Critical path in construction

Definitions

AACE International's glossary defines the critical path as "the longest path of activities through the project schedule to a milestone, such as: interim milestone(s); substantial completion; final completion" (AACE 10S-90). The US Government Accountability Office puts it the same way: "the longest continuous sequence of activities in a schedule. It defines the program's earliest completion date or minimum duration" (GAO Schedule Assessment Guide).

The critical path method (CPM) is how the path is found. Each activity gets a duration and links to the activities before and after it. A forward pass from the start date gives every activity its early start and finish; a backward pass from the finish gives its late start and finish. The difference between late and early dates is the activity's total float, and the activities with the least float form the critical path. James Kelley and Morgan Walker published the method in 1959 ("Critical-path planning and scheduