JP7629826B2 - concrete - Google Patents

Description

The present invention relates to concrete that uses aggregate that is reactive to alkali-silica reactions.

Among the aggregates used in concrete, there are reactive aggregates that are known to be reactive to alkali-silica reactions. Concrete that uses reactive aggregates is at risk of deterioration due to alkali-silica reactions (expansion and cracks caused by expansion, etc.). Alkali-silica reactions are caused by the water absorption and expansion of alkali silica gel, which is produced by a chemical reaction of alkaline components dissolved in an aqueous solution.
Methods for inhibiting the alkali-silica reaction in concrete include reducing the total amount of alkali in the material, using mixed cement that has an inhibitory effect, using aggregate that is recognized as safe, and injecting lithium nitrite chemicals onto the surface or inside the concrete.
However, even when the total amount of alkali is controlled, deterioration due to alkali-silica reaction may occur.
In addition, blended cement with suppressive effects is expensive. In addition, when using special admixtures, it takes time and effort to determine the mixing ratio, and the plant needs to be redesigned, which is time-consuming and costly.
In addition, the use of aggregates that are deemed safe may require the transport of the aggregates from distant locations, which increases costs and hinders efforts to reduce the environmental impact.
Furthermore, lithium nitrite is an expensive material, and it takes time and effort to supply it to concrete.
Therefore, Patent Document 1 discloses a method of suppressing the alkali-silica reaction in concrete by supplying relatively inexpensive chemicals such as calcium nitrite or calcium nitrate to the surface of the concrete or by injecting them into the concrete under pressure.
However, even if calcium nitrite or calcium nitrate is used, it takes time and effort to apply it to concrete.

JP 2017-128496 A

From this perspective, the present invention aims to propose a concrete that can suppress deterioration caused by alkali-silica reaction that occurs when reactive aggregate is used.

The present invention for solving the above problems provides a concrete containing reactive aggregate that is determined to be non-harmful according to the alkali-silica reaction test method for aggregate specified in JIS A 1145 , the concrete having an air content of 10% or more, a water-cement ratio of 45% or less, and a fine aggregate ratio of 45% or less .
The inventors have found that by increasing the amount of air in concrete, the expansion pressure caused by the alkali-silica reaction can be reduced, and cracks in the hardened body can be suppressed. According to the present invention, it is possible to effectively use reactive aggregate, reduce costs, and enable local production and consumption of aggregate. In addition, since no special admixtures or chemicals are required, there is no increase in the effort and cost required to suppress the alkali-silica reaction.
Even if the proportion of reactive aggregate in the aggregate is within the range of 30% to 100%, the expansion reducing effect can be obtained.
Since the water-cement ratio is 45% or less and the fine aggregate rate is 45% or less, a slump of 12 cm or more can be ensured and the necessary strength can be secured against the effects of durability due to an increase in air content.

The concrete of the present invention can suppress deterioration due to alkali-silica reaction, even when reactive aggregate is used as the aggregate.

1 is a graph showing the change in creep strain over time, which is a result of a test carried out on concrete according to an embodiment of the present invention. 1 is a graph showing creep strain per unit stress, which is a result of a test performed on concrete according to an embodiment of the present invention. 1 is a graph showing chloride ion concentration, which is a test result for confirming salt penetration resistance of concrete according to an embodiment of the present invention. 1 is a graph showing the results of a reactivity evaluation test of reactive aggregate by a scientific method. 1 is a graph showing the results of a reactivity evaluation test of reactive aggregate by a mortar bar method. 1 is a graph showing the relationship between the acceleration period and the expansion rate, which is a result of a test carried out on the concrete according to the embodiment of the present invention.

In an embodiment of the present invention, a concrete capable of suppressing deterioration due to alkali-silica reaction is described, even when using as aggregate a reactive aggregate that is determined to be non-harmful according to the aggregate alkali-silica reaction test method specified in JIS A 1145. Specifically, in the range where the dissolved silica amount Sc is 10 mmol/L or more and the alkali concentration reduction amount Rc is less than 700 mmol/L, according to JIS A 1145, if the dissolved silica amount Rc is less than the alkali concentration reduction amount Rc, the aggregate is determined to be non-harmful, and if the dissolved silica amount Rc is equal to or greater than the alkali concentration reduction amount Rc, the aggregate is determined to be non-harmful.
The concrete of the present embodiment is made by mixing cement, fine aggregate, coarse aggregate, admixtures such as an air-entraining water-reducing agent or an air-entraining agent, and water. The water-cement ratio of the concrete is set to 45% or less.
Examples of the cement that can be used include ordinary Portland cement, early-strength Portland cement, moderate-heat Portland cement, low-heat Portland cement, blast-furnace cement (types A to C), fly ash cement (types A to C), silica cement (types A to C), and ecocement.
The aggregate used is reactive aggregate or a mixture of reactive and non-reactive aggregate. The proportion of reactive aggregate in the aggregate (fine aggregate and coarse aggregate) is within the range of 30% by mass to 100% by mass. In this embodiment, the fine aggregate and the coarse aggregate each contain reactive aggregate. The fine aggregate ratio is 45% or less. Furthermore, the maximum dimension of the coarse aggregate is 25 mm or 20 mm. The air content of the concrete is 12%. The amount of air, the size of the air bubbles, the number of the air bubbles, etc. in the concrete are adjusted by the amount of admixtures such as AE agents added.

