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During our teleconference this
|Other Keywords||Epoxy Anchors|
|Date||July 30, 2014|
During our teleconference this morning, we had a discussion regarding appropriate epoxy anchorage depths for the F-shape PCB tie-down system and related research on epoxy adhesive anchorages in Wisconsin.
First, the barrier system we are discussing was a redesigned F-shape PCB that incorporated a three-loop connection that provided double shear at two locations on each pin. The bolt-through, tie-down system consisted of three 1â…›-in. diameter, ASTM A307 anchor bolts with heavy hex nuts and 3-in. x 3-in. x ½-in. thick washers spaced evenly across the traffic side of each PCB segment. Each anchor bolt was epoxied into the concrete with an embedment depth of 12 in. The test installation consisted of sixteen 12-ft 6-in.) long, redesigned F-shape PCB segments placed adjacent to a simulated bridge deck edge with a total system length of 204 ft. During test no. KTB-1, a 4,448-lb (2,018-kg) pickup truck impacted the system 5 ft – 5 in. upstream from the joint between barrier nos. 8 and 9 at a speed of 62.0 mph (99.8 km/h), and at an angle of 25.3 degrees. The system contained and redirected the vehicle with maximum lateral dynamic and permanent set deflections of 11.3 in. and 3½ in., respectively, and was considered successful according to TL-3 of NCHRP Report No. 350.
In the past, we have often been asked what embedment depth was required for the epoxy anchorage of the A307 rods used in that system. Adhesive anchorage capacity depends on many factors, including anchor size, anchor embedment, concrete strength, adhesive bond strength, spacing effects, edge effects, and other factors. Thus, we have typically recommended that the embedment for the anchor rods should be selected to develop the ultimate shear and tensile capacities of the anchorage. For the 1 1/8” dia. A307 rod, the ultimate shear and tensile capacities are 26.4 kips and 45.8 kips, respectively.
MwRSF has also done some recent work to investigate epoxy adhesive anchors for permanent concrete barriers. As part of that research, MwRSF conducted static and dynamic testing of threaded rod and rebar with shallow embedment and attempted to determine design procedures for the epoxy adhesive anchors. The full report can be downloaded at the following link. http://mwrsf.unl.edu/researchhub/files/Report14/TRP-03-264-12.pdf
In that report, we tested the 1 1/8” dia. A307 rod used in the tie-down system in concrete with an f’c = 6,454 psi, an epoxy with a nominal bond strength of 1,800 psi (1,904 psi based on threaded anchor diameter effects in manufacturer literature). In this test, the rod developed 45.3 kips in tension loading and over 40 kips in shear loading prior to anchor fracture. Based on these results, we made the following comments.
“The ultimate tension and shear capacities were calculated to be 45.9 kips (203.6 kN) and 26.4 kips (117.6 kN), respectively. The average ultimate tension and shear loads observed from the dynamic testing program of the 1 1/8 in. (29 mm) diameter A307 rods were 45.3 kips (201.5 kN) and 40.6 kips (180.8 kN), respectively. The failure mode in tension consisted of a pullout of the adhesive core accompanied by a 2 ¾ in. (70 mm) deep concrete cone breakout. The ultimate shear value obtained during the component test is an estimated minimum value because the anchor did not fail in the test and the load was governed by the equipment. Nonetheless, the ultimate shear capacity was determined to be far greater that the nominal shear capacity of the anchor and the ultimate tension capacity was within one percent of the nominal tension capacity for the concrete strength in the component tests. Therefore, the anchorage design with 5 ¼ in. (133 mm) embedment depth utilizing the Hilti HIT-RE 500-SD epoxy adhesive was considered an adequate alternative anchorage design for the 1 1/8 in. (29 mm) diameter A307 rods used in the tie-down temporary concrete barrier developed by MwRSF because the tested capacities met the nominal capacities of the anchorages used in the full-scale crash test. However, the failure in the tension test created significant concrete damage. This concrete damage would be expected to occur to the bridge decks of real-world installations during severe, high-energy impacts. In addition, the compressive strength of the concrete used in these component tests may be higher than the typical strength of concrete bridge decks. Thus, some decrease in the capacity of the anchors would be expected for lower strength concrete. This decrease in strength would likely be offset to some extent by the presence of reinforcing steel in the bridge deck. Thus, it is believed that using the A307 rod with Hilti HIT-RE 500 or Hilti HIT-RE 500 SD epoxy adhesive with a 5 ¼-in. embedment depth should provide similar anchorage to the tested system, but some increased deflection and increased deck damage may result. It should also be noted that epoxy adhesive manufacturer recommendations for torque requirements on threaded anchors should be closely followed for these types of anchors to prevent concerns for anchor creep and associated reductions in anchor capacity.”
So while component level testing did indicate that the shallow embedment had the potential to meet the desired loads, it was noted that reduced concrete strengths would reduce the loads and that damage and release of the anchors could occur in high-energy impacts.
MnDOT has different embedment depths listed in their standards. The bridge standard suggests an embedment of 5.5” due to deck thickness concerns, while the roadside standards suggest 9” of embedment. In order to shed more light on the issue, I reviewed the anchor design procedures suggested in TRP-03-264-12. In this report, we calculated anchor capacities based on two methods:
1. A factored, as-tested procedure based on the ACI-11 code that applied dynamic increase factors for the steel and concrete, used as-tested values for the material strengths, and without strength reduction factors. This was used to compare the analytical procedure to tested values as closely as possible.
2. A design procedure based on the ACI-11 code that applied dynamic increase factors for the steel and concrete, used published values for the material strengths, and included standard strength reduction factors. These were more conservative and recommended for design values.
If the factored, as-tested procedure is used to estimate the anchor tensile capacity in the component test, the procedure returns a value of 43.6 kips. This corresponds very well to the test value of 45.3 kips and predicts the failure mode (concrete failure). Using the factored, as-tested procedure for concrete with strengths of 4,000 psi and 3,000 psi yields tensile loads of 34.3 kips and 29.7 kips respectively. Thus, reduction in concrete strength would be expected to reduce tensile capacity significantly. Shear capacities for all concrete strengths were acceptable.
If the design procedure is used to estimate the anchor tensile capacity in the component test, the procedure returns a value of 27.3 kips and predicts the concrete failure mode. Using the design procedure for concrete with strengths of 4,000 psi and 3,000 psi yields tensile loads of 22.3 kips and 19.3 kips respectively. Shear capacities for all concrete strengths were acceptable. Thus, the design procedure provides more conservative estimates on anchor capacity.
We typically design our hardware and anchor systems near the edge of the ultimate capacities without reduction factors or factors of safety. However, we generally test those systems to verify their capacity. Thus, the method of estimating anchor loads may be dependent on what level of conservativeness the DOT wants to have in their spec.
It should be noted that higher bond strengths won’t improve performance as the failure are concrete controlled. In addition, cracked concrete may be very difficult to design a reasonable anchorage for. Published values for bond strength tend to decrease over 50% for cracked concrete. That does not include reductions in strength for the concrete. Thus, it is difficult to recommend anchoring in cracked concrete. The numbers above also consider only individual shear and tensile loads and not combined loading. Estimation of the effect of combined loads on anchor performance is difficult. Thus, the best avenue for addressing this issue may be full-scale testing of the tie-down system with shallow anchor embedment to evaluate its performance.
I also analyzed the 9” embedment listed in the roadside spec. For 3,000 psi to 6,000 psi concrete, 9” embedment is sufficient to generate the ultimate steel rod capacities using both the factored, as-tested procedure and the design procedure.
Please review this information and contact me the any questions and or comments.
|Date||July 30, 2014|
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