Surface measurements, reinforced concrete

To check the analytical dependence conformity to experimental results, a series of reinforced concrete beams measuring 160 x 40 x 40 mm, reinforced by one bar 0 6 mm was prepared. After vibrating, the coating is applied to a surface of a ready concrete mixture, following which curing takes place in natural conditions (Figure 5.6). 

The use of corrosion inhibitors is of increasing interest as they are claimed to be useful in reinforced concrete not only as preventative measure for new structures (as addition to the mixing water) but also as surface-applied inhibitors for preventative and restorative purpose. Apphcation on the concrete surface could be a promising technique to protect existing structures from corrosion or increase the lifetime of structures that already show corrosion attack. 

From the point of view of the apphcation, corrosion inhibitors can be used as a preventative or as a curative measure to increase the service life of a reinforced concrete structure. In preventative apphcations inhibitors are used as admixtures to the fresh concrete (calcium nitrite, organic inhibitor blends) or appHed on the surface of hardened concrete (MFP, organic inhibitor blends), in which case the inhibitor has to penetrate the concrete cover to reach the steel surface. In curative appHca-tions inhibitors are applied on the surface of hardened concrete with the goal to reduce the corrosion rate of the rebars. In the following sections the two modes of action are presented. 

Surface treatments are applied to new structures as a preventative measure, to existing structures where the need for future protection is anticipated, and to repaired structures in order to improve the service life of the repairs (as well as to mask the visible effect of repairs). Of the many types of surface treatment of concrete, only those aimed at providing protection against corrosion of reinforcement will be mentioned here. This places coatings for protection against chemical attack outside the present scope. 

Measurements at the concrete surface. All methods for on-site measurement of concrete resistivity involve at least two electrodes (of which one may be a reinforcing bar). A voltage is superimposed between the electrodes and the resulting current is measured, the ratio gives a resistance (measured in O). The resistivity is obtained by multiplying the measured resistance by a geometrical conversion factor, the cell constant. This approach is valid only for a homogeneous material. 

Figure 20.3c shows the effect of application of cathodic protection on carbonated concrete. The applied cathodic current density, even if it brings about only a small lowering of the steel potential, can produce enough alkalinity to restore the pH to values higher than 12 on the reinforcement surface and thus promote passivation. The effectiveness of cathodic protection in carbonated concrete was studied with specimens with alkaline concrete, carbonated concrete and carbonated concrete with 0.4% chloride by cement mass that were tested at current densities of 10, 5, and 2 mA/m (of steel surface) [45]. Carbonated concrete specimens polarised at 10 mA/m showed that, although initially protection was not achieved since the four-hour decay was slightly lower than 100 mV, after about four months of polarization, the protection criterion was fulfilled and higher values, in the range 200-300 mV of the four-hour potential decay were measured (Figure 20.6). The same results were obtained on carbonated and slightly chloride-contaminated concrete.

The potential of the corroding surface can be monitored periodically by means of a reference electrode. One such example is the corrosion potential measurement of reinforced steel rebar in concrete structures. Corrosion of the steel in reinforced concrete is a major factor in the deterioration of highway and bridge infrastructure. A survey of the condition ofa reinforced concrete structure is the first step toward its rehabilitation. A rapid, cost-effective, and nondestructive condition survey offers key information to evaluate the corrosion, aids in quality assurance of concrete repair and rehabilitation. 

To prevent steel corrosion in reinforced concrete eiqiosed to chlorides, measures must be taken to minimize the migration of chloride ions from the surface towards the steel reinforcement. Such measures include a sufficiently high cement content, a low... 

Condition Evaluation of Concrete Bridges Relative to Reinforcement Corrosion. Vol. 7 Method for Fietd Measurement of Concrete Permeabilitt/. Evaluates a prototype surface air flow (SAP) device for the estimation of concrete surface permeability. A portable field device was constructed that obtains readings at one per minute allowing a large amount of information to be developed at close intervals across a given concrete member. 87 pages. SHRP-S-329, 10... 

The reasons described above lead to a conclusion that the interface may be the weakest element in the repair work, and as such, a source of future destruction. That is why there are many practical methods available to reinforce the interface and to ensure its high quality. Among the various known measures, special interface layers of the Portland cement paste, neat or reinforced with thin grids of steel or textile wires may be mentioned. Careful preparation of the surface of old concrete is always needed by cleaning and moistening. Intermediary layers may be prepared with increased Portland cement content, admixtures of polymeric materials or silica fume, or composed with epoxy resins. 

