Degradation Of Concrete Structures Research Articles

The case for air-entrainment in high-performance concrete

Freezing–thawing cycles are a major cause of degradation of concrete structures in cold climates. The use of air-entraining admixtures to induce an adequate network of closely spaced air bubbles in concrete has been proven to dramatically decrease its susceptibility to frost action. However, with the advent of high-strength high-performance concrete (HPC), the need for air-entrainment in such concrete has become a controversial issue since the 1980s. Some researchers advocate using air-entrainment in HPC, while others argue that this practice is unnecessary owing to the very low porosity and amount of freezable water in HPC. In this paper, an extensive review of the available literature indicates that there is substantial scatter and irreproducibility of data regarding this issue. There is also evidence of vulnerability of HPC to both micro and macrocracking in service and a demonstrated reduction in performance under combined ageing mechanisms. This could make non-air-entrained HPC vulnerable to freezing–thawing degradation even though such vulnerability may not be detected using classical frost durability tests. Moreover, there are practical limitations to the use of very low water/cement ratio concrete in field conditions, which make the production of HPC with negligible freezable water content difficult to achieve in full-scale structures. This paper presents evidence that supports the argument that the use of air-entrainment in HPC is a practice that provides enhanced safety, reliability and serviceability, and is therefore recommended for concrete practitioners.

Theoretical Modeling of Signal Loss versus Crack Opening for a Novel Crack Sensor

The degradation of concrete structures is always accompanied by the formation of cracks. Crack monitoring is hence useful in assessing the “health condition” of the structure. In our previous investigation on an optical-fiber crack sensor, we have successfully demonstrated the possibility of crack detection and monitoring without requiring prior knowledge of crack locations. Also, a single fiber can be employed to monitor a number of cracks. In practical applications, the signal loss versus crack opening relationship of the sensor needs to be properly “designed” to satisfy performance requirements on sensitivity to small cracks and/or the total number of detectable cracks (which is limited by the maximum loss at each crack). In this paper, a theoretical model for the signal loss versus crack opening relationship is developed through a combination of mechanical and optical analyses. Using concrete beams with embedded sensors, crack monitoring experiments are conducted. Test results are found to be in reaso...

Test methods for on-site corrosion rate measurement of steel reinforcement in concrete by means of the polarization resistance method

1. SCOPE This Recommendation covers the description of nondestructive electrochemical test methods for the estimation in large size concrete structures of the instantaneous corrosion current density, ioorr, expressed in gA/cm 2, by means of the so-called Polarization Resistance technique, R o, in order to assess the condition of embedded steel reinforcement related to its corrosion. The values of i~orr, Can be used to assess the rate of degradation of concrete structures affected by reinforcement corrosion. However, they cannot give information on the actual loss in steel cross section which, at present, only can be assessed by means of direct visual observation. Values of the free corrosion potential or half-cell potential, Ecorr [V], of the embedded reinforcing steel and of the electrical concrete resistance, Re [f)], are obtained as preliminary steps of the Rp measurements. Values of the concrete resistivity, P [~m], can be calculated from Re values providing the geometrical arrangement of the electrodes enables this calculation. Both parameters, Ecorr and Re (or P) may be used to complement the reliability of the ico~r measurements.

Potentiostatic determination of chloride threshold values for rebar depassivationExperimental and statistical study

The presence of chlorides in concrete is the main cause of corrosion of reinforcements, responsible for most of the damages and cost losses attributed to the degradation of concrete structures. In spite of this, there are not enough data when defining chloride threshold determination. Most experimental work has been devoted to identify the variables involved in depassivation: type of cement, water/cement (w/c) ratio, pore humidity, type of steel, etc. Present situation considers a comparative value, in general, of 0.4% by weight of cement. In the present work, tests on mortar specimens were performed using several types of cements, covering a region of potentials from +250 to −650 mV (SCE) (+150 to −800 mV Mn/MnO 2 ). Series of identical specimens made with different types of cement were used. A statistical treatment of the results of chloride threshold is made. Results indicate that two regions of potential are identified: (1) The region of potentials more positive to −200 mV (SCE). The mean values of chloride threshold for 95% probability of corrosion vary between 0.497 ± 0.126 and 0.569 ± 0.177 for free chlorides and 0.632 ± 0.112 and 0.771 ± 0.346 for total chlorides. The lowest limits of chlorides for corrosion initiation, from a normal distribution (characteristic values) are 0.283% for free chlorides and 0.316% for total chlorides (wt.% of cement). Differences in chloride threshold with the type of cement are identified after the statistical study of the experimental results. (2) For potentials more negative to −200 mV (SCE) the experimental results fit well with a log-normal function. A mean value for total chlorides of 1.53%, with a variation coefficient of 0.36 is found. A mean potential for protection of −500 mV (SCE) could be identified.

