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Under the condition of drying and wetting circulation, ordinary concrete, modified concrete containing fly ash, and double-admixture concrete containing fly ash and polypropylene fiber were corroded in the solution of Na_{2}SO_{4} and (NH_{4})_{2}SO_{4}. The sulfate concentration of the solution was designed to be 1000 g/L. The compressive strength and splitting tensile strength of the concrete were tested after different number of drying and wetting cycles (0, 2, 4, 6, 8, and 10). The results indicate that the strength of concrete increases in the early stages of corrosion and decreases gradually later. The admixture significantly improves the resistance to sulfate erosion of the modified concrete, while polypropylene fiber plays a less important role on anticorrosion properties. When suffered 10 times of drying and wetting cycle, the compressive strength and splitting tensile strength of modified concrete increase by 28% and 19%, respectively. Based on the test results, the service life of the modified concrete corroded by sulfate was predicted.

There are many saline soil areas containing a large amount of strong corrosive medium, such as sulfate and chloride, in Western China. For example, the sulfate concentration is more than 4200 mg/L in saline soil of Qinghai Province, which belongs to the strong corrosion level and results in severe corrosion of concrete structures [

Concrete corrosion mechanism varies greatly with different sulfate conditions [

The purpose of researching mechanical properties of concrete subjected to sulfate attack is to improve the anticorrosion capacity of the concrete. Therefore, it is of great importance to explore new corrosion-resistant modified concrete materials. In engineering, most of concrete structures work in service under drying and wetting conditions instead of completely dry or wet sulfate environments, which makes the concrete corroded rapidly and severely. In this study, the ordinary concrete, fly ash concrete, and double-mixture concrete (including fly ash and polypropylene) corroded by Na_{2}SO_{4} and (NH_{4})_{2}SO_{4} were tested in drying and wetting conditions with 1000 g/L SO_{4}^{2−} in solution. Mechanical properties of modified concrete suffered from sulfate corrosion were researched, and the variation trend of the compressive strength of fly ash concrete was analyzed. Based on the test results, the service life of the modified concrete was predicted.

A Chinese standard (GB175-2007) [^{3} and 2720 kg/m^{3}, respectively. The slump of concrete was 35∼50 mm. The designed strength grade of concrete was C25. Three types of concrete specimens were cast, including ordinary concrete, fly ash concrete, and double-admixture concrete (containing fly ash and polypropylene). A Chinese standard (JGJ 55-2011) [

Mix proportions of concrete composites.

Mix proportion (kg/m^{3}) | |||||||
---|---|---|---|---|---|---|---|

Types of concrete | Water/binder | Cement | Water | Fine aggregate | Coarse aggregate | Fly ash | PP fiber |

Ordinary | 0.57 | 342.0 | 195 | 663 | 1180 | 0 | 0 |

Fly ash | 0.52 | 273.6 | 195 | 600 | 1180 | 102.6 | 0 |

Double-mixture | 0.52 | 273.6 | 195 | 600 | 1180 | 102.6 | 0.9 |

A series of cubic concrete specimens with the size of 100 mm × 100 mm × 100 mm were cast including ordinary concrete, fly ash concrete, and double-mixture concrete. Two solutions of Na_{2}SO_{4} and (NH_{4})_{2}SO_{4} were compounded in the experiment, and the sulfate concentration was designed to be 1000 g/L. Under the condition of each corrosion solution, 12 groups of specimens were casted for each type of concrete, which include 6 groups for compressive strength test and 6 groups for splitting tensile strength test. The total numbers of specimens was 288. The specimens were cured for 28 days at ambient temperature with straw mats and watered twice a day to ensure the moisture of specimens. The size of the solution box is 1000 mm (l) × 600 mm (w) × 600 mm (h).

During the course of the experiment, the concentration of SO_{4}^{2−} in the solution was monitored on time to maintain the concentration unchanged basically. The corrosion solution was reconfigured to replace the original solution when the color of solution was changed or the content of other impurities was excessive. The bottom of the corrosion box was dredged in time so that the corrosion precipitations were not too thick.

