TECHBRIEF

Research, Development, and 

Technology

Turner-Fairbank Highway 

Research Center

6300 Georgetown Pike

McLean, VA 22101-2296

www.fhwa.dot.gov/research

Slip and Creep of Thermal

Spray Coatings

Publication No. FHWA-HRT-14-083

FHWA Contact: Justin Ocel, HRDI-40, (202) 493-3080, 

justin.ocel@dot.gov

Introduction

All steel bridge systems and their components need some level 

of corrosion protection to assure a serviceable life. One of two 

approaches is typically used: either the bridge component is 

fabricated from a corrosion-resistant alloy, or the steel is coated 

for protection. The most common coating practice is use of a 

multilayered paint system over a zinc-rich primer. Other coating 

alternatives for corrosion protection are hot-dip zinc galvanization 

and thermal spray coatings (TSC). Both galvanization and TSCs 

offer better long-term corrosion protection than zinc-bearing 

paint systems in severe environments. For this reason, these 

alternative-coating systems need to be mainstreamed for the 

protection of steel bridges.

In addition to corrosion resistance, the coating must be compatible with use in high-strength bolted connections. The American 

Association of State Highway and Transportation Officials 

(AASHTO) Load and Resistance Factor Design (LRFD) Bridge 

Design Specifications require bolted connections be designed as 

“slip-critical” if the connection is subjected to “...stress reversal, 

heavy impact loads, severe vibration or located where stress and 

strain due to joint slippage would be detrimental to the serviceability of the structure...”(1) Slip-critical connections rely on the 

clamping force from the bolts to develop frictional shear stresses 

as the means to transfer force from one element to the next. This 

construction is in contrast to bearing connections, in which the 

individual connection elements bear on the bolt and the force is 

transferred through shear stresses in the bolt itself. In the design 

of a slip-critical connection, the engineer must select a “frictional 

slip coefficient” between the layers of a connection to calculate the 

slip resistance. AASHTO refers to this frictional value as a “surface 

condition factor,” although in this TechBrief, it will be referred to as 

the “slip coefficient.” The engineer does not specify an exact slip 

coefficient; rather, the AASHTO LRFD Bridge Design Specifications

provide three different categories (Class A, B, and C) from which 

the engineer can choose. 

Class A surfaces have a minimum slip coefficient of 0.33, which 

can be achieved with unpainted, clean mill scale. Class B surfaces 

have a minimum slip coefficient of 0.50, which can be achieved with 

unpainted, blast-cleaned surfaces. In lieu of having bare steel on 

the slip surface, certified coatings applied over a blast-cleaned surface that demonstrates Class A or B performance may also be used. 

Class C surfaces also have a minimum slip coefficient of 0.33 but 

are only applicable for hot-dip galvanized coatings and are outside 

the scope of this TechBrief. 
2

Coatings applied over blast-cleaned surfaces must 

be demonstrated through testing to achieve either 

Class A or B slip resistance and be certified as such. 

From the perspective of the bridge fabricator, there 

may be advantages to using slip-certified coatings 

in the faying surfaces of slip-critical connections. 

For instance, if the bridge will be painted, then it 

will have to be blast-cleaned prior to paint application, and primers should be applied shortly after 

blast-cleaning before the steel can flash rust. If the 

primer has been certified to provide a certain slip 

coefficient, then the entire piece can be primed 

without masking off the areas of the faying surfaces, a time-consuming step that adds cost to 

the overall fabrication of the bridge. The AASHTO 

LRFD Bridge Design Specifications say nothing 

about the use of TSCs on the faying surface. That 

is not to say they cannot be used, but because 

they are not directly referenced, there may be an 

aversion to specifying their use because of their 

unknown slip resistance. 

