Concrete composition and molded product thereof

Abstract

To provide a mortar or concrete composition that has short fiber and is excellent in freeze-thaw resistance and its molding.SOLUTION: A mortar or concrete composition containing a binder containing cement, fine aggregate, short fiber, water and thickener, in which the freeze-thaw resistance by the predetermined freeze-thaw cycle in a mortar or concrete test sample prepared from the mortar or concrete composition, due to freeze-thaw test based on the freeze-thaw test method described in JIS A 1148, is excellent than the freeze-thaw resistance according to the predetermined freeze-thaw cycle in the mortar or concrete test sample prepared from the mortar or concrete composition different only in not containing the thickener from the mortar or concrete composition.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a mortar or concrete composition containing short fibers and a molded article thereof. [Background technology]

[0002] There are approximately 730,000 road bridges in Japan that are 2 meters or longer, of which approximately 25% are currently bridges that have been in operation for more than 50 years, and this will reach approximately 39% by 2023 (see, for example, Non-Patent Document 1). Regarding expressways, since the opening of the Ritto IC to Amagasaki IC section of the Meishin Expressway in 1963, the current expressway network stretches to approximately 9,000 km, with bridges that have been in operation for more than 30 years accounting for approximately 40% of the total. Since the time of construction, the volume of large vehicle traffic on all bridges has increased, and the total vehicle weight is also on the rise, which, combined with the aging mentioned above, is a factor that is exacerbating the deterioration of bridges.

[0003] Against this backdrop, construction work to replace aging reinforced concrete decks on highway bridges is becoming increasingly common. In these cases, prestressed concrete (PC) decks are often used to minimize the increase in the deck weight borne by the existing steel girders and to withstand increased design loads compared to the original bridge construction. Furthermore, to shorten the road closure period required for the replacement work, precast (PCa) PC decks are often constructed on-site. Depending on the crossing conditions under the bridge, measures to prevent concrete spalling are required for PC decks. Recently, while triaxial aramid mesh sheets are embedded in the concrete surface, concrete containing synthetic short fibers (short fiber concrete) is increasingly being used to improve productivity at manufacturing plants.

[0004] Short fiber concrete has traditionally been widely used in concrete structures, and while previous research has often shown that its freeze-thaw resistance is equivalent to that of concrete without short fiber (see Non-Patent Document 2), there have also been reports that it is sometimes inferior (see Non-Patent Document 3).

[0005] On the other hand, it is also known that the addition of a viscosity improver reduces the freeze-thaw resistance of concrete (see, for example, Non-Patent Document 4). [Prior art documents] [Non-patent literature]

[0006] [Non-Patent Document 1] Ministry of Land, Infrastructure, Transport and Tourism (MLIT): Part II, Chapter 2, Section 2, Measures to address aging of social capital, 2019 Edition, Land, Infrastructure, Transport and Tourism White Paper, p.110, 2019 [Non-patent document 2] Masuo Yabuki: Study on the effect of preventing spalling of polypropylene fiber for concrete reinforcement, Doctoral dissertation, Okayama University, pp.37-39, 2004 [Non-patent document 3] Yuki Yoshida, Fumio Taguchi, Isao Yamazaki: Compressive Strength and Freezing-Thawing Resistance of Polypropylene Reinforced Fiber Concrete, Monthly Report of the Public Works Research Institute of Hokkaido, No. 608, 2004 [Non-patent document 4] Yuji Sudo, Koichi Ayuda, Haruya Sahara, Haruyuki Takeshita: Fundamental Study on the Frost Resistance of High-Fluidity Concrete with Viscosity Agent, Proceedings of the Japan Concrete Institute Annual Conference, Vol. 14, No. 1, pp. 1003-1008, 1992 Summary of the Invention [Problem to be solved by the invention]

[0007] In factory-made products such as PC floor panels, which require a shorter manufacturing process, high-early-strength Portland cement is often used in combination with steam curing. However, the effect of these mix proportions and manufacturing methods on the freeze-thaw resistance of short-fiber concrete is unknown.

[0008] Therefore, the present inventor conducted a freeze-thaw test using salt water on concrete for PCaPC products containing short polypropylene (hereinafter referred to as PP) fibers, and found that even when 3% or more entrained air was mixed in, the freeze-thaw resistance of the concrete containing short PP fibers was inferior to that of concrete without short fibers, and that this was presumably due to excess water trapped in the concrete by the short fibers, and that adding a thickener was an effective way to improve freeze-thaw resistance. Based on the above findings, the present inventor arrived at the following invention.

[0009] The present invention has been made in view of the above, and an object of the present invention is to provide a mortar or concrete composition containing short fibers and having excellent freeze-thaw resistance, and a molded article thereof. [Means for solving the problem]

[0010] In order to solve the above-mentioned problems and achieve the object, the mortar or composition for concrete according to the present invention is a mortar or composition for concrete comprising a binder containing cement, fine aggregate, short fibers, water, and a thickener, characterized in that the freeze-thaw resistance of a mortar or concrete specimen made from the mortar or composition for concrete in a predetermined freeze-thaw cycle, as determined by a freeze-thaw test based on the freeze-thaw test method described in JIS A 1148, is superior to the freeze-thaw resistance of a mortar or concrete specimen made from a mortar or composition for concrete that differs from the mortar or composition for concrete only in that it does not contain a thickener, in the predetermined freeze-thaw cycle.

[0011] In addition, another mortar or concrete composition according to the present invention is characterized in that, in the above-mentioned invention, the index representing the freeze-thaw resistance is a relative dynamic modulus of elasticity, a durability index, a mass reduction rate, or a compressive strength ratio.

[0012] Another mortar or concrete composition according to the present invention is characterized in that, in the above-mentioned invention, the short fibers are polypropylene short fibers.

[0013] Another composition for mortar or concrete according to the present invention is characterized in that in the above-mentioned invention, the fineness of the short fibers is 30 to 2000 dt.

[0014] The molded article according to the present invention is a molded article obtained by molding the above-mentioned mortar or concrete composition. [Effects of the Invention]

[0015] The composition for mortar or concrete according to the present invention has the effect that the freeze-thaw resistance can be improved by adding a thickener even to mortar or concrete containing short fibers. [Brief explanation of the drawings]

[0016] [Figure 1] FIG. 1 is a table showing the mix proportions of the concrete used in the freeze-thaw test. [Figure 2] FIG. 2 is a table showing the mix proportions of the concrete used in the bleeding test. [Figure 3] Figure 3 is a photograph showing the two types of PP short fibers used in the test: (a) 2,000dt-30mm, and (b) 30dt-12mm. [Figure 4] FIG. 4 is a diagram showing the effect of air content on the freeze-thaw resistance of short fiber concrete. [Figure 5] FIG. 5 is a diagram showing the effect of adding short PP fibers on freeze-thaw resistance (relative dynamic modulus of elasticity of rectangular column specimens). [Figure 6] FIG. 6 is a diagram showing the effect of adding short PP fibers on freeze-thaw resistance (mass reduction rate of prismatic specimens). [Figure 7] FIG. 7 is a diagram showing the effect of adding short PP fibers on bleeding. [Figure 8] FIG. 8 is a photograph showing an enlarged image from an internal flaw detection test. [Figure 9] FIG. 9 is a diagram showing the effect of adding short fibers on freeze-thaw resistance (relative dynamic modulus of elasticity of cylindrical specimens). [Figure 10] FIG. 10 is a diagram showing the effect of adding short PP fibers on compressive strength. [Figure 11] FIG. 11 is a graph showing the freeze-thaw resistance (relative dynamic modulus of elasticity of a prismatic specimen) when a thickener is added. [Figure 12] FIG. 12 is a graph showing the freeze-thaw resistance (mass loss rate of a prismatic specimen) when a thickener is added. [Figure 13] FIG. 13 is a graph showing the freeze-thaw resistance (relative dynamic modulus of elasticity of cylindrical specimens) when a thickener is added. [Figure 14] FIG. 14 is a graph showing the change in compressive strength when a thickener is added. [Figure 15] Figure 15 shows the appearance of the rectangular column specimens at the end of the freeze-thaw test, where (1) shows the case without the addition of a thickener and (2) shows the case with the addition of a thickener. DETAILED DESCRIPTION OF THE INVENTION

[0017] Hereinafter, embodiments of a mortar or concrete composition and a molded article thereof according to the present invention will be described in detail with reference to the drawings, although the present invention is not limited to these embodiments.

