Possibilities of using alizarin and neutral red indicators to determine the neutralized layer of concrete in the field

. The article concerns with the prospects of using the phenolphthalein test solution in the practice of surveying concrete and reinforced concrete building structures in planta. The problem of the study is the limits of application of the phenolphthalein test solution on the pH level of the determined concrete (the indicator works only at 8 ≤ rn ≤ 10). There is a need of expanding the phenolphthalein test solution application. In order to determine the different pH zones of concrete we have to modernize the method by adding other indicators to phenolphthalein. The standard phenolphthalein test solution does not allow a high degree of accuracy in determining the most sensitive to carbonation boundary zones of concrete. We proposed to modernize the phenolphthalein sample solution by using additional acid-base indicators alizarin and neutral red. We presented the results of experiments on measuring the surface neutralized layer of concrete by alcoholic solutions of acid-base indicators alizarin and neutral red on different age and size concrete samples. As a result, one of the proposed indicators (neutral red) allows to expand the phenolphthalein test solution application. We compared the results obtained by the traditional and modified methods. According to it, the proposed modified method is more accurate one.


Introduction
The areas of damage to the protective concrete layer are revealed during the inspection of reinforced concrete building structures more than 10-15 years old, operated in conditions of open air and atmospheric humidity [1,2]. Having less adhesion, these areas provide the penetration of atmospheric moisture and, consequently, the corrosion of the reinforcement of unconfined concrete.
The hydrogen pH value influences on the passivation properties of the protective concrete layer significantly. According to various studies, the concrete potential of hydrogen at loss of passivation is 8˂rH˂9.5 [2][3][4][5]. Loss of passivation of steel reinforcement to concrete begins at pH˂11 [4].
A neutralized concrete layer is formed by carbon dioxide and water, which are the negative components of carbonation and wetting. The process is long-term (takes decades) and involves the following reactions: (1) рН˃11 рН˃8 СаСО3↓ + СО2 +Н2О → Са(НСО3)2 (2) By reaction (1), the resulting calcium carbonate is a low-soluble compound, as a result, it bleeds on the concrete surface and pores in the form of salt [3,5]. As a consequence, there is a neutralization of the alkaline environment of the concrete. The excessive carbon dioxide in a humid environment (2) provides the formation of calcium hydrogen carbonate, a soluble compound, is able to be washed away by ambient moisture and removed away by atmospheric water [2][3][4][5].
The carbonation causes decreasing of the reinforced concrete structures durability, especially of supporting and bending elements, due to concrete pH decreasing. The carbonation also passivates steel reinforcement properties and reduces the cross-sectional area of the reinforcement, as a result of the formation of corrosion products [3,5]. It can be clearly seen on examining structures operated in a humid environment for a long time (Fig.  1a,b) [4]. Figure 1 clearly shows the concrete protective layer failing as a result of the concrete carbonation; the horizontal prestressed steel reinforcement is bared and covered with corrosion products. Nowadays, phenolphthalein test solution is the most widespread method for detecting carbonation and determining its depth in planta [3]. The single working pH-transition coloring interval of phenolphthalein solution, as well as the absence of consensus on concrete pH are their significant disadvantages. [4,5]Under these conditions concrete pH begins to lose its passivation properties relative to the steel reinforcement. But, in spite of the concrete carbonation studies, experts do not achieve a consensus on this issue [6][7][8].
The urgent tasks of the study are the modernization of the phenolphthalein test solution, increasing its ergonomical properties, creating alternative methods of detecting carbonation in planta along with the increasing of accuracy and cost-effectiveness when examining building structures. The availability of alternative methods will enhance the capacity of survey specialists in planta.
The most significant disadvantages of the phenolphthalein test solution are in the detection of carbonation in the initial and intermediate stages of the process [4,5]. It can be shown by the study of carbonation at pH˃10 and pH˂8 [4,5,9]. The operating range of the phenolphthalein solution is approximately 8≤p≤10.1, which initially defines the limits of its application. Thus, at the pH˃10 and pH˂8 range the method will not be valid. There will be no colour change in the indicator (colourless staining).
The purpose of the study is to modify the phenolphthalein test method by using additional solutions of acid-base indicators in order to expand the boundaries of application, accuracy and ergonomics.
The objective of the study is the theoretical and practical substantiation of the effectiveness of the introduction of solutions of neutral red and alizarin by comparing the technical and economic indicators of the modernized method in comparison with the typical one and the accuracy of work under given conditions.

