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TEST METHOD and RESULTS for FORTRESS STABLIZATION SYSTEMS
Carbon Fiber Straps Anchored to Concrete
Saginaw Valley State University
Independent Testing Laboratory
March 22, 2005
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Specimen Description
Carbon/Kevlar 2x2 fiber weave and unidirectional carbon fiber was received from Fortress Stablization Systems. Fifteen concrete blocks with nominal dimensions of 13” long x 7.7” wide x 3.6” thick were purchased from a local home & garden shop. The blocks were broken under three loading. All fifteen blocks were repaired using Emecole 101 medium viscosity paste, a standard construction grade 1:1 grey paste, as shown in Figures 1 & 2. An anchor strip of Carbon/Kevlar weave was glued perpendicular to the crack in ten blocks using Fortress Toughened #4020 Fast Epoxy. The anchor strip was seated into place by inserting the strip tabs in two 1” pre-drilled holes (see Figures 3 & 4.) Five of these ten blocks were wrapped with unidirectional carbon fiber cloth as shown in Figures 5 & 6. The carbon cloth strips were 4” x 42” wetted out using E-Bond 2-part resin and 12767PL hardener. The E-Bond and toughened epoxy were allowed to dry for 3 days at room temperature before testing.
Test Method
A three point flex test approach was applied as shown in Figures 7 & 8. In this case, an Instron 1125 with MTS ReNew Upgrade package was used to apply the load at 1mm/minute crosshead speed. The blocks were broken using a twelve inch span with the load being placed in the middle covering an area of 6 x 0.75 as shown in Figures 7 & 8.
Test Results and Discussion
Ten cement blocks with nominal dimensions of 13” long x 7.7” wide x 3.6” thick were broken and the results are summarized in the table below.
| Specimen |
Peak Load (lbs) Virgin Block |
Stress (PSI) Virgin Block |
Peak Load (lbs) Repaired Block |
Stress (PSI) Repaired Block |
Peak Load (lbs) Anchored Block |
Stress (PSI) Anchored Block |
| 1 | * | * | 2335 | 421 | | |
| 2 | 2726 | 492 | 2006 | 362 | | |
| 3 | 2830 | 510 | 2601 | 469 | | |
| 4 | 2699 | 487 | 2708 | 489 | | |
| 5 | 2873 | 518 | 3190 | 576 | | |
| 6 | 2673 | 482 | | | 3439 | 620 |
| 7 | 3335 | 602 | | | 3280 | 592 |
| 8 | 3085 | 556 | | | 3119 | 563 |
| 9 | 2671 | 482 | | | 3576 | 645 |
| 10 | * | * | | | 3624 | 654 |
| * data point not captured |
An additional 5 specimens were tested with the results shown below.
| Specimen |
Peak Load (lbs) Virgin Block |
Stress (PSI) Virgin Block |
Peak Load (lbs) Anchored and Wrapped Block |
Stress (PSI) Anchored and Wrapped Block |
| 1 | 2112 | 381 | 4884 | 893 |
| 2 | 1992 | 359 | 4897 | 895 |
| 3 | 2642 | 477 | 5795 | 1059 |
| 4 | 2160 | 390 | 4771 | 872 |
| 5 | 2256 | 407 | 4166 | 761 |
Failure modes of the various blocks are shown in Figures 9 through 14. The blocks repaired with the grey paste broke at similar loads just slightly to the side of the first crack line. The blocks repaired with Fortress Anchors required higher loads with the crack following around the carbon strip. The wrapped blocks showed higher loads with similar cracks.
Testing Conclusions
The purpose of these tests is to show that Carbon-Kevlar staples improve crack control, and actually create a structural repair.
Although Epoxy is considered a structural repair, it is limited in that it returns fracture to as designed condition. In our experience, regardless of the strength of the Epoxy resin (this resin 5,700 p.s.i. in tensile), it provides no extra strength to typical concrete/masonry.
The average break of C.M.U. was 2,600 p.s.i. at breaking point. The average break of same bonded with Epoxy was 2,600 p.s.i. next to glue line. By adding Carbon-Kevlar staple the average break was 3,100 p.s.i. However the important factor is that the break occurred outside of the glue line and outside of the staple area, also the failure was compression shear.
A third test was performed to increase compression strength of the C.M.U. in an effort to take the staple and/or the glue line to failure. In order to achieve this, the c.m.u. was wrapped with 2 layers of 300 g.s.m. carbon tow sheet to improve compression shear. Average breaking point was 4,900 p.s.i. or 886 tensile, once again c.m.u. began to fail inside of wrap yet outside of glue line and staple in a crushing manner.
It has been discussed to use 5,000 p.s.i. concrete reinforced to increase numbers even much higher, however we wanted to keep scope as close to real life application with weaker/standard substrates.
Final conclusion of scope: By utilizing Carbon-Kevlar Grid Staples in conjunction with crack repair materials, the Carbon-Kevlar will carry loads away from fractured area therefore keeping glue line intact. We believe this will be beneficial wherever concrete is repaired from walls, slabs, beams, etc.
Finally, if under-designed concrete exists away from fracture, adding carbon reinforcement to these areas would bring concrete up to par with repaired area.
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Figure 14 |
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