[0018] A mortar or concrete composition according to an embodiment of the present invention is a mortar or concrete composition comprising a cement-containing binder, fine aggregate, short fibers, water, and a thickener, and the freeze-thaw resistance of a mortar or concrete specimen made from the mortar or concrete composition over a predetermined number of freeze-thaw cycles (e.g., 300 cycles) as determined by a freeze-thaw test based on the freeze-thaw test method described in JIS A 1148 is superior to the freeze-thaw resistance of a mortar or concrete specimen made from a mortar or concrete composition that differs from the mortar or concrete composition only in that it does not contain a thickener. Here, freeze-thaw resistance can be expressed by indices such as the relative dynamic modulus of elasticity, durability index, mass loss rate, and compressive strength ratio.

[0019] Furthermore, the freeze-thaw resistance of this embodiment is superior to the freeze-thaw resistance of mortar or concrete specimens made from a mortar or concrete composition that differs from the above-mentioned mortar or concrete composition only in that it does not contain a thickener.

[0020] Examples of cement that can be used include ordinary Portland cement, high-early-strength Portland cement, extra-high-early-strength Portland cement, moderate-heat Portland cement, and low-heat Portland cement. The binder may contain materials other than cement as long as the effects of the present invention are not impaired.

[0021] The fine aggregate may be, for example, crushed hard sandstone. The coarse aggregate may be, for example, crushed hard sandstone. The fine aggregate and coarse aggregate may contain other materials as long as the effects of the present invention are not impaired.

[0022] The short fibers may be, for example, polypropylene short fibers (PP short fibers), but other short fibers may also be used. When using PP short fibers, for example, those with a fineness of about 30 dt to 2,000 dt may be used. Furthermore, for example, when using PP short fibers with a fineness of 2,000 dt and a length of 30 mm, the PP short fibers may be added at a volume ratio of about 0.5% based on the volume of the mortar or concrete composition. When using PP short fibers with a fineness of 30 dt and a length of 12 mm, the PP short fibers may be added at a volume ratio of about 0.05% based on the volume of the mortar or concrete composition.

[0023] The thickener may be, for example, one containing an alkyl aryl sulfonate and an alkyl ammonium salt as its main components. However, other thickeners may also be used as long as they do not impair the effects of the present invention.

[0024] Among the compositions for mortar or concrete, the composition for concrete usually further contains coarse aggregate, and the mortar or concrete composition hardens to obtain mortar or concrete. The amount of water (W) used in the composition for mortar or concrete is preferably such that the mass ratio (W / B) of water (W) to binder (B) is 0.25 to 0.40, that is, 25 to 40 parts by mass of water (W) per 100 parts by mass of binder (B).

[0025] The mortar or concrete composition of the present embodiment may further contain other components, such as chemical admixtures such as high-performance water-reducing agents and air-entraining agents, as long as the effects of the present invention are not impaired.

[0026] The mortar or concrete composition of this embodiment has excellent freeze-thaw resistance. Therefore, it is particularly effective for construction of buildings and other structures that require freeze-thaw resistance, and for locations such as highways and bridges in mountainous areas where snow-melting agents are sprayed in winter. In addition to these applications, it is also suitable for use in locations that require freeze-thaw resistance, such as coastal structures, marine structures, waterway structures, road structures, and retaining wall structures in cold regions. In this case, the mortar or concrete composition of this embodiment may be molded in advance and applied as a molded product, or the mortar or concrete composition may be used as a repair material to repair a mortar or concrete surface.

[0027] When a molded product is produced using the mortar or concrete composition of this embodiment, it can be produced in the same manner as when a general molded mortar or concrete product is produced. That is, the mortar or concrete composition of this embodiment is poured into a formwork and cured for a predetermined period of time to harden. The curing method is not particularly limited, and may be, for example, steam curing or wet curing.

[0028] (1. Overview of the Example) Next, an embodiment of the present invention will be described. This example assumes concrete to be used for replacement precast road bridge decks in an environment where antifreeze agents are sprayed. In this example, concrete using high-early-strength Portland cement and steam curing is used, to which short polypropylene fibers are added.