As shown in figure 1, application of TSCs is analogous to painting, but the spray is droplets of 

molten metal. At the application gun, wire stock is 

melted with either a flame or an electric arc, and 

compressed air sheds the molten pool into a spray 

of droplets. The droplets are propelled toward the 

surface, where the molten droplets land on the 

surface and solidify. Hot-dip galvanizing produces 

a solid layer of zinc chemically bonded to the 

steel substrate, but a TSC of zinc leaves a porous 

layer of zinc that is only adhered to the steel 

substrate. For this reason, it is critical that TSCs 

are applied over a blast-cleaned surface with an 

angular anchor profile so the droplets can interlock with the roughened steel surface. Because 

the droplets also randomly form over each other, 

TSCs are inherently porous, and current practice 

recommends using topcoat sealant to fill the voids 

and prevent moisture infiltration.

This TechBrief introduces limited data on the 

slip coefficients developed by both sealed and 

unsealed TSCs. 

Testing for Slip Resistance

The testing of coatings for slip resistance 

is described by the Research Council of 

Structural Connections (RCSC) document 

Specification for Structural Joints Using HighStrength Bolts.(2) In accordance with their 

specification, two tests are required to certify a 

coating for either Class A or B slip resistance: 

a short-term compression test and a long-term 

tension creep test. The short-term test establishes 

the slip coefficient. The long-term creep test 

determines whether the coating will slip over a 

period of time or reduce the bolt clamping force.

Short-Term Compression Test

The short-term compression test specimen is 

constructed from three separate plates, all with 

the same geometry. Each plate is 4 inches square, 

is 5/8 inch thick, and has a 1-inch-diameter hole 

drilled on one centerline of the plate but offset 

1.5 inches from a plate edge parallel to the other 

plate centerline. The plates are stacked together 

so that the holes are aligned, and the middle 

plate is rotated 180 degrees from the two outer 

plates. A threaded rod is inserted through the 

holes and tensioned to 49 kips to represent the 

clamping force from an A490 bolt. A vertical load 

is slowly applied at a rate not exceeding either 

0.003 inches/min or 25 kips/min until slip occurs 

between the two plates. The slip load is determined 

as either the peak load or the load at 0.02 inches of 

slip. Five replicate specimens are tested, and the 

mean slip coefficient is reported. The RCSC specification provides additional guidance on loading 

configuration, instrumentation, and loading protocols. A schematic of the load system used for this 

effort is shown in figure 2. 

Long-Term Creep Test

The long-term creep testing uses a bolted chain 

of three specimens in the series illustrated in 

figure 3. Because the specimens are bolted in 

parallel, the plate size is increased to 4 by 7 inches, 

and each plate has two holes instead of one. The 

bolts between individual specimens are left loose, 

and the A490 bolts clamping individual specimens 

Figure 1. TSC application.
3

Figure 2. Short-term compression test.

are fully tensioned. The whole chain is then placed 

under a tensile load. Creep slip displacements for 

each specimen are monitored manually with a dial 

gauge magnetically affixed to each specimen.

The tension load applied is in accordance with the 

RCSC defined service load, based on the assumed 

slip coefficient factor, using the real bolt pretension. 

The real bolt tension was the average of three bolts 

from the lot verified with a bolt-tension calibration 

device. The slip-critical service load is maintained 

for 1,000 h, and if the slip exceeds 0.005 inches for 

any of the three specimens, then the creep test is 

considered a failure. If it passes 1,000 h with no 

more than 0.005 inches of slip, then the load is 

increased to the RCSC factored load and checked 

to ensure the slip does not exceed 0.015 inches. 

The RCSC specification provides more detailed 

guidance for test configuration, instrumentation, 

and loading protocols.(2)

Test Matrix

The study considered two TSC alloy compositions under sealed and unsealed conditions. One 

alloy was 100-percent zinc, and the other was an 

85/15-percent zinc/aluminum alloy; these alloys 

are the two most common wires used in the 

industry. The seal coat used in this project was a 

commercially available epoxy penetrating sealer. 

Table 1 is an outline of the test matrix.

