HOW TO ESTIMATE THE COST OF HOT MIXED ASPHALT CONCRETE PAVING FOR COMMERCIAL CONSTRUCTION PROJECTS
Costing is the determination of the estimate of the cost of all inputs that are used in any form of construction or installation. This involves determining the total costs of capital, materials, labor and any professional charges that may be required in the construction of any proposed project. The cost of an installation project is usually based upon the totals in the construction contract/s plus other direct installation based costs. These do not include fees paid to professional consultants and engineers, right-of-way, cost of land or any other form of costs defined as the owner’s responsibility in the contract document.
Proper documentation gives support to the credibility of cost estimates, helps in analysis and adjustment of the program cost. It also enables assessors to effectively review the cost estimate. These records could also form a basis for future cost estimation. The accuracy levels of installation cost estimates differ in various steps of project implementation. This ranges from ball-park figures in earlier stages to relatively dependable figures for control of the budget before the actual installation. Thus, because decisions reached at the onset of a project’s life cycle are often more tentative than decisions in later stages, the estimates on cost made at the onset are expected to be of lesser accuracy. This is mainly due to the fact that the cost estimate’s accuracy is reflective of the availability of information on whose basis the estimate is drawn.
A. CSI Division:
Construction Specifications Institute 2004
Division 32 EXTERIOR IMPROVEMENTS
B. CSI Subdivision:
Construction Specifications Institute 2004
Division 321216 Asphalt Paving
$11. Description of the Work:
This composition paper will discuss the take-off methodology of the Construction Estimator from the perspective of the General Contractor, as a scope of work within the overall project scope. It will not attempt to delve deeply into technical aspects of raw materials, manufacturing and mixing requirements or techniques of the actual product or the various production rates, efficiencies and cost of the individual pieces of machinery or operating crew by whom the actual work will be performed.
Generally speaking, there are two types of asphalt mixes; hot-mix and cold-mix. Hot Mixed Asphalt (HMA) is commonly used for heavier traffic areas while cold-mix asphalt is used for secondary roads, and maintenance of roads.
HMA is a manufactured product, created in a gas fired batch plant, blended from an engineered design mixture of suitable large aggregate, small aggregate and occasionally special additives such a recycled rubber from tires, other man made materials and bituminous asphalt emulsion (hydrocarbons). (Fig 1)
Figure 1 – Example of various graded aggregates and waste materials used to manufacture the HMA
Typical paving asphalt consists of 90 to 95 percent by weight of aggregate and 5% to 10 % of bitumen. These materials are combined in specific quantities then heated to a temperature between 280 and 325 deg. Fahrenheit, to obtain sufficient fluidity of the asphalt cement for proper mixing and workability (Fig.2),then loaded on trucks, hauled to the paving site, dumped into a paving spreader (Fig 3) and evenly distributed on to the area to be paved. Newer designs of Warm mixed asphalt (WMA) have lower temperature requirements but we will leave that as a topic for another paper.
Steel wheeled vibratory roller/compactor machines (Fig. 4) are employed to evenly roll and compact the mixture to the required thickness and density. The material must remain at or above 175 deg. Fahrenheit to spread and compact properly. The application of the asphalt base and the surface course is done in separate operations allowing the base to cool and harden prior to applying the final surface course. Depending on the design there may be several layers applied over the base course. Paving designs are constantly being improved by innovative designs using green earth technology, an emphasis on reducing the carbon foot print as well as improving the life cycle, noise reduction and wear ability of the finish product.
Laying and compaction in process
Figure 4 Steel wheel vibratory roller compactors compacts the fresh laid HMA to the correct final thickness and density
2. METHODS OF MEASUREMENT:
All take-off quantities and calculations used in this composition are based on the Imperial unit of measure. The units used in this paper are as follows:
$1· Square feet (SF) = length in feet x width in feet
$1· Square yards (SY) = (SF) / 9
$1· Tons (TNS) = pounds (lb) / 2000
Method of take-off is by electronic digitizer using on-screen take-off (OST)
$13. FACTORS THAT AFFECT TAKE-OFF AND PRICING
When preparing an estimate of Probable Cost, it is important to know distances to the nearest Hot Mix Asphalt producer. Trucking cost is a large percentage of the installation cost for paving, and therefore; knowing the probable haul distance will obviously have a bearing on the unit price estimated. Prepare a rough but reasonable time-line schedule for the project. Determine if paving will occur in the summer or winter months.
