The challenges of testing Fiber Reinforced Concrete (FRC) go well beyond what we would normally think of with respect to concrete testing. The difference lies in the fact that most of the valuable information from an FRC test comes after the concrete fails. The ability of FRC to resist crack growth after the initial crack is determined by measuring the toughness (typically area under the curve) of the sample as it is deflected well past the first crack. All of these tests must be run under servo-control and one of the more common tests must be run using the measured beam deflection as the servo feedback. This can be very challenging as the specimen goes from being very stiff to very compliant after the first crack.
FRC is one of the fastest growing segments in the concrete industry as more and more engineers, architects, owners, specifiers, and concrete contractors are turning to the use of fibers to supply their reinforcing needs in their concrete applications. In many cases, fibers are replacing traditional rebar. Although still a small part of overall concrete production, some estimates have the North American market growing at 20% annually.
A US-based company is currently in the "Pre-Feasability" phase of excavating a copper mine where they are boring a shaft vertically to a depth of nearly 7000 ft (2100 m). Once they get to this depth they will bore horizontal shafts under the copper deposit and mine the ore from beneath. They do not expect to start producing ore until 2020. Talk about a long-term investment!
As they bore the shaft it is being lined with Shotcrete, which is basically a fiber reinforced concrete product that is shot out of a tube onto the walls of the shaft. As they apply the Shotcrete they also make 3 beam samples for testing to ASTM C1609. The samples are placed in a climate-controlled storage area until they are about 12 hrs old and ready for testing. The samples are then tested on the custom 300 KN testing system to determine the strength and toughness of the concrete. If the results indicate adequate strength they can allow people to re-enter the shaft to continue excavating. If not, they must tear the old shotcrete down and re-apply. The time requirements and critical nature of the testing make it impossible for them to use a test lab.
As this was our first experience with FRC testing, we had a steep learning curve with respect to how the specimens react. The test standards certainly do not tell the whole story here. Our biggest challenge came on the ASTM C1609 test, which is a Third Point beam loading of a 150 x 150 x 450 mm (6 x 6 x 18) in sample. During the test, deflection of the sample is measured using 2 LVDT's fixtured to the specimen; one on either side of the specimen to get the average deflection.
The ASTM standard requires the machine to be in servo-control using the LVDT's as feedback to maintain a deflection rate of approximately 0.05 mm/min (0.002 in/min). This is already looking challenging. But wait, there are more challenges. Unlike a typical concrete beam test that ends after the concrete cracks, this test must run well past the initial crack and maintain the same deflection control mode. The testing machine must be able to maintain servo-control through the large deflection change that occurs during the initial crack and it must also accommodate the very large change in specimen compliance that occurs when the concrete fails and the fibers begin to take load.
Potential projects suited to the use of fiber reinforced concrete:
- Residential: driveways, sidewalks, pool construction with shotcrete, basements, colored concrete, foundations, and drainage
- Commercial: exterior and interior floors, slabs and parking areas, and roadways
- Warehouse / Industrial: light to heavy duty loaded floors and roadways
- Highways / Roadways / Bridges: conventional concrete paving, SCC, white-toppings, barrier rails, curb and gutter work, pervious concrete, and sound attenuation barriers
- Ports and Airports: runways, taxiways, aprons, seawalls, dock areas, and parking and loading ramps
- Waterways: dams, lock structures, channel linings, ditches, and storm-water structures
- Mining and Tunneling: precast segments and schotcrete, which may include tunnel lining, shafts, slope stabilization, and sewer work
- Elevated Decks: commercial and industrial composite metal deck construction and elevated formwork at airports, commercial buildings, and shopping centers
- Agriculture: farm and animal storage structures, walls, silos, and paving
- Precast Concrete and Products: architectural panels, tilt-up construction, walls, fencing, septic tanks, burial vaults, grease trap structures, and bank vaults and sculptures