Specimen Preparation

Plate Preparation

All samples were solvent-cleaned in accordance 

with the Society for Protective Coatings’ (SSPC)

SSPC-SP 1 guide, Solvent Cleaning, to remove oils 

prior to blast cleaning.(3) The faying surfaces on 

each plate were then blast cleaned to SSPC-SP 5, 

Figure 3. Long-term creep test.
4

Test Type Alloy Sealer Number of Specimens

Short-term 100-percent Zn Yes 5

Short-term 100-percent Zn No 5

Short-term 85-percent Zn/

15-percent Al Yes 5

Short-term 85-percent Zn/

15-percent Al No 5

Long-term 100-percent Zn Yes 3

Long-term 100-percent Zn No 3

Long-term 85-percent Zn/

15-percent Al Yes 3

Long-term 85-percent Zn/

15-percent Al No 3

Table 1.Test matrix.

Al = aluminum

Zn = zinc

White Metal Blast, condition with G40 steel grit to 

a target 3.5- to 4.5-mil surface profile.(4) The plates 

were visually inspected in accordance with SSPC 

VIS 1, and the surface profile was verified with 

replica tape according to ASTM D4417-11.(5,6) The 

measured profile on the faying surfaces ranged 

from 3.2 to 3.9 mils. 

Separate 4- by 6- by 1⁄4-inch-thick panels were 

prepared under the same conditions as the slip 

and creep specimens. These panels were coated 

along with the specimen that could be subjected 

to a quality control adhesion test of just the TSC. 

Three adhesion test panels were prepared for 

each of the TSC alloys.

TSC Application

An experienced contractor applied the TSC in 

accordance with SSPC-CS 23.00/AWS C2.23M/

NACE No. 12 to all samples on the same day.(7)

Typical TSC is 12 mils thick, although RCSC 

requires an additional 2 mil of thickness for slip 

evaluation, so the contractor was asked to target 

a 14-mil thickness. A separate spray unit was used 

for the two different alloy wires; each machine 

was operated at 350 amps current. The spray distance varied between 4 and 6 inches. The thickness of the TSC was measured on both sides of 

each specimen with a calibrated type 2 (electronic) 

non-destructive dry film thickness (DFT) gauge. 

Because of the relatively small panel size, two spot 

measurements were obtained from each panel 

face. The average TSC thickness for each surface

of each specimen is reported in the slip and creep 

testing results sections. 

Seal Coating

The specimens with a sealer were sealed on the 

same day that the TSC was applied. The epoxy 

penetrating sealer was applied using an airless 

sprayer fitted to a semi-automated robotic arm 

to ensure uniform coverage. A wet film target 

thickness of 2 mils was targeted and verified with 

a smooth witness panel sprayed at the same 

time the TSC panels to quantify the volume 

of sealer applied to the TSC panels. Because this 

type of sealer penetrates into the porous TSC 

surface, a DFT of the applied sealer over the TSC 

cannot be attained. The DFTs reported in the 

results sections for the sealer are from the 

witness panels, and those results only give 

an indication of the volume of sealer that was 

applied to the TSC, not the actual film thickness 

of the sealer. The epoxy sealer was allowed to 

cure for 7 days before testing.

Quality Control Adhesion Testing

The adhesion tests were conducted in triplicate in 

accordance with ASTM D4541-09e1 with a type IV 

(self-aligning, pneumatic) test device.(8) The results 

of the adhesion testing are reported in table 2. 

The average adhesion strength of zinc TSC and 

zinc/aluminum TSC was 1,032 and 1,297 psi, 

respectively. Table 2 also reports the percentage 

of adhesive and cohesive failure. Adhesive 

failures occur between the steel and TSC interface; cohesive failures are confined within the TSC.
5

