A numerical system used to predict applied costs of coating products for steel bridge protection
 

By Wayne A. Senick, Steel Reclamation Specialist, Termarust, Inc., Montreal, Qc
Paul D. Carter, P. Eng., Ried Crowther & Partners Ltd., Edmonton, AB
Charles N. Bradford, P. Eng., Alberta Transportation & Utilities, Edmonton, AB
Dr. Reed Ellis, Ph. D., P. Eng., SLG Stanley Consultants Inc., Edmonton, AB

Introduction

How can one predict the total costs of a steel bridge coating as an aid in budgeting?

The applied costs include both materials and labour, but these vary significantly from one coating product to another and include factors such as the number and thickness of coats.

This article reviews a system originally refined for use by Alberta Transportation & Utilities (AT&U) to analyse the interaction between labour and coating material costs and to predict total painting costs for different types of typical Alberta bridges.[1]

This data was designed to help make cost-effective predictions of likely maintenance expenditures for future painting projects, particularly when bid modification factors were taken into account. Bid modification factors relate to the estimated remaining life of the bridge and the expected longevity of each coating type. For example, it often is more cost-effective to pay for a premium coating product if a structure is expected to be in service for a number of years. On the other hand, it would not be justifiable to use a premium product to paint a bridge that will be replaced in a short period of time.

However, the cost model presented here is limited to a comparison of installed costs (materials and application) of different coating systems and does not examine life cycle costs.

Furthermore, surface preparation and coating equipment costs are treated as constants for all coating systems.

The figures used in this article reflect real world numbers in the Province of Alberta at the time the cost model was developed.
 
 

Methodology

The first step was to find a point of reference to allow comparison of cost data between currently used and new, proposed coatings.

AT&U had painted more than 500 bridges with the M50 alkyd (lead-based) system over a period of 30 years, so there was significant historical data on the cost of applying this system in Alberta.

The data showed significant variation in applied costs for different types of bridges. In recent years, AT&U has coated bridges only with low VOC, nonlead-based systems. As a result, the need to more accurately predict the applied costs for contract painting was identified. The formula presented here uses the historical costs for the M50 alkyd system as a reference. However, the M50 reference used in the AT&U spread-sheet could be replaced to reflect whatever baseline material an owner chose to use.

A number of new and proposed coating systems were evaluated in the laboratory for AT&U and its partners, including other departments of transportation, municipal governments, and an independent testing facility. This evaluation involved a series of accelerated test procedures for predicting the durability or potential service lives of these paints in different types of field exposure environments.2 The testing protocol was designed to emulate the physical conditions in which the coating systems would have too function in the field. PowerTechLabs of Surrey, BC, was commissioned as the independent testing agency to test and evaluate the systems.

The coating systems included in the AT&U applied cost spreadsheet were tested by PowerTech in 1993 and 1994.3

The spreadsheet was developed using Microsoft Excel. The format allowed for variables to be input from manufacturers’ product data sheets or other information known from field experience.

Variables included material and labour costs, waste factors depending on the type of bridge structure being coated, rate of coverage, and cost of the reference coating (M50 alkyd) for both overcoat and recoat applications. The analysis was based on three categories of bridge structures: pony trusses, through or desk trusses, and plate girder-type bridges

The figures used were derived by polling contractors and considering historical data on field performance. There may be other costs, such as cleaning between coats to ensure intercoat adhesion. These additional costs could be included by decreasing the application rates for the coats.1
 

Assumptions

For the purpose of this analysis, the following assumptions were made.
 

• One litre of a 100 percent solids coating will cover 36.4 square metres (m2) at 1 mil dry film thickness (DFT) at 100 percent transfer efficiency (standard coverage).
• Surface preparation costs are constant for all coating systems. Washing followed by an SSPC-SP 6 commercial blast cleaning preparation (equivalent to ISO Sa 2) is AT&U’s standard procedure.
• Coating equipment costs are assumed to be the same for all coating systems on an hourly basis.
• Worker application rates are variable and depend on factors such as the applied DFT and percent solids of the liquid paint.
• Typical waste factors (for overspray, wind drift etc.) may vary from site to site. AT&U’s generally accepted percentage transfer efficiency for spray applications on bridge projects is 50 percent loss for truss-type structures and 30 percent loss for girder-type structures. These numbers can be adjusted in the spreadsheet for special conditions and types of structures.
• Standard application rates for one painter may vary from coating to coating and site to site. For truss bridges, the numbers assumed are 18 square metres per hour (m2/hr) for the first coat and 22.5 m2/hr for subsequent coats. For girder-type bridges, the assumed coverage rates are 24 m2/hr for the first coat and 30 m2/hr for subsequent coats.
• Worker’s wages may vary from site to site and from time to time.
• Environmental, worker safety, and containment factors are assumed to be constant within the Province of Alberta.

