1.1 Coatings and sealers

Coatings and sealers present the advantage of providing a physical barrier on the concrete surface, which isolates it from the aggressive agents of the external environment. These coatings require a homogeneous and smooth substrate with pores of 0.1 mm width at most. This means that they are capable of covering a 0.1 mm wide crack. However, the film breaks if the concrete structure cracks after painting. Moreover, coatings do not allow the concrete to dry if it is wet. This can cause the deterioration of the film, causing the formation of bubbles due to the vapor pressure of the internal humidity. From the architectural point of view, this group of surface treatment modifies the aesthetics of the structure adding brightness or color to the concrete surface, which is sometimes desired (^{Helene, 2000}).

This group of surface treatment (coating and sealers) has been intensely studied in the last fifteen years. ^{Delucchi et al. (1998)} studied the importance of the parameters E (modulus of the material) and ( (viscosity) in the crack-bridging ability of the coating. ^{Seneviratne et al. (2000)}, using mechanical thermal analysis, suggested that the most successful coating is able to maintain its elastomeric properties over the required period of exposure and over a wide range of operational temperatures. ^{Uemoto et al. (2001)} showed a correlation between the paint pigment volume content and water permeability. ^{Al-Zahrani et al. (2002)} showed that the accelerated corrosion performance of the four coating systems studied correlates well with the performance results obtained from the physical properties, in particular, water absorption, water permeability, and chloride penetration. ^{Medeiros and Helene (2009)} suggested that the determination of the chloride diffusion coefficient allows a quantitative comparison of the protection systems and, therefore, needs to be made possible in migration tests. This kind of protection is probably the most used by the construction industry and that would be the reason why they are widely studied.

1.2 Water repellent

Water repellents or hydrophobic agents always result from silicon resins that are chemically bound to the concrete base. Currently, the most commonly used are silanes, siloxanes oligomerics and a mixture of these two components (^{Medeiros; Helene, 2008}).

Chemically, silanes are formed of small molecules that have one silicon atom and siloxanes are short chains of a few silicon atoms in which the molecules have alkoxy groups (organics) connected to the silicon atom. Silanes and siloxanes react with the silicate of the concrete, forming a stable bonding (^{Vries and Polder, 1997}) (Figure 2). ^{Medeiros et al. (2009)} showed that the penetration of the hydrophobic agent is better in finished faces than in formwork faces, due to the higher permeability of the second. ^{Jacob; Hermann (1998)}, ^{Batista (1998)} and Moriconi et al. (2002) presented an ample review on surface hydrophobic agents. The use of hydrophobic agents in the construction industry is increasing at an interesting rate and they have been reasonably studied. However, there are some items that are still being studied, such as the influence of the water penetration mechanism (capillary suction and permeability) in the efficacy of the hydrophobic treatment (^{Medeiros; Helene, 2008}).

Figure 2 Chemical bond of the hydrophobic agent with concrete (adapted from ^{Vries and Polder, 1997}). 

1.3 Pore blockers

Pore Blockers, such as sodium silicate, are used in the cement industry for some applications. For example, as slag cement activator; as treatments of vegetal fibers used for cement reinforcement, as surface treatment for concrete and as hardening/densifying of surface concrete floors (^{Mehta, Monteiro, 2008}; ^{Brough, Atkinson, 2002}; ^{Jones, 2002}; Toledo et al., 2003; ^{Melo Neto, 2009}).

Although this group of treatment has been known for several years, little technical and scientific information about this subject has been published. On the other hand, many products of this type are commercialized and indicated for the protection of concrete surfaces. ^{Thompson et al. (1997)} explained the 3 theories on how silicates act to improve the performance of concretes:

The latter theory is currently the most accepted. In this way, theoretically, the pore blockers are products composed by silicate, which penetrate the superficial pores of the concrete and react with portlandite forming C-S-H. Sodium silicate is the mostly used silicate applied in this way.

According to ^{Thompson et al. (1997)}, Eq. 1 represents what happens when the sodium silicate solution penetrates the pores of the concrete.

1

In this way, this treatment forms a less porous layer in the concrete surface modifying the water penetration into the concrete.

This treatment requires that sodium silicate reacts with portlandite [Ca(OH)₂]. Therefore, it is easy to understand that, in carbonated concrete, the reaction above fails to occur. This is why in the superficial layer the Ca(OH)₂ already reacted with CO₂ and originated CaCO₃. In this case, it is necessary to make an impregnation with hydroxyl ions before the application of sodium silicate. This explains why ^{Kagi and Ren (1995)} affirmed that a significant reduction in the permeability of concrete due to silicate solution impregnation can only be achieved if the concrete is very fresh and repeated impregnations are carried out to fill all the concrete capillaries. Another contribution of ^{Kagi and Ren (1995)} is related with water vapour permeability of the substrate which is not significantly affected by silicates because they only cover capillary walls without blocking them fully. ^{Ibrahim et al. (1997)} focused their work on measuring the time taken for reinforced concrete specimens coated with sodium silicate to crack by corrosion and concluded that the results were similar to that of the uncoated concrete specimens. However, their results are not in accordance to those of ^{Thompson et al. (1997)} regarding water absorption. It is important to emphasize that ^{Ibrahim et al. (1997)} do not inform the concentration of sodium silicate solution used in their study. The use of a low concentration may have influenced their results. ^{Thompson et al. (1997)} compared some concentrations of sodium silicate solutions and the value around 26% presented greater efficiency.

