In this study, a procedure for interpreting impact-echo data in an automated, simple manner for detecting defects in concrete bridge decks is presented. Such a procedure is needed because it can be challenging for inexperienced impact-echo users to correctly distinguish between sound and defective concrete. This data interpretation procedure was developed considering the statistical nature of impact-echo data in a manner to allow impact-echo users of all skill levels to understand and implement the procedure. The developed method predominantly relies on conducting segmented linear regression analysis of the cumulative probabilities of an impact-echo data set to identify frequency thresholds distinguishing sound concrete from defective concrete. The accuracy of this method was validated using two case studies of five slab specimens and a full-scale bridge deck, each containing various typical defects. The developed procedure was found to be able to predict the condition of the slab specimens containing shallow delaminations without human assistance within 3.1 percentage points of the maximum attainable accuracy. It was also able to correctly predict the condition of the full-scale bridge deck containing delaminations, voids, corrosion damage, concrete deterioration, and poorly constructed concrete within 3.5 percentage points of the maximum attainable accuracy.

Chloride diffusivity and steel corrosion are two major factors in the durability characteristics of concrete structures. It is possible to use the electrical resistivity (ER) of concrete as a measure of concrete’s ability to resist the movement of ions within the material. In this study, surface electrical resistivity (SR) and bulk electrical resistivity (BR) of concrete cylinders were measured from 3 to 161 days for concrete mixtures with four varying water-cement ratios (w/c) (0.45 to 0.60) and three distinct cement types. The study investigated the influence of important durability parameters such as cement type, long-term curing period, and w/c on concrete electrical resistivity. In addition, the impact of cylinder size on SR of concrete was observed. The findings show that both SR and BR of concrete decrease with increasing w/c, except for concrete with cement Type-I/II, which showed a minor increase in resistivity with a w/c of 0.55. Concrete with Type-V cement showed the highest electrical resistance. Moreover, a strong linear relationship between the two types of resistivity was established, and a new equation was introduced in terms of cement type, w/c, and long-term curing period. The correlation between SR and BR was validated by determining the mean absolute error (MAE) of the proposed equation for the three types of cement, which were 0.41 (Type-I/II), 0.65 (Type-III), and 0.35 (Type-V). For all three cement types, the mean absolute percentage error (MAPE) and coefficient of variation (COV) were within acceptable limits, and the 95% confidence interval (CI) indicated a small error margin for the proposed equation when estimating BR from SR using experimental data. Statistical analysis showed that the new equation was less reliable for Type-III cement than the other two types, possibly due to its rapid strength increase property.

The objective of this paper is to investigate the microscopic pore characteristics and macroscopic mechanical properties of concrete under different curing conditions. Ultrasonic nondestructive testing technology was used to measure the ultrasonic sound velocity of specimens of different ages, and the compressive strength and splitting tensile strength were obtained through indoor mechanical performance tests. The pore-size distribution characteristics and internal microstructure were observed using nuclear magnetic resonance (NMR) technology and scanning electron microscopy (SEM) testing, respectively. The results revealed that, compared with standard curing conditions, the decrease of the curing temperature and humidity can result in the volume and proportion of macropores and microcracks being larger, which results in the deceleration of the ultrasonic wave speed inside the concrete and the decrease of the mechanical properties. Under the same curing condition, a lower water-binder ratio (w/b) enables the internal pore surface area of the material to increase, and the mechanical properties are improved. With the decrease of the curing temperature and relative humidity, the stress-strain curve appeared delayed in the initial compaction stage and presents more obvious brittleness characteristics in the failure stage. By fitting the relationship between the concrete strength and the porosity under different curing conditions, an extended model that can be applied to cement-based materials was obtained. Additionally, it was found that the porosity is negatively correlated with the ratio of the compressive strength to splitting tensile strength of the concrete.

Blast-furnace slag (BFS) has been increasingly used in cement production and has shown great influence on the electrical resistivity of concrete. The objective of this paper is to compare the theoretical values of electrical resistivity obtained through a mathematical model with experimental values for concrete with BFS. Reference concrete mixtures with ordinary portland cement were also studied. Results indicate higher electrical resistivity and smaller porosity for concretes with CEM III/A. The electrical resistivity of the CEM III/A concrete does not have a well-defined correlation with the water-binder ratio (w/b) or with the compressive strength, unlike CEM I concretes. The correlation between calculated and experimental resistivity requires a correction factor for the CEM III/A concretes. In this study, the correction factor was obtained empirically by reducing the theoretical tortuosity of concrete by 15%. Therefore, the model should be used in cements with BFS with the application of a correction factor.

Cement paste is the primary constituent of concrete that keeps all other constituents together and gives concrete its strength. During curing, the cement is developed as a binder by going through various chemical reactions. In the present study, ultrasonic testing is carried out on concrete samples during curing in transmission mode. The acoustic signals are generated using lead zirconate titanate (PZT) transducers which are excited by a sweeping frequency signal. Nondestructive testing and evaluation were carried out at various stages of curing for concrete with two different watercement ratios (w/c). The obtained signals were processed to analyze the change in signal characteristics during the different stages of curing. It was found that the nonlinear ultrasonic technique called the side band peak count (SPC) index, which is derived from the frequency spectra, exhibits a clear distinction among various concrete specimens at different stages of curing. Linear ultrasonic parameters, however, do not show such consistency. Therefore, the nonlinear ultrasonic technique provides an easy and effective way for monitoring the degree of concrete curing.