Carbonation depth measurement

Carbonation depth is easily measured by exposing fresh concrete and spraying it with phenolphthalein indicator solution. The carbonation depth must then be related to the cover (the average and its variation) so that the extent to which carbonation has reached the rebar can be estimated, and the future carbonation rate estimated. 

Splitting cores to expose a fresh surface should be done carefully to prevent dust from carbonated areas contaminating the uncarbonated surface and vice versa. 

The best indicator solution for maximnm contrast of the pink colouration is a solution of phenolphthalein in alcohol and water, usually 1 g indicator 

Phenolphthalein changes colour at pH 9. The passive layer breaks down at about pH 11. If the carbonation front is 5-10 mm wide the steel can be depassivated 5 mm away from the colour change of the indicator as shown 

Some aggregates can confuse phenolphthalein readings. Some concrete mixes are dark in colour and seeing the colour change can be difficult. Care must be taken that no contamination of the surface occurs from dust and the phenolphthalein sprayed surface must be freshly exposed or it may be carbonated before testing. 

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There is a draft CEN standard on carbonation depth measurement DRAFT prEN 14630 (2003). The Concrete Society (2004) report is similar to the CEN standard. BRE Digest 405 (1995) specifically discusses carbonation and its measurement in Portland cement concrete. BRE Digest 444 (2000) Part II and BRE IP 11/98 (1998) discuss measurement in high alumina cement concrete (HAC). 

Most work on linear polarization probes has been done in chloride corrosion condition. Ho vever, the only methods of assessing carbonated concrete are destructive drilling or coring for carbonation depth measurement and trying to interpret half cell potentials which is difficult (Section 4.7.2). Linear polarization is therefore very useful in asse.ssing carbonated structures, particularly as half cell potentials are so difficult to interpret for carbonation induced corrosion. 

Shinozaki, S. Yoshida, Y. Eko, N. Carbonation depth measuring method of concrete structure. Jpn. Kokai Tokkyo Koho JP 2005233819, 2005 Chem. Abstr. 2005,143, 252462. 

After installation in concrete, it is almost impossible to check the potential accurately. Measurement of the potential of the embedded electrode with a reference electrode placed on the concrete surface may give highly variable results, depending on the condition of the concrete cover (moisture, carbonation depth and chloride content). A better practice is to install two reference electrodes in approximately the same position in the concrete and compare their mutual potential difference. 

Fig. 39 Location of wells used in second phase of block-cyclic-steaming of petroli.ferous formation. Wells 1-production 2-injection 3-abandoned 4-observation S-stnicture contours drawn on top of massive carbonate bed (contour figures indicate depths measured from a surface datum level).

Our most recent work on fatty acids in Conception Bay [117] has provided additional details to the overall observations made for bulk carbon and individual hydrocarbons discussed above. We combined detailed molecular characterization with carbon isotopic measurements to describe both the temporal and depth variations of primary production during the spring bloom. The isotopic compositions of the fatty acids are summarized in Fig. 9. Bulk spring bloom particulate organic matter in Conception Bay has a 6 C value between -24%o and -26%o [116]. Since lipids are normally expected to be depleted relative to... 

For existing structures the carbonation depth of any part of the structure can be measured after a given period of time and thus the value of K and its spatial variation can be calculated. By assuming that the average exposure conditions will not change in the future, these values of K may be utilised to extrapolate the carbonation depth to a later point in time. 

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. 

This is easy for carbonation. A simple measurement of carbonation depth will show when it has been reduced to zero. However, it has been pointed out that the phenolphthalein indicator turns from clear to pink as the pH rises above about nine. This is still an unpassivated condition. Miller (1989) recommends ensuring that the indicator is bright pink. Universal indicator or an indicator with a colour change closer to 12 may be required to be sure that alkalinity has been fully restored. 

A number of empirical calculations have been used to derive values of A and n based on such variables as exposure conditions (indoors and outdoors, sheltered, unsheltered), 28 day strength and water cement ratio. A wider range of empirically derived equations is given in Table 3.1. These cover different exposure conditions, curing and concrete properties. The easiest solution for a given structure is to take some measurements of carbonation depth, assume n = 1/2 and calculate A. This can be used to predict the rate of progression of the carbonation front. The time taken to reach the steel can then be estimated and the rate of depassivation calculated. 

As stated in the previous section, the easiest way to predict the carbonation rate of an existing structure is to measure the carbonation depths at representative locations and use equation (9.1) to calculate the coefficient A, assuming n = 1/2 or a chosen value. A simple spreadsheet programme for doing this can be found at http //projects.bre.co.uk/rebarcorrosioncost/ and is described in Trend (2000) et al. (2001). 

The diffusion models work reasonably well for predicting the initiation time. The chloride profile and the carbonation depth can be measured in the field or from cores in the laboratory. However, it is far more difficult to look at the next step in our model. Corrosion rate measurements are now being taken in the field with linear polarization instruments and empirical estimates have been made with different instruments for the time to spalling. 

When presented with a corroding structure we can determine its condition by measuring the chloride profiles, carbonation depths and cover depths. From this we can calculate diffusion rates of the carbonation front or the chloride threshold and estimate the initiation time To. Both an average and a distribution of Tq values can be derived. 

For a new concrete mix or structure, the prediction of carbonation rate is complicated by the lack of data to extrapolate. In a series of papers (Parrott and Hong, 1991 Parrott, 1994a and b), a methodology was outlined for calculating the carbonation rate from air permeability measurements with a specific apparatus, Parrott analysed the literature (Parrott, 1987), see Section 3.U1, and suggested that the carbonation depth D at time t is given by ... 

Provided that the concrete is not water-saturated, it may be reasonable to assume that the initiation phase is considerably longer than the propagation period and that the end of the initiation period alone is a useful indicator of service fife. Clifton and Pommersheim have reviewed simple models based on this approach. For chloride-induced rebar corrosion, one of these is the use of Fick s second law of diffusion and the concept of a critical chloride concentration. Limitations and simplifying assumptions of this approach have been discussed in previous sections. Actual chloride concentration profiles can be measured on structures, to estimate parameters such as the diffusion coefficient used in the model. For carbonation, it has been proposed that the depth of carbonation is proportional to the square root of the exposure time. Again, the measurement of actual carbonation depth with time can be used to estimate a proportionality constant for a specific structure. 

The cured mortar specimens 100x100x100mm were placed in a non-pressurizing carbonation test chamber for 28 days, in which temperature, humidity and CO2 gas concentration were controlled to be 30°C, 60%R.H. and 5% respectively. After accelerated carbonation, the mortar specimens were split, and the split crosssections were sprayed with 1% phenolphthalein alcoholic solution. The depth of the rim of each crosssection changed to white color was measured with slide calipers as a carbonation depth as shown in Figure 1. 

Prompted by the success, TOFD measurements were conducted on a fatigue crack in a stainless steel compact tension specimen. Test and system parameters were optimised following the same procedure used for carbon steel specimens. A clear diffracted signal was observed with relatively good SNR and its depth as measured from the time-of-flight measurements matched exactly with the actual depth. 

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