Failure mechanism on sulfate attack and dissolved corrosion of diseased tunnel lining structure

3.1. Groundwater analysis

In order to analysis the salts concentration of groundwater. Water samples at the arch foot and initial support that suffered the most severe sulfate attack were collected. The ion chromatography instrument of Thermo Scientific (DIONEX AQUION RFIC) was used to analyze the ion concentration of the water samples.

3.2. Compression test

The rebound method and core sampling drilling were carried out to examine the concrete intensity and the degree of carbonization. Using LR-TH10 digital carbonization depth measuring instrument with 1 % phenolphthalein reagent to determine the concrete carbonation depth and ZC3-B rebound instrument was used to measure the concrete rebound strength. According to the relevant regulations of Technical Specification for Testing Concrete Strength with Drilling Core Method (JGJ/T 384-2016), the core samples ($d=$$\text{100 mm}$) were processed into cylindrical specimens with a height-diameter ratio equal to 1. Then, a computer-controlled flexural compressive testing machine (YAW-300C) was used to test the concrete compressive strength.

3.3. SEM, EDS, XRD and FTIR analysis

In the site investigation process, samples of the soft crumbly degraded concrete, travertine and white crystals were carefully chosen and preserved for laboratory investigations. The samples preparation process of corrosion products is shown in Fig. 1. Firstly, the samples were placed in a dry crucible and put them in an oven at 50°C for 5 days. And then, the dried samples were grinded into powder by agate mortar. After grinding, passed through a sieve with an aperture of 0.074 mm to obtain powder with a particle size no larger than 0.074 mm. Thus, samples used for SEM, EDS, XRD and FTIR had been prepared.

Fig. 1Samples preparation process

4.1. Ion concentration of groundwater

The salts concentrations of groundwater were shown in Table 2.

The results showed that the groundwater contains substantial concentrations of salts, especially sulfate ion. The SO₄^2- content of sample 3# reached 1180 mg/L. The HCO₃^- content of the groundwater was almost 100 mg/L-200 mg/L. The content of Mg²⁺ was relatively low and there was no Cl^-, Na⁺ and K⁺ basically. According to the environmental effects grade specified in Code for Durability Design of Concrete Structures in Highway Engineering (JTG/T 3310-2019), the tunnel was in medium-severe groundwater chemical corrosion environment and should be designed based on medium corrosion protection. Standard Standard for Design of Concrete Structure Durability (GB/T 50476-2019) ruled the strength grade of tunnel concrete should not be lower than C40 and corrosion resistant concrete needed to be applied, while the actual lining concrete was C25 ordinary concrete. It showed that the hydrogeological conditions were not ascertained accurately when the tunnel was constructed, which was one of the main reasons for the serious lining concrete corrosion.

4.2. Concrete compressive strength

The compression test results of lining concrete cylinder specimens provided in Table 3.

From Table 3, we could see that only in individual locations, the compressive strength of concrete met its design requirements, the compressive strength of most specimens was lower than its design value basically, especially at K0+613, the strength was only 13.07 MPa, which was much lower than 25 MPa and reduced by about 50 %. It could be seen that the lining structure deteriorated seriously, and the safety grade of lining structure reduced greatly.

5.1. Thaumasite

The tunnel lining concrete suffered serious sulfate attack, especially at the arch foot, soft crumbly degraded concrete was found in several places. The powder particle was used for SEM, XRD and FTIR analysis. Fig. 2 showed SEM images of the soft crumbly degraded concrete at two resolutions. It could be seen that there was little trace of the products of normal cement hydration such as calcium silicon hydrate (C-S-H) in the XRD trace due to the deterioration. Fig. 2(a) showed that a lot of club-shaped or needle crystals embedded irregularly in the pulpy material with large internal pores. And there was little CH crystal. Fig. 2(b) was the SEM image under higher magnification, showed that plenty of needle crystals up to 0.5 μm in diameter and 3-4 μm in length. It could be found the characteristics of crystal distribution was parallel orientation predominantly, and the surface was smooth. It was judged as ettringite or thaumasite preliminary.

The EDS spectrum of the soft crumbly degraded concrete was shown in Fig. 3. EDS analysis of O, S, Si and Ca indicated the presence of thaumasite. A higher Si and little Al peak indicated that these needle crystals were predominantly thaumasite and ettringite was almost absent. It could be preliminarily determined that the tunnel appeared thaumasite-type sulfate attack. But it was possible that the amorphousphase was closely associated with the thaumasite/ettringitecrystals and interferes with the analytical results obtained by EDS.

Fig. 2SEM image of the soft crumbly degraded concrete

a) Club-shaped crystals

b) Needle-like crystals

The XRD analysis result was shown in Fig. 4. It shown that several crystalline phases exist in the sample, including thaumasite, ettringite, gypsum and quartz. The characteristic peaks (0.956n m, 0.551 nm, 0.467 nm, 0.387 nm, et al.) of thaumasite were similar to those of ettringite (0.972 nm, 0.561 nm, 0.486 nm, 0.387 nm, et al.) [5]. The diffraction peaks positions of thaumasite and ettringite were similar on the pattern, so it was difficult to identify them by XRD method.

