METHOD TO REDUCE SCALING DUE TO FREEZING AND THAWING IN CONCRETE

Abstract: A concrete, more especially a non-reinforced concrete has at least a residual amount of sodium fluorophosphate which reduces scaling of the concrete, when the concrete is subjected to freezing and thawing. A method to reduce scaling of concrete due to freezing and thawing comprises contacting the concrete with a system containing at least sodium fluorophosphate.

Description: METHOD TO REDUCE SCALING DUE TO FREEZING AND THAWING IN CONCRETE TECHNICAL FIELD This invention relates to a method to reduce scaling due to freezing and thawing of concrete, particularly when a deicer, such as sodium chloride, is used to deice the surface of a concrete. BACKGROUND ART U.S. Patent 5,071,579, issued December 10, 1991, of Domtar Inc., describes the use of sodium fluorophosphate, also known as "sodium monofluorophosphate" and abbreviated as " FP", to prevent corrosion of reinforced concrete containing steel rebars when such concrete is exposed to a corrosive environment, for example, chloride ions. As described in the U.S. Patent establishment of galvanic corrosion cells at the steel rebars results in corrosion of the steel, with creation of expansive forces which destroy the concrete. Concrete is used in the construction of buildings and in the manufacture of articles including paving slabs, structural members, curbing, gutters, pipes and other cast articles. A particular problem with concrete is that it deteriorates when subjected to repeated cycles of freezing and thawing. The poor freeze/thaw durability of concrete is thought to be due to pressures generated by moisture movement through pores inherent in the concrete structure. The migration of the water in freeze/thaw cycles results in pressure build-up w thin the concrete structure, and the pressure is relieved by formation of fissures or fractures in the concrete, with consequent failure of the concrete. Failure of concrete due to freeze/thaw cycles is unrelated to failure due to corrosion of steel rebar reinforcement in concrete, and arises with both reinforced and non-reinforced concrete. DISCLOSURE OF THE INVENTION Broadly stated, the invention is directed to the reduction of scaling in concrete due to freezing and thawing by contacting the concrete, more especially a surface of the concrete, with a system containing sodium fluorophosphate. The invention is also directed to concrete having at least a residual amount of sodium fluorophosphate. DESCRIPTION OF PREFERRED EMBODIMENTS The concrete may be contacted with the system by numerous means, including but not limited to: laying, spraying, brushing, rolling, painting, soaking, immersing, impregnating or powdering; or by raising a dike or a dam around a concrete to treat a concrete slab with a solution of MFP; or by any other means of contacting a concrete surface with sodium fluorophosphat . The invention is preferably directed to a method of spraying concrete with an aqueous solution of sodium fluorophosphate. Preferably, the concrete is dried before contacting or treating it with a system containing aqueous sodium fluorophosphate, in accordance with the invention. When deicers for the concrete are needed, the system may include a deicer and particularly sodium chloride. The word "system" as employed herein contemplates the inclusion of solvents, deicers and carriers, that may be used with the sodium fluorophosphate in the treatment of the concrete. The invention has application to the treatment of concrete, including a concrete surface. The concrete may be a non-reinforced concrete or a reinforced concrete, for example, concrete reinforced with rebars, particularly steel rebars. Suitably the sodium monofluorophosphate is applied in an aqueous solution containing up to 35%, by weight, of the sodium monofluorophosphate, more especially 0.5 to 20%, by weight. BRIEF DESCRIPTION OF DRAWING The invention is illustrated in particular and preferred embodiments by reference to the accompanying drawings in which: Figure 1 is a plot of weight loss due to scaling in a concrete, against a number of freeze/thaw cycles for untreated concrete and concrete treated in accordance with the invention. EXAMPLES The following examples serve to illustrate the invention. Examples 1-6 The Cubes Concrete cubes or blocks were made by mixing one part of Portland cement concrete (type 10 or ASTM type 1, a general use cement), 0.52 parts of water, 2.25 parts of sand and 2.75 parts of aggregates having a particle size 4 and 20 mesh, said parts being parts by weight. The size distribution of the aggregates were as shown in Table I. The concrete was cast into cubes, and covered with plastic for 3 days, then cured for 28 days in distilled water. The concrete cubes were allowed to dry for a period of 24 hours: at room temperature for Examples 1 to 4, Table II; for Examples 5 and 6, Table III, the cubes were dried at elevated temperature by placing them in an oven at 50°C. The cubes were then treated as follows: Treatment with a Scaling Inhibitor The concrete cubes were immersed in sodium monofluorophosphate aqueous solution for a period of 4 hours, in order to allow for penetration