Other Variables

There are other factors that affect the unit cost for coating bridge structures. These factors are
 

1.  percent volume solids of paint,
2. recommended DFT,
3. number of coats,
4. price per litre by tender or open offer, and
5. type of bridge structure:
• pony truss – a truss with no overhead members,
• through truss – a truss with overhead members,
• deck truss – a truss with no structure above the desk, or
• girder-type structure – any structure with supporting girders beneath the deck, including riveted girder structures, rigid frame structures, rolled beam structures, and welded girder structures.

The following algorithm was used to calculate the total cost of applying each of the coating products.

1: Material cost:

    1.a: Material coverage:

The data used in this formula comes from coating suppliers’ material data sheets and the standard coverage mentioned above.

 1.b: Total material unit cost:

2: Application unit cost:

    2.a: Base application unit cost:
 

Fig. 1
 
 

    2.b: Modified application   cost for percentage of   solids and mil thickness is     based on the formula in    Fig.1.

3. Total unit cost for all coats applied, including modified application cost for percent of costs not associated with number of coats (i.e., mobilisation, preparation, etc.), is based on the formula in Fig. 2.
 

Abbreviations:
 

 
Overcoating vs. Recoating

There generally is a difference in the number of coats as well as the DFT required for overcoating aged alkyd versus recoating blast-cleaned bare steel. For example, while an overcoating job may require two coats at a total DFT of 6 mils, a recoating project may require three coats and a total of 9 mils DFT. In addition, more labour may be required for recoating jobs, because instead of spot-cleaning a structure for overcoating, the entire structure would be cleaned for recoating.

There also may be additional costs for containment requirements, disposal of hazardous waste, etc. Inclusion of these costs when selecting a material to be tendered on a contract can provide a more accurate estimate of the budget needed for the project.

Overview of the Cost Model

In the following listing, fields 1-15 are the adjustable parameters in the cost model that can be altered for a particular project; fields 16-24 are the calculated fields on the spreadsheet.

Adjustable Parameters:

Fields 1-15
 

• Truss bridge overspray percent – Overspray variables were separated into two categories, because in field situations it was found that rates of loss and overspray were greater on a truss with lattice work than a girder beam structure, which has more flat surfaces. This rate can be adjusted to any rate that fits the actual situation.
• Girder bridge overspray percent – As noted, the loss rate on a girder structure usually is less than on a truss bridge, so a second loss factor can be placed in this cell to reflect loss of material on this type of structure.
• Painters’ wages per hour ($/hr) – The applicable charge-out rate can be placed in this cell. It can be the actual paid rate or a contract charge-out rate. Either will provide a valid cost per square metre.
• Truss coverage, m2/hr (first coat) – The figure in this example is 18 m2 covered by one painter in one hour applying M50 primer at 2 mils DFT and 59 percent volume solids on a truss bridge. The figure was obtained by polling contractors and averaging the covering rates AT&U has experienced in field-coating these structures, but it is adjustable for any specific project.
• Truss coverage, m2/hr (subsequent coats) – The figure in this example is 22.5 m2 covered by one painter in one hour applying a second coat of M50 at 2 mils DFT and 46 percent volume solids. In an overcoat situation, this would be the topcoat(s). In a recoat situation, this figure would be used for each additional coat. This figure may be adjusted to reflect extra time required to clean between coats, mix catalysing material, clean up hazardous material, flush equipment, or wait for multiple coats to dry. These are all factors in accurately predicting the installation cost. The figure used in this example includes only the actual time required to coat and does not make allowances for these variables.
• Girder coverage, m2/hr (first coat) – The figure in this example is 24 m2/hr covered by a painter applying M50 alkyd primer at 2 mils DFT and 59 percent volume solids. This figure is based on polling contractors and averaging the covering rates AT&U has experienced in field-coating these structures. It is adjustable for any specific project.
• Girder coverage, m2/hr (subsequent coats) – The figure in this example is 30 m2/hr covered by one painter applying a second coat of M50 at 2 mils DFT and 46 percent volume solids.
• Percent influence of percent solids and mils on productivity (first coat) – This figure reflects extra costs in applying the primer coat due to more time being required if the coating needs additional passes to get proper film build or a tack coat so it will not run. For example, setting the percent influence of percent solids and mil thickness to 100 percent means it is believed an 80 percent solids material could be applied twice as quickly as a 40 percent solids material at the same mil thickness. Conversely, it also means a material would require twice as much time to apply at 8 mils than a material with the same percent solids would require at a 4-mil thickness.

