# Determination of pH in Powdered Concrete Samples or in Suspension
## 1 Introduction  
The majority of urban infrastructure such as buildings, bridges, roads and tunnels are built from reinforced concrete. Reinforced concrete structures may deteriorate due to exposure too severe conditions related with the environment--carbonation, chloride ingress, freeze-thaw effect, abrasion/erosion or chemical action. Although different deterioration mechanisms can lead to degradation of the concrete itself, reinforcement corrosion is the biggest durability issue for reinforced concrete structures, and can lead to the cracking, loss of bearing capacity and catastrophic failures. To maintain the structural integrity of our extensive urban concrete infrastructure and prevent catastrophic situations it is very important to implement regular inspection regimes that include the measurement of pH [1]. The pH value (defined as the negative logarithm of the hydrogen-ion concentration) in this domain represents the level of alkalinity of concrete under different conditions. Portland cement paste in fresh and hardened concrete has a pH value usually between 12.5 and 13.8 [2], and in this high alkalinity environment a thin passivation layer is formed above the steel surface that prevents the rebar from corrosion [3]. The carbonation process, in which calcium hydroxide in concrete is transformed into calcium carbonate, can decrease the pH of concrete and active corrosion will begin near the rebar once the alkalinity level (pH) is reduced to values 9.0-9.5. As one of the key parameters influencing the corrosion rate together with the temperature (T) and relative humidity (RH), the precisemethods of measuring pH in concrete are of great interest to science and industry. The in situ leaching method for measuring the pH in concrete was based on inserting a glass micro pH electrode into a hole drilled into the concrete sample [4]. An Ag/AgCl electrode was used as the reference electrode. The potential difference between the working and the reference electrode was measured and translated to pH values. However, this method is time-consuming and destructive [5]. Hence, considerable effort has recently been invested in developing non-destructive techniques based on the application of different embedded sensors [6]. A multifunctional sensor consisting of pH and chloride sensors was reported in [7]. The sensor was embedded in the concrete at a depth of 10 mm and a drop in pH was noted in as little as 120 days. However, embedded sensors cannot be calibrated in situ and they can be broken inside the concrete. Elsewhere, fibre optic sensors for measuring pH in concrete were proposed [8]. Necessary components for performing this measurement are a light source, a coupler, a fibre switch, a spectrometer, a sensor and a PC [9]. A disadvantage of optics sensors is that they are chemically instable. Phenolphthalein is a commercial indicator that can be used for visual estimation of carbonation because it changes colour from red-violet (non-carbonated areas) to colourless (carbonated areas) within a pH range of 8.9 to 9.8 [10]. Alizarin yellow-R and indigo carmine were also applied as pH indicators in concrete [11]. These indicators are spread onto a cross-section of the specimen; thus, they are also able to detect the carbonation depth [12]. However, the information provided by these indicators regarding the actual carbonation status of concrete is limited by their very narrow pH range. In [13] the authors proposed the application of pH strips for verification of concrete carbonation status, mixing concrete powder with a few drops of water. Elsewhere, a halochromic method was applied for mapping pH values of Portland cement concrete [14]. The measuring system was composed of a white light source, a camera, and two optical filters.