A method for evaluating the deterioration grade of concrete aggregate under sulphate attack

Abstract

The present application belongs to the technical field of concrete, and discloses a method for evaluating the degradation grade of concrete aggregate caused by sulphate attack. The method comprises the following steps: first, preparing fine aggregate samples from the sulphur-containing aggregate to be evaluated; then, mixing the samples with cement and water to prepare mortar expansion rate test pieces and mortar flexural strength test pieces; next, testing the 84d expansion rate of the mortar expansion rate test pieces and the 84d flexural strength retention rate of the mortar flexural strength test pieces; finally, evaluating the grade according to the test results: if the 84d expansion rate is greater than 0.2%, the sulphate attack is determined to be strong; if the 84d expansion rate is less than or equal to 0.2% and the flexural strength retention rate is less than 80%, the sulphate attack is determined to be moderate; if the 84d expansion rate is less than or equal to 0.2% and the flexural strength retention rate is greater than or equal to 80%, the sulphate attack is determined to be weak. The present application solves the problem of the lack of a unified test method standard for sulphate attack on concrete aggregate, provides a complete test method, establishes an evaluation system for the damage grade of sulphate attack, and promotes the use of aggregate with excessive sulphur content.

Description

Technical Field

[0001] This invention relates to the field of concrete technology, and in particular to a method for assessing the degree of sulfate erosion degradation of concrete aggregates. Background Technology

[0002] Sulfate attack within concrete is a critical issue affecting structural durability, primarily stemming from excessive sulfide or sulfate content in aggregates (exceeding 0.5% as SO3). Its destructive mechanism involves the coupling of chemical attack and physical expansion. While existing standards such as GB / T 14684-2022 specify limits for sulfate content in sand and gravel aggregates (e.g., SO3 content in fine aggregates ≤ 0.5%), a unified testing method is still lacking for assessing the dynamic process and damage level of internal concrete attack.

[0003] Traditional testing methods primarily focus on static analysis of the content of harmful substances in aggregates, failing to simulate the accelerated erosion effects of complex environments such as wet-dry cycles and freeze-thaw cycles in actual engineering projects. Studies have shown that sulfate attack triggers the formation and volume expansion of ettringite, leading to concrete cracking, strength degradation, and steel corrosion. The degree of damage is closely related to sulfate concentration, water-cement ratio, and curing conditions. However, existing evaluation systems have significant limitations: firstly, sulfate attack resistance tests (such as the KS90 test) often employ external solution immersion methods, making it difficult to accurately reflect the long-term penetration behavior of sulfates within the aggregates; secondly, the classification of damage levels lacks quantitative indicators, resulting in inconsistent project acceptance and quality control standards. For example, some methods rely solely on qualitative assessments based on visual damage descriptions, without establishing grading standards for key parameters such as compressive strength and corrosion resistance coefficients.

[0004] Furthermore, according to the current national standards "Construction Sand" (GB / T14684-2022) and "Construction Gravel and Crushed Stone" (GB / T14685-2022), the sulfate content (based on SO3 mass) in aggregates should be ≤0.5%. Aggregates that do not meet this requirement are considered unusable. However, in actual production, it has been found that some aggregates with sulfate (based on SO3 mass) content greater than 0.5% also have application potential. The different forms of sulfur in aggregates, as well as the type and source of the aggregates, all affect their properties. Related research indicates that when the SO3 content in aggregates is no more than 1.5%, its adverse effects on the long-term durability of concrete are relatively limited. If aggregate quality is judged solely based on the current national standard limit of 0.5%, a large amount of usable aggregate will be excluded from engineering applications, exacerbating the supply-demand imbalance of high-quality aggregates and failing to meet the requirements of green and low-carbon development.

[0005] Therefore, how to provide a method for assessing the degradation level of sulfate attack on concrete aggregates, form a unified test method standard, and establish a grading evaluation system for the degree of attack damage is an urgent problem to be solved in this field. Summary of the Invention

