Chloride content/profiles

The chloride content profile of different mixes of 28-day cured concretes subjected to 60-day exposure to NaCl solution is presented in Figure 11.9. The figure shows the measured chloride concentration at different depths, and the chloride concentration inside of the concrete is measured by the percentage of the concrete s weight. In the chloride content profile, the rate of chloride diffusion can be determined from the steepness of the curve. A less steep curve indicates that the concentration of chloride inside of the concrete equals the concentration on the concrete s surface. On the other hand, the steep curve indicates the slow chloride ion diffusion inside of the concrete due to very slow chloride penetration from the concrete s surface. 

Figure 11.9 Chloride content profile of different concretes cured at 60 days of exposure (Shaikh and Supit, 2014).

Another application of Tick s second law of diffusion is the analysis of chloride-penetration profiles in cores taken from structures that were actually exposed to chloride penetration from the outer surface. A profile of the chloride content is determined experimentally as a function of depth (Section 16.3.2). The values of Cj and D are then determined mathematically, by fitting Eq. (4) to the experimental data [19]. An example is given in Figure 2.6. The diffusion coefficient D (which may typically vary from 10 to 10 m /s as a function of the concrete s characteristics) can be used as the main parameter that describes the rate of chloride penetration. Together with the fitted surface content and using Eq. (4) the chloride penetration can be extrapolated to longer times. The limits of this approach will be discussed in Chapter 6. 

Furthermore, Eq. (1) is also used for the prediction of long-term performance of structures exposed to chloride environments, e. g. during the design stage or the evaluation of the residual life of existing structures. In principle, if D pp and Q are known and can be assumed to be constant in time, it is possible to evaluate the evolution with time of the chloride profile in the concrete and then to estimate the time t at which a particular chloride threshold will be reached and corrosion will initiate. For example. Figure 6.6, plots chloride profiles calculated for a surface chloride content of 5 % by mass of cement and a time of 10 y as a function of the apparent difiusion coefficient. Figure 6.7 shows the initiation times that can be estimated from those profiles as a function of the thickness of the concrete cover when a chloride threshold of 1 % by mass of cement is assumed. 

Interpretation. The presence of an above-critical amount of chloride ions at the rebars leads to depassivation and in the presence of oxygen and water to corrosion attack. From chloride profiles information on the transport of chlorides into the concrete (Chapter 6) can be obtained. In combination with results from potential mapping, the critical chloride content for the specific structure can be obtained. On chloride-contaminated structures an empirical correlation between chloride content and half-cell potential could be established, thus the chloride distribution can be roughly obtained from the potential map. 

In case a, future penetration of chlorides has to be evaluated in order to assess if the chloride threshold will be reached at the surface of the reinforcement before time tf. If this is expected, the concrete with a chloride content higher than the critical value has to be removed (and replaced with a chloride-free material that prevents further penetration of chloride). Equation (1), Chapter 6, may be used to evaluate the future penetration of chlorides. By fitting the present chloride profile, the surface content and the apparent diffusion coefficient Dj j, may be calculated at time Since these parameters are evaluated on the actual structure and usually after a long time of service, it is often reasonable to assume that they will not change significantly in the future (unless the conditions of exposure of the structure will change). Equation (1), Chapter 6, can then be used to plot the expected chloride profile at time tf. 

From the profiles of remaining chloride in specimens subjected to chloride extraction in the laboratory with various amounts of charge, the relationship between the durable cover depth values and the charge was established [67]. Before CE testing, the 144 specimens studied were subjected to chloride ponding that resulted in the penetration of about 2.5 % chloride by mass of cement in the outermost 15 mm and about 0.6% from 15 to 30 mm depth. The results of the CE tests were interpreted as follows. It was considered that it should take at least 10 y after treatment before the chloride content at the rebar surface would exceed 0.4% chloride by mass of cement If no new chloride would penetrate, it was concluded that [67] ... 

Figure 12.8 presents the chloride concentration profiles with higher surfece chloride content, (7.36 kg/m ). A comparison reveals chloride concentration throughout the entire deck is higher with an increase in surface chloride content. 