Panel Replicate Alloy Adhesion (psi) Failure Description

1A 1 Zn 1,223 75-percent adhesion 

25-percent cohesion

1A 2 Zn 1,039 50-percent adhesion 

50-percent cohesion

1A 3 Zn 999 50-percent adhesion 

50-percent cohesion

2A 1 Zn 1,059 75-percent adhesion 

25-percent cohesion

2A 2 Zn 1,100 50-percent adhesion 

50-percent cohesion

2A 3 Zn 917 85-percent adhesion 

15-percent cohesion

3A 1 Zn 937 75-percent adhesion 

25-percent cohesion

3A 2 Zn 1,018 60-percent adhesion 

40-percent cohesion

3A 3 Zn 999 60-percent adhesion 

40-percent cohesion

4A 1 Zn/Al 1,263 100-percent adhesion

4A 2 Zn/Al 1,345 90-percent adhesion 

10-percent cohesion

4A 3 Zn/Al 1,406 100-percent adhesion

5A 1 Zn/Al 1,223 100-percent adhesion

5A 2 Zn/Al 1,304 100-percent adhesion

5A 3 Zn/Al 1,223 100-percent adhesion

6A 1 Zn/Al 1,365 100-percent adhesion

6A 2 Zn/Al 1,365 95-percent adhesion

5-percent cohesion

6A 3 Zn/Al 1,182 100-percent adhesion

Table 2. Adhesion results of TSC.

Al = aluminum

Zn = zinc

Results of Slip Testing

Table 3 and table 4 report the individual specimen DFTs and results of the short-term slip tests 

of unsealed and sealed specimens, respectively. 

The unsealed zinc and zinc/aluminum TSCs had 

slip coefficients greater than 0.75. In table 3, many 

of these results are reported with a greater-than 

symbol because the vertical load was nearing 

the limits of the load frame and the test was 

terminated for safety reasons. Regardless, these 

results far exceed the 0.50 requirement for Class B 

classification. The higher slip values might be 

attributed to the interlocking of the rougher TSC 

surface. A typical faying surface of an unsealed 

TSC after slip testing is shown in figure 4; an 

adhesion failure rather than a slip shear failure 

between the plates is observable. 

Results of the slip test for the epoxy-sealed specimens were much lower, at 0.414 for the zinc alloy 

and 0.439 for the zinc/aluminum alloy. Because 

the coefficients are less than 0.50 and greater 

than 0.33, they are classified as Class A. A typical 

faying surface of a sealed TSC after slip testing 

is shown in figure 5; a smear failure between the 

interfaces of the plates is observable. Although 

both the sealed and unsealed surfaces had a 

rough texture, it is speculated that the sealer 

added lubricity to promote slippage.
6

Specimen Alloy Left Outer 

Panel (mils)

Middle Plate, 

Left Face 

(mils)

Middle Plate, 

Right Face 

(mils)

Right Outer 

Panel (mils) Slip Coefficient

Slip 1 Zn 14.2 14.9 14.7 14.6 > 0.857

Slip 2 Zn 14.2 15.2 14.9 14.7 0.743

Slip 3 Zn 15.6 14.8 14.9 14.6 0.641

Slip 4 Zn 15.1 13.5 15.0 13.9 > 0.824

Slip 5 Zn 14.3 14.1 14.2 15.0 > 0.833

Slip 6 Zn/Al 13.9 13.1 14.5 13.9 > 0.837

Slip 7 Zn/Al 14.5 14.5 14.2 14.0 > 0.857

Slip 8 Zn/Al 14.4 14.5 13.5 13.9 0.702

Slip 9 Zn/Al 14.5 14.2 15.1 15.0 > 0.865

Slip 10 Zn/Al 15.0 14.2 14.6 14.8 0.596

Table 3. Unsealed specimen DFTs and slip results.

Al = aluminum

Zn = zinc

Specimen Alloy Left Outer 

Panel (mils)

Middle Plate, 

Left Face 

(mils)

Middle Plate, 

Right Face 

(mils)

Right Outer 

Panel (mils) Slip Coefficient

Slip 11 Zn 16.4 + 1.5 14.9 + 1.7 15.5 + 1.6 14.7 + 1.5 0.327

Slip 12 Zn 15.1 + 1.9 14.7 + 1.7 13.8 + 1.6 14.1 + 1.4 0.339

Slip 13 Zn 14.1 + 1.4 14.7 + 1.7 15.6 + 1.6 16.9 + 1.4 0.416

Slip 14 Zn 16.4 + 1.0 15.5 + 1.7 15.1 + 1.6 16.2 + 1.4 0.522

Slip 15 Zn 16.4 + 1.0 14.9 + 1.9 15.1 + 1.5 15.3 + 1.0 0.465

Slip 16 Zn/Al 15.5 + 1.2 14.0 + 1.1 15.0 + 1.2 14.2 + 1.1 0.502

Slip 17 Zn/Al — — — — —

Slip 18 Zn/Al 14.3 + 1.1 17.4 + 1.1 14.5 + 1.2 14.3 + 1.2 0.437

Slip 19 Zn / Al 16.0 + 1.2 14.8 + 1.2 14.1 + 1.1 14.2 + 1.4 0.522

Slip 20 Zn / Al 15.1 + 1.4 17.7 + 1.2 16.3 + 1.1 14.9 + 1.4 0.294

Table 4. Sealed specimen DFTs (TSC + sealer) and slip results.

—One test plate coated with sealed zinc was inadvertently used in this specimen, making the test result invalid.

Al = aluminum

Zn = zinc

Results of Creep Testing

The individual specimen DFTs and results of the 

long-term creep tests are shown in table 5 and 

table 6, respectively, for the unsealed and sealed 

specimens. For the unsealed specimens, all of the 

creep displacements were less than 0.005 inches 

after 1,000 h for each of the alloys. Each of the two 

chains slipped less than 0.015 inches after 1,000 h 

when the load was increased to the design slip 

load. Therefore, both unsealed TSC zinc and zinc/

aluminum alloys passed the creep test.

Conversely, the creep results for the sealed specimens were much different. The tension loads on 

these creep chains were lower because the slip 

tests exhibited Class A slip performance. Despite 

the lower tension load, the sealed zinc/aluminum 

alloy failed upon initial loading to the service load. 

The sealed zinc alloy passed the 1,000-h test but 
7

Figure 4. Typical post-test faying surface of unsealed TSC. Figure 5. Typical post-test faying surface of sealed TSC.

Specimen Alloy Left Outer 

Panel (mils)

Middle Plate, 

Left Face 

(mils)

Middle Plate, 

Right Face 

(mils)

Right Outer 

Panel (mils)

1,000-h Creep 

Displacement

(inch)

Creep 1 Zn 14.6 14.0 13.8 14.6 0.0035a

Creep 2 Zn 14.2 14.1 14.7 14.1 0.0036a

Creep 3 Zn 13.8 15.1 14.9 14.2 0.0040a

Creep 4 Zn/Al 13.5 14.0 14.4 13.4 0.0060b,c

Creep 5 Zn/Al 14.8 14.5 13.9 13.8 0.0029c

Creep 6 Zn/Al 14.2 14.3 14.7 14.5 0.0033c

Table 5. Unsealed specimen DFTs and creep results.

aChain of three specimens slipped 0.0040 inches when loading to the design load.

bTesting on neighboring chain failed, causing movement. Chain passed final loading, and slip in excess of 0.005 inches is 

thought to be the result of shock loading from neighboring chain.

c

Chain of three specimens slipped 0.0058 inches when loading to the design load.

Al = aluminum

Zn = zinc

Specimen Alloy Left Outer 

Panel (mils)

Middle Plate, 

Left Face 

(mils)

Middle Plate, 

Right Face 

(mils)

Right Outer 

Panel (mils)

1,000-h Creep 

Displacement

(inch)

Creep 7 Zn 14.7 + 0.8 15.2 + 1.5 14.4 + 2.2 16.0 + 0.8 0.0017a

Creep 8 Zn 15.6 + 1.4 14.9 + 0.9 15.8 + 0.9 15.4 + 1.7 0.0014a

Creep 9 Zn 15.1 + 1.7 15.7 + 1.2 14.6 + 1.4 14.0 + 0.9 0.0039a

Creep 10 Zn/Al 15.0 + 1.5 14.4 + 1.7 15.1 + 1.5 15.0 + 1.1 > 0.25b

Creep 11 Zn/Al 14.1 + 1.0 16.5 + 1.3 13.9 + 1.1 14.5 + 1.0 > 0.25b

Creep 12 Zn/Al 16.7 + 1.1 16.6 + 1.3 15.9 + 1.1 13.7 + 1.1 > 0.25b

aChain of three specimens slipped more than 0.25 inches when loading to the design load.

bAll three specimens failed the creep test under service load, and there was no need to load the chain to the design load.