[0006] In view of this, the present invention provides a method for evaluating the deterioration level of sulfate attack on concrete aggregates. Addressing the problem that a unified test method standard has not yet been formed for sulfate attack inside concrete, the present invention provides a complete test method for sulfate attack inside concrete and establishes a corresponding evaluation system for the degree of attack damage.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: A method for assessing the sulfate attack degradation level of concrete aggregates includes the following steps: 1) Prepare fine aggregate samples using the sulfur-containing aggregate to be evaluated; 2) Mix cement and fine aggregate samples with water to prepare mortar expansion rate specimens and mortar flexural strength specimens, respectively; 3) Test the 84-day expansion rate of mortar expansion rate specimens and the 84-day flexural strength retention rate of mortar flexural strength specimens; When the 84-day expansion rate is greater than 0.2%, it is considered strong erosion; When the 84-day expansion rate is ≤0.2% and the 84-day flexural strength retention rate is <80%, it is judged as moderate erosion; When the 84-day expansion rate is ≤0.2% and the 84-day flexural strength retention rate is ≥80%, it is judged as weak erosion.

[0008] Preferably, the sulfur-containing aggregate in step 1) is an aggregate containing sulfides or sulfates.

[0009] The sulfur content in the sulfur-containing aggregate, calculated as SO3, is >0.5% by mass.

[0010] Preferably, the particle size distribution of the fine aggregate sample in step 1) is 2.5~5.0mm:1.25~2.5mm:0.63~1.25mm:0.315~0.63mm:0.16~0.315mm=100:350:250:200:100.

[0011] Preferably, the mass ratio of cement, fine aggregate sample and water in step 2) is 1:2.5:0.5.

[0012] Preferably, the cement comprises low-heat silicate cement and / or high sulfate-resistant silicate cement; The mass content of tricalcium aluminate in the cement is 2.9-3.1%.

[0013] Preferably, the size of the mortar expansion rate test specimen is 25 mm × 25 mm × 280 mm; The dimensions of the mortar flexural strength test specimen are 40 mm × 40 mm × 160 mm.

[0014] Preferably, the test method for the 84d expansion rate is as follows: After the mortar expansion rate specimens were molded, they were cured for the first time along with the mold. The temperature of the first curing was 20±2℃, the relative humidity was ≥95%, and the time was 24h±15min. After the first curing, the mortar expansion rate specimens were demolded under water-sealed conditions and subjected to a second curing. 24h±15min after the start of the second curing, the length of the mortar expansion rate specimens was measured using a length comparator and recorded as the initial length. After curing for 84 days, the length of the mortar expansion rate specimens was measured again using a length comparator and recorded as the 84-day curing length. The temperature for the second curing was 60±2℃. Then, using Equation 1, the expansion rate is calculated, and the average value of three tests is recorded as the 84-day expansion rate. Formula 1: ; In the formula: P t —Inflation rate, % L t —Length of 84 days, mm; L 0 —Initial length, mm; 250—Effective length of specimen, mm.

[0015] Preferably, the test method for the 84d flexural strength retention rate is as follows: After the mortar flexural strength test specimens are molded, they are cured for the first time along with the mold; the temperature of the first curing is 20±2℃, the relative humidity is ≥95%, and the time is 24h±15min. After the first curing, the mortar flexural strength specimens were demolded under water-sealed conditions and subjected to a second curing; the temperature for the second curing was 60±2℃. During the second curing, the flexural strength was tested at 28 days and 84 days respectively. Then, the flexural strength retention rate was calculated using Equation 2. The average value of the three tests was recorded as the flexural strength retention rate at 84 days. Formula 2: ; In the formula: P Ct — Flexural strength retention, % C t —Flexural strength at 84d, MPa; C 28 —Flexural strength at 28 days, MPa.

[0016] As can be seen from the above technical solution, compared with the prior art, the present invention has the following beneficial effects: 1. This invention solves the problem of the lack of unified test method standards for sulfate attack on concrete aggregates, provides a complete set of test methods, and establishes a corresponding evaluation system for attack damage levels, which can accurately evaluate sulfur-containing aggregates.

[0017] 2. This invention provides guidance for the use of sulfur-containing aggregates. Aggregates with SO3 content exceeding 0.5% are considered to have excessive sulfate content. The innovative discovery of this invention shows that aggregates with excessive sulfate content that meet the evaluation criteria of this invention also have application potential. Detailed Implementation

[0018] This invention provides a method for assessing the sulfate attack degradation level of concrete aggregates, comprising the following steps: 1) Prepare fine aggregate samples using the sulfur-containing aggregate to be evaluated; 2) Mix cement and fine aggregate samples with water to prepare mortar expansion rate specimens and mortar flexural strength specimens, respectively; 3) Test the 84-day expansion rate of mortar expansion rate specimens and the 84-day flexural strength retention rate of mortar flexural strength specimens; When the 84-day expansion rate is greater than 0.2%, it is considered strong erosion; When the 84-day expansion rate is ≤0.2% and the 84-day flexural strength retention rate is <80%, it is judged as moderate erosion; When the 84-day expansion rate is ≤0.2% and the 84-day flexural strength retention rate is ≥80%, it is judged as weak erosion.

