Search

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.