According to the concrete of this embodiment, the proportion of reactive aggregate in the aggregate is within the range of 30% to 100%, but the air content is set to 12%, which reduces the expansion pressure caused by the alkali-silica reaction and thus suppresses cracking of the hardened body. This allows for effective use of reactive aggregate, reducing costs and enabling local production and consumption of aggregate. In addition, since special chemicals such as lithium nitrite and admixtures such as fly ash and slag are not required, there is no increase in the effort and cost required to suppress the alkali-silica reaction. In addition, while the air content in standard concrete is about 4.5%, the air content in the concrete of this embodiment is significantly increased to 12%.
Furthermore, by keeping the water-cement ratio at 45% or less and the fine aggregate rate at 45% or less, a slump of 12 cm or more can be ensured and the necessary strength can be secured.

Although an embodiment of the present invention has been described above, the present invention is not limited to the above-described embodiment, and each of the above-described components can be appropriately modified without departing from the spirit of the present invention.
In the above embodiment, the air content of the concrete is 12%, but the air content in the concrete is not limited as long as it is 10% or more.
In the above embodiment, a case has been described in which both the fine aggregate and the coarse aggregate contain non-reactive aggregate and reactive aggregate, but reactive aggregate may be contained in only either the fine aggregate or the coarse aggregate.

The following describes the test results for verifying the creep characteristics (durability), salt penetration resistance, and alkali-silica reaction suppression effect of the concrete of this embodiment. In addition, as a comparative example, measurements were also made for a normal concrete with an air content of 4.5%.
Table 1 shows the materials used in this verification test.

(1) Creep properties The creep properties of the concrete of the examples and comparative examples were confirmed according to JIS A 1157. The unit cement content was kept constant, and the water-cement ratio was adjusted so that the 28-day compressive strength was ⁴⁰ N/mm2. The mix was then determined by adjusting the fine aggregate ratio so that the slump was 12 cm. Table 2 shows the concrete mixes of the examples and comparative examples. The load stress for the creep test was determined based on the results of the compressive strength test carried out at the start of the test.

Figure 1 shows the change in creep strain over time, and Figure 2 shows the creep strain per unit stress. As is clear from the test results at 150 days of loading, the absolute values of the creep strain and creep strain per unit stress in the example were smaller than those in the comparative example. The results showed that the creep characteristics were improved by the mix selection method in the example.

(2) Salt penetration resistance For the comparative example and the example with the mix ratio in Table 2, the resistance to chloride ion penetration was examined in accordance with the Japan Society of Civil Engineers standard "Test method for the apparent diffusion coefficient of chloride ions in concrete by immersion (JSCE-G 572-2018)". The specimens of the comparative example and the example were immersed in a 10% sodium chloride aqueous solution for three months, and the chloride ion concentration was measured at four depths at 10 mm intervals from the penetration surface.
The measurement results are shown in Figure 3. The surface chloride ion concentration _C0 (kg/ ^m3 ) and the apparent chloride ion diffusion coefficient _Dap ( ^cm2 /year) calculated by regression analysis of the measurement results are shown in Table 4. From the test results, it was confirmed that the penetration of chloride ions in the depth direction tends to be suppressed in the examples compared to the comparative examples.

(3) Effect of reducing expansive pressure due to alkali-silica reaction The study was conducted according to the concrete alkali-silica reactivity test method (JCI-S-010-2017) defined by JCI (Japan Concrete Institute). For Comparative Example B with an air content of 4.5% and Example B with an air content of 12%, sodium hydroxide was added to the mixing water so that the total alkali content was 5.5 kg/ ^m3, and concrete bars were prepared and subjected to accelerated curing in a humid environment at 40°C.
First, a reactivity evaluation test was conducted by the chemical method (JIS A 1145) for the reactive aggregate and the non-reactive aggregate (coarse aggregate) used in Example B and Comparative Example B. The results are shown in Figure 4. In addition, a reactivity evaluation test was conducted by the mortar bar method (JIS A 1146) for the test specimens containing reactive aggregate. The results are shown in Figure 5.
For the mortar bar method, tests were conducted with the ratio of reactive aggregate to the total aggregate (hereinafter referred to as the mixture ratio) set to 10%, 30%, 50%, and 100%, and the Pesimum mixture ratio was confirmed. The test results showed that Pesimum showed high reactivity in the chemical method and the mortar bar method, and was judged to be non-harmful.
The measurement results using the mortar bar method showed that the expansion rate was highest at a mixing ratio of 30%, so we decided to use reactive aggregate at a mixing ratio of 30% in the concrete bar method as well to confirm the effect of air volume.

Table 5 shows the mix ratios of Comparative Example B and Example B. In order to confirm the effect of an increase in the amount of air on the alkali-silica reaction, the unit water amount was kept constant so that there would be no difference in the amount of water used in the reaction between Comparative Example B and Example B, and the unit cement amount was adjusted so that the 28-day compressive strength would be 40 N/ ^mm2 . In addition, the unit coarse aggregate amount (reactive aggregate amount) and the total alkali amount were kept constant between Comparative Example B and Example B. The total alkali amount was adjusted by adding granular sodium hydroxide to the mixing water.

The measurement results are shown in Figure 6. As shown in Figure 6, during the accelerated period of six months, the expansion rate of the concrete bar was 0.22% in Comparative Example B, while it was 0.18% in Example B. It was confirmed that increasing the amount of air had the effect of reducing expansion by about 20%.

Claims (1)

Concrete containing reactive aggregate that is determined to be non-harmful by the alkali-silica reaction test method for aggregates specified in JIS A 1145,
1. A concrete characterized in that the air content is 10% or more, the water-cement ratio is 45% or less, and the fine aggregate ratio is 45% or less.

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