Fig. 3EDS spectrum of the soft crumbly degraded concrete

Fig. 4XRD pattern of the soft crumbly degraded concrete

FTIR was carried out and the result was shown in Fig. 5. The wave numbers of important infrared absorption bands which was identified by comparison with previously published data shown in Table 4.

Firstly, a peak was detected at 497 cm^-1which was assigned to the presence of SiO₆ bonds. Si was such an extremely rare coordinate sate for mineral silicates. The presence of this peak must be indicative of either thaumasite or a thaumasite-containing solid solution. Secondly, peaks at 1390 cm^-1 could be assigned to C-O stretching bending which attributable to the presence of the CO₃^2-, the peak at 1092 cm^-1 was related to SO₄^2-, but there was no sign of AlO₆ at 850 cm^-1. The above indicated that the soft crumbly degraded concrete contained thaumasite and little ettringite. Existing research showed that thaumasite-type sulfate attack mainly occurred at the interface between lining concrete and ground, corresponded to the site investigation results.

Sulfate, silicate, carbonate, sufficient water and low temperature environment were the necessary conditions for the formation of thaumasite [6]. The ion concentration test results in Table 2 showed that sulfate content in groundwater was sufficient. The second lining concrete of the tunnel adopted Portland cement. Calcium silicon hydrate (C-S-H) that produced by hydration of Portland cement were the primary source of silicate. The main source of carbonate, forming thaumasite, not only from aggregate but also from carbonated concrete, especially carbonated concrete. In addition, due to the existence of limestone in tunnel site, the flashing of groundwater would increase the content of carbonate ions. Water leakage from lining surface and arch foot provided sufficient water for the formation of thaumasite. Existing studies shown that thaumasite was easily formed at temperatures below 15 °C, especially 0-5 °C. However, some scholars found that thaumasite could also be formed at temperatures above 15 °C, the annual average temperature in the tunnel area was about 16.4 °C. The tunnel environment created proper conditions for the formation of thaumasite. It was further confirmed that thaumasite-type sulfate attack occurred at the arch foot.

Fig. 5FTIR spectrum of the soft crumbly degraded concrete

5.2. Drying-wetting cycle corrosion

During the rainy season, the lining surface was moist. In the dry season, there was less seepage or no seepage on the secondary lining and the lining surface was relatively dry. The lining structure was in the state of socking and drying repeatedly, which made lining concrete suffer drying-wetting cycle sulfate attack and white crystals formed on the lining surface (As 5# shown in Table 1). The SEM image and XRD pattern of white crystals were shown in Fig. 6.

From SEM image, the morphology of sodium sulfate crystal was cake-shaped with a diameter of 0.2-2 μm. XRD pattern showed that the substances mainly contained sodium sulfate crystals including thenardite (Na₂SO₄) and mirabilite (Na₂SO₄·10H₂O), and Na₂SO₄ was in the majority. When the crystallization pressure exceeded the tensile strength of concrete, the concrete structure would be destroyed. At different temperatures and relative humidity (RH), Na₂SO₄ and Na₂SO₄·10H₂O were interchangeable. Na₂SO₄ and Na₂SO₄·10H₂O would transform each other at about 32.4 ℃. When Na₂SO₄ was converted to Na₂SO₄·10H₂O, its volume increased by 315 % [7], but that didn’t produce crystallization pressure. When the saturation was constant, the crystallization pressure of Na₂SO₄ was much higher than that of Na₂SO₄·10H₂O.

Fig. 6SEM image and XRD pattern of white crystals

a) SEM image

b) XRD pattern

5.3. Dissolved corrosion

The SEM image and EDS spectrum of travertine were shown in Fig. 7. From Fig. 7(a), it could be seen that these crystals had scaly shapes and were stacked upon each other. And most of calcium silicon hydrate (C-S-H) had been decomposed without obvious distribution of C-S-H gel. The result of EDS spectrum shown that the main elemental composition of travertine was Ca, Si, O and C, and also contained a small amount of Mg, Al and Na. XRD pattern of travertine was shown in Fig. 8, it showed that the main phase was calcite and also contained a small amount of gypsum and quartz, which was corresponded to the EDS result.

Fig. 7SEM image and EDS spectrum of travertine

a) SEM image

b) EDS spectrum

Fig. 8XRD pattern of travertine

Under the groundwater seepage action, calcium hydroxide [Ca(OH)₂] in cement stone dissolved continuously [8], which reduced the density of cement stone. HCO₃⁻ in groundwater reacted with Ca(OH)₂ and formed CaCO₃. When the water pressure dropped, decarbonization occurred and CO₂ escaped from water, Ca²⁺ and HCO₃^- precipitated because of CaCO₃ dissolving in the surface layer, but when they were brought to the exterior of lining concrete by seepage action, CaCO₃ would form again due to the sudden drop of pressure (As 3# shown in Table 1). The dissolution and crystallization of CaCO₃ repeated again and again. When the crystallization pressure exceeded the concrete tensile strength, concrete on the lining surface would peel off.