[0029] As will be described later, freeze-thaw tests using saltwater have shown that concrete with insufficient entrained air may have low freeze-thaw resistance, even if the air content is within the specified range. In contrast, in this example, the addition of a thickener can improve the freeze-thaw resistance of concrete with a low air content that contains short fibers. An experiment to demonstrate this effect is described below.

[0030] (2. Experimental Overview) (2.1 Materials used and composition) The mix proportions of the concrete used in the freeze-thaw test are shown in Figure 1 (mix proportions 1 to 6), and the mix proportions of the concrete used in the bleeding test are shown in Figure 2 (mix proportions 7 to 9). Mix proportions 5 and 6 correspond to the present example, and the others correspond to comparative examples.

[0031] The binder (C) is high-early-strength Portland cement (density: 3.13 g / cm 3 , Blaine value: 4,600 cm 2 The fine aggregate (S) was made of crushed hard sandstone (surface dry density: 2.65 g / cm 3 , water absorption rate: 1.53%, coarse particle ratio: 2.93), and hard crushed sandstone (maximum size: 20 mm, surface dry density: 2.75 g / cm) was used as coarse aggregate (G). 3 The concrete used had a water-cement ratio of 35% and a water content of 155 kg / m3 (water absorption: 0.45%, coarse particle ratio: 6.86). The chemical admixtures used were a high-performance water-reducing agent, an air-entraining agent, and a thickener mainly composed of alkylarylsulfonate and alkylammonium salt. 3 Two types of PP short fibers were used: one with a fineness of 2,000 dt and a length of 30 mm, and the other with a fineness of 30 dt and a length of 12 mm, as shown in Figure 3. The density of the PP short fibers was 0.91 g / cm 3 The air content of the concrete measured when fresh was in the range of 3.1% to 4.6%.

[0032] (2.2 Curing method) After pouring, the concrete was steam cured in the formwork for 18 hours. During steam curing, the concrete was left to stand at 20±2°C for four hours after pouring, and then the temperature was raised to 50°C at a rate of 15°C / hour. After maintaining the maximum temperature of 50°C for four hours, the concrete temperature was lowered by natural cooling. After demolding, the concrete was cured in air at 20±2°C until it reached an age of seven days.

[0033] (2.3 Test Method) (1) Freeze-thaw test The freeze-thaw test was conducted using one 100 x 100 x 400 mm rectangular column specimen and three φ75 x 150 mm cylindrical specimens, in accordance with the underwater freeze-thaw method (Method A) specified in JIS A 1148:2010, "Freeze-thaw Test Methods for Concrete." However, the frozen water used was a 5% by mass sodium chloride solution (salt water). Measurements of the relative dynamic modulus of elasticity and mass loss rate were conducted at intervals not exceeding 36 freeze-thaw cycles. The test results for the cylindrical specimens were the average of the three specimens.

[0034] (2) Compressive strength test after freezing and thawing Cylindrical specimens measuring 75mm diameter x 150mm were used for the compressive strength tests after the freeze-thaw cycle. The specimens were subjected to 0, 50, 100, 150, 200, 250, and 300 cycles of freeze-thawing, and then the compressive strength tests were conducted. Three specimens were tested for each number of cycles, and the average value was used as the test result.

[0035] (3) Breeding test The bleeding test was conducted in accordance with JIS A 1123:2012 "Concrete Bleeding Test." A container with an inner diameter of 250 mm and an inner height of 285 mm was used. After the sample was placed in the container, a vibrator was inserted and the concrete was vibrated for 20 seconds to compact it, the surface of the sample was leveled, and the sample was left to stand in the air. Immediately after leveling the surface of the sample, water that had seeped out onto the surface of the concrete was absorbed every 30 minutes until no more bleeding was observed, and the mass was measured to determine the bleeding rate.

[0036] (4) Internal flaw detection test For the internal flaw detection test conducted to observe cracks inside the concrete, rectangular column specimens that had completed the freeze-thaw test were cut with a wet cutter. Fluorescent paint used for rock detection, etc., was applied to the cut surface, which was then polished after drying, and the cracks were observed by irradiating them with ultraviolet light using an ultraviolet fluorescent lamp.

