Abstract:
A method using concentrated aqueous solutions of sodium cobaltinitrite and a rhodamine dye is described which can be used to identify concrete that contains gels formed by the alkali-silica reaction (ASR), and to identify degraded concrete which results in a porous or semi-permeable paste due to carbonation or leaching. These solutions present little health or environmental risk, are readily applied, and rapidly discriminate between two chemically distinct gels; K-rich, Na--K--Ca--Si gels are identified by yellow staining, and alkali-poor, Ca--Si gels are identified by pink staining.

Description:
This application is a continuation-in-part of copending application Ser. No. 08/745,118, which was filed on Nov. 07, 1996, and issued as U.S. Pat. No. 5,739,035 on Apr. 14, 1998. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to detection of structural weaknesses in concrete and, more particularly, to a method using sodium cobaltinitrite and a rhodamine dye for identifying gel formation in concrete from the alkali-silica reaction (ASR) and for identifying concrete degradation which results in a porous or semi-permeable paste due to carbonation or leaching. The invention was made with government support under Contract No. W-7405-ENG-36 awarded by the U.S. Department of Energy to the Regents of The University of California. The government has certain rights in the invention. 
     BACKGROUND OF THE INVENTION 
     The alkali-silica reaction (ASR) is a primary cause of premature degradation of concrete structures, particularly for highways, bridgedecks, runways, and sidewalks. Such degradation occurs throughout the world; however, some regions have more frequent occurrences. The reaction occurs between concrete pore water, some aggregate constituents (e.g., poorly crystalline silica phases), and alkali cations (Na and K) released by the cement and/or aggregate. As the alkali content of the pore water increases, dissolution of the silica phases increases, resulting in a release of silica to form the gel. Gel formation results in a volume increase, which causes internal pressure in the concrete and eventually forms fractures. This process can be exacerbated by freeze-thaw cycles and by salting of road surfaces. Thus, ASR products found in concrete may be understood as gelatinous silica, and its crystalline modifications, having adsorbed sodium, potassium and calcium ions. 
     Important steps in the control of ASR include the recognition of potentially reactive aggregates and early diagnosis of ASR-affected structures. For these and other reasons, it is important to identify the presence of ASR gel in a structure as early as possible. Current approaches to the identification of ASR in concrete rely heavily on petrographic analyses that are expensive and time-consuming (often requiring days or weeks before an answer is obtained). 
     Recently, the Federal Highways Administration (FWHA) endorsed a staining procedure that was initially proposed in &#34;In Situ Identification Of ASR Products In Concrete&#34; by K. Natesaiyer and K. C. Hover, Cement and Concrete Research 18, 455 (1988) and subsequently tested and refined as part of the Strategic Highways Research Program (SHRP), in &#34;Alkali-Silica Reactivity: An Overview Of Research&#34; by Richard Helmuth et al., SHRP-C-342, National Research Council, Washington, D.C. (1993). This procedure uses uranyl acetate  (UO 2 ) ++  (C 4  H 6  O 4 ) =  ! to treat the concrete surface, whereupon the uranyl ion, (UO 2 ) ++ , is adsorbed in areas where silica gel is present, replacing previously adsorbed Na + , K +   or Ca ++   ions and may be detected by its yellow-green fluorescence under ultraviolet (UV) light. 
     The uranyl-acetate technique presents several disadvantages. Although the amounts of uranyl acetate employed are small, the technique results in a potential exposure to uranium which is hazardous as both a heavy metal and a radioactive element. Additionally, liquid and solid waste generated by this process is contaminated with uranium, so disposal as mixed waste is costly. Due to the small amounts of uranium used in a typical test, disposal of the materials in the sewer system is attractive. However, this approach unnecessarily adds to system contamination. The use of the uranyl-acetate method also requires a source of UV radiation and a light-tight enclosure for viewing the fluorescence generated by adsorbed uranyl species, making the application of this procedure difficult in a field situation where the use of light-tight enclosures limits the area under investigation, and rendering the observation of a typical road surface tedious and slow. 