Numerical assessment and prediction method for the chemico-mechanical deterioration of ASR-affected concrete structures

The degradation of concrete structures by the alkali–silica reaction (ASR) is highlighted in recent years in civil engineering. To quantify the deterioration of concrete structures, one has the behavior modeling of affected concrete proposed by multiple approaches, on one side, and the expansion of the structure measured by the in situ engineers, on the other side. The task is to determine the progressive structure degradation from these two supports by a systematical method. Nevertheless, this method should not depend on the employed model so that a better one can always be added to the assessment. Moreover, one should also be able to analyze the credibility of the assessment result. All these points differentiate between a laboratory expertise and a structural assessment. This article proposes a time-scaled evaluation method and formulates it into an inverse problem. With an adopted chemico-mechanical model for ASR expansion and the analyzed structure observation data, the authors give the numerical solution for the established inverse problem. An ASR-affected suspension bridge is then evaluated using a finite element code for its durability by the achieved method. Some further discussions are given in the last section of this article. Key words: alkali–silica reaction, modeling, inverse problem, assessment, suspension bridge, durability.

Potentiostatic determination of chloride threshold values for rebar depassivation: Experimental and statistical study

The presence of chlorides in concrete is the main cause of corrosion of reinforcements, responsible for most of the damages and cost losses attributed to the degradation of concrete structures. In spite of this, there are not enough data when defining chloride threshold determination. Most experimental work has been devoted to identify the variables involved in depassivation: type of cement, water/cement (w/c) ratio, pore humidity, type of steel, etc. Present situation considers a comparative value, in general, of 0.4% by weight of cement. In the present work, tests on mortar specimens were performed using several types of cements, covering a region of potentials from +250 to −650 mV (SCE) (+150 to −800 mV Mn/MnO 2). Series of identical specimens made with different types of cement were used. A statistical treatment of the results of chloride threshold is made. Results indicate that two regions of potential are identified: (1) The region of potentials more positive to −200 mV (SCE). The mean values of chloride threshold for 95% probability of corrosion vary between 0.497 ± 0.126 and 0.569 ± 0.177 for free chlorides and 0.632 ± 0.112 and 0.771 ± 0.346 for total chlorides. The lowest limits of chlorides for corrosion initiation, from a normal distribution (characteristic values) are 0.283% for free chlorides and 0.316% for total chlorides (wt.% of cement). Differences in chloride threshold with the type of cement are identified after the statistical study of the experimental results. (2) For potentials more negative to −200 mV (SCE) the experimental results fit well with a log-normal function. A mean value for total chlorides of 1.53%, with a variation coefficient of 0.36 is found. A mean potential for protection of −500 mV (SCE) could be identified.

Coupled chemo-mechanical deterioration of cementitious materials Part II: Numerical methods and simulations

The paper is concerned with numerical solution procedures for coupled chemo-mechanical simulations of concrete structures subjected to calcium leaching and mechanical damage. The analyses are based upon the chemo-mechanical damage model for cementitious materials presented in the first part of the paper [Int. J. Solids Struct., in press]. These analyses are characterized by a non-linear transient two-field problem described by the displacement field, the pore fluid calcium concentration field and its rate as state variables. The related non-linear first order initial boundary value problem is solved numerically by a semi-discretization strategy. For the temporal discretization the second order accurate implicit generalized mid-point rule is used. The spatial discretization is based upon hierarchical finite elements. The non-linear algebraic system of equations is solved by consistent linearization and Newton–Raphson iteration. Representative simulations of calcium leaching and the coupled chemo-mechanical degradation of concrete structures are presented. In particular, the process of calcium leaching is studied in detail by means of a one-dimensional model problem. Chemo-mechanical interactions on the structural level are investigated in the numerical simulation of a concrete beam subjected simultaneously to mechanical loads and deionized water. It is demonstrated that chemical damage not only reduces the limit load and the stiffness of concrete structures but also their structural behavior.

Assessment of broadband ultrasonic attenuation measurements in inhomogeneous media

In this paper, we address the problem of evaluating the acoustic attenuation of “difficult” media, i.e. highly attenuating and scattering media. In a broadband, through transmission setup, the signals acquired from such media are characterized by a poor signal-to-noise ratio. Therefore, an accurate estimate of attenuation cannot be obtained from a single measurement, but multiple measurements must be combined. Two methods are considered to yield a single estimate of attenuation from multiple measurements. The first one, the “average attenuation” (AA) method, consists in a simple average of individual attenuation estimates. The second one, the “cross spectrum” (CS) method, is based on a system identification approach. In order to evaluate the estimation errors for these methods, ultrasonic signals transmitted through a material of known attenuation were simulated and mixed with both coherent and incoherent noise. In all tests performed, the “CS” method was found to yield the most accurate estimate. This method, combined time delay compensation, is then applied to real signals measured from a concrete slab. A valid frequency band for the attenuation estimate can be defined based on the coherence function. Results from this research are being applied to characterize the degradation of concrete structures using high-frequency ultrasound.