[0037] (3. Experimental Results and Discussion) (3.1 Effect of adding short PP fibers on freeze-thaw resistance) Figure 4 shows the results of measurements of the relative dynamic modulus of elasticity in freeze-thaw tests conducted on rectangular prism specimens made from concrete containing 30 dt short PP fibers. After steam curing, the specimens were air-cured for up to 7 days before the test began. The squares in the figure represent the results for concrete with an air content of 3.1% (mixture number 3), and it can be seen that the relative dynamic modulus of elasticity decreased with fewer cycles. On the other hand, the black squares represent the results for concrete with an air content increased to 6.1% by adjusting the amount of admixture. It is well-known that concrete with sufficient entrained air has high freeze-thaw resistance, and this figure, consistent with previous studies, shows that concrete with sufficient entrained air has high freeze-thaw resistance, even with the addition of short PP fibers. However, the standard for air content in concrete is generally set at 4.5±1.5%, and the concrete with an air content of 3.1% mentioned above is within the standard. Therefore, it is important to note that the air content significantly affects the freeze-thaw resistance of short-fiber concrete. The concrete used in the freeze-thaw tests described below had air contents ranging from 3.1% to 4.6%, as shown in Figures 1 and 2.

[0038] Figure 5 shows the results of measurements of the relative dynamic modulus of elasticity in freeze-thaw tests using prismatic specimens made of concrete with and without short PP fiber. In the figure, "No Short Fiber" corresponds to mix number 1, "2000dt Short PP Fiber" corresponds to mix number 2, and "30dt Short PP Fiber" corresponds to mix number 3. No viscosity enhancer was added to either concrete. This figure shows that the relative dynamic modulus of elasticity of the concrete without short fiber remains approximately 80% even after 300 cycles. In contrast, the relative dynamic modulus of elasticity of the two types of concrete with short PP fiber fell below 60% after approximately 220 cycles for the 2000dt fineness and approximately 80 cycles for the 30dt fineness. Figure 6 shows the results of measurements of the mass loss rate in freeze-thaw tests using the prismatic specimens shown in Figure 5. This figure shows that the mass loss of the two types of concrete with short PP fiber was greater than that of the concrete without short fiber. Furthermore, the test results for both the relative dynamic modulus of elasticity and the mass loss indicate that deterioration progressed in fewer cycles in the specimen with 30 dt PP short fiber added than in the specimen with 2,000 dt PP short fiber added.

[0039] Figure 7 shows the results of bleeding tests conducted on concrete with and without PP short fiber. In the figure, "No Short Fiber" corresponds to mix number 7, "2000dt PP Short Fiber" corresponds to mix number 8, and "30dt PP Short Fiber" corresponds to mix number 9. From this figure, it can be seen that the bleeding rates of concrete with 2000dt and 30dt PP short fiber were reduced to about half of those of concrete without short fiber. This is presumably because the PP short fiber trapped excess water in the concrete as it hardened. While there was no significant difference in the bleeding rates between the 2000dt and 30dt PP short fiber additions, the 2000dt PP short fiber addition rate was 0.5% by volume, while the 30dt PP short fiber addition rate was 0.05%, or one-tenth of the 2000dt rate. Taking into account this difference in addition rate, it is thought that 30dt PP short fibers have a greater effect of retaining excess water within the concrete than 2,000dt short fibers.

[0040] Figure 8 shows a photograph taken when a rectangular column specimen (mixture number 2) made from concrete containing 2,000 dt PP short fibers was cut after a freeze-thaw test and internal flaw detection testing was carried out. The photograph in this figure shows that cavities have formed around the PP short fibers arranged perpendicular to the cut surface, and these appear to be the starting points of cracks.

[0041] Therefore, the decrease in freeze-thaw resistance due to the addition of short PP fibers may be due to various factors, such as the addition of short fibers, the mix proportions, curing, and test conditions. However, the decrease in bleeding rate and the formation of voids around the short fibers suggest that excess water in the concrete may have accumulated around the short fibers, forming voids, which then froze and became the starting point for cracks.

