Introduction
The application of functionally graded material in concrete has attracted more and more attention over the last years. Maalej et al. [
1] investigated the corrosion resistance and failure characteristic of functionally graded concrete beam and ordinary concrete beam. The experiment reveals that, compared to ordinary concrete beams, functionally graded reinforced concrete beams have better resistance to corrosion, and thus more durable. Yang et al. [
2,
3] studied the preparation of cement-based graded material. The impact of graded distribution of components and density on the mechanical and heat-conduction properties in cement-based materials was investigated. The results show that the continuous gradient change of the component can be basically ensured, its thermal insulation and bending and compressive strength have been obviously increased by the method of multi-layer vibrating formation.
However, the existing study only focused on the impact of this technique on the performance of concrete. Reports on the exploration of related theory and the application of functionally graded concrete are unavailable. In this paper, the layer-by-layer casting method was adopted to prepare functionally graded concrete segment (FGCS). The performance test revealed that the durability of FGCS is apparently higher than that of ordinary segment.
Design of FGCS
For the environment of the Wuhan Yangtze River Tunnel, the segments used in shield tunneling lining structure should have high durability and long service life. Based on the principle of functionally graded materials (FGMs), the FGMs for reinforced concrete segment were designed and optimized. By the method of multi-layer vibrating formation, the segments with characteristics such as high impermeability of covering layer, high strength of main structural layer and fire-resistance of inner layer, were prepared. Furthermore, good manufacturing technique of the interface was adopted to ensure the homogeneous transition of different material layers, and the integrated design effects of function and structure were achieved.
The structure of FGCS is shown in Fig. 1. The outer surface of the FGCS is a high anti-permeability covering. A meso interfacial transition zone free of cement-based material (MIF) [
4] was used as high anti-permeability covering layer. The C50 high performance concrete was used for preparing the structural layer. The fire-retardant coating was sprayed on the inner surface of the structure as fire-resistant layer.
Preproduction of FGCS
Production flow
The production of FGCS has the following stages: 1) mould assembly; 2) adjustment of mould; 3) reinforcement cage fixing; 4) casting of structural layer; 5) casting of high anti-permeability covering; 6) steam curing; 7) demoulding; 8) examining and patching of FGCS; 9) water curing; 10) spraying of waterproof coating; 11) spraying of fire-retardant layer.
Procedure and quality control
Preparation work
Clear the surface of mould, and then brush the demoulding agent. Assemble the mould following the setting procedure. After assemblage, the measurements were committed to decide whether the thickness, length of arc and position of handhold meet the requirements. The adjustment should be done for the size below grade. The micrometer calipers and steel tape were adopted in the measurement, and the corresponding errors were+0.2/-0.4 mm and±1 mm.
After the preparation of mould, the reinforcement cage was fixed in the mould. The impact between the reinforcement cage and the mould should be avoided in lifting. The end of the screw was inserted in the handhold so that there is no gap and paste leak. The bearer supporting bracket of the covering layer of the reinforcement cage should be placed properly to ensure the outboard and inboard thickness of the covering layer of the reinforcement cage are 50 mm and 40 mm respectively.
Casting of structural layer
The mix proportions of structural layer concrete are shown in Table 1. J0 is the concrete mix proportion of ordinary segment. J1 is the concrete mix proportion of structural layer of FGCS.
When casting, fresh concrete was poured into the mould several times. After each pouring, the concrete was vibrated. The concrete was first poured into the end of the mould, then the middle. After adding appropriate quantity of the concrete, the interface treating board was used to cover the concrete. The concrete was quivered with the inserted bar.
Casting of high anti-permeability covering layer
The mix proportion of the MIF is shown in Table 2. The slump of the MIF was controlled in the range of 140±10 mm. The compressive strength of the MIF is above 15 MPa after 12 h steam curing, and 35 MPa after 7 d water curing. The 28 d compressive strength is above 60 MPa. The chloride diffusion coefficient is below 0.8×10-13 m2/s. The seepage resistance grade is above S40.