Table 1: Overcoating Unit Costs Per Square Metre

 
Please click here to see table.
 

Description of Systems:
 

A = Alkyd (M50 System)
B = Calcium Sulfonate Alkyde
C = Moisture-Cured Micaceous Iron Oxide / Urethane
D = Water-borne Vinyl
E = Water-borne Acrylic
F = Zinc-Rich Epoxy / High-Build Vinyl
G = 2-Component Epoxy / Water-borne Acrylic
H = Water-borne Modified Calcium Sulfate Alkyd
I = 2-Component, High-Build Epoxy / 2-Component Urethane
J = High-Solids Zinc / Epoxy Micaceous Iron Oxide / Moisture-Cured Urethane
K = 2-Component Epoxy / 2-Component, High-Solids Urethane
L = Water-borne Acrylic
M = 2-Component, High-Build, Surface-Tolerant Epoxy
N = Water-borne Vinyl / Water-borne Acrylic
O = 2-Component Zinc-Rich / Moisture-Cured Urethane
P = 100% Solids Epoxy Sealer / Water-borne Latex / Water-borne Acrylic

Table 2: Recoating Unit Costs Per Square Metre

 
Please click here to see table.
 

Description of Systems:
 

A = Alkyd (M50 System)
B = Calcium Sulfonate Alkyde
C = Moisture-Cured Micaceous Iron Oxide / Urethane
D = Water-borne Vinyl  E = Water-borne Acrylic
F = Zinc-Rich Epoxy / High-Build Vinyl
G = 2-Component Epoxy / Water-borne Acrylic
H = Water-borne Modified Calcium Sulfate Alkyd
I = 2-Component, High-Build Epoxy / 2-Component Urethane
J = High-Solids Zinc / Epoxy Micaceous Iron Oxide / Moisture-Cured Urethane
K = 2-Component Epoxy / 2-Component, High-Solids Urethane
L = Water-borne Acrylic
M = 2-Component, High-Build, Surface-Tolerant Epoxy
N = Water-borne Vinyl / Water-borne Acrylic
O = 2-Component Zinc-Rich / Moisture-Cured Urethane
P = 100% Solids Epoxy Sealer / Water borne Latex / Water-borne Acrylic

Table 3: Summary of Input Data
 

Please click here to see table.
 

PT = Pony Truss   TH = Through Truss   DT = Deck Truss   WG = Welded Girder

• Percent influence of percent solids and mils on productivity (subsequent coats) – This figure reflects extra costs in applying the mid-coat and topcoat(s) (i.e., additional passes might be required to get proper film
• Percent of costs not associated with number of coats – This figure represents costs such as surface preparation, mobilisation, containment, environmental constraints, etc.
• Manufacturer’s specified mil thickness – This figure is the mil thickness from the manufacturer’s data sheet for each coat.
• Percent solids by volume (%S) – The manufacturer’s data sheet also provides the percent volume solids for each coat.
• Cost per litre of material ($/L) – This figure is the cost per litre of material either from actual tenders, open offers, or quoted pricing.
• Adjustable base line applied cost for each category of structure for the M50 alkyd system for overcoating – This figure is the total cost per square metre for the installed system for overcoating each individual type of structure.
• Adjustable base line applied cost for each category of structure for the M50 alkyd system for replacement – This figure is the cost per square metre for recoating a structure with total removal of the existing coating on each individual type of structure.