[0042] Figure 9 shows the results of measurements of the relative dynamic modulus of elasticity in freeze-thaw tests using φ75 x 150 mm cylindrical specimens made from concrete with and without added short PP fibers. This figure also shows that the relative dynamic modulus of elasticity of the specimen with added short PP fibers deteriorates more quickly than that of the specimen without added short fibers. However, the effects of the addition of short fibers and the type of short fibers are smaller than those of the test results using the rectangular specimen shown in Figure 5. This is presumably because the cross-sectional area of the rectangular specimen with a side length of 100 mm differs by approximately 2.3 times from that of the 75 mm diameter cylindrical specimen, and the freeze-thaw effect had a greater effect on the 75 mm diameter cylindrical specimen.

[0043] Figure 10 shows the results of freeze-thaw tests on concrete with and without added short PP fibers, conducted using the same cylindrical specimens as in the freeze-thaw tests shown in Figure 9. The tests were conducted up to 300 cycles, with the specimens removed every 50 cycles to examine their compressive strength. Cylindrical specimens measuring 75 mm in diameter and 150 mm in length were used for the compressive strength tests, with the average value of three specimens calculated and expressed as a ratio to the compressive strength before the freeze-thaw cycle. This figure shows that the difference in compressive strength between the two types of concrete with and without added short fibers is not as great as the difference in the relative dynamic modulus of elasticity obtained from the tests on the rectangular column specimens, but that the compressive strength of the two types of concrete with added short PP fibers declines at a greater rate than that of concrete without added short fibers.

[0044] These results show that in the above test specimens, which were prepared to simulate PCaPC products, if the entrained air is insufficient even when the air volume is within the standard, the freeze-thaw resistance of concrete containing PP short fibers is inferior to that of concrete without the addition of short fibers.

[0045] (3.2 Measures to improve freeze-thaw resistance of short fiber concrete) Figure 11 shows the relative dynamic modulus of elasticity in a freeze-thaw test using prismatic specimens made with concrete containing a thickener. In the figure, "no short fiber" corresponds to mix number 4, "short PP fiber (2000 dt)" corresponds to mix number 5, and "short PP fiber (30 dt)" corresponds to mix number 6. Mix numbers 5 and 6 correspond to this example. From this figure, it can be seen that with the addition of a thickener, the relative dynamic modulus of elasticity of the short fiber concrete did not decrease even after 300 cycles, a significant improvement over the results for the case without thickener, as shown in Figure 5. On the other hand, the relative dynamic modulus of elasticity of the specimens without short fiber but with thickener added fell below 60% before reaching 300 cycles. Figure 12 shows the mass loss rate obtained by a freeze-thaw test using prismatic specimens made with concrete containing a thickener, as shown in Figure 11. This figure also shows that when a thickener is added, the mass of the specimens containing 2,000dt and 30dt PP short fibers barely decreases even after 300 cycles, a significant improvement over the results for the specimens containing no thickener, as shown in Figure 6. On the other hand, the specimens containing thickener but no short fibers show a linear decrease in mass from around the 50th cycle, indicating that they deteriorate in fewer cycles than the specimens without thickener.

[0046] Figure 13 shows the relative dynamic modulus of elasticity in freeze-thaw tests using cylindrical specimens measuring 75 mm in diameter and 150 mm in length made from concrete containing a thickener. This figure shows that when a thickener was added, the relative dynamic modulus of elasticity of specimens containing 2,000 dt and 30 dt short PP fibers did not decrease even after 300 cycles. As mentioned above, the fact that no decrease in performance was observed despite the use of cylindrical specimens with small cross-sectional areas confirms the high effectiveness of thickeners in improving freeze-thaw resistance. Meanwhile, the relative dynamic modulus of elasticity of specimens containing no short fibers but with added thickener fell below 60% after around 110 cycles.

[0047] Figure 14 shows the change in compressive strength over time when a freeze-thaw test was conducted using cylindrical specimens of concrete to which a viscosity improver had been added, similar to the test shown in Figure 10. This figure shows that when a viscosity improver was added, the compressive strength of the concrete to which short PP fibers had been added did not decrease, but that of the concrete to which no short fibers had been added did decrease.

[0048] Figures 15(1) and (2) show the appearance of the rectangular prism specimens at the end of the freeze-thaw tests shown in Figures 5 and 11, respectively. These photographs also show that for concrete without short fibers, the specimen with added viscosity improver showed more surface deterioration, whereas for concrete with added short fibers, the specimen with added viscosity improver showed less surface deterioration.

