Cover depth

Conventional covermeters operating on the principle of electro-magnetic fields are capable of determining depth of cover to reinforcement down to about 100 mm with an accuracy of 5 % provided the bar diameter is known and the spacing between individual bars is greater than about 150 mm. If bars are spliced then the covermeter will underestimate the cover depth. 

A number of individual radar scans have been joined together Variations in cover depth give rise to variations in arrival time for the wave reflected from the reinforcing bars... 

Seawater. A half dozen analyses of Mo isotopes in the Pacific, Atlantic and Indian oceans, covering depths to -3000 m (Barling et al. 2001, Siebert et al. 2003), reveal two important facts. First, there is no detectable 5 Mo variation in the oceans with location or depth. Second, d Mo in the oceans is similar to the heaviest of euxinic sediments, and is heavier than in igneous rocks or ferromanganese sediments by -1.5%o and -2%o, respectively. A uniform isotopic composition in the oceans is consistent with the 10 -1 O " year ocean residence time. The explanation for the heavy isotopic composition is discussed further below. 

The processes of infiltration and evaporation of ground water depend strongly on the vertical profile of the soil layer. The following soil layers can be selected saturated and unsaturated. The saturated layer usually covers depths >lm. The upper unsaturated layer includes soil moisture around plants roots, the intermediate level, and the level of capillary water. Water motion through these layers can be described by the Darcy (1856) law, and the gravitation term KZ(P) in Equation (4.31) can be calculated by the equation ... 

The habitat of this species in the modem Aral covered depths from 0 to 25-30 m in the Western basin, down to 6.2 m in the channel and down to 3-4 m - in the north of the Eastern basin (2002-2005). 

Besides concrete quality, a minimum value of the concrete cover also has to be specified. Eurocode 2 [3] fixes minimum values ranging from 10 mm for a dry environment up to 55 mm for prestressing steel in chloride-bearing environments, as shown in Table 11.5. It should be kept in mind that these values are minimum values that should be increased to obtain nominal values by 10 mm, to also take into consideration construction variability. Besides the protection of steel to corrosion, further requirements of minimum cover depth are fixed to ensure adequate transmission of mechanical forces and fire resistance. 

Table 11.5 Minimum vaiues for concrete cover depth, simplified from Eurocode 2 with regard...

As the environmental aggressiveness increases, it is theoretically possible to maintain a constant level of durability by increasing the thickness of the concrete cover. In reality, however, the cover thickness cannot exceed certain limits, for mechanical and practical reasons. In particular a very high cover may have less favourable barrier properties than expected. In extreme cases, a thick unreinforced layer of concrete cover may form (micro)cracks due to tensile forces exerted by drying shrinkage of the outer layer, while the wetter core does not shrink. In practice, having cover depths above 70 to 90 mm is not considered realistic. 

In the early years, service-life models for reinforcement corrosion basically followed the square-root-of-time approach or shght modifications, such as the example illustrated in Figure 11.2. Based on empirical data on the rate of carbonation or chloride ingress, a minimum cover depth was determined that was expected to delay the onset of corrosion for a required period. Following this approach, more and more data were collected from existing structures and exposure sites, and the influence of various factors such as cement type and local environment became more clear [1,13). 

A Hmit state related to corrosion propagation considered in DuraCrete is the appearance of a crack with a width of 1 mm, which is taken as the start of spaUing. Equations are given [21] for the crack width caused by a corroding bar based on the amount of corrosion present, the amount needed to produce a crack, and some factors including the cover depth, the rebar diameter and the tensile strength of the concrete. After corrosion initiation, the amount of corrosion is given by the corro-... 

In the design process, the DuraCrete approach works as follows. The chloride surface content and the intended cover depth for the structure are identified. As a first step, concrete compositions are selected for appropriateness using input values for chloride-penetration resistance from a database. 

The resistance of the selected concrete mix against chloride penetration is determined using this test and if necessary, the composition is modified until satisfactory values are obtained. At this stage, increasing the cover depth may also be possible. 

It appears that the probability of corrosion initiation even with 70 mm cover depth is high (about 50 %) within fifty years. The probability of corrosion initiation after 12 y is about 1 % (corresponding to a serviceability limit state) for steel at 70-mm cover depth. The agreement with the results given in Table 11.4 is reasonable. 

Table 11.15 Times-to-initiation of corrosion at different cover depths caiculated with the DuraCrete model using input from Table 11.14...

Other types of resistance measurements, especially also involving the rebar network, have been apphed. Commercially available instruments combine half-cell potential mapping with resistance measurements between the electrode and the rebars. This results in resistance maps, however, conversion to true resistivity is much more difficult because the cell constant is also influenced by the cover depth to the steel bars and the size of the external electrode. 

M. Pentti, The accuracy of the ex-tent-of-corrosion estimate based on the sampling of carbonation and cover depths of reinforced concrete facade panels , Tampere University of Technology, Publication 274, 1999. 

Variability. The examples of Figures 19.1 and 19.2 summarise the process of evaluation of the depth of concrete removal when a single rebar is considered and measurement of carbonation, chloride and cover thickness are available locally. In a real structure carbonation and chloride penetration may vary due to spatial variation of exposure conditions (microclimate) and of concrete properties (e. g. cracking or had compaction), etc. The cover depth may also be very variable. 

Execution. In the execution phase, the CP system is applied to the structure following the design. During the work, the complete concrete surface is checked for cracking, delaminations, cover depth, steel continuity and the presence of metal objects that might cause short circuits. If necessary, continuity is provided and metal objects are connected to the reinforcement. Subsequently the cracked and spalled areas are removed and repaired. The reference electrodes and other monitoring probes are embedded. Then the anode is applied, with overlay or top coat as designed. All electrical coimections are made and the power source is installed. 

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] ... 

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. 

Figure 4.9 Phenolphthalein applied to a concrete window mullion showing approximately 10 mm carbonation depth (clear area at surface) and an uncorroded bar in uncarbonated concrete with a cover depth of approximately 40 mm.

To measure the concrete cover depth required to prevent carbonation from reaching the steel it is therefore necessary to measure the air permeability and the relative humidity, and then calculate D. This can be done with a proprietary apparatus developed by Parrott and available commercially. Gas and air permeability measurements are discussed in Kropp and Hilsdorf (1995). 

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. 

By measuring corrosion rates over a period of time we can estimate the time to cracking and knowing the distribution of corrosion rates, cover depths, etc. a cracking rate can be established by adding the time to cracking to the initiation time. An empirical condition curve can be calculated and the time taken to reach an unacceptable level can be determined. 

Concrete Bridge Protection and Rehabilitation Chemical and Physical Techniques—field Validation. Covers the field application and short-term corrosion performance of six trial installations of two inhibitor-modified concrete systems. The installations were applied to both deck and substructure components in a range of environments. Both pre- and posttreatment corrosion assessments v/ere performed to estimate the corrosion performance of inhibitor mcdified concrete systems, including visual inspections, delamination surveys, cover depth sur eys, chloride contamination levels, corrosion potential measurements, and corrosion current measurements. 67 pages. SHRP-S-658... 

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