Experimental Investigations on Degradation of Concrete Structures

ABSTRACT Most models on Durability of concrete structures have focussed on chemical aspects of the degradations. The last years have been at the origin of the development of Durability Mechanics in order to be able to predict the time-dependent evolution of structural properties of a construction. Experimental techniques have been proposed to assess coupling data between physico-chemistry and mechanics. This paper introduces recent data and techniques on leaching and steel corrosion effects on mechanical behavior of concrete materials and structures.

Chemical and microbiological tests to simulate sulfuric acid corrosion of polymer-modified concrete

In certain industrial activities sulfuric acid is used during the production process, which may cause degradation of concrete structures. Another important phenomenon where sulfuric acid is responsible for concrete corrosion is biogenic sulfuric acid corrosion, which occurs often in sewer systems. Because previous investigations have already pointed out the difference between purely chemical sulfuric acid corrosion and biogenic sulfuric acid corrosion two different tests were performed: a chemical test and a microbiological test. Five different concrete compositions were used in the tests, including a reference mixture with high sulfate resistant portland cement and four different polymer cement concrete with a styrene–acrylic ester polymer, an acrylic polymer, a styrene butadiene polymer and a vinylcopolymer, respectively. The concrete composition with the styrene–acrylic ester polymer showed in both tests a higher resistance than the reference mixture while the compositions with the acrylic polymer and the styrene butadiene polymer had a lower resistance than the reference mixture. The concrete composition with the vinylcopolymer did not induce the same results in both tests. The results of the chemical test indicated a slight increase in resistance compared with the reference mixture while the opposite was noticed for the microbiological test.

Modelling of the Interaction Between an Engineered Clay Barrier and Concrete Structures in a Deep Storage Vault

AbstractThe degradation of concrete structures in a storage vault will release an alkaline plume influencing the geochemical evolution of clay engineered barriers or plugs. The studied configuration has a representative scale and is composed of a rectangular clay barrier interfaced on one side with a cement block and on the other side with an external granitic field. A clay model including major and accessory minerals is used; cement material is supposed to be made of portlandite and CSH minerals. The clay barrier and the cement block are assumed to be initially saturated and in equilibrium with their respective interstitial water. Diffusive transport of aqueous species coupled to chemistry is simulated in a one dimension space up to 10000 y. The feed-back of the porosity evolution on the transport properties is not taken into account. The temperature is supposed to be constant (25°C). Calculations are performed with the TRIO-EF coupled transport-chemistry code.The main results deal with the evolution of the pH profile across the clay barrier, the dissolution of clay minerals and the precipitation of new solid phases in the clay such as albite, gibbsite and CSH. These simulations show that the pH excursion includes only a minor part of the clay barrier at 10 000 y and that the CSH precipitation retards the alkaline plume progression. The appearance of local accumulations of newly precipitated minerals might reduce strongly the porosity. Future developpements of this work include: (i) the completion of the geochemical model by introducing other minerals in concrete (other CSH phases and sulfo-aluminates) and ion exchange sites within the clay, (ii) the implementation of a precipitation/dissolution model in order to describe on a realistic way the porosity profile evolution and possible clogging phenomena.

Another look at the deterioration of calcium aluminate cement concrete

Potential degradation of concrete structures made of calcium aluminate cement (CAC) is well known and is caused by transformation (conversion) of the thermodynamically metastable into stable calcium aluminate hydrate phases. This recrystallization is influenced by temperature and humidity; the structural degradation of the concrete itself thus its loss of strength, is strongly related to the pore structure of the hydrated cement paste, the critical parameters being the total volume of pores below 15-20 nm and the median micropore radius. This constitutes a novel procedure for evaluation of existing CAC concrete structure.

Investigation of Bond Behavior Between Glass Fiber Composite Reinforcements and Concrete

Abstract Worldwide there is a need for the renewal of infrastructure because of age, deterioration, misuse, lack of repair, use of improper materials and techniques, and even due to changing needs. The corrosion of steel reinforcing bars is one of the major causes of degradation of concrete structures. Composites are increasingly being considered as a potential replacement for steel rebar, but there exists a critical lack of fundamental data and understanding of their behavior in relationship to traditional civil engineering materials such as concrete. This paper presents a preliminary investigation on the bond and pullout behavior of glass fiber reinforced composite reinforcement in concrete. The pullout behavior of the system was studied using a concentric pullout test and results are compared with theoretical predictions. The study is aimed at the development of a fundamental understanding of bond behavior and materials-configuration interrelationships needed to further the use of composites as reinforcements for application in areas where corrosion and electromagnetic interference make the use of steel reinforcement inefficient.