Al = aluminum

Zn = zinc

Table 6. Sealed specimen DFTs (TSC + sealer) and creep results.
8

then exhibited excessive slip displacement when 

reloaded to the design slip load level. Therefore, 

both sealed TSC alloys failed the creep test. 

Conclusions

Because of their rough textures, unsealed zinc 

and zinc/aluminum alloy TSCs had no problems 

passing Class B slip performance requirements in 

accordance with the RCSC specification. However, 

once the surface was sealed, neither coating 

system could meet the RCSC criteria for failing 

the creep test (despite achieving Class A shortterm slip resistance). Until further research can 

demonstrate slip-critical performance of sealed 

TSCs, it is recommended that slip-critical faying 

surfaces be either masked off in fabrication or 

assembled before application of TSC sealers. 

Future Research Needs

Additional parametric studies should be performed to understand the essential variables that 

influence the slip and creep performance of TSCs. 

Because this project demonstrated that the sealer 

has a dominant influence, researchers should 

test other sealers or other sealing techniques for 

compatibility with TSCs to attain Class A or B slip 

performance.

References

1. AASHTO (2012), AASHTO LRFD Bridge Design 

Specifications, 6th Edition, Washington, DC.

2. RCSC (2009), Specification for Structural Joints 

Using High-Strength Bolts—2009 Edition, 

Chicago, IL.

3. SSPC (2004), Surface Preparation Specification 

No. 1, Solvent Cleaning (SSPC-SP 1), Pittsburgh, 

PA.

4. SSPC (2007), Joint Surface Preparation 

Standard—SSPC-SP 5/NACE No. 1—White Metal 

Blast Cleaning, Pittsburgh, PA.

5. SSPC (2004), Guide and Reference Photographs for 

Steel Surfaces Prepared by Dry Abrasive Blast 

Cleaning—SSPC-VIS 1, Pittsburgh, PA.

6. ASTM International (2011), ASTM D4417-11—

Standard Test Methods for Field Measurement 

of Surface Profile of Blast Cleaned Steel, West 

Conshohocken, PA.

7. SSPC (2003), SSPC-CS 23.00/AWS C2.23M/NACE 

No. 12 Specification for the Application of Thermal 

Spray Coatings (Metallizing) of Aluminum, 

Zinc, and Their Alloys and Composites for the 

Corrosion Protection of Steel, Pittsburgh, PA.

8. ASTM International (2011), ASTM Standard 

D4541-09e1—Standard Test Method for Pull-Off 

Strength of Coatings Using Portable Adhesion 

Testers, West Conshohocken, PA.

Researchers—This research was conducted by KTA-Tator in Pittsburgh, PA, with the 

collaboration of Rampart, LLC, funded through contract DTFH61-10-D-00017-T-11002.

Distribution—This TechBrief is being distributed according to a standard distribution. Direct 

distribution is being made to the Divisions and Resource Center.

Availability—This TechBrief may be obtained from the FHWA Product Distribution Center by 

e-mail to report.center@dot.gov, fax to (814) 239-2156, phone to (814) 239-1160, or online at 

http://www.fhwa.dot.gov/research.

Key Words—Thermal spray coatings, metalizing, slip-critical, high strength bolts, steel bridge, 

connections. 

Notice—This document is disseminated under the sponsorship of the U.S. Department of 

Transportation in the interest of information exchange. The U.S. Government assumes no liability 

for the use of the information contained in this document. The U.S. Government does not endorse 

products or manufacturers. Trademarks or manufacturers’ names appear in this report only 

because they are considered essential to the objective of the document.

Quality Assurance Statement—The Federal Highway Administration (FHWA) provides highquality information to serve the Government, industry, and public in a manner that promotes 

public understanding. Standards and policies are used to ensure and maximize the quality, 

objectivity, utility, and integrity of its information. FHWA periodically reviews quality issues and 

adjusts its programs and processes to ensure continuous quality improvement.

SEPTEMBER 2014 FHWA-HRT-14-083

HRDI-40/09-14(WEB)E