Materials and methods
The objects of the study are 5 concrete cubic samples of 10x10x10 cm, strength grade of concrete is B30, tightness to water is W6 (Fig.2, a) and 10 fine grain concrete cubic samples of 3x3x3 cm of Portland cement CEM I 42,5 with water-cement ratio equal to 0.3 (Fig.2, b). The deviation in geometric dimensions of the samples did not exceed ±1.5 mm, the age of 10x10x10 cm samples was 3~5 years and that of 3x3x3 cm samples did not exceed 180 days. The different sample ages were chosen in order to determine the performance of the acidbase indicator solutions under study more accurately.
During experimental studies, preparation of acid-base indicator solution and sample preparation the following instruments and equipment were used: moisture meter di-elcometricTesto 606-1, portable electrometric pH-meter Testo 206-рН1 (certificate of State Register of the Russian Federation DE.C.31.010.A#43924/1), digital SLR camera with a fast shooting capability Canon 1200D, purified and distilled water carbonator Oursson OS1000SK, battery perforator DeWALT DCH133M1.
A new developed technique was used to create artificial conditions for the neutralisation of "free" calcium hydroxide (approximating the conditions of real structural operation) in order to create a concrete carbonation process and to identify areas of damaged concrete in laboratory conditions [5]. According to this technique, the laboratory samples were premoistened by distilled water and immersed into the open plastic vessel.  The plastic vessel was then placed in a closed, sealed, larger vessel. The active corrosive medium -distilled (purified) water saturated with carbon dioxide (carbonic acid solution) was in the vessel. The samples were kept in carbon dioxide for 28 days, after they were removed and placed directly into a carbonic acid solution for 120 days. The carbonic acid solution was changed every 2 days to maintain the correct pH level of the solution (the solution itself was also acidified) and the moisture content of the concrete samples was checked and controlled daily. The samples under study in an aggressive environment were 3 pcs (size 10x10x10 cm) and 6 pcs (size 3x3x3 cm).
At normal atmospheric pressure, carbon dioxide was released from the carbonic acid solution; its molecules were adsorbed into the pores of the concrete samples; slow process of neutralisation of "free" calcium hydroxide began in a humid environment by reactions (1) and (2). We realised the acceleration of the neutralisation of the concrete alkaline medium after the placement of the samples into the corrosive medium [15,16].
Samples 2 pcs (size 10x10x10 cm) and 4 pcs (size 3x3x3 cm) were kept in open air without access to carbonized corrosive medium to compare the values of the actual depth of the carbonized (neutralized) concrete layer.
We totally dried the samples after they were in the aggressive carbon dioxide environment for the specified period. In order to guarantee the samples dryness and provide their moister control, a portable moisture meter, operating by dielcometric method, was used [17,18].
A portable pH meter operating on the principle of electrometric pH determination was used to measure the pH value of an aggressive medium (carbonic acid solution) [19,20].
The dried samples were pre-treated by line -marking (Fig. 3, a). Samples (3x3x3 cm) were split along the median line (Fig.3, b), the actual depth of carbonized layer was determined with acid-base indicator solutions at the nick points (Fig.3, c, d). Samples 10x10x10 cm were pre-treated by line -marking, too. In the centre of the intersection of the centrelines, inspection hole with a diameter of d=24 mm and a depth of 36 mm were drilled (Fig.4,5). The holes were thoroughly cleaned of dust (by distilled water). The acid-base indicator solutions were applied to the cleaned holes. The characteristic colouring of the indicator solution was recorded with a camera at the carbonized layer locations. Alizarin and neutral red indicator solutions were prepared in accordance with the requirements for the preparation of alizarin-containing and other indicators according to national standards used for the determination of pH of various media [21][22][23]. 0.1 and 1% solutions of alizarin and neutral red in ethyl alcohol were used. Pure (non-technical) alcohol was used to prepare the solutions in order to avoid possible negative side effects on impurities of the technical product. Also 0.1% and 1% solutions of alizarin/neutral red in ethyl alcohol were applied to the concrete surface at the nicks after the first phenolphthalein solution applying.

Main part
The alcoholic alizarin solution has 3 colour transition intervals. It allows to define the boundaries of zones of high alkalinity with 12≥pH≥10.1 and those of neutralised and acidified concrete with pH˂7.5. According to theoretical data, the alizarin colour tint is darker. So the areas of carbonised concrete should be visible when the 2 indicators are alternately applied to the surface. Consequently, the use of this indicator solution in theory will increase the versatility of the phenolphthalein test solution.
The alcohol solution of neutral red has one colour transition interval, but towards the lower pH value of the concrete with 6.8≤pH˂8.0. The change of the indicator colouration is very distinct. It allows us to control the condition of the concrete surface with ease (Fig. 6). The actual depth of carbonation (neutralisation) was determined with acid-base indicator solutions by the areas of visible areas of characteristic staining (Fig. 7). The actual carbonation depth of [24][25][26] several concrete samples was also determined by using the previously known phenolphthalein method in order to compare the accuracy and limits of the methods. A 1% solution of phenolphthalein in ethyl alcohol was used.
A solution of an additional indicator (neutral red) allows you to expand the range of applicability of the phenolphthalein solution, since the color transition occurs at 6.5<pH<8. As a result, the ergonomics (convenience of use) of the phenolphthalein test method in the field increases, since the zones of highly carbonized concrete will be clearly and contrastingly defined (yellow color against a crimson background) compared to zones of normal concrete.  The expirimental results provide following conclusions: modernized phenolphthalein test solution has an increased accuracy (in some cases more than 8-10%) of detecting areas of carbonised concrete in compare with the conventional one; neutral red solution helps to extend the use of phenolphthalein test solution to determine the areas of concrete carbonation at 6 < pH < 10.5; Experimentally showed, it is difficult to use alizarin in addition to the phenolphthalein solution because of the implicit solution colouring (Fig. 7). The detection of strongly alkaline concrete areas is inaccurate (the phenolphthalein colouring overlaps the alizarin colouring).

Conclusions
Theoretically, both test indicators were identified as ideal for extending the scope of phenolphthalein test solution, but in practice the colouring of the alizarin solution proved to be rather dim both alkaline and neutralized concrete areas.
To summarize the experimental results, the phenolphthalein test solution has several disadvantages may hinder the work of surveyors in planta. There is a need to continue the search for indicators to replace phenolphthalein for these purposes.