Chloride profiling in marine concrete - methods and tools for sampling

Abstract: This project focuses on how to determine the chloride content in concrete structures exposed to marine environment in an accurate way. When analyzing dust from dry drilling in concrete, it is very often a fact that the chloride profiles show extensive variations even if the sampling has been performed within a very concentrated area with the sampling points close to each other. Several authors have observed this phenomenon in earlier research and some of these are referred to in this thesis.

The aim of this project has been to find reasons to these extensive variations and to find inspection methods that decrease the variations. Both laboratory studies and field studies have been performed within the line of this work. The laboratory study has consisted of concrete sampling by dry drilling and by profile grinding 100 mm cores on a concrete slab that had been submerged in saline water in the laboratory. The chloride profiles from the cores have been used as a reference to all other samples collected on the slab.

The study shows that profile grinding of cores is the most accurate method to perform sampling on concrete with the purpose of analyzing the chloride content, and that when using small bore diameters the sampling should be performed as mixed samples from several bore holes. In all cases, samples should not only be analyzed for chloride but also for binder content. The results from the dry drilling in laboratory shows that the binder content becomes over estimated in the dust samples compared to the analyzed cores on
the same concrete slab. This means that the analyzed dust samples seams to contain a higher chloride content than the cores analyzed by profile grinding. This phenomenon became obvious when simulating chloride profiles using data from EPMA. Most probably the drilling follows an easy path between the large aggregate particles.

One field study has been performed in the port of Trelleborg in southern Sweden and another one in the port of Malmö, also in southern Sweden within some 50 km distance but facing the open sea in different directions. The field study in Trelleborg has consisted of an inventory of existing concrete structures in the harbour and climate studies of the wind and temperature in the area followed by a more detailed inspection including
sampling on selected parts of a quay. This study shows that the exposure to open sea clearly influences the chloride content in concrete structures exposed to marine environment more than the main wind direction. The field study in the port of Malmö also confirms this finding.

The conclusions of this work is that when sampling concrete on structures in marine environment it is important not only to study the surrounding
environmental loads such as the dominating wind direction, but also the exposure of the structure to the open sea which has shown to be of great importance in the field studies. If it is possible, the sampling should be performed by profile grinding cores. The studies in the laboratory have shown that sampling cores gives small variations in the analyzed chloride content. If dry drilling is used, the diameter of the drill bit should not be smaller than 20 mm. In all cases the analyzes should include the binder content, since it is the binder who contains chlorides. Presenting the chloride content in % by binder content has been shown to give the smallest variations in this study.