In the rural Southeast where many of the Asphalt producers make the Lion’s share of their sales from Federal, State or Local highway and road contracts, the producers may close their plants during the coldest periods of the year; these are usually January through March. The Producer will use this period to do major maintenance on the plant and his equipment. If the project schedule is such that it is a must that paving be constructed during these periods, it may necessitate an escalation in the anticipated unit price. A call to the producer will answer these questions and get you the minimum quantity that the plant requires to remain in operation. Small volumes will be more costly than larger volumes, for instance an in-filled area inside a parking lot or an added turn-out or infill between existing asphalt and say a new concrete gutter.
$14. OVERVIEW OF EQUIPMENT, MATERIALS, LABOR AND EQUIPMENT
The purpose of this paper is to provide guidance to the Estimator in his role to do take-off and to
Estimate the total cost to the General Contractor, for the as-specified in-place Asphalt Paving. As most General Contractors do not self-perform Asphalt Paving with its own forces, materials, labor and equipment are of little interest to the estimator. Of course it is of paramount importance that the estimator understands the process of the paving activity and the capabilities of prospective sub-contractors. All sub-contractors are not equal. It is prudent to visit the location of unknown or prospective sub-contractors that have never been employed by the firm. Knowing or familiarizing oneself to the age and condition of the sub-contractor’s equipment, management staff and plant facilities etc; will give insight to any potential contingencies that should be included with one sub-contractor’s bid over another.
$15. Costs Associated with Hot Mixed Asphaltic Concrete Paving in Commercial Projects.
The installation capital cost for projects of laying asphalt paving includes expenditure related to initial establishing of the installation. The expenses include feasibility and planning expenses, engineering designs and documentation, construction including labor, material and equipment. Other costs may also include field supervision for installation, construction financing, taxes and insurance involved in the installation, owner’s office overheads, furnishing and equipment not included in the installation as well as testing and inspection fees.
The total amount of each cost component is dependant on the size, nature and location of the project in question as well as the management’s organizational structure-just to mention but a few. However, the ultimate goal any project’s owner is to obtain the lowest possible total installation cost that falls within the objectives of the investment. In budgeting for installations, an allowance for un-expected costs is always made (contingencies). This is due to the fact that deviant variations may occur in the budget’s estimate. These costs may be included in each item’s cost or in other cases they may be incorporated into a single group of installation contingency. Contingencies are deduced from historical records and experience as well as the anticipated complexity of the installation. Adjustments by contingencies inclusions may be necessitated by design changes, differences in the nature of the site from the expected, schedule changes and general administrative changes such as changes in wage rates. Third party impositions such as permit fees that may come up during installation could also necessitate contingencies.
$16. SPECIAL RISK CONSIDERATIONS
The typical Paving Sub-contractor will be at the mercy of the fuel and oil market therefore; he will want to limit his risk by guaranteeing his quote for only a short period usually 30 to 60 days if possible. There are times when conditions dictate an even shorter price hold period due to the volatility of the oil market. Sub-contractors realize that usually a major part of the project has to be constructed before the work begins and this will escalate his price. In order to handle this issue, an educated guess has to be made as to how the market will do during that period. The Construction Estimator should be aware of these same conditions. He should not double dip on the subcontractors escalation.
