PSI - Issue 64

Said Debbakh et al. / Procedia Structural Integrity 64 (2024) 130–136 S. DEBBAKH and al. / Structural Integrity Procedia 00 (2024) 000 – 000

131

2

1. Introduction Concrete is considered to be the most cost-effective and efficient material of construction. However, traditional preparation and pouring methods continue to be used and adopted in many countries due to a great concern of access to housing. Consequently, hand-mixing method and old-fashioned pouring technique strongly contribute on decreasing the concrete quality. The concrete quality downgrading makes the decision to make about ranking concrete class as a tedious operation (Benidir and al., 2023). The quality degradation of the concrete leads to major strength variability. Due to high strength variability, the buildings can experience irregular seismic responses (Pucinotti and al., 2009; De Stefano and al., 2013). Different testing methods ranging from destructive, non destructive, and semi-destructive methods allow the assessment of the strength and condition of concrete (Frappa and al., 2022; Karahan and al., 2020; Nobile, 2015). The destructive tests (crushing) are very easy to carry out, and yield direct interpretation, but involve breaking the samples and are limited as for identifying detailed characteristics. On the other hand, non-destructive methods including Rebound Hammer test (RH) (El-Mir and al., 2023; Kumavat and al., 2021; Sanchez and al., 2015), Ultrasonic Pulse Velocity (UPV) (Bungey and al., 2001; Kouddane and al., 2023), etc. as well as partially destructive methods including Pull-off test (PF) (NF EN 1542, 1999), Pull-Out test (PO) (Bungey and al., 2001), etc. are widely experienced. These methods are becoming more likely adopted regardless of the cost of their equipment’s due to the more extensive data they provide (microstructure, uniformity, deterioration, etc.). In addition, these methods can be applicable in new as well as in existing buildings. The drawback of these methods is that they require a great deal of skills and experience, and would be effective only at limited dimension ranges. The partially destructive methods including the bond strength pull-off test (NF EN 1542, 1999) allows the preservation of the material through the possibility of concrete repairing. This test involves bonding a steel dolly using a rapid hardening adhesive on a previously cored surface of concrete. The bond strength to concrete or either the tensile strength of concrete can be derived according to the failure mode induced from the pull-off action, provided that the failure occurs within the range of mechanisms referenced in the standard (NF EN 1542, 1999). The non-destructive methods including Rebound Hammer test (RH) (NF EN 12504-2, 2021) preserves the material, and would be the most widely employed due to its several features (movable, cheap and easy to use). The device includes an elastic mass whose rebound defines the hardness of concrete (NF EN 12504-2, 2021). The accuracy of estimating the compressive strength through rebound readings has been much discussed among researchers, where it has been revealed a sensitivity of these readings to near surface properties. Various factors, such as surface smoothness, concrete age, moisture content, carbonation, presence of aggregates, air voids, steel reinforcement, temperature, and calibration of the rebound hammer, have been identified as influential in determining surface hardness (NF EN 12504-2, 2021). The influence of these factors has demonstrated that the measurements obtained from rebound hammer tests would not be consistent, and that the test results would be dependent on the specific properties of the concrete tested, which in turn fluctuate based on various construction parameters. In this work, an investigation is performed on reinforced concrete columns after one year of exposure to natural environment. The pre-characterization at 28 days using destructive and non-destructive tests have already revealed a zone effect related to the location of the specimens along the high of the columns (upper, medium and lower zones) (Benidir and al., 2023). A complementary characterization is performed based on two methods involving the rebound hammer test, and the bond strength pull-off test according to (NF EN 1542, 1999) and (NF EN 12504-2, 2021) standards respectively. The aim of the study is to assess the efficiency of both methods in rating the quality of concrete as well as to check the persistence of the zone effect on the concrete strength after one year of natural exposure. 2. Materials and methods 2.1. Materials and mix proportions Two reinforced concrete columns with a cross-section of 40 cm x 40 cm and a height of 300 cm, were used for the purposes of the experiment. Both columns have been previously characterized in compression by the mean of destructive and non-destructive tests, see Fig. 1(a). Results of these tests were reported in the following references