The concrete specimens cured for 28 d were immersed in the solutions of Na_{2}SO_{4} and (NH_{4})_{2}SO_{4}, respectively, and separated by some wooden bars each other. The drying and wetting cycle system was designed as one week for immersing in solution and one week for air drying. Two weeks was one drying and wetting cycle. The cubic compressive strength _{cu} and splitting tensile strength _{st} of three types of concrete specimens were tested after 0, 2, 4, 6, 8, and 10 cycles, respectively. The strength tests were performed in accordance with Chinese standard (SL 352-2006) [

The electrohydraulic pressure testing machine (Type YA-3000; SCHIAK Company, Shanghai, China) was used in the experiment of compressive strength of concrete cubes. According to the Chinese standard (SL 352-2006), loading should be loaded uniformly and continuously at the speed of 0.3 MPa/S to 0.5 MPa/S. The universal testing machine (Type WE-X00; SCHIAK Company) was used in the experiment of splitting tensile strength of concrete cubes. Loading speed was 0.04 MPa/S to 0.06 MPa/S. The picture of splitting tensile experiment is shown in Figure

Splitting tensile strength experiment of concrete.

Due to different cations, the degrees of concrete corrosion in sulfate solutions are various. The compressive strength and splitting tensile strength of ordinary concrete and modified concrete corroded by Na_{2}SO_{4} and (NH_{4})_{2}SO_{4} varied to dry-wet cycles are presented in Figures

Changes in the strengths of corroded ordinary concrete varied to dry-wet cycles. (a) Compressive strength. (b) Splitting tensile strength.

Changes in the strengths of corroded fly ash concrete varied to dry-wet cycles. (a) Compressive strength. (b) Splitting tensile strength.

Changes in the strengths of corroded double-admixture concrete fly ash and polypropylene concrete varied to dry-wet cycles. (a) Compressive strength. (b) Splitting tensile strength.

In Figures _{4}^{2−} react with hydration of cement to form gypsum and ettringite, which is a volume expansion process [

The experimental results show that two different corrosion solutions make little difference on the compressive strength of modified concrete undergoing the same drying and wetting cycles. However, the splitting tensile strength varies to the different solution. At the initial strengthening stage, the splitting tensile strength of modified concrete corroded by sodium sulfate is generally higher than that in ammonium sulfate solution. The splitting tensile strengths of fly ash concrete and double-admixture concrete in sodium sulfate solution increase 10.9% and 6.8%, respectively, compared with those in the ammonium sulfate solution after four dry-wet cycles. At performance deterioration stage, the splitting tensile strength of the modified concrete decreased more rapidly in Na_{2}SO_{4} to be lower than that in (NH_{4})_{2}SO_{4}. For example, the splitting tensile strengths of fly ash concrete and double-mixture concrete in Na_{2}SO_{4} are 93% and 94% of that in (NH_{4})_{2}SO_{4}, respectively, after 8 dry-wet cycles.

At the beginning of corrosion, the interaction between NH_{4}^{+} and Ca(OH)_{2} neutralizes the hardened cement paste, which results in weakening of bond force of the cement gel and aggravation of SO_{4}^{2−} corrosion [_{2}SO_{4}·10H_{2}O crystal will be precipitated in the internal pores of concrete when the content of Na_{2}SO_{4} exceeds its solubility [

In order to intuitively describe the influence of the admixtures on properties of concrete attacked by sulfate corrosion, the relative compressive strength ratio _{c} and relative splitting tensile strength ratio _{t} are defined to characterize the variation of the strength of the modified concrete:_{c} and relative splitting tensile strength ratio _{t} changes of modified concrete corroded by Na_{2}SO_{4} and (NH_{4})_{2}SO_{4} are presented in Figures

Relative compressive strength ratio of modified concrete corroded by Na_{2}SO_{4} solution.

Relative splitting tensile strength ratio of modified concrete corroded by Na_{2}SO_{4} solution.

Relative compressive strength ratio of modified concrete corroded by (NH_{4})_{2}SO_{4} solution.

Relative splitting tensile strength ratio of modified concrete corroded by (NH_{4})_{2}SO_{4} solution.

Figures _{2}SO_{4}. Figures _{4})_{2}SO_{4}.

It is clear that the relative strength ratio curve of modified concrete always exceeds 1.00, which indicates that the addition of admixture improves the ability of sulfate resistance of concrete. Physically speaking, the addition of fly ash reduces the porosity ratio of the concrete and improves the porosity characteristic by replacing a large quantity of cement and fine aggregate, so the impermeability of concrete is increased [_{4}^{2−} in pores. In addition, the active effect of fly ash can consume calcium hydroxide, which reduces the amount of calcium hydroxide reacted with sulfate, to slow down the process of sulfate attack and reduce the degree of erosion. Addition of polypropylene fibers effectively controls the development of microcracks of concrete caused by solid plastic shrinkage, drying shrinkage, and temperature change. The performances of seepage proofing and crack resistance of concrete are improved [

As can be seen from _{c} curve, the relative compressive strength ratio of the modified concrete roughly presents growth trend. Due to the faster growth of concrete strength mixed with fly ash, the growth rate of compressive strength of the modified concrete is higher than that of the ordinary concrete in the early stage of corrosion. At the deterioration stage, the strength of the ordinary concrete drops faster. The value of _{c} reaches its maximum after 10 dry-wet cycles. The compressive strengths of fly ash concrete and double-mixture concrete increase 27.7% and 28% than those of ordinary concrete in Na_{2}SO_{4}, respectively, and increase 28.9% and 29.3% than those in (NH_{4})_{2}SO_{4}. The results show that the admixture materials significantly improve the ability of concrete against sulfate corrosion.