$18. ratios and anaylysis – testing the bid
the Actual Cost Estimation Procedures-Approaches to estimation of costs
One of the biggest management task for an asphalt pavement installation project is cost estimation. Cost estimation activities lays the baseline of the costs of the project in the sequential steps of the installation of the paving. The estimation of cost at a specific stage of the installation reflects a deduction provided by the estimator or engineer based on data that is available. The (AACE)-American association of cost engineers- defines cost engineering as an area in which the practice of engineering experience and judgement is used in the implementation of scientific techniques and principles to the challenges of estimating costs, control of cost and profitability. Almost all cost estimating is done by a combination or one of the following general costing approaches:
Unit costs for bill of quantities.
A cost unit is dedicated to every task and each facility component depending on the representation in the bill of quantities. Therefore, in order to get the total costs; unit costs are multiplied by the quantities estimated and summed up to give the total cost figure. This is a parametric model of costing that employs the use of project parameters (characteristics) in a mathematically desingned model that predicts the costs of aproject. The costing model could be as simple as using the cost per square metre of asphalt paving to approximate costs for large areas to be covered by the paving. In some instances this could be as complex as the use of multiple varying factors to determine the cost of installation-such as the number of drainage holes and kerb sides for the paving. This method is straightforward in its simpler form of principle, though; it it a very tedious and laborious method (‘Cost estimation’, n.d).
The initial step in the use of this method would involve the differentiation of a project into broken down, singular tasks. In this case these tasks can be exemplified by tasks such as grading, excavation of drainages trenches, establishment of drainage holes and channels, compaction, leveling and laying of the asphalt as well as its compaction. Tasks that may come later in the sequence may involve building of side-kerbs, installation of signs and recovery of vegatation around the site-just to mention but a few. These tasks have to be accomplished in order for the installation to be complete, and thus; form the scope of works. The determination and defination of these tasks is followed by the assessment of quantities of elements such as material or labor required for each task. All defined tasks and their assigned costs are then summed, and thus; form the total cost of the installation project. Task disaggregation/decomposition varies greatly depending which estimate level is selected.
Top-down Estimation or Estimation by use of Historical Cost Information
This estimation method utilizes data from previous actual projects to draw projected estimations of cost for current projects. This methodology is useful in instances where there is little or no information about the current installation project. Thus, installation estimations may be drawn based on historical records of similar projects. The data used in this method can only be be useful if it was organized during collection in a format that could make it applicable as a tool for future projections. As a result, data used in cost estimation should always be collected and stored in a manner that will render it useful for future use in the exercise of estimation. This should include its occassional update that will ensure it will always be up-to-date with the current times. In this project’s case cost data published in some print sources could be useful. These are printed by various organizations and could acts as resourceful references. These sources may include periodicals that print construction cost data details, commercial manuals with cost data details, digest that have actual installation project costs and catalogs.
Cost data from historical records must be used with care because changes occur in relative pricing over time and these could cause substantial impacts to the costing process. Another unfortunate fact is that it is dificult to predict the changes that may occur or have occurred on the pricing of tasks and material as a person reviews the material. Additionally, analysis may be carried out erroneously and thus introduce new errors in the whole costing system
Therefore, data and information retrieved from these records should not be directly applied in the determination of the estimates of costs in this kind of installation. Instead these records should be used as a basis to make a preliminary cost analysis that will later be used to draw finer estimates based adjustments that take into account the current trends and pricing the construction industry. As a result, it should be noted that this is never an effective method on its own. Therefore, it should be used alongside another method in order to draw the closest possible estimate.
Experience in estimating this work scope tells us that the following factors are usually required for input:
$1· An bituminous tack coat or “Binder” is spray applied to the subbase prior to installing the first layer or “base layer” of hot mixed asphalt. Bituminious tack coat usually costs around $1.00 per square yard of contact coverage for labor, equipment and material depending on the specified rate per gallon of coverage.Photo complements of Division of Construction, California Department of Transportation, State of California,
$1· Hot mixed asphalt is sold by the ton to the applicator, who then adds his labor, equipment and mark-up and resales it as an in-place product. He may elect to propose the work on a per ton in-place price or per square yard in-place price. Most subcontractor applicators will provide a scope letter that indicates the price of their work by one or the other of these means. with a stated approximate number of tons or square yards of each product along with an extended value for the total cost of the job.