     A method for differentially staining K-feldspar (yellow) and plagioclase feldspar (red) on slab surfaces or uncovered thin sections is described in &#34;Selective Staining of K-Feldspars And Plagioclase On Rock Slabs And Thin Sections,&#34; by Edgar H. Bailey and Rollin E. Stevens in Am. Mineral. 45, 1020 (1960). K-feldspar is stained yellow using sodium cobaltinitrite according to the method described in &#34;A Staining Method For the Quantitative Determination Of Certain Rock Minerals,&#34; by A. Gabriel and E. P. Cox in Am. Mineral. 14, 290 (1929). The feldspar is first treated by etching the surface to be stained with hydrofluoric acid vapor to form etch residues, since such residues are subsequently stained, whereas the feldspar itself is not visibly stained. Residual potassium from the K-feldspar reacts with the sodium cobaltinitrite to form yellow potassium cobaltinitrite. Staining of plagioclase is accomplished by replacing the calcium ion in the etched feldspar with barium from barium chloride solution, which reacts with the rhodizonate reagent subsequently applied to form red insoluble barium rhodizonate. 
     Accordingly, it is an object of the present invention to provide a selective process for identifying concrete containing gels formed by the alkali-silica reaction. 
     Another object of the invention is to provide a selective process for identifying concrete containing gels formed by the alkali-silica reaction that uses environmentally benign chemicals. 
     Yet another object of the present invention is to provide a selective process for identifying concrete containing gels formed by the alkali-silica reaction that uses environmentally benign chemicals and does not require chemical pretreatment of the concrete. 
     Still another object of the invention is to provide a selective process for identifying concrete containing gels formed by the alkali-silica reaction that uses environmentally benign chemicals, that does not require chemical pretreatment of the concrete, and that does not require a fluorescence-excitation light source. 
     A further object of the invention is to provide a selective process for identifying concrete containing calcium-rich gels formed by the alkali-silica reaction. 
     Yet a further object of the present invention to provide a selective process for identifying degradation of concrete which results in a porous or semi-permeable paste due to carbonation or leaching. 
     Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims. 
     SUMMARY OF THE INVENTION 
     To achieve the foregoing and other objects, and in accordance with the purposes of the present invention, as embodied and broadly described herein, the method for identifying concrete containing gels formed by the alkali-silica reaction hereof includes the steps of: contacting the surface of the concrete under investigation with a concentrated aqueous solution of sodium cobaltinitrite for a chosen period of time; contacting the surface of the concrete under investigation with a concentrated aqueous solution of a rhodamine dye for a chosen period of time; rinsing the concrete so treated with water; and searching the rinsed surface of the concrete for regions of yellow staining and for regions of pink staining, whereby K-rich, Na--K--Ca--Si gels generated from the alkali-silica reaction are identified by yellow staining and alkali-poor, Ca--Si gels generated from the alkali-silica reaction are identified by pink staining. 
     Preferably, the step of contacting the surface of the concrete under investigation with a concentrated aqueous solution of sodium cobaltinitrite for a chosen period of time occurs before the step of contacting the surface of the concrete under investigation with a concentrated aqueous solution of the rhodamine dye for a chosen period of time. 
     It is also preferred that the concrete under investigation be permitted to dry after the application of the solutions before viewing the staining. 
     Preferably also, rhodamine B base is employed as the rhodamine dye. 
     Benefits and advantages of the present invention include selectivity for two types of gels produced by the alkali-silica reaction, the use of environmentally benign chemicals, the absence of a requirement for chemical pretreatment of the concrete, and the absence of a requirement for a fluorescence-excitation light source. Additionally, degradation of concrete which results from carbonation or leaching may be detected. 
     DETAILED DESCRIPTION 
     Briefly, the present invention includes the sequential application of solutions of each of two water soluble compounds to the concrete under investigation. Concentrated solutions of sodium cobaltinitrite and a rhodamine dye such as rhodamine B base, rhodamine B, and rhodamine 6G, in water are prepared. The exact amounts of solutes are unimportant, but best results were obtained when the final solutions have undissolved solid. The concrete surface to be examined is treated by pre-rinsing with water and subsequently applying each solution to the surface. After 30-60 seconds, the concrete is rinsed thoroughly with water. The treated surface will show yellow and pink regions where ASR gel is present. Yellow regions indicate the presence of K-rich, Na--K--Ca--Si gels, while pink regions indicate alkali-poor gels. The final rinse step is required, since the yellow sodium cobaltinitrite solution will coat the entire concrete surface as will the pink rhodamine dye solution, thereby obscuring the stained gel regions. Best results are obtained when the sample is treated first with the sodium cobaltinitrite solution; however, the application of the rhodamine dye solution first gives adequate results. Moreover, although solutions containing both sodium cobaltinitrite and rhodamine dye were found to selectively identify K-rich and alkali-poor gels, respectively, the intensity of the staining appears to be reduced from that produced when the individual solutions are used. The visibility of the colors improves as the concrete dries, and a previously treated sample may be re-treated with either solution to enhance the intensity of any staining already present. It has been observed that the staining intensity fades over the course of days to weeks. However, restaining the sample according to the method of the present invention restores the original intensity. 