[0019] In this invention, the erosion damage evaluation level is shown in Table 1: Table 1 Evaluation levels of concrete aggregate erosion damage to concrete

[0020] In this invention, the expansion rate is used as the primary control index, and the flexural strength retention rate is used as an auxiliary index. When the corrosion is determined to be moderate, measures to inhibit aggregate sulfate corrosion must be taken during engineering applications; when the corrosion is determined to be severe, it is generally not recommended for engineering applications.

[0021] In this invention, the sulfur-containing aggregate in step 1) is an aggregate containing sulfides or sulfates; the mass content of sulfur in the sulfur-containing aggregate, calculated as SO3, is >0.5%, specifically 0.6%, 0.8%, 1%, 1.2%, 1.4%, or 1.5%.

[0022] In this invention, the particle size distribution of the fine aggregate sample in step 1) is 2.5~5.0mm:1.25~2.5mm:0.63~1.25mm:0.315~0.63mm:0.16~0.315mm=100:350:250:200:100.

[0023] In this invention, the mass ratio of cement, fine aggregate sample and water in step 2) is 1:2.5:0.5.

[0024] In this invention, the cement includes low-heat silicate cement and / or high sulfate-resistant silicate cement.

[0025] In this invention, the mass content of tricalcium aluminate in the cement is 2.9-3.1%, preferably 3%.

[0026] In this invention, when the mass content of tricalcium aluminate in cement is lower than 2.9-3.1%, tricalcium aluminate ore can be added externally for adjustment, wherein the purity of the tricalcium aluminate ore is ≥99%.

[0027] In this invention, the size of the mortar expansion rate specimen is 25 mm × 25 mm × 280 mm; In this invention, the dimensions of the mortar flexural strength test specimen are 40 mm × 40 mm × 160 mm.

[0028] In this invention, the test method for the 84d expansion rate is as follows: After the mortar expansion rate specimens are molded, they are cured for the first time along with the mold. The temperature of the first curing is 20±2℃, preferably 20℃; the relative humidity is ≥95%, specifically 96%, 97%, 98%, or 99%; and the time is 24h±15min, preferably 24h.

[0029] After the first curing, the mortar expansion rate specimens are demolded under water-sealed conditions and subjected to a second curing. The second curing begins 24h±15min, preferably 24h. The length of the mortar expansion rate specimens is measured using a length comparator and recorded as the initial length. After curing for 84 days, the length of the mortar expansion rate specimens is measured again using a length comparator and recorded as the 84-day curing length. The temperature for the second curing is 60±2℃, preferably 60℃.

[0030] Then, using Equation 1, the expansion rate is calculated, and the average value of three tests is recorded as the 84-day expansion rate.

[0031] Formula 1: ; In the formula: P t —Inflation rate, % L t —Length of 84 days, mm; L 0 —Initial length, mm; 250—Effective length of specimen, mm.

[0032] In this invention, although the size of the mortar expansion rate specimen is 25 mm × 25 mm × 280 mm, the actual effective length of the specimen is 250 mm due to the presence of the probe.

[0033] In this invention, the test method for the 84-day flexural strength retention rate is as follows: After the mortar flexural strength test specimens are molded, they are cured for the first time along with the mold. The temperature of the first curing is 20±2℃, preferably 20℃; the relative humidity is ≥95%, specifically 96%, 97%, 98%, or 99%; and the time is 24h±15min, preferably 24h.

[0034] After the first curing, the mortar flexural strength test specimens are demolded under water-sealed conditions and subjected to a second curing; the temperature of the second curing is 60±2℃, preferably 60℃.

[0035] During the second curing, the flexural strength was tested at 28 days and 84 days respectively. Then, the flexural strength retention rate was calculated using Equation 2. The average value of the three tests was recorded as the 84-day flexural strength retention rate.

[0036] Formula 2: ; In the formula: P Ct — Flexural strength retention, % C t —Flexural strength at 84d, MPa; C 28 —Flexural strength at 28 days, MPa.

[0037] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0038] Example 1

[0039] Aggregate #1 was taken from the excavation material of the ventilation and safety tunnel of the Yongchang Pumped Storage Power Station, and its sulfate and sulfide content (based on SO3 mass) was 0.8%. This initial aggregate was processed by crushing and screening, and then blended to form fine aggregate samples with a specific gradation. The particle size distribution of the fine aggregate samples was 2.5~5.0mm: 1.25~2.5mm: 0.63~1.25mm: 0.315~0.63mm: 0.16~0.315mm = 100:350:250:200:100 (totaling 1000 samples).

[0040] Prepare Jiahua low-heat silicate cement for later use, with the tricalcium aluminate content adjusted to 3%.

[0041] Then, cement, fine aggregate samples and water were mixed with mortar in a ratio of 1:2.5:0.5 (mass ratio). The mortar was then used to prepare mortar expansion rate specimens (25 mm × 25 mm × 280 mm) × 3 and mortar flexural strength specimens (40 mm × 40 mm × 160 mm) × 3.

[0042] After the mortar expansion rate specimens were molded, they were placed in a standard curing room (curing temperature 20℃, relative humidity 95%). After curing for 24 hours, the mortar expansion rate specimens were demolded under water-sealed conditions and cured under water-sealed conditions at a curing temperature of 60℃. During the water-sealed curing process, the length of the mortar expansion rate specimens was measured using a length comparator at 24 hours and 84 days. The length at 24 hours was recorded as the initial length, and the length at 84 days was recorded as the 84-day length. Then, the expansion rate of the three specimens was calculated using Equation 1, and the average value was taken to obtain the 84-day expansion rate. The calculation method for the expansion rate at other ages was the same as that for the 84-day expansion rate. The test results are shown in Table 2.

[0043] After the mortar flexural strength specimens were molded, they were placed in a standard curing room (curing temperature 20℃, relative humidity 95%). After curing for 24 hours, the mortar expansion rate specimens were demolded under water-sealed conditions and cured under the same conditions at a curing temperature of 60℃. During the water-sealed curing process, the flexural strength was tested at 28 days and 84 days. The flexural strength at 28 days was recorded as the 28-day flexural strength, and the flexural strength at 84 days was recorded as the 84-day flexural strength. Then, using Equation 2, the flexural strength retention rate of the three specimens was calculated, and the average value was taken to obtain the 84-day flexural strength retention rate. The calculation method for the flexural strength retention rate at other ages was the same as that for the 84-day flexural strength retention rate. The test results are shown in Table 2.

[0044] Flexural strength test method: GB / T 17671-2021.

[0045] Table 2 Evaluation levels of concrete aggregate erosion damage to concrete

[0046] Example 2

[0047] Aggregate #2 was taken from the excavation material of the traffic tunnel of Niushoushan Pumped Storage Power Station. The grade evaluation of aggregate #2 (the mass content of sulfate and sulfide is 0.7% as SO3) was carried out. The specific operation was the same as in Example 1. The test results are shown in Table 3.

[0048] Table 3 Evaluation levels of concrete aggregate erosion damage to concrete

[0049] Example 3

[0050] Aggregate #3 was taken from natural sand and gravel in the lower reservoir area of ​​Zhangye Pumped Storage Power Station. The grade evaluation of aggregate #3 (the mass content of sulfate and sulfide is 1.2% as SO3) was carried out. The specific operation was the same as in Example 1. The test results are shown in Table 4.

[0051] Table 4 Evaluation Levels of Concrete Aggregate Erosion Damage to Concrete

[0052] Example 4

[0053] Aggregate #4 was taken from natural sand and gravel in the lower reservoir area of ​​Zhangye Pumped Storage Power Station. The grade evaluation of aggregate #4 (the mass content of sulfate and sulfide is 1.5% as SO3) was carried out. The specific operation was the same as in Example 1. The test results are shown in Table 5.

[0054] Table 5 Evaluation Levels of Concrete Aggregate Erosion Damage to Concrete

[0055] In summary, the aggregate tested in Example 1 of this invention has a sulfur content greater than 0.5% based on SO3, but it is evaluated as weakly corrosive. When actually used to prepare concrete, the erosion damage to the concrete meets the relevant technical requirements. Although the sulfur content of the aggregate in Example 2 is lower than that in Example 1, the erosion damage to the concrete is more severe, and it is evaluated as moderately corrosive. The aggregates in Examples 3 and 4 are both highly corrosive aggregates, which have a significant impact on the performance of concrete.