$1· The estimator must be acutly aware that the subcontractor’s quote will more times than not, exclude a gurantee of the stated quantities and thus limit the applicators responsibility to guranteeing the unit price only. For this reason, it is impreitive that the estimator familiarize himself throughly with the toal scope of the project, make a comprehensive estimate of quanities that will include an appropriate waste factor based on his company’s history of over and under runs for projects they have completed in recent years. It is also important for the estimator to profile the typical bidding subcontractos as to the consistantsy of their estimates and whether they typically hit the mark on quanity or histoirically run over or under. In the bidding wars of today, there are many tactics used to effect the low bid, unfortunantly for the General Contractor, many Subcontractors will under state the quantities in order to seemingly have a lower bid, beware of this tactic! One should realize that Asphalt Paving contractors are typically localized to the area of construction except for major highway construction. This is because of the vast expense in building or setting up an asphalt mixing plant. Unlike most concrete redi-mix suppliers, most asphalt batching plants are there to provide materials to their own installation crews. Out-of-towners bidding against locals will effectively be required to buy their materials from their respecitive competitors. Unless there is no local participation anticipated for the impending bid, the non-locals may use the aforementioned tactic to lure a general contractor into using his price.
$1· Spacific gravity of materials will of course determine the total weight of the installed mix as mentioned earlier and the specified compaction and design mix will likely vary from geographical area to geographical area. For this estimator, the rule of thumb for caculating inplace tonage in the Southeastern states of Tennessee, Arkansas, Mississippi, Alabama and Lousiana, is 110 pounds per square yard inch. That means, for every square yard placed that is 1” thick will require an input of 110 lbs., this figure may be devided or multiplied as necessary to achieve the total weight for the specified thickness. For example, the project documents call for two thicknesses of paving, one for standard auto parking and access and the other for heavy traffic such as delivery and maintenance trucks. The standard paving might be labled “Light Duty” and the other “Medium Duty”. The Light duty paving in this estimate requires a 2” compacted asphalt base and a 1-1/2” asphalt surface course for a total of 3-1/2” of aspahlt paving. The Medium Duty paving requires 1-1/2” of hot mixed asphalt surface course over a 2-1/2” of hot mixed asphalt base for a total of 4” of asphalt paving. It is important that the estimate take-off includes quantities for both the base material and the surface material as these material are priced differently. The surface course material will typicall cost slightly more than the base due to the increase quantity of asphalt and seive size of the large aggregate in the mix.
$1· Asphaltic binder ot tack coat is calculated by taking the total coverage area times one unless the paving specifications require a second application between the base and surface course which sometimes is the case.
The General Contractors Take-off
For the everyday construction estimator, calculating the cost for constructing a parking lot, street or drive as we do on practuically every commercial construction project we pursue, the average General Contractor will not actually perform this work. The estimator however; must determine in advance of bid day, what his firm can resonably expect to pay for this scope of work. To do so, he must first determine the point of beginning and the point of ending for this scope of work. For the benefit of this paper, we will conclude that the point of beginning is from the top of the prepared subgrade which, may include material other than compact earth such as compacted stone subbase. Since the topic of this techinical paper is determining the cost of Hot Mixed Asphaltic Concrete Pavement, we will exclude any other related cost. The asphalt paving estimate will include the following scope:
To determine the quanity of asphalt required for an asphalt paving scope, one must fist determine the total area to be paved. Depending on the shape of the paved area, this determination may be as simple as calculating the length by the width but unless you are building a tennis court or a long straight drive, your estimate will require much more input than that. Since most paving projects encountered on a commercial project will likely include turn-outs, planter areas, cul-de-sacs and areas where the shape is irregular.
If one has the use of an electronic digitizer, calculating the volumn of these areas is a piece of cake but if not, the take-off may at first seem impossible. By sectioning off all the various areas and dividing them into simple geometric shapes, the task can be made much easier while maintaining accuracy in quantity take-off.
