Shamsad Ahmad

Reinforcement corrosion in concrete structures, its monitoring and service life prediction––a review

Reinforcement corrosion has been widely reported in the literature over the last two to three decades. It is one of the major durability problems, mainly when the rebar in the concrete is exposed to the chlorides either contributed from the concrete ingredients or penetrated from the surrounding chloride-bearing environment. Carbonation of concrete or penetration of acidic gases into the concrete, are the other causes of reinforcement corrosion. Besides these there are few more factors, some related to the concrete quality, such as w/c ratio, cement content, impurities in the concrete ingredients, presence of surface cracks, etc. and others related to the external environment, such as moisture, oxygen, humidity, temperature, bacterial attack, stray currents, etc., which affect reinforcement corrosion. The assessment of the causes and extent of corrosion is carried out using various electrochemical techniques. Prediction of the remaining service life of a corroding RC structure is done with the help of empirical models and experimental methods. In this paper a review is presented on the mechanism of reinforcement corrosion, techniques utilized to monitor reinforcement corrosion and methodologies that are utilized for the prediction of remaining service life of structures.

Residual strength of corrosion-damaged reinforced concrete beams

In this work, an effort has been made to first observe the effect of reinforcement corrosion on flexural behavior of reinforced concrete beams and then to develop a model based on the test data to predict their residual flexural strength. Test data were gathered from the testing of 56 reinforced concrete beam specimens that were subjected to a varying degree of accelerated corrosion. It has been observed that the product of corrosion current density and corrosion period I corr T is the most significant factor affecting the flexural strength of a corroded beam. Based on the experimental data, a two-step approach is proposed to predict the residual flexural strength of a corroded beam. First, the flexural strength is calculated using the reduced area of corroded bars, and then this value is multiplied by a correction factor that is formulated through a regression analysis of test data to take into account bond, slip, and other applicable factors.

A Statistical Approach to Optimizing Concrete Mixture Design

A step-by-step statistical approach is proposed to obtain optimum proportioning of concrete mixtures using the data obtained through a statistically planned experimental program. The utility of the proposed approach for optimizing the design of concrete mixture is illustrated considering a typical case in which trial mixtures were considered according to a full factorial experiment design involving three factors and their three levels (33). A total of 27 concrete mixtures with three replicates (81 specimens) were considered by varying the levels of key factors affecting compressive strength of concrete, namely, water/cementitious materials ratio (0.38, 0.43, and 0.48), cementitious materials content (350, 375, and 400 kg/m3), and fine/total aggregate ratio (0.35, 0.40, and 0.45). The experimental data were utilized to carry out analysis of variance (ANOVA) and to develop a polynomial regression model for compressive strength in terms of the three design factors considered in this study. The developed statistical model was used to show how optimization of concrete mixtures can be carried out with different possible options.

Chloride-Induced Corrosion of Steel in Concrete: An Overview on Chloride Diffusion and Prediction of Corrosion Initiation Time

Initiation of corrosion of steel in reinforced concrete (RC) structures subjected to chloride exposures mainly depends on coefficient of chloride diffusion, , of concrete. Therefore, is one of the key parameters needed for prediction of initiation of reinforcement corrosion. Fick’s second law of diffusion has been used for long time to derive the models for chloride diffusion in concrete. However, such models do not include the effects of various significant factors such as chloride binding by the cement, multidirectional ingress of chloride, and variation of with time due to change in the microstructure of concrete during early period of cement hydration. In this paper, a review is presented on the development of chloride diffusion models by incorporating the effects of the key factors into basic Fick’s second law of diffusion. Determination of corrosion initiation time using chloride diffusion models is also explained. The information presented in this paper would be useful for accurate prediction of corrosion initiation time of RC structures subjected to chloride exposure, considering the effects of chloride binding, effect of time and space on , and interaction effect of multidirectional chloride ingress.

Prediction of residual flexural strength of corroded reinforced concrete beams

This study aims to make an effort to develop a model to predict the residual flexural strength of reinforced concrete beams subjected to reinforcement corrosion.,For generating the required data to develop the model, a set of experimental variables was considered that included corrosion current density, corrosion duration, rebar diameter and thickness of concrete cover. A total of 28 sets of reinforced concrete beams of size 150 × 150 × 1,100 mm were cast, of which 4 sets of un-corroded beams were tested in four-point bend test as control beams and the remaining 24 sets of beams were subjected to accelerated rebar corrosion inducing different levels of corrosion current densities for different durations. Corroded beams were also tested in flexure, and test results of un-corroded and corroded beams were utilized to obtain an empirical model for estimating the residual flexural strength of beams for given corrosion current density, corrosion duration and diameter of the rebars.,Comparison of the residual flexural strengths measured experimentally for a set of corroded beams, reported in literature, with that predicted using the model proposed in this study indicates that the proposed model has a reasonably good accuracy.,The empirical model obtained under this work can be used as a simple tool to predict residual flexural strength of corroded beams using the input data that include rebar corrosion rate, corrosion duration after initiation and diameter of rebars.

Effect of Steel Fibers and Thermal Cycles on Fracture Properties of Ultra-High-Performance Concrete

The advent of ultra-high-performance concrete (UHPC), a new generation of cementitious material having excellent material properties, has generated a great deal of interest in the field. In this study, the fracture properties of UHPC mixtures reinforced with steel fiber (fiber content varying from 0 to 6.2 % by weight) and subjected to thermal cycles were investigated to examine the effect of heat–cool cycles. The standard prism of 100 × 100 × 400 mm3 with a central notch was used in a three-point bend test to determine fracture properties that include critical stress-intensity factor (Kic), critical crack tip opening displacement (CTODc), energy release rate, and total fracture energy. All specimens were water-cured for 28 days. Two exposure conditions were used: a 6-month thermal cycling and a 6-month laboratory exposure for further self-curing. The thermal cycling comprised heating in an oven at 60°C for 2 days and then cooling them at room temperature for the next 2 days over a period of 6 months. Test results show that UHPC reinforced with 6.2 % steel fiber exhibited excellent fracture properties with significant ductility. Both thermal cycling and prolonged self-curing of water-cured UHPC specimens enhance fracture properties because of more complete hydration of cement in UHPC. This improvement in properties signals an additional advantage of water-cured UHPC for its application in hot climatic conditions.

Development of an optimum mixture of ultra-high performance concrete

Existence of ultra-high performance concrete (UHPC) is the result of one of the latest advances in concrete technology. As compared to the normal concrete mixtures, the mixtures of UHPC are reported to have several times more strength, elastic modulus, fracture toughness, ductility and durability. UHPC is prepared by mixing high amounts of cement and silica fume with fine quartz sand, quartz powder and fibres. The water to binder ratio is kept very low and therefore a high dosage of superplasticizer is needed to maintain the required flowability. Need for a fine quartz sand and quartz powder for producing UHPC mixtures increases the cost of UHPC in places where these ingredients are not naturally available. In the present study, an attempt was made to develop an optimum mixture of UHPC using readily available local fine dune sand. The local dune sand was used collectively as a natural mixture replacing fine quartz sand and quartz powder. To select an optimum UHPC mixture, several trial mixtures were considered by varying the proportions of the ingredients of UHPC. An optimum mixture of UHPC was selected based on optimum flow (above 180 mm), strength (above 150 MPa) and lower amounts of silica fume and steel fibres to reduce the unit cost. A series of tests were conducted on the specimens prepared with the optimally selected mixture of UHPC for determining its mechanical properties and durability characteristics including resistance against reinforcement corrosion and sulphate attack. Test results indicate an excellent performance of the optimum mixture of UHPC developed through the present work.

A Simple and Reliable Setup for Monitoring Corrosion Rate of Steel Rebars in Concrete

The accuracy in the measurement of the rate of corrosion of steel in concrete depends on many factors. The high resistivity of concrete makes the polarization data erroneous due to the Ohmic drop. The other source of error is the use of an arbitrarily assumed value of the Stern-Geary constant for calculating corrosion current density. This paper presents the outcomes of a research work conducted to develop a reliable and low-cost experimental setup and a simple calculation procedure that can be utilised to calculate the corrosion current density considering the Ohmic drop compensation and the actual value of the Stern-Geary constants calculated using the polarization data. The measurements conducted on specimens corroded to different levels indicate the usefulness of the developed setup to determine the corrosion current density with and without Ohmic drop compensation.

Effect of the Key Mixture Parameters on Shrinkage of Reactive Powder Concrete

Reactive powder concrete (RPC) mixtures are reported to have excellent mechanical and durability characteristics. However, such concrete mixtures having high amount of cementitious materials may have high early shrinkage causing cracking of concrete. In the present work, an attempt has been made to study the simultaneous effects of three key mixture parameters on shrinkage of the RPC mixtures. Considering three different levels of the three key mixture factors, a total of 27 mixtures of RPC were prepared according to 33 factorial experiment design. The specimens belonging to all 27 mixtures were monitored for shrinkage at different ages over a total period of 90 days. The test results were plotted to observe the variation of shrinkage with time and to see the effects of the key mixture factors. The experimental data pertaining to 90-day shrinkage were used to conduct analysis of variance to identify significance of each factor and to obtain an empirical equation correlating the shrinkage of RPC with the three key mixture factors. The rate of development of shrinkage at early ages was higher. The water to binder ratio was found to be the most prominent factor followed by cement content with the least effect of silica fume content.

An experimental study on correlation between concrete resistivity and reinforcement corrosion rate

Purpose – The purpose of this paper was to explore the possibility of establishing an empirical correlation between concrete resistivity and reinforcement corrosion rate utilizing the experimental data generated by measuring corrosion current density of reinforced concrete specimens subjected to chloride-induced corrosion at different levels of concrete resistivity. Design/methodology/approach – To generate concrete resistivity vs corrosion current density data in a wide range, ten reinforced concrete specimens were prepared and allowed to corrode under severe chloride exposure. After significantly corroding the specimens, they were removed from the chloride exposure and were subjected to different moisture levels for achieving variation in the resistivity of concrete so that reasonably good number of resistivity vs corrosion rate data can be obtained. Resistivity and corrosion current density tests were conducted for all the ten specimens and their values were measured in wide ranges of 0.8-65 kΩ·cm and ...