[0056] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on the differences from other embodiments. The same or similar parts between the various embodiments can be referred to each other.

[0057] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for assessing the sulfate attack degradation level of concrete aggregates, characterized in that, Includes the following steps: 1) Prepare fine aggregate samples using the sulfur-containing aggregate to be evaluated; 2) Mix cement and fine aggregate samples with water to prepare mortar expansion rate specimens and mortar flexural strength specimens, respectively; 3) Test the 84-day expansion rate of mortar expansion rate specimens and the 84-day flexural strength retention rate of mortar flexural strength specimens; When the 84-day expansion rate is greater than 0.2%, it is considered strong erosion; When the 84-day expansion rate is ≤0.2% and the 84-day flexural strength retention rate is <80%, it is judged as moderate erosion; When the 84-day expansion rate is ≤0.2% and the 84-day flexural strength retention rate is ≥80%, it is judged as weak erosion.

2. The method for assessing the sulfate attack degradation level of concrete aggregates according to claim 1, characterized in that, The sulfur-containing aggregate mentioned in step 1) is aggregate containing sulfides or sulfates; The sulfur content in the sulfur-containing aggregate, calculated as SO3, is >0.5% by mass.

3. The method for assessing the sulfate attack degradation level of concrete aggregates according to claim 2, characterized in that, The particle size distribution of the fine aggregate sample mentioned in step 1) is 2.5~5.0mm:1.25~2.5mm:0.63~1.25mm:0.315~0.63mm:0.16~0.315mm=100:350:250:200:

100.

4. A method for assessing the sulfate attack degradation level of concrete aggregates according to any one of claims 1 to 3, characterized in that, The mass ratio of cement, fine aggregate sample and water in step 2) is 1:2.5:0.

5.

5. The method for assessing the sulfate attack degradation level of concrete aggregates according to claim 4, characterized in that, The cement includes low-heat silicate cement and / or high sulfate-resistant silicate cement; The mass content of tricalcium aluminate in the cement is 2.9-3.1%.

6. The method for assessing the sulfate attack degradation level of concrete aggregates according to claim 5, characterized in that, The dimensions of the mortar expansion rate test specimen are 25 mm × 25 mm × 280 mm; The dimensions of the mortar flexural strength test specimen are 40 mm × 40 mm × 160 mm.

7. A method for assessing the sulfate attack degradation level of concrete aggregates according to claim 4 or 5, characterized in that, The test method for the 84d expansion rate is as follows: After the mortar expansion rate specimens were molded, they were cured for the first time along with the mold. The temperature of the first curing was 20±2℃, the relative humidity was ≥95%, and the time was 24h±15min. After the first curing, the mortar expansion rate specimens were demolded under water-sealed conditions and subjected to a second curing. 24h±15min after the start of the second curing, the length of the mortar expansion rate specimens was measured using a length comparator and recorded as the initial length. After curing for 84 days, the length of the mortar expansion rate specimens was measured again using a length comparator and recorded as the 84-day curing length. The temperature for the second curing was 60±2℃. Then, using Equation 1, the expansion rate is calculated, and the average value of three tests is recorded as the 84-day expansion rate. Formula 1: ; In the formula: P t —Inflation rate, % L t —Length of 84 days, mm; L 0 —Initial length, mm; 250—Effective length of specimen, mm.

8. The method for assessing the sulfate attack degradation level of concrete aggregates according to claim 7, characterized in that, The test method for the 84d flexural strength retention rate is as follows: After the mortar flexural strength test specimens are molded, they are cured for the first time along with the mold; the temperature of the first curing is 20±2℃, the relative humidity is ≥95%, and the time is 24h±15min. After the first curing, the mortar flexural strength specimens were demolded under water-sealed conditions and subjected to a second curing; the temperature for the second curing was 60±2℃. During the second curing, the flexural strength was tested at 28 days and 84 days respectively. Then, the flexural strength retention rate was calculated using Equation 2. The average value of the three tests was recorded as the flexural strength retention rate at 84 days. Formula 2: ; In the formula: P Ct — Flexural strength retention, % C t —Flexural strength at 84d, MPa; C 28 —Flexural strength at 28 days, MPa.

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