Experimental Study on Mechanical Properties of Repair Materials for Underwater Rapid Construction

Several strength grades of sulphoaluminate cement concrete were designed, and the mechanical properties with different mix ratios were studied. This paper mainly analyzes the mechanical properties of sulphoaluminate cement concrete with the setting time test, compressive strength test and flexural test. The test results show that the setting time of cement concrete can be controlled by mixing different admixtures. One hand, the initial setting time increases from 34 min to 340 min, and the final setting time increases from 57 min to 580 min when the incorporated borax content changes from 0 to 1.0 %. Other hand, the initial setting time decreased from 34 min to 11 min, and the final setting time increased from 57 min to 18 min, when the incorporated borax content changed from 0 to 0.5 %. Furthermore, the compressive strength can reach 40 MPa, and the flexural strength can reach more than 2.5MPa after 6 hours of curing. The experiment results illustrate that the setting time and the mechanical properties satisfy the needs of the rapid construction requirements under normal temperature conditions.


Introduction
The number and scale of hydraulic structures in China have caught up the world level. What's more, the Three Gorges Dam, Baihetan Hydropower Station and the South-to-North Water Diversion Project under construction have become the largest water conveyance and diversion projects in the world. However, natural disasters occur frequently in our country. Landslides and breaches caused by flood disasters, and long-term use often destroy the underwater parts of deep-water buildings, most of them begin to leak. The leakage of underwater concrete structure is mainly divided into point leakage, linear leakage and flow leakage. Point leakage is divided into one or more points, most of which are multipoint leakage. Most leakage points are dispersed and the range is relatively large. The flow leakages usually appear in the contact area between the water conveyance building and the adjacent building. Due to the long-term washing and erosion, leakage occurs in the contact area, and leakage channels are formed along the edge of the water conveyance building. Linear leakage can be divided into leakage of disease cracks, contact and expansion joints. The crack-leakage is mainly caused by penetrating cracks inside the concrete structure, while the contact joint and expansion joint leakage is mainly caused by the failure of waterproof structure used in the construction process of water conveyance buildings. The conventional plugging methods include direct plugging method, lower pipe plugging method, wood wedge plugging method and grouting plugging method. Different plugging methods are used according to different construction environments and conditions. The direct plugging method is mainly aimed at the leakage point with water pressure less than 1 m. The lower pipe plugging method is mainly aimed at the leakage channel corresponding to the water head of 1 m -4 m, when the wood wedge plugging method is mainly aimed at the leakage channel with depth greater than 4 meters. The grouting method can be used to seal holes with large water pressure, large holes and large water leakage. The leakage passages of underwater concrete structures in deep-water buildings are often located in the deep underwater position, especially for the leakage passages at 300 m water depth. Under the conditions of low temperature and high pressure, the grouting materials should meet the requirements of rapid hardening at low temperature, rapid development of strength and micro expansion.
Therefore, it is of great significance to develop an early-strength, high-strength and microexpansion material for the repairment of underwater leakage channel at low temperature conditions. To overcome technical difficulties for the micro-expansion grouting materials, which can rapidly harden under low temperature conditions of underwater engineering, the project team mainly studies the theory and practice of grouting materials. The setting time, early strength, high strength and microexpansion of grouting materials are studied, because these indexes are important to repair leakage points under low temperature conditions [1]. Usually, different types of leakage channel grouting materials have different characteristics. Ordinary Portland cement-based grouting materials are suitable for general ground engineering due to their poor water resistance and chemical corrosion resistance. Thanks to the short setting time, Sulphoaluminate cement grouting material are often used to repair the emergency construction, shotcrete anchor support, slurry anchor node, cementing plugging, winter construction application. What's more, Sulphoaluminate cement grouting material can meet the requirements of fast hardening and early strength under the condition of low temperature [2]. Compared with the ordinary Portland cement-based grouting materials, sulphoaluminate cementbased grouting materials can be rapidly constructed at low temperature. However, the setting time is so fast that the sulphoaluminate cement concrete have been hardened before the construction is finished. Due to this reason, the sulphoaluminate cement can't be directly used to construct. Otherwise, the only sulphoaluminate cement can't achieve the effect of rapid construction. Instead, it blocks equipment and repair pipelines, affecting the construction period. In order to control the setting time of sulphoaluminate cement, the heating of sulphoaluminate cement should be reduced [3]. Meanwhile, the indexes of early strength, high strength, no shrinkage, and micro expansion should meet the requirements. Most researchers mixed silicate cement and sulphoaluminate cement to prepare the cement-based grouting material of silicate cement and sulphoaluminate cement. Similar preparation methods also include the cement-based grouting material composed of silicate cement and aluminate cement, but the performance is not stable when two or more types of cement are mixed, and the production processes are complex and the cost is high, which is not the best choice [4].
Firstly, the setting time of the sulphoaluminate cement should satisfy the needs of construction. Simultaneously, the subsequent strength of grouting material can increase stably. Furthermore, the construction technology of grouting material should be improved for leakage passage of underwater concrete structure. In this paper, early strength and high strength of the sulphoaluminate cement grouting material is developed. The setting time of cement clinker is reasonably controlled by the retarder and accelerator, and the preparation technology of early strength grouting material at normal temperature is developed.
Na2SiO39H2O is colorless orthogonal bipyramidal crystal or white to gray-white block or powder. At 100 °C, 6 molecules of crystal water were lost, as shown in The aluminium sulfate (Al2 (SO4)318H2O), the maximum impurity content isn't more than 1 %, and the result of pH is 2.7(2.5, 25g/L, 25℃), see table 2.

Compressive Test.
In this experiment, a total of five groups of new rapid hardening retarding concrete were prepared, and the effects of water-cement ratio and water reducer content on rapid hardening concrete were studied.

Bending Test.
Dimensions of 100mm100mm400mm concrete specimen were used in the bending test. The concrete specimens should be cured in the standard curing room, and the dimensions of these concrete specimens should be checked before the bending test. The test device is shown in figure 1. During the bending test, if the fracture position of the lower edge of the specimen is between two concentrated load action lines, the flexural strength (MPa) of the specimen should be calculated as follows:  Figure 1. Bending test device.   From figure 4 and table 7, it can be seen that the initial setting time and final setting time increase first and then decrease when the sodium silicate content changes from 0 to 1.0 %. When the sodium silicate content is 0.2 %, the initial setting time is 42 min. Compared with the setting time without sodium silicate, the change was not significant. The test results indicate that the sodium silicate was not stable for the setting time of sulphoaluminate cement.   As shown in figure 5 and table 8, it can be seen that the initial setting time reduces from 34 min to 11 min, and the final setting time reduces from 57 min to 18 min when the borax content changes from 0 to 0.5 %. During the setting time test, it is found that the sulphoaluminate cement concrete specimen is prone to cracks in the later curing process, if the lithium carbonate in the amount of admixture increases so much. Therefore, it is necessary to reasonably control the content of lithium carbonate when the lithium carbonate is considered as the accelerator of sulphoaluminate cement.

Compressive Test
Dimensions of 100 mm100 mm100 mm cube specimens were used in the compressive strength test.

Bending Test
The experimental results of the influence of different water-cement ratios on the flexural strength of low sulfur aluminate cement concrete are shown in figure 7 and table 10. As can be seen from figure 7, the concrete specimens remain integrated after the bending test. The test results indicates that the sulphoaluminate cement concrete has good toughness. As can be seen from table 10, with the increase of water-cement ratio, the flexural strength shows an increasing trend with the curing age.