Two-dimensional potential measurements on the concrete surface serve to determine the corrosion state of the reinforcing steel. This method has been proved for one-dimensional systems (pipelines), according to the explanation for Fig. 3-24 in Section 3.6.2.1 on the detection of anodic areas. 

The activities of the Working Parties cover corrosion topics associated with inhibition, education, reinforcement in concrete, microbial effects, hot gases and combustion productions, environment sensitive fracture, marine environments, refineries, surface science, physico-chemical methods of measurement, the nuclear industry, the automotive industry, computer based information systems, coatings, tribo-corrosion and the oil and gas industry. Working Parties and Task Forces on other topics are established as required. 

Monitoring of the electrochemical potential of steel reinforcement in concrete is a well established technique for assessing the severity of corrosion and for controlling cathodic protection systems. A reference electrode is the electrochemical device used for measuring these potentials. The reference electrode is either placed on the concrete surface during the measurements or permanently embedded in the concrete in close proximity to the steel. The latter technique enables remote long-term monitoring. 

Polarization curves. The rate at which the anodic or the cathodic process takes place depends on the potential ( ). The corrosion behaviour of the reinforcement can be described by means of polarization curves that relate the potential and the anodic or cathodic current density. Unfortunately, determination of polarization curves is much more complicated for metals (steel) in concrete than in aqueous solutions, and often curves can only be determined indirectly, using solutions that simulate the solution in the pores of cement paste. This is only partly due to the difficulty encountered in inserting reference electrodes into the concrete and positioning them in such a way as to minimize errors of measurement. The main problem is that diffusion phenomena in the cement paste are slow (Chapter 2). So when determining polarization curves, pH and ionic composition of the electrolyte near the surface of the reinforcement may actually be altered. 

Corrosion rate measurements on-site. The main difference in measuring the polarization resistance Rp on site and not in the laboratory is the geometrical arrangement of the electrodes. In the laboratory a uniform geometry and thus uniform current distribution can be achieved usually with small specimens, whereas on site a there is a non-uniform current distribution between the small counter-electrode (CE) on the concrete surface and the large rebar network (WE). The current fed by the external counter electrode (Figure 16.15) not only polarizes the surface of the reinforcement below the CE itself but also spreads laterally or may reach... 

Electrochemical monitoring methods have also been developed for application on steel reinforcement in concrete. These methods include potential measurement on the concrete surface, linear polarization (LPR) and determination of polarization curves [9.17]. Electrical resistance probes (ER) and probes embedded in the concrete for measuring galvanic current have also been used. 

Recently, Kobayashi et al. (2010) also studied the chloride penetration into the cracked HPFRCC material. In their study, several concrete beams were prepared, and the bottom layer surrounding the reinforcing steel was repaired using HPFRCC and repair mortar (polymer cement mortar [PCM]). Cracks were introduced in the specimens by uniaxial tension. The maximum crack widths in the HPFRCC repair layer and PCM layer were 0.06 and 0.43 mm, respectively. The specimens were then subjected to saltwater (containing 3% NaCl) spray for 5 min every 6 h and continned for 2 months. Similar to Miyazato and Hiraishi (2005), the AgNOj solution was also sprayed on the fractured surface to measure the chloride penetration depth. Reduced chloride penetration in cracked HPFRCC compared to PCM was reported in their study (Figure 6.10). The above results on the permeability of HPFRCC show that it has very low water and chloride permeability even in the crack state. 

An alternative approach using a single electrode on the surface and the rebar network can be used to measure the resistivity of the concrete cover. This is available as part of a corrosion rate measuring device (see Section 4.12 and Figure 4.13) and uses the reinforcement cage as one electrode and a small surface probe as the other electrode. The advantage of this approach is that it measures the resistivity of the cover concrete only. The disadvantage is that it suffers from contact resistance problems. 

An alternative approach measures the resistivity of the cover concrete by a two electrode method using the reinforcing network as one electrode and a surface probe as the other tubular sensor B in Figure 4.13 (Feliu et ai, 1988 Broomfield et aL, 1993, 1994). 

Chlorides concentration of 0.6% by mass of concrete is reducing the resistivity two times [60], Therefore, the progress of reinforcement corrosion in concrete can be assessed from measurements of surface resistivity [60],... 

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