Finally, the high-temperature deterioration mechanism of AR-glass TRC material was determined by comparing the changes in the matrix–textile interface and the textile fracture after exposure to different temperatures. [ABSTRACT FROM AUTHOR]

Textile reinforced concrete (TRC) has a higher sustainable potential than conventional reinforced concrete, with a significantly lower CO2 footprint. To study the dynamic flexural behavior of TRC composites, the Alkali Resistant (AR)-glass textile reinforced concrete (TRC) plate specimens was investigated with the three-point bending test under low-velocity impact. This study examined the influences of the textile layer number (1, 2, and 3 layers) and the impact velocity (1.29–4.04 m/s) on the dynamic behavior of TRC composites, and compared that with the quasi-static test results. The results show that the Young's modulus and initial peak impact stress of the AR-glass TRC generally increase with the impact velocity, while its influence on the ultimate flexural stress is insignificant. It is found that the ultimate flexural stress can better represent the dynamic flexural capacity of AR-glass TRC materials than the initial peak stress. The failure modes were also analyzed in terms of crack patterns and crack widths, which were measured through the digital image correlation (DIC) method. It was found that rebound and rupture are the two main failure scenarios of the AR-glass TRC under low-velocity impact loading. The cracks of AR-glass TRC specimens could be mainly categorized into flexural crack, shear-flexural crack, and interfacial debonding crack. The study of stress-crack width relationship is helpful to determine the design strength of TRC material to meet the requirement on crack width. [ABSTRACT FROM AUTHOR]

Foamed concrete is an attractive material for structural use and insulation of buildings, which can replace flammable organic insulating materials on the market. This study investigated the compressive and thermal properties of the plain, fly ash and expanded polystyrene (EPS) foamed concrete ranging widely in density from 180 to 1800 kg/m3. Furthermore, the compressive strength of the plain foamed concrete was analyzed at four density levels of 180, 325, 450, and 595 kg/m3 and at four temperatures of room temperature, 300, 600, and 900 °C to quantify its fire resistance. The compressive strength of the foamed concrete dramatically declined, and it suffered severe damage when the temperature was higher than 300 °C. A constitutive stress–strain model incorporating the compressive strength–density relationship and the temperature effects was also proposed for the plain foamed concrete. The findings may provide new perspectives on the existing design codes for energy efficiency and fire protection. [ABSTRACT FROM AUTHOR]

The laboratory testing of concrete has shown that elevated temperatures cause air permeability index (API) increase and elastic modulus E decrease. Indeed, the API response typically spans several orders of magnitude making it a sensitive indicator of elastic modulus reduction. However, the API versus E data has been historically assumed to be both event and concrete composition specific, thus limiting its applicability for predictive or modeling purposes. In response, a theoretical model is developed here by (1) establishing that the API is equivalent to the Darcy permeability; (2) developing a microcrack flow model accounting for tortuosity, crack density, and microcrack geometry; and (3) interpreting the Giordano and Colombo elastic modulus damage formulas in terms of the corresponding increase in permeability. The resulting general model depends on stress conditions (plane stress versus plane strain) and a single microcrack flow parameter. For materials with small Poisson's ratios, however, the model becomes independent of stress condition and both equations converge on the same simple solution. The model is verified with both field and laboratory data of concrete response to typical fire elevated temperatures and shown to be comparable in accuracy to previous empirical approaches requiting many more degrees of freedom. Moreover, the data on fire damaged concrete were found to collapse to a single curve suggesting that the microcrack flow parameter is sensitive to elevated temperature but insensitive to concrete composition. The success of the model implies certain limits to microcrack geometry and airflow turbulence in damaged concrete, both of which are discussed. DOI: 10.1061/(ASCE)MT.1943-5533.0000074 CE Database subject headings: Elasticity; Concrete; Material tests; Temperature effects; Fluid dynamics; Fire safety; Fire resistance; Forensic engineering. Author keywords: Air flow; Flow; Analysis; Elastic analysis; Stress analysis; Concrete; Concrete tests; Cracking; Fracture; Fluid dynamics; Elasticity; Fires; Fire safety; Fire resistance; Forensic engineering; Heating; Laboratory tests; Tests; Flow measurement; Nondestructive tests; Permeability; Permeability tests; Physical properties; Plain strain; Strain; Pore water; Microporosity; Portland cements; Thermal resistance; Spalling; Thermal stress; Thermal analysis.