     In the parent patent, the efficacy of rhodamine B for identifying Ca-rich, susceptible materials, primarily Ca-rich forms of ASR gel, was disclosed. More recently, however, it has been discovered that in some situations rhodamine B also stains regions of concrete that are not affected by ARS, including areas of carbonation and areas of general paste deterioration. It has also recently been found that rhodamine 6G, as an alternative cationic dye, and rhodamine B base as a neutral species dye can be used, the latter dye having been found not to interfere with the action of Na-cobaltinitrite, while being a more specific indicator of the presence of Ca-rich ASR gel. 
     Thus, a variety of rhodamine compounds may be utilized with Na-cobaltinitrite to effectively document the presence of ASR. Since Na-cobaltinitrite is the primary indicator of ASR-generated gel, any stain that does not interfere with Na-cobaltinitrite may be used in a staining methodology to illustrate ASR gel, and different rhodamine compounds show different specificity for Ca-rich ASR gel. Rhodamine B base shows less tendency to stain non-ASR materials (e.g., areas of carbonation), and is therefore preferred in a methodology for identifying ASR. Rhodamine 6G was the least effective and in some situations was found to interfere with the action of Na-cobaltinitrite. 
     As currently understood by the inventors of the subject technology, the staining mechanisms of Na-cobaltinitrite and rhodamine compounds in concrete are different. 
     Na-cobaltinitrite solution is commonly used as an analytical reagent for potassium because K-cobaltinitrite is much less soluble than Na-cobaltinitrite. Hence, a small amount of K +   introduced into a solution saturated with Na-cobaltinitrite causes a yellow precipitate of K-cobaltinitrite to form. ASR gels become yellow (that is, become coated with K-cobaltinitrite precipitate) following exposure to saturated Na-cobaltinitrite solution for two reasons: first, ASR gels are typically in enriched in K +  ; and second, ASR gels can cation exchange, allowing the K +   to be released into solution. There are few materials in concrete that are likely to possess both of those characteristics (high K content and the ability to cation exchange), which makes the Na-cobaltinitrite stain especially diagnostic for ASR. Normal cement paste has too low a K concentration to allow it to be stained significantly by a saturated solution of Na-cobaltinitrite. Potassium-rich minerals that might be present in the aggregate (e.g., feldspars or micas) generally do not have the cation-exchange characteristics that would allow K +   to be released into solution; hence, they are not stained by a saturated solution of Na-cobaltinitrite as applied according to this procedure. Minerals that are exceptions to this include some clay minerals and some zeolites; however, these are not likely to be confused with ASR gel for a number of reasons, including (1) they will become K-poor early in the setting of the concrete by exchange with Ca 2+   in the initial pore fluids; and (2) the distribution of these minerals in the aggregate is unlikely to mimic the distribution of ASR gel. 
     The rhodamine compounds work by bonding to susceptible materials and are not primarily cation-specific stains. However in combination with Na-cobaltinitrite, they can be used to identify calcium-rich forms of ASR gel as well as other forms of deteriorated cement paste because of their ability to stain susceptible (i.e., porous and/or semi-permeable) materials. Rhodamine B and rhodamine 6G are basic dyes, that is, they act primarily as a cationic species. Rhodamine B base, however, is classified as a solvent dye, which means that it is a neutral species and works by dissolving into porous materials. 
    
    
     Having generally described the invention, the following examples illustrate more detailed characteristics thereof. 