of the solution into the concrete cubes. This treatment is defined herein throughout the disclosure and claims as a "MFP treatment cycle". The cubes were weighed before and after treatment. Before any other cycle, the cubes were left at room temperature for 24 hours. Freeze/Thaw Determination In order to determine the freeze/thaw characteristics of each cube, the following procedure was followed for concrete cubes subjected to the MFP treatment cycle and cubes which were not treated (for comparison purposes): The cubes, treated or not with sodium monofluorophosphate, were washed with water and allowed to dry on a board for 3 days or until their weight was constant. The cubes were then placed in a recipient containing a cellulose sponge. A 4% aqueous NaCl solution was added to half the height of the sponge. The recipient was then tightly closed and placed in a freezer during 16 to 18 hours at -10°C. The recipient was then kept at room temperature during 6 to 8 hours so that a "freeze/thaw cycle" was a day. Tests of 5 to 7 freeze/thaw cycles were conducted: the number of cycles was determined by a visual examination of the cubes i.e.until in appearance a certain amount had disappeared. The cubes were then washed with distilled water and dried for 24 hours to measure the weight loss and average the cube weight loss. The recipient was kept tightly closed during the freeze/thaw cycles. Table II illustrates the effect of sodium monofluorophosphate treatment on concrete blocks after treatment and drying at room temperature (R.T.) during a day. Tablβ II - Scaling of concretes, dried at R.T. and then treated with MFP Example No.: Sample A 1 2 3 4 MPF treatment None 1x5% 1x20% 3x20% 5x20% (cycle(s) x concentration) % of MPF re¬ tained, as deter¬ mined by weight gain 0 0 0.4 1.5 3.7 FREEZE/THAW TEST IN 4% NaCl SOLUTION Weight loss (%) 32 12 10 7 0 As is easily seen from Table II, the non- treated cubes (sample A) had a weight loss of 32% weight during about a week of treatment while in Example 1, which had been treated once, a 12% loss had occurred. In Example 2, where cubes had been treated with one cycle at 20% MFP, only 10% a weight loss of average had occurred. In Example 3, where cubes had been treated with 3 cycles of 20% MFP each cycle, a 7% average loss in weight of the cubes had occurred. In Example 4, where cubes had been treated with 5 cycles, at 20% MFP each cycle, the cubes of concrete lost no weight. Examples 5 and 6 The MFP was found to have a greater synergistic effect on concrete, when the concrete was dried, as is evident from Table III, where the drying was conducted at 50°C, with all other conditions remaining the same as in Examples 2 and 4, except that for the freeze/thaw determination, one group of the cubes were treated in water and another group in 4% NaCl solution. Table III illustrates that the freeze/thaw test showed absolutely no deterioration in water, demonstrating that MFP has no negative effect on the resistance of concrete to freeze/thaw scaling. Furthermore, even concrete cubes treated with only one cycle of MFP at a concentration of 20%, suffered no weight loss. Also the MPF concentration retained in the concrete, as determined by weight gain, was higher. As is shown in Table III, dried concrete absorbs a larger amount of MFP solution. The concrete, dried at this higher temperature absorbs about 30% more MFP, as is shown in Table III. Table III - Scalinσ of concrete, dried at 50°C. and Treated with MFP Example No. 5 6 MFP TREATMENT (eyelets) x concentration) 1x20% 5x20% % of MFP retained as determined by weight gain 0.5 5.9 Freeze/thaw test in water (weight loss %) 0 0 Freeze/thaw test in 4% NaCl solution (weight loss in %) Tables II and III demonstrate that higher MFP concentrations in the concrete produce a greater resistance to freeze/thaw scaling, if one compares the weight gain indicative of the % MFP retained versus the weight loss. Also, if one compares Table II against Table III, the weight gain which is the MFP retained, for a given concentration and cycle is higher when a concrete is dried at a higher temperature, demonstrating that the concrete acts as a sponge, drawing a greater amount of MFP solution, when it is dry. This also indicates that the contacting of the sodium fluorophosphate should preferably be conducted immediately following setting of the concrete in order to prevent any scaling. Examples 7-14 Concrete slabs were made in accordance with ASTM C-672 standard. The slabs were designed to have a low resistance to freezing and thawing and were subjected to impregnation with MFP in aqueous solution. The slabs were maintained at 40°C. for 24 hours before the first impregnation and at 23°C. and 50% relative humidity for 48 hours between subsequent impregnations. A first batch A of the slabs was subjected to two impregnations each of 5 minutes duration, and a second batch B of cubes was subjected to five impregnations each of 30 minutes. The MFP aqueous solution had a concentration of 20%, by weight, MFP. The slabs were stored at 23"C. and 50% relative humidity. The results are recorded in Table IV: Table IV Level of Impregnation of MFP as a Function of the Number of