This paper presents the diagnostic investigation of a reinforced concrete jetty after 15 years of service. The main cause of the deterioration was reinforcement corrosion, which initiated within few years after the completion of the jetty structure. The structure underwent major corrosion rehabilitation after 7 years of service. Despite the use of high-strength concrete in the construction of the jetty structure, inadequate cover thickness for reinforced concrete elements intensified corrosion rate and caused early age corrosion problems to reappear. Other contributing factors included high salinity of the seawater by simultaneous action of climatic factors such as exposure to high relative humidity and air temperature. After the initial repair work, a new investigative procedure was carried out to reevaluate the repaired and surrounding unrepaired areas. Results indicate that the deterioration progressed even in concrete elements that were undamaged during the first step of the investigation. No sign of steel corrosion appeared on repaired areas; but shrinkage cracking and incipient corrosion around repaired areas were indications of dimensional and electrochemical incompatibility between repair concrete and substrate. The repair strategy was reevaluated through the repair index method proposed by Andrade and Izquierdo. The results showed that the patching repair method was more suitable and feasible in comparison with other techniques. DOI: 10.1061/(ASCE)CF.1943-5509.0000112 CE Database subject headings: Sea water; Corrosion; Cracking; Durability; Reinforced concrete; Rehabilitation; Jetties; Case studies; Marine terminals. Author keywords: Sea water corrosion; Cracking; Durability; Reinforced concrete; Reinforcement; Jetties; Repair; Concrete durability; Rehabilitation; Case reports; Corrosion; Chlorides; Marine terminals.

Concrete is the most widely used material in construction industries. Therefore, it is necessary to understand and when possible, improve the efficiency of a concrete production process. Statistical process control is generally applied to gain information about variation in the manufacturing process. Control charts can be implemented to monitor the various processes involved in the production, the delivery and construction of concrete. When historical data are available on various projects, better insight into operational procedures can be obtained through the use of control charts. This paper studies a series of statistical analysis procedures to analyze the compressive strength of concrete. The proposed method is based on combining the cumulative sum (CUSUM) control chart and a run chart (CUSUM-run chart) for early detection of shifts in the process mean. The combined charts address both the consumers' and the producers' perspectives. The CUSUM-run chart can aid the consumer in making decisions about accepting or rejecting a strength test. In addition, the producers (concrete manufacturers) can use the chart to determine if the monitored process is out-of-control and subsequently attempt to identify the possible causes for the out-of-control situation. By identifying assignable causes of the out-of-control process, the producer can then improve the manufacturing process by reducing product variation, unnecessary waste, or over-designed concrete mixtures. The CUSUM-run chart is also beneficial in that it can often indicate when the strength of mixture is less than the minimum acceptable level very quickly. The delay in detecting an unacceptable strength can result in more penalties, project delays and increased associated costs. CE Database subject headings: Fatigue; Hybrid methods; Energy dissipation; Seismic design; Concrete; Monitoring; Active control. Author keywords: Low-cycle fatigue; Hybrid frame; Hysteresis; Energy dissipation; Seismic design; PRESSS; ASTM A706; ASTM A615. DOI: 10.1061/(ASCE)0899-1561(2010)22:1(77)

A study is presented to predict the degradation of cement-based materials due to external sulfate attack. Parameters of the model are chosen based on the mix design parameters, degree of hydration, and exposure conditions of concrete. A solution of the diffusion equation with a term for a second-order chemical reaction is proposed to determine the sulfate concentration and calcium aluminate profile as a function of time and space. The crystallization pressure of hydration products such as ettringite can lead to internal stresses. Using the volumetric information, the model predicts the generation of internal stresses, evolution of damage, reduction in stiffness, and thus expansion of a matrix phase. The theoretical expansion-time responses are obtained and compared with a variety of available data in the literature. The most important parameters are the w/c ratio, internal porosity, diffusivity of the cracked and uncracked material, and available calcium aluminates. The importance of controlling the pH of the test solution is clearly observed. Model simulations indicate a reasonable agreement with experimental expansion-time data available in the literature. CE Database subject headings: Cements; Sulfates; Damage; Models; Concrete.