It is important to emphasize that the pore blocker should be applied after the structure is in service. This is after it has taken all the projected loads and cracks and microcracks have been produced. However, environmental effects can cause more cracks. If there are cracks (large enough to be visible) before the application of the silicates, it is necessary to repair them prior the treatment application. If the cracks (large enough to be visible) appear after the silicates treatment, they need to be repaired immediately because the low porosity layer formed by the treatment is ruptured. This is one of the main problems of the surface treatment (Pore liners, pore blockers and coatings): they do not absorb the deformation of the concrete of substrate. This means that the formed protection layer breaks if the concrete cracks, causing the prompt imperfection of the protection system.

It is important to remember that, independently of the crack width, the remaining superficial pores will still be smaller than if they do not have the pore blocker on the surface. Then, the environment attack will be isolated to the cracked area and the other areas will keep being protected.

In general, it can be said that more information is needed about the protection capacity of silicates applied on surfaces of reinforced concrete. On the other hand, the use of these types of concrete protection is wide and their application is contributing to many of the several concerns about service life prediction since they can be applied as preventive or corrective methods. The service life approach has significantly changed during the last years (^{Castro-Borges, Helene, 2007}) and this is mainly due to the recent introduction of more quantitative parameters than a few years ago. These parameters include new concepts such as serviceability, functionality, security, reliability and durability.

Part of this work focuses on some additional contributions to the study of concrete surface treatments in the sense that they can constitute a useful tool in the field of protection of new reinforced concrete structures. However, based on the real concerns promoted by their application, the main purpose of this paper is to indicate a way to estimate the influence of concrete surface treatments in the service life of a reinforced concrete structure exposed to chloride contamination. This type of results presentation was made possible with the use of the diffusion theory developed by Fick.

2.1 Substrate concrete

The substrate concrete was produced with a Brazilian Portland cement designated CPII E-32, which is equivalent to ASTM C 595 (Slag-modified Portland cement). This cement has blast furnace slag admixture and is one of the most widely used cements in the state of São Paulo, Brazil. The chemical composition and physical properties of the cement are listed in Table 1. Coarse aggregate was a dense, crushed granitic stone. Fine aggregate was natural siliceous sand.

Table 1 Chemical compositions and physical properties of cement CP II E-32 (ASTM C 595). 

The mix proportions details are summarized in Table 2. After mixing, a vibrating table was used to ensure appropriate compaction. The curing adopted was conditioning the specimens in wet chamber with relative humidity of 100 % and 24oC temperature for 91 days. This curing period was chosen to produce a high hydration level and eliminate the influence of an additional curing that could occur in the specimens exposed to water. This could especially affect the control series.

2.2 Surface protection materials

The surface treatment systems investigated in this study are presented in Table 3. Table 4 shows the curing type, number of coatings and the manufacturer's recommended consumption rate for each surface treatment material.

2.3 Estimation of chloride diffusion coefficients

The method used to estimate the chloride diffusion coefficient followed the procedure described by ^{Luping and Nilsson (1992)}, which involves measuring the depth of color change of a freshly broken concrete surface in the direction of the chloride flow using 0.1 M AgNO₃ aqueous solution (^{Andrade et al., 1999}) and in the application of Eq. (2) in a migration test. fixed chlorides are ignored and it is possible to release a portion of the fixed chlorides among the Since the method is based on the depth of penetration of free chlorides, it should be noted that the time, becoming free. Thus, the method may produce inaccurate results. However, this practice was implemented due the ease of testing and because the free chlorides are the ones that effectively influence the steel depassivation.

2

Where: D is the diffusion coefficient (cm²/s), R is the gas constant (J/molK), T is the temperature (K), z is the valency of the ion, F is the Faraday constant (J/Vmol), E is the field density (V/m), xd is the penetration depth (m), t is the test time (s), a and b are test constants. For chloride ion z = -1 and, when E = -600 V/m and T = 298 K, then, a = 1.061 and b = 0.589. The model used is based on Fick's second law because the test is developed in a non-steady state condition.

Slices of 50 mm in thickness were obtained by sawing the mid-portion of the cylindrical specimen (10 cm diameter and 20 cm length). 3 concrete slices of each case were used in this test and these slices were extracted from the same specimen, as suggested previously (^{Medeiros et al., 2009}). Then, the slices were kept in an oven at 60 ^oC until obtaining constant mass. Then, the surface protection materials were applied on one of the circular surfaces of specimens. Seven days after the application of protection, the specimens were saturated using the procedure of ASTM C 1202/12. The manufacturer guidelines for the amount of material per unit of area were followed in all cases.

The 50 mm thick slice was placed between two acrylic cells. One cell was filled with a 0.3 N NaOH solution and the other with a 3.0% NaCl solution. The cells were connected to a 30 V power source and maintained for 30 h. The schematic presentation of the accelerated chloride migration test cell is illustrated in Figure 3.

Figure 3 Experimental arrangement of chloride migration test cell. 

Next, the slices were broken, and the depth of chloride penetration was determined as illustrated in Figure 4. ^{Andrade et al. (2000)}, comparing methods for determining the chloride diffusion coefficient in non-steady state using the accelerated chloride migration test, indicated that the ^{Luping and Nilsson (1992)} procedure presents better reproducibility of results, thus justifying the test method here presented.

Figure 4 Colorimetric method being applied.