     EXAMPLE 1 
     A concrete chip was broken off from a concrete core. The chip was placed in a beaker containing a concentrated sodium cobaltinitrite solution in water for approximately 1 min., removed, and subsequently rinsed with water. The chip was then placed in a beaker containing rhodamine B solution for approximately 1 min., removed, and rinsed with water. After drying with an air gun, the concrete chip exhibited a pink hue over large regions of the paste surface and a yellow hue in isolated pockets and in rings circling some aggregate particles. Upon examination using a binocular microscope, it was apparent that both the pink and yellow regions involved gels which showed shrinkage cracking. Examples of both gels were extracted from the concrete and prepared for examination by scanning electron microscopy which showed Na, Si, K, Ca, and Co in the yellow stained gel (the Co resulting from the sodium cobaltinitrite), and Si and Ca in the pink stained gel. Since scanning electron microscopy cannot detect hydrogen and the detector employed does not detect oxygen or nitrogen, the elements set forth in the previous sentence were the only elements observed. The concrete sample was also examined petrographically in order to verify the presence of ASR gel the location of which was found to be consistent with the staining results. 
     EXAMPLE 2 
     A concrete chip was broken from a concrete core. The chip was placed in a beaker containing a concentrated solution of sodium cobaltinitrite in water for approximately 1 min., removed, and subsequently rinsed with water. The chip was then placed in a beaker of rhodamine B solution for about 1 minute, removed and subsequently rinsed with water. After drying with an air gun, the concrete chip exhibited a yellow hue in isolated pockets and in rings circling some aggregate particles. No regions of pink staining were observed. Upon examination using a binocular microscope, it was apparent that both the yellow regions contained a gel that showed shrinkage cracking. 
     EXAMPLE 3 
     A core was taken from a concrete structure apparently affected by ASR. The core was treated immediately in the field using the procedure set forth hereinabove, and was found to test positive for yellow ASR gel but negative for pink ASR gel. The core was subsequently examined petrographically to confirm the presence of ASR gel, the location of which was consistent with the staining results. 
     EXAMPLE 4 
     A core was removed form a section of road showing no apparent signs of significant distress. The core was treated in the field using the method of the present invention, but showed no signs of yellow or pink gel. Yellow and pink patches were observed at locations where gravel has become dislodged from the concrete surface. 
     EXAMPLE 5 
     A core was removed from a concrete structure apparently affected by ASR. The core was treated using the method of the present invention except that a single solution was prepared which contained both sodium cobaltinitrite and rhodamine B. The treated core showed both yellow and pink gel following rinsing. However, the intensity of the staining appeared to be less dramatic than that which results from sequential staining using individual concentrated solutions. 
     EXAMPLE 6 
     A concrete core was taken from a curb having extensive ASR, identified from previous petrographic studies. The core was broken in three pieces. One piece was treated using the procedure described in EXAMPLE 1 hereof. The other two pieces were treated according to the method set forth hereinbelow. The detailed process involved the application of a concentrated solution of Na-cobaltinitrite to each piece using an eyedropper and allowing to stand for about 30 seconds. Each piece was then rinsed thoroughly with water. Each of the pieces was then stained with a different rhodamine compound: rhodamine B, rhodamine B base, and rhodamine 6G. In each case, a concentrated solution of the rhodamine compound was applied using an eyedropper and allowed to stand for about 30 seconds. Each of the pieces was then thoroughly rinsed with water. The pieces were then allowed to dry. Upon visual examination, all three samples showed distinct regions stained yellow by Na-cobaltinitrite that included aggregates and air voids filled with white material. These areas were confirmed as containing ASR gel by petrographic examination and by the comparison with results set forth hereinabove. The latter results also demonstrated that a K-rich form of the ASR gel was identified with these methods. All three pieces also showed pink-staining by the rhodamine compounds. However, the amount of pink-staining was not the same. The method disclosed hereinabove involving the use of rhodamine B showed the most extensive amount of pink-staining. Areas of gel with desiccation cracks were clearly identified. As previously disclosed, these were Ca-rich forms of ASR gel. Similarly, the method using rhodamine B base identified Ca-rich forms of ASR gel that were adjacent to aggregates, existed along fractures, or were present in the concrete matrix. The pink-staining due to rhodamine B base was less intense in color than that of rhodamine B. The rhodamine 6G did not stain Ca-rich ASR gel. It did stain areas within the aggregates containing ASR gel. The pink staining had a different pattern than that observed for rhodamine B and rhodamine B base and was present in much less abundance. In summary, all three methods (the previously disclosed method and methods based on rhodamine B base and rhodamine 6G) all identified the presence of K-rich forms of ASR gel as distinctly stained yellow regions. Rhodamine B and rhodamine B base were equally able to identify Ca-rich forms of ASR gel. Rhodamine 6G did not effectively identify CA-rich forms of ASR gel. 