A chemomechanical mathematical model is presented to simulate the response of concrete exposed to external sulfate solutions. The model is based on the diffusion-reaction approach, and several mechanisms for the reaction of calcium aluminates with sulfates to form expansive ettringite are considered. Fick's second law is assumed for diffusion of the sulfate ions. A second-order chemical reaction between reacting calcium aluminates and ingressing sulfates depletes the sulfate concentration. The products of the second-order reaction between the aluminates and sulfates are chosen among several competing mechanisms, and a rule-of-mixtures approach is used to relate the expansive nature of the products with the prescribed specific gravity of the compounds. It is furthermore assumed that the crystallization pressure of products of reaction results in a bulk expansion of the solid. The constitutive response of the matrix and the expansive stresses are calculated from the imposed volumetric strain. Microcracks are initiated when the strength of the matrix is reached, leading to changes in the diffusivity and a reduction in the elastic properties of the matrix. The variation of diffusivity is linked to the scalar damage parameter due to cracking of the matrix. Due to the changes in the diffusivity, the problem is treated as a moving boundary problem, and a methodology is proposed to adapt the solution of the 1D case to the 2D problem of a prismatic specimen. Theoretical expansion-time responses are obtained and compared with a variety of data available in the literature. CE Database subject headings: Damage; Models; Sulfates; Cements.

Current failure criteria of asphalt pavements are either empirical or assume linear elastic material response and use a single load level to relate the number of load repetitions to fatigue failure. To better understand the crack propagation properties of asphalt pavements, laboratory tests and nonlinear analysis were performed to evaluate the low-temperature fracture parameters of conventional asphalt concrete and asphalt-rubber mixture. Two approaches based on nonlinear fracture mechanics, the compliance approach and the R-Curve approach, were used. Beam specimens were prepared with different binder contents and tested under three-point bending flexural conditions at two test temperatures. A closed-loop servohydraulic test system was used with the crack mouth opening as the control parameter. Nonlinear fracture parameters were obtained at different stages of crack propagation. Results show that the asphalt-rubber mixture has higher fracture toughness and consequently larger resistance to cracking than asphalt concrete. Also, the asphalt-rubber mixture is less sensitive to temperature than asphalt concrete. Increasing the binder content increased the toughness values for both asphalt concrete and asphalt-rubber mixture in most cases. The R-Curve approach provides a good measure of characterizing the fracture behavior of asphalt mixtures.

The increasing demands of sustainable design and construction with economical sections, reduced cover, and more efficient time schedule require more flexibility in the design methodologies. The development of ultra‐high performance concrete (UHPC) have gained increasing interests as an attractive option for structural members with lightweight and superior performances. Concrete members reinforced with steel bars and fibers, generally known as hybrid reinforced concrete (HRC), offer a feasible solution in terms of reducing reinforcing materials and achieving desired structural performance. This paper proposes an analytical model to predict the flexural behavior of hybrid reinforced UHPC with steel reinforcements. Moment–curvature solutions are derived for reinforced sections based on parameterized tension‐compression constitutive models. The approach is applicable to customized cross section and derivation of T‐section is demonstrated. The moment–curvature response is further simplified as a tri‐linear model, which is used for the development of full‐range displacement solutions in analytical form. The proposed model is validated with the experimental data from literature covering a range of materials and member sizes. The full‐range solutions may provide insights into the serviceability design approach based on the criterion of maximum crack width or allowable deflection. [ABSTRACT FROM AUTHOR]