GRID-TYPE STRUCTURES

Although a majority of developments in the area of composite reinforcement for concrete have focused on the investigation of composite rebar, it must be stressed that the specific form is not the best application of composites, due to reasons related to bond development. In this section, we describe a representative 2-D grid structure called NEFMAC New Fiber Composite Material for Advanced Concrete) as shown in Figure Figure 4.1


Figure 4.1. Schematic of NEFMAC Grid

NEFMAC is a grid-type reinforcement for concrete structures. It consists of high- performance fibers such as glass, carbon, aramid, and hybrids, impregnated with resin systems ranging from vinylesters and other thermosetting resin systems to thermoplastics. Besides the inherent corrosion resistance, the grid form itself is advantageous in that the intersections provide anchorage and mechanical interlock in the concrete, facilitating good stress transfer. Again, due to the non-corrosive and alkali resistant (based on resin selection) nature of the grid, cover requirements are reduced, resulting in lighter slabs and other concrete elements. A listing of applications is given at the end of this section.

NEFMAC is produced by the NEFCOM corporation -- a cooperative venture between Shimizu Corporation and Asahi Glass Matex Company (formerly Dainihon Glass Industrial Company). The actual method of production is shown in the schematic in Figure 4.2, and is termed "pin winding." The process is similar to filament winding in that individual fibers/tows are placed in prespecified patterns after being impregnated in a wet-bath. The process is a semi-batch type operation, unlike the continuous operation of forming NESTEM geosynthetic material for soil reinforcement. In the NEFMAC process a series of continuous fibers are dispensed from individual creels by a mechanical system, through a wet-bath to be deposited by two orthogonal traveling (winding) heads. The heads are moved at synchronized speeds that define the size of the grid. Successive movement of the heads results in fiber cross-over and placement of interlocking layers until the desired content/cross-sectional area is achieved. The process is currently capable of line speeds as high as 2 m/min in continuous mode, with about 200 m(2) of grid produced per hour. Cure is achieved through the use of either infra-red or ultraviolet heat sources assisted by peroxide catalysts in the resin system. Post-cure is conducted at room temperature. A list of primary characteristics and their resulting features is given in Table 4.1.


Figure 4.2. Schematic of the Pin-Winding Process

Four basic types of NEFMAC are available based on the type of fibers used. A comparison of gross properties is given in Table 4.2 and representative stress-strain profiles are shown in Figure 4.3. A wide variety of fiber types are used within these broad classes, the details of which are given in Table 4.3.

Table 4.1
Principal Characteristics of NEFMAC

Table 4.2
Main Types of NEFMAC Based on Reinforcement Type
(Vinylester Resin Based)


Figure 4.3. Typical Stress-Strain Profiles

It is interesting to note that the range is wide and includes fibers manufactured in Japan (Torayca - Toray Industries, Besfight - Toho Rayon Company, Technora - Teijin) and abroad (Kevlar 49 - DuPont). The hybrid, using a combination of glass and carbon, is designed to have a proportional limit and bi-linear stress-strain profile, similar to that of steel. Other hybrids such as high strength or high-modulus carbon and aramids are also available, based on specific needs. The size of a NEFMAC grid is given by specifying the area of the bars and the interval spacing. Figure 4.4 gives a detailed schematic of the grid with details of geometrical specifications.

The cross-sectional area varies from 32 - 3806 mm(2) for glass, carbon and aramid based NEFMAC grids, and from 284 - 3806 mm(2) for hybrid grids. Typical interval spacings are 25, 50, 75, 100 and 150 mm (nominally 1, 2, 3, 4 and 6 inches). Although the standard profiles are flat panel-type structures, L and curved profiles are also available, as are full 3-D cage type structures that include shear reinforcement. Standard specifications of the NEFMAC grid are given in Table 4.4.

Table 4.3
Details of Fiber Type


Figure 4.4. Geometrical Features of NEFMAC

Within the limits of this chapter we will briefly review the use of NEFMAC in three applications: (1) slabs, (2) shotcrete reinforcement in tunnels, and (3) 3-D reinforcement in beams.

Slabs

NEFMAC reinforcement has wide application in concrete slabs due to its corrosion and chemical resistance, its light weight, and its need for significantly less cover. A comparison of the behavior of the reinforcement types used in one study is shown in Table 4.5 and a schematic of the slab reinforcement is given in Figure 4.5.

Table 4.4
Standard Grid Specifications (Available Commercially)

Table 4.5
Comparison of Reinforcements


Figure 4.5. Details of the Test Slab

An overall comparison of behavior is given in Figure 4.6, which also shows the effect of temperature. It was concluded that deflections in NEFMAC were comparable to those with steel until 600C, with almost no recognizable differences. The Japanese code for allowable deflection is also met by the NEFMAC grids at the one-hour fire resistant levels under specific design details.

Shotcrete Reinforcement in Tunnels

NEFMAC shows considerable potential for use as reinforcement of shotcrete in tunnels because of its corrosion and chemical resistance, its light weight, and its ease of forming to fit curvatures. A typical M- curve comparing NEFMAC reinforced and welded wire fabrics reinforced shotcrete panels is shown in Figure 4.7.

Since the reinforcing material is placed along the center plane in such applications and the reinforcement ratio is small, the maximum load coincides with cracking load, irrespective of the kind of reinforcement. NEFMAC would appear to be better because of the advantages stated earlier. Table 4.6 gives a listing of the applications of NEFMAC in shotcrete between May 1986 and June 1987. It is significant that no failures have been observed so far, and it should be noted that under some of the prevalent conditions, metallic reinforcement would have degraded due to corrosion and/or chemical attack resulting in overall failure/cracking of the structural element.

3-D Reinforcement in Beams

It is possible to create 3-D cages of NEFMAC that are analogous to the steel reinforcement cages comprised of longitudinal and shear reinforcement as in Figure 4.8. Tests in fatigue have demonstrated that NEFMAC has a fatigue strength equal to or greater than that of a reinforcing steel bar.

Applications

NEFMAC is currently claimed to be effective for use in:

  1. Tunnel supports and supports for storage containers
  2. Airport facilities such as runways and aprons
  3. Roads and bridge structures
  4. Marine and offshore structures
  5. Power plant facilities
  6. Architectural features and structures such as exterior walls, handrails, etc.


Figure 4.6. Stress/Strain Behavior of NEFMAC as a Function of Temperature/Time


Figure 4.7. Comparison of M- Behavior


Figure 4.8. NEFMAC Grid

Table 4.6
Representative Use of NEFMAC in Existing Structures in Shotcrete

Table 4.7 gives a comprehensive overview of the applications of NEFMAC, and shows its versatility of use. A number of claims have been made regarding the overall cost-effectiveness of NEFMAC as compared to steel grids. An example of this is the construction of the new Shimizu building, where it was claimed that although the cost of materials was higher, significant systems-level savings were achieved due to the factors of weight (i.e., no need for specialized lifting equipment, the increased ease of placement of the structure, improved life cycle, and lower overall structural weight). However, no hard evidence has yet been seen to prove the claims as such. It must be stated that the advantages and potential for systems-level cost savings (even on a purely acquisition cost basis) make this a very attractive use of composites in the civil engineering area.

Table 4.7
Applications of NEFMAC

Table 4.7 Applications of NEFMAC (Continued)

Table 4.7 Applications of NEFMAC (Continued)

Table 4.7 Applications of NEFMAC (Continued)

Table 4.7 Applications of NEFMAC (Continued)


Published: April 1994; WTEC Hyper-Librarian