As can be seen from _{t} curve, the relative splitting tensile strength ratio of fly ash concrete and double-mixture concrete has the maximum value of 1.554 and 1.700, respectively, at the beginning of corrosion. The trends of _{t} of modified concrete decline at first and then increase later in the two solutions. Compared with the modified concrete, the splitting tensile strength of the ordinary concrete grows faster in the early stage of corrosion and decreases faster in the later stage.

Comparing _{c} and _{t} curves of the modified concrete, the strength of double-mixture concrete is higher than that of fly ash concrete at initial corrosion period. The compressive strength and splitting tensile strength of double-mixture concrete are 1.105 times and 1.091 times as that of fly ash concrete, respectively. However, the strength ratio curves of two kinds of modified concretes are almost coincided after the second cycle. According to the test data, the strength difference between the two kinds of concrete is not more than 10%. Therefore, polypropylene fiber plays a role in improving the performance of seepage and crack resistance of concrete but has little effect on increasing the ability of concrete against sulfate corrosion compared with fly ash concrete. Considering the economic efficiency and resource conservation, it is suggested not to add polypropylene fiber into fly ash concrete.

As mentioned above, fly ash concrete is used as an example to predict the failure time of concrete corroded by sulfate solution. The relative compressive strength is adopted as an index to evaluate the damage process of concrete under sulfate attack._{c} is the relative compressive strength of concrete specimen suffering drying and wetting cycles, _{cn} is the compressive strength of concrete specimen after _{c0} is the compressive strength of concrete specimen without corrosion.

The relationship between compressive strength of fly ash concrete and number of drying and wetting cycles can be derived from the test results when the sulfate concentrate is 1000 g/L.

When concrete corroded by Na_{2}SO_{4},

When concrete corroded by (NH_{4})_{2}SO_{4},_{2}SO_{4} and (NH_{4})_{2}SO_{4}. It is reliable to use the above two formulas to predict reduction of strength and service life of concrete.

According to “Standard for Test Methods of Long-Term Performance and Durability of Ordinary Concrete” in the Chinese standard [_{c} is adopted as an evaluation parameter, and the calculation formula is as follows:_{c} is the failure coefficient of compressive strength. When the relative compressive strength is less than 75%, concrete specimen is regarded as damaged.

Based on (_{2}SO_{4} and (NH_{4})_{2}SO_{4} reduce less than 75% after 18 and 16 circulations of drying and wetting; namely, the compressive strength of fly ash concrete reaches its failure bound.

The two different corrosion solutions make little difference on the compressive strength of modified concrete after the same drying and wetting cycles. However, the splitting tensile strength suffering from Na_{2}SO_{4} is generally higher than that in (NH_{4})_{2}SO_{4} at initial strengthening stage. At performance deterioration stage, the splitting tensile strength of modified concrete decreased more rapidly in Na_{2}SO_{4} to be lower than that in (NH_{4})_{2}SO_{4}.

The addition of admixture significantly improves the ability of sulfate resistance of ordinary concrete. When suffered 10 times of drying and wetting cycle, the compressive strength and splitting tensile strength of modified concrete increase by 28% and 19%, respectively.

Polypropylene fiber improves the sulfate resistance ability of fly ash concrete to a small degree. Considering construction cost, it is not advised to add polypropylene fiber in fly ash concrete to resist sulfate attack.

Life prediction formula for fly ash concrete corroded by sulfate under the designed sulfate concentration was proposed. Based on the formula, fly ash concrete reaches its compressive strength failure bound and is not suitable for continuous service after 18 and 16 cycles of drying and wetting corroded by Na_{2}SO_{4} and (NH_{4})_{2}SO_{4}, respectively.

The authors declare that there are no conflicts of interest in this paper.

The authors would like to acknowledge the financial support received from the National Natural Science Foundation of China (Grant no. 51679221), sponsored by Program for Science & Technology Innovation Talents in Universities of Henan Province (Grant no. 16HASTIT012).