[0049] As mentioned above, previous research has reported that the addition of a thickener reduces the freeze-thaw resistance of concrete (see Non-Patent Document 4). In this experiment, as in previous research, the addition of a thickener reduced the freeze-thaw resistance of concrete without short fibers. However, in concrete with short fibers, as in this example, the addition of a thickener improved the freeze-thaw resistance even when entrained air was insufficient. If the decrease in freeze-thaw resistance is due to excess water accumulating around the short fibers, it is presumed that the addition of a thickener dispersed the excess water throughout the concrete without accumulating around the short fibers, thereby improving the freeze-thaw resistance.

[0050] (4. Summary) In this experiment, we evaluated the freeze-thaw resistance of concrete that is intended for precast products when short PP fibers are added. The results of the experiment are summarized as follows:

[0051] (1) If sufficient entrained air is not entrained in concrete containing short PP fibers, its freeze-thaw resistance may be inferior to that of concrete without short fibers, even if the air content is within the standard range. (2) Freeze-thaw tests were conducted on concrete containing two types of PP short fibers with different fineness, and the relative dynamic modulus of elasticity, mass loss, and compressive strength were evaluated. These were found to be inferior to concrete without short fibers. (3) When short PP fibers were added, the bleeding rate of fresh concrete was lower than that of concrete without short fibers. Therefore, short PP fibers have the effect of retaining excess water in the concrete, and this excess water may be a weakness during the freeze-thaw test. (4) The freeze-thaw resistance of concrete without short fibers decreases with the addition of thickeners, but in the case of concrete with PP short fibers, adding thickeners has the effect of significantly improving the freeze-thaw resistance.

[0052] In the above examples, the experimental results for concrete made from the concrete composition were used as examples, but the present invention is not limited to this, and it is believed that the same tendency will be observed in mortar made from concrete excluding coarse aggregate.

[0053] As described above, the mortar or concrete composition according to the present invention contains a cement-containing binder, fine aggregate, short fibers, water, and a thickener. In a freeze-thaw test based on the freeze-thaw test method described in JIS A 1148, a mortar or concrete specimen prepared from the mortar or concrete composition exhibits superior freeze-thaw resistance over a predetermined freeze-thaw cycle compared to a mortar or concrete specimen prepared from a mortar or concrete composition that differs from the mortar or concrete composition only in that it does not contain a thickener. Therefore, the freeze-thaw resistance of mortar or concrete containing short fibers can be improved by adding a thickener.

[0054] Furthermore, when the molded product of the present invention is applied as a PCaPC product to a precast road bridge deck for renewal, which is used in an environment where antifreeze agents are sprayed, its excellent freeze-thaw resistance ensures durability. Therefore, when the molded product of the present invention is used, the service life of the deck can be extended compared to when a general PCaPC product is used. This makes it possible to reduce repair and renewal costs. [Industrial Applicability]

[0055] As described above, the mortar or concrete composition and molded product thereof according to the present invention are useful for improving the freeze-thaw resistance of mortar or concrete containing short fibers, and are particularly suitable as materials for structural members that are subject to repeated loads, such as precast road bridge decks for renewal, which are used in environments where antifreeze agents are sprayed.

Claims

1. A concrete composition comprising a binder containing cement, fine aggregate, coarse aggregate, polypropylene short fibers, water, and a thickener containing an alkyl aryl sulfonate and an alkyl ammonium salt as two main components, A composition for concrete characterized in that the freeze-thaw resistance of a concrete specimen made from the composition for concrete in a predetermined freeze-thaw cycle, as determined by a freeze-thaw test based on the freeze-thaw test method described in JIS A 1148, is superior to the freeze-thaw resistance of a concrete specimen made from a composition for concrete that differs from the composition for concrete only in that it does not contain the thickener, in the predetermined freeze-thaw cycle.

2. 2. The concrete composition according to claim 1, wherein the index representing the freeze-thaw resistance is a relative dynamic modulus of elasticity, a durability index, a mass reduction rate, or a compressive strength ratio.

3. A concrete composition as described in claim 1 or 2, characterized in that the fineness of the polypropylene short fibers is 30 to 2000 dt.

4. A molded article obtained by molding the concrete composition according to any one of claims 1 to 3.

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