     EXAMPLE 7 
     A concrete core was taken from a pavement with minor evidence of ASR. The coarse aggregate was predominantly a limestone. The core was split in two places using a hydraulic press, producing surfaces for study. One surface was treated with the procedure described in EXAMPLE 1 hereof. Two other surfaces were treated according to the methods described in this continuation. The detailed process involved the application of a concentrated solution of Na-cobaltinitrite to each surface using an eyedropper and allowing to stand for about 30 seconds. Each surface was then rinsed thoroughly with water. Each of the surfaces was then stained with a different rhodamine compound: rhodamine B, rhodamine B base, and rhodamine 6G. In each case, a concentrated solution of the rhodamine compound was applied using an eyedropper and allowed to stand for about 30 seconds. Each of the surfaces was then thoroughly rinsed with water. The surfaces were then allowed to dry. All three methods identified the presence of minor K-rich ASR gel in the form of yellow (Na-cobaltinitrite) staining of fine-aggregate particles most of which were shale particles. In the case of the method using rhodamine B base, there was also evidence for minor K-rich ASR gel in the concrete matrix as documented by yellow staining adjacent to areas with abundant yellow-stained aggregates. The pink-staining was distinctly different for each of the rhodamine compounds. The rhodamine B stained much of the surface with a uniform pink color and was not indicative of ASR gel. Petrographic examination demonstrated in this case carbonated or deteriorated concrete paste was stained by rhodamine B. In contrast, rhodamine B base did not significantly stain the cement paste and imparted only a weak pink color to some of the limestone aggregates. The rhodamine 6G method stained the surface in a blotchy pink that did not reflect the presence of ASR gel. In summary, all three methods correctly identified the presence of K-rich ASR gel. Both rhodamine B and rhodamine 6G gave false indications of the presence of a Ca-rich form of ASR gel. However, rhodamine B base provided an accurate indication of very minor Ca-rich ASR gel. 
     EXAMPLE 8 
     A concrete core was taken from a bridge deck and was suspected of having ASR. The core was split twice using a hydraulic press to create fresh surfaces. One surface was tested using the method describe in EXAMPLE 1 hereof. The other two surfaces were tested according to the method described in this continuation. The detailed process involved the application of a concentrated solution of Na-cobaltinitrite to each surface using an eyedropper and allowing to stand for about 30 seconds. Each surface was then rinsed thoroughly with water. Each of the surfaces was then stained with a different rhodamine compound: rhodamine B, rhodamine B base, and rhodamine 6G. In each case, a concentrated solution of the rhodamine compound was applied using an eyedropper and allowed to stand for about 30 seconds. Each of the pieces was then thoroughly rinsed with water. The surfaces were then allowed to dry. All three staining methods identified significant amounts of K-rich ASR gel as yellow-stained regions, particularly covering aggregate surfaces. The staining from rhodamine B was modest in extent and identified CA-rich forms of ASR gel and some areas of cement paste without ASR gel. The rhodamine B base stained the surface less intensely than rhodamine B. Almost all of the material identified by rhodamine B base was Ca-rich ASR gel, although minor regions of cement paste free of ASR gel were also stained. The rhodamine 6G, like rhodamine B base, showed a restricted degree of staining. However, rhodamine G stained areas possessing K-rich ASR gel resulted in a combined yellow and pink staining of such regions. Although this did not prevent identification of K-rich ASR gel, it represents an interference with the action of the Na-cobaltinitrite. The rhodamine 6G also identified CA-rich ASR gel and minor regions of deteriorated or porous cement paste. The foregoing description of the invention has been presented for purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously many modifications and variations are possible in light of the above teaching. For example, it would be apparent to one having ordinary skill in the art, after having studied the subject disclosure that the present method could be utilized to detect degradation of concrete which results in a porous or semi-permeable paste due to carbonation or leaching. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.