After finishing the casting of the structural layer for about 20 min, the fresh concrete of the covering was poured on the structural layer. The concrete was vibrated by inserted bar for 10–20 s, then the bar was drawn out slowly and the roof of mould was covered on the concrete.
Finishing
The roof of the mould was uncovered after having finished the casting for about 45 min. An A-alloy nose bar was employed to level the surface of the concrete, and then trowel finished. After casting for about 1 h, the anti-cracking curing agent was sprayed over the surface of the segment.
Steam curing
Curing by steam improves the strength and shortens the curing time. After the initial setting of concrete, the roof of mould was covered again and the mould was coated with an air-tight hook for steam curing.
After curing, the compressive strength of the specimen prepared under same condition was tested. If the strength were above 15 MPa, demould the segment. When demoulding, loosen the fixing screw of the grout hole first, uncouple the side board of mould next, then uncover the roof, connect hanger to the segment and demould.
Spraying of waterproof coating
After demoulding, the segment was cured in water for 7 d. The next procedure is the spraying of waterproof coating on the external cambered surface. Before that, clear up the oil soil and dust the surface. Spray nozzle should be close to the surface of the segment to ensure that the micropore and microcrack on the surface would be coated. The second spraying was done 4 h after the first. If the surface was dry, water should be sprayed to avoid cracking.
Quality control
Dimension examination for single segment
The Vernier calipers with the range of 0-2100 mm and 0–510 mm were used to measure the width and thickness of the segment. The steel measuring tape with a length of 5 m was adopted to measure the length of arc. The torsion of the segment was measured by a nylon wire with the diameter of 1 mm and length of 7 m. The quality control specification is shown in Table 3.
Dimension examination for segment erection
The quality control specification of segment erection is shown in Table 4.
Cracking observation of segment appearance
Cracking of the ordinary segment and FGCS was observed by naked-eye and width gage. The results of observation show that there are distinct macroscopic cracks on the surface of the ordinary segment. Measurement by width gage indicates that the width of the crack was 0.15–0.3 mm. However, no crack could be observed by the naked-eye on the surface of the FGCS. The width of the crack measured by width gage was only 0.02–0.1 mm. Moreover, MIF is a self-repairing material. Cracking with a width less than 0.1 mm is acceptable for FGCS.
Mechanical test of segment
Conventional test method for mechanical properties of concrete can not attain the performance of segment due to the huge size. The rebound method and comprehensive method of ultrasonic rebound was adopted to detect the 28 d compressive strength of the ordinary segment and FGCS, the compressive strength of the specimen prepared for conventional test was considered at the same time. The results of the test are shown in Fig.2. It is indicated that the compressive strength of the FGCS is higher than that of the ordinary segment, regardless if it’s in the inner side or outer side.
Impermeability test
The initial water pressure of impermeability test is 0.2 MPa. The water pressure should be increased by 0.2 MPa every 15 min, then maintain the pressure for 4 h after it reaches 0.8 MPa. Observation on the concrete after the test showed that the water penetration depth in the ordinary segment is about 15 mm, whereas the depth in the FGCS is 0.
Summary
1) In this paper, the structure and material design method was proposed for FGCS. The two-layer or multi-layer gradient composite of different cement-based material can improve the performance of concrete, and meet the special requirement.
2) The interface treating board was designed, processed and used in the FGCS casting. The structural layer and covering layer were inserted at the interface between two layers after treatment, thus enhancing the interface of the FGCS.
3) The performance test was conducted on the FGCS. The results revealed that the FGCS has outstanding performance. The 28 d compressive strength of the outer side (covering layer) of the FGCS is over 80 MPa, and the strength of the structural layer can meet the requirement of concrete strength grade of C50. The crack-resistance of the FGCS is excellent, and there is no macroscopic crack on the surface of the FGCS. The water penetration depth under 0.8 MPa of the 28 d FGCS is 0.
4) The preparation technique of large diameter concrete segment is improved by the preproduction of FGCS. A large number of field data were obtained. It is helpful to perfect the design method of FGCS.
Higher Education Press and Springer-Verlag Berlin Heidelberg
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