The purpose of the detailed survey is to ensure a cost-effective repair in line with the client s requirements. This is done by accurately defining and measuring the cause, extent and severity of deterioration. In Chapter 7, we will discuss how test measurements may be used to model the deterioration rate, time to corrosion and life cycle costing. We will need to know how much damage has been done and what has caused the damage. Quantities for repair tenders will probably be based on the results of this survey, so a full survey of all affected elements may be required. Alternatively a full visual survey may be required, with a hammer (delamination) survey of all accessible locations. A number of representative areas may be selected for a detailed survey of cover depths, carbonation depths, chloride content or profile, half cell potentials and other techniques described in the following sections of this chapter. 

For chloride ingress life is more complex as we are dealing with atmospherically exposed structures that are exposed to variable chloride contents. Also, we are not dealing with a front that moves through the concrete but a chloride profile that builds up in the concrete. 

This is an S shaped curve that approximates to the two straight lines for To and in Figure 9.1. The curve could therefore be calculated if there are two sets of data. These can be derived from two sets of measurements of cracking, spalling and chloride content separated by several years, or taking one set of measurements at the present time and back calculating data for an earlier time This may be to the time of the first spall or a back calculation of the time to depassivation from the chloride profiles (approximately Tq). We can therefore derive values for A and B. These can be used to project forward the delamination rate and show how costs will escalate if work is deferred or how repair quantities will increase between the survey and the start of patch repair work. 

The goals of the electrochemical repair methods is to check ongoing corrosion of the rebars by increasing the alkalinity of the pore solution and to extract chlorides from the concrete cover. No official standard for the acceptance of the treatments exists, but at the end of the treatment the effectiveness should be checked. This can be done by direct measurements (half cell potential mapping) or by indirect means (chloride content, sodium profiles, total charge flow, etc.). 

Chloride content. Samples for determining the chloride level in concrete are collected in the form of powder produced by drilling or by the extraction of cores, sections of which are subsequently crushed. The latter method can provide a more accurate chloride concentration depth profile. The chloride ion concentration, used as a measure of the risk of corrosion damage and degree of chloride penetration, is subsequently determined by potentiometric titration. Two distinctions are made in chloride ion concentration testing Acid-soluble chloride content (ASTM C 114) refers to the total chloride ion content, while the water-soluble content represents a lower value. 

Since humid concrete is a conductive medium, experience shows that the more humid the concrete and the higher its chloride content, the more will be the galvanic corrosion of aluminium in contact with steel [8]. Tests performed in corrosion testing stations at the seacoast on concrete blocks with embedded aluminium profiles and steel reinforcement rods (Figure G.4.2) have shown that severe galvanic corrosion occurs when aluminium is... 

A diagnosis of possible damage should be made before beginning repairs with other construction measures [48,49]. There should be a checklist [48] of the important corrosion parameters and the types of corrosion effects to be expected. Of special importance are investigations of the quality of the concrete (strength, type of cement, water/cement ratio, cement content), the depth of carbonization, concentration profile of chloride ions, moisture distribution, and the situation regarding cracks and displacements. The extent of corrosion attack is determined visually. Later the likelihood of corrosion can be assessed using the above data. 

Assuming oxidized glutathione as the disulfide responsible for the polarographlc wave at -0.59 V, the disulfide concentrations were calculated as 5 uM and 139 uM for the 0-3 and 3-6 cm depths respectively. Below 12 cm, the pore water profile is dominated hy inorganic sulfide. Inorganic sulfide is not present when thiols are present. The thiol concentration Increases with depth until just above the onset of the inorganic sulfide zone which shows increased sulfide content with depth. The highest concentration of thiol corresponds to the pH minimum and to sulfate production as evidenced by the excess sulfate values relative to open ocean sulfate chloride ratios ( ASO in Tables IV and V). 

Fig. 14.16 Comparison of AT anomalies (blue lines) to gas hydrate content estimated dissolved chloride distribution (red lines), and given as percent occupancy of the pore space. Leg 204. Green lines denote estimates based on data from pressure core barrel deployments, the depth of seismic reflectors corresponding to the bottom of the GHSZ (BSR). Location of Insert B shows the temperature profile derived from an infrared image in the vicinity of from Site 1245, and the corresponding chloride concentration in closely-spaced pore water depth between the two graphs is due to the removal of core as gas expansion voids between collected and the pore water samples were taken (modified from Trehu et al. 2004a).

Nevertheless, experience shows that, even in these cases, chloride profiles can be mathematically modelled with good approximation using an equation formally identical to (4), Chapter 2. In this equation, the total content of chlorides is usually considered, hence including bound chlorides ... 

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