Impregnations Amount of Impregnation of MFP g of MFP solution/ -- concrete Impregnation Cycle Batch A 1 365 2 700 3 4 5 The impregnated slabs of batches A and B as well as similar slabs which were not treated were subjected to compressive strength and stress trials, 9 days and 3 months after the impregnation of batches A and B, after storage at 23°C. and 50% relative humidity. The compressive strength results after 9 days are recorded in Table V: The compressive strength results after 3 months are recorded in Table VI: Table VI The stress results after 9 days are recorded in Table VII: Table VII Example Comparison With Impregnation No. (MPa) (MPa) (MPa) 2 5 4.86 4.87 5.00 The stress results after 3 months are recorded in Table VIII: Table VIII Example Comparison ... With Impregnation No. (MPa) (MPa) (MPa) 2 5 5.94 6.46 6.42 Figure 3 shows graphically the weight loss of the concrete slabs. by scaling, with number of freeze/thaw cycles for concrete slabs of batches A and B, and similar slabs of a batch C not subjected to impregnation with MFP. These tests were conducted after 3 months storage at 23°C. and 50% relative humidity. The lowest weight loss is for batch B which were subjected to the 5 impregnations and the highest weight loss is for batch C not subjected to impregnation with MFP. Example 15 Concrete slabs were made as described in Examples 7 to 14. A batch D of the slabs was soaked in an MFP solution and a batch E was soaked in water. In each case the slabs were subjected to five soakings each for 10 days at 40°C, the slabs being dried, in a drying oven at 40°C. between each soaking. The resulting batches were subjected to freeze/thaw cycles and the weight loss resulting from soaking was recorded. The results are set out in Table IX. Table IX No. of Freeze/ Weight Loss (kg/m^) Thaw Cycles Batch D Batch E 0.05 0.2 0.76 ..1.7 2.3 2.8 3.3 3.6 Although the present invention has been explained hereinabove by way of preferred embodiments thereof, it should be pointed out that any modifications to these preferred embodiments, within the scope of the appended claims, is not deemed to change or alter the nature and scope of the invention.

Claims: CLAIMS 1. A method for reducing scaling of a concrete due to freezing and thawing, comprising: contacting the concrete with sodium fluorophosphate. 2. A method according to claim 1, wherein said sodium fluorophosphate is dissolved in an aqueous medium. 3. A method according to claim 1, wherein said sodium fluorophosphate is present in a system which comprises a deicer. 4. A method according to claim 1, wherein said sodium fluorophosphate is present in a system which comprises sodium chloride. 5. A method according to claim 1, wherein said contacting comprises spraying said concrete with an aqueous solution of sodium fluorophosphate. 6. A method according to claim 1, wherein said contacting comprises soaking said concrete with an aqueous solution of sodium fluorophosphate. 7. A method according to claim 1, 2, 3, 4, 5 or 6, wherein said concrete is non-reinforced concrete. 8. A method according to claim 1, 2, 3, 4, 5 or 6, wherein said concrete is a reinforced concrete having steel rebars therein. 9. A method according to claim 1, 2, 3 or 4, which comprises contacting a surface of the concrete with sodium monofluorophosphate. 10. A method according to claim 2, which includes a step of drying said concrete to increase the capacity of said concrete to receive said sodium fluorophosphate, prior to contacting the concrete with sodium fluorophosphate. 11. A method according to claim 1, wherein said contacting is carried out immediately following setting of said concrete. 12. A method according to claim 1, wherein said solution contains up to 35%, by weight, sodium monofluorophosphate. 13. A method according to claim 2, wherein said solution contains from 0.5 to 20%, by weight, sodium monofluorophosphate. 14. A method according to claim 1, wherein said sodium fluorophosphate is sprayed on said concrete. 15. A method according to claim 1, wherein said sodium fluorophosphate is brushed on said concrete. 16. A method according to claim 1, wherein said concrete is soaked with said sodium fluorophosphate. 17. A method according to claim 1, wherein said concrete is immersed in said sodium fluorophosphate. 18. A method according to claim 1, wherein said concrete is powdered with said sodium fluorophosphate. 19. A method according to claim 16, wherein the immersion is obtained by placing a dike around . said concrete 20. A non-reinforced concrete having a concrete surface with at least a residual amount of sodium fluorophosphate. 21. A non-reinforced concrete having a concrete surface with at least a residual amount of sodium fluorophosphate, as obtained by the method of claim 1, 2,, 3, 4, 5, 6, 10, 11, 12, 13, 14, 15, 16, 17, 18 or 19. 22. Use of sodium monofluorophosphate for reducing scaling of a concrete when the concrete is subjected to freezing and thawing.

Inventors: BEAUDOIN REJEAN, BERTHELOT CHANTAL, MALRIC BERNARD, BEAUDOIN, REJEAN, BERTHELOT, CHANTAL, MALRIC, BERNARD
Applicants: DOMTAR INC, DOMTAR INC.
Application year: 1992
Main classification: C23F11
IPC codes: C04B41, C09K3