Fields 16-24
 

• Girder coverage, square metres per litre (m2/L) at the specified mil thickness – This is the amount of material required to cover 1 square meter at the specified DFT for a girder-type structure.
• Truss coverage, m2/L at the specified mil thickness – This is the amount of material required to cover 1m2 at the specified DFT for a truss-type structure.
• Girder material cost per square metre ($/m2) at the specified mil thickness – This figure is the cost of the material to cover 1 m2 of girder at the specified DFT.
• Truss material cost ($/m2) at the specified mil thickness – This is the cost of

Table 4: Summary of Total Material and Application Costs Per System ($/m2)

 Please click here to see table.
 

Key:    WB = Water-borne  MC = Moisture-Cured  HB = High-Build  MIOX = Micaceous Iron Oxide
PT = Pony Truss  TH = Through Truss  DT = Deck Truss  WG = Welded Girder

• material to cover 1 m2 of truss at the specified DFT.
• Girder application cost ($/m2) at the specified mil thickness – This figure is the cost of application of the material at the specified DFT to 1 m2 of a girder-type bridge.
• Truss application cost ($/m2) at the specified mil thickness – This is the cost of application of the material at the specified DFT to 1 m2 of a truss bridge.
• Order total cost ($/m2), including material and application cost differences at the specified mil thickness – This figure is the total unit cost of application, including both materials and application per square metre, for the coating system at the specified DFT for a girder-type structure.
• Pony truss total cost ($/m2), including material and application cost differences at the specified mil thickness – This is the total unit cost of application, including both materials and application per square metre, for the coating system at the specified DFT for a pony truss-type structure.
• Through truss and deck truss total cost ($/m2), including material and application cost at the specified mil thickness – This is the total unit cost of application, including both materials and application per square metre, for the coating system at the specified DFT for through trusses and deck trusses.

Discussion

Table 1 is a breakdown of the calculations to determine the unit cost for overcoating various types of bridge structures using a variety of coating systems. For comparison, the M50 alkyd baseline material is listed first. Table 2 is a comparable breakdown of the calculations for determining the unit cost for recoating various types of bridge structures. Table 3 summarises the assumptions and the variables used for the calculations. Table 4 is a summary output of the various coating systems, the structure types, and the unit costs for overcoating or recoating these structures, including material and application costs.

The figures in Table 4 illustrate that a large factor affecting the installed cost of these coating systems is the number of coats. This is substantiated by the fact that the installed cost of the one-coat system is notably less than the installed cost of the multi-coat systems.

Knowledge of the material and application costs is essential for predicting an accurate overall cost for coating any structure.
Furthermore, combining bid modification factors, such as the expected remaining life of a structure and the expected longevity of a coating system (i.e., cost/m2 years of service), with accurate predictions of installed costs for various coating products can be a powerful planning tool.

Summary

This article describes an interactive computer programme used for predicting applied bridge painting costs for a variety of paint systems with different application and material characteristics. Fifteen different material and site variables can be adjusted for their impact on the final unit cost. One-coat systems of differing thicknesses and solids contents can be compared with multiple-coat paint systems for the final cost. This programme is a useful tool, and when used in conjunction with service life prediction data and proper planning, it can simplify the process of selecting the most cost-effective paint systems for a wide variety of situations.

References
 

1. 1. Wayne A. Senick, «Establishing a Value for Material and Application Costs per Applied Square Metre for Coatings», Unpublished, 1993.
2. M. Chichak and P. Carter, «Accelerated Testing of Low VOC Paint Systems for the Development of an Approved Product List», International Bridge Conference, St. Louis, MO, US, 1995.
3. J. Inch, «Test Program to Approve Paint Systems for Use on Steel Highway Bridges and Other Structures», ABTR/RD/RR – 94/07, PowerTech, Surrey, BC.

Author Wayne Senick can be reached by e-mail: wsenick@Termarust.com or by phone: 1-888-279-5497 .

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A version of this article was originally published in 'Balancing Economics and Compliance for Maintaining Protective Coatings' (SSPC 95-09).
The proceeding of the SSPC 95 seminars in Dallas Texas U.S.
It is published here with permission from SSPC.