Even if one has the luxury of these modern day tools, he should learn the basic method of take-off as well; who knows, just when you need that software tool the most, suddenly it is nonfuncting.
The National Association of Asphalt Paving (NAPA) suggest that the following geometric shapes and basic formulas be used to calculate the areas of the difficult shapes.
SAMPLE SKETCH AND DETAILS
The Quanity Survey
The following illustration indicates an asphalt paved area that includes an entry drive, automobile parking and a service drive passing through the project. Light Duty and Medium Duty paving is indicated by the darker and lighter shaded areas and notations on the drawing.
This estimate is broken into the two types paving, Light Duty and Medium Duty Paving.
As noted in the take-off, there is a total of approximatly 3,432 square yards of paved area, of this there is 1,086 square yards of Light Duty and 2,346 square yards of Medium Duty paving.
Light Duty Paving
The quanity survey for light duty paving yeilds 9,774 square feet of area.
The design specifies a 2” compacted asphalt base “often times called Black Base” placed over a bituminous binder coat therfore:
Black Base : (9,774 / 9) = 1,086 so (1,086 sqyd x 2 inch deep) = 2,172 sqyd-inch
and (2,172 x 110# / 2000) = 119.46 + 5% = 125 tons @ 77.70 = $9,712.50
Surface Course: (9,774 / 9) = 1,086 so (1,086 sqyd x 1.5 inch deep) = 1,629 sqyd/inch
and (1,629 x 110# / 2000) = 90 + 5% = 95 tons @ $86.55 = $8,222.00
Binder coat: 1086 x 1 application = 1086 sqyds @ $1.00 = $1,086.00
Medium Duty Paving
The quanity survey for medium duty paving yeilds 21,114 square feet of area.
The design specifies a 2.5” compacted asphalt base “often times called Black Base” placed over a bituminous binder coat therfore:
Black Base : (21,114 / 9) = 2,346 so (2,346 sqyd x 2.5 inch deep) = 5,865 sqyd-inch
and (5,865 x 110# / 2000) = 323 + 5% = 339 tons @ 77.70 = $26,340.30
Surface Course: (21,114 / 9) = 2,346 so (2,346 sqyd x 1.5 inch deep) = 3,519 sqyd/inch
and (3,519 x 110# / 2000) = 194 + 5% = 204 tons @ $86.55 = $17,656.00
Binder coat: 2,346 x 1 application = 2,346 sqyds @ $1.00 = $2,346.00
$110. pricing sheets
It is of paramount importance that all estimators and costing engineers take note of the fact that there is no single costing method that can be effective on its own. Therefore, they should learn to do costing using the best combination of methodologies that should give an estimate that is closest to the actual cost. This is important because poor costing could lead to subsequent poor budgeting that may cause the installation projects to grind to a halt.
Additionally, over-estimations or under-estimations will influence the computation of payable fees to designers, engineers and conslutants involved in the project. This is because their payments are based on the scope of works and overall cost of the project.
It is not easy to foresee every problem that may arise in the installation and operations of constructing an asphalt pavement. It is been evident that estimates in construction activities’costs have more often than not been underestimated in comparison to their actual costs at completion. This could be attributed to the non-anticipated rise in costs, over-optimism and changes in the initial design that come up during the construction process. Therefore, since prices of future installed facilities are affected by a lot of uncertain factors, its essential for all involved parties to be cognizant of the fact that the risk must be borne by everyone to some extent. These parties include the design engineers, financiers and construction contractors.
$1· ‘Cost Estimation’. (n.d). Cost Estimation: Approaches to cost estimation. Retrieved from, http://pmbook.ce.cmu.edu/05_Cost_Estimation.html#5.2%20Approaches%20to%20Cost%20Estimation, viewed on 5th June 2010.
$1· National Asphalt Pavement Association – How to determine quantities, http://www.hotmix.org/index.php?option=com_content&task=view&id=144&Itemid=227, viewed on 8th June, 2010.
$1· Photo complements of Division of Construction, California Department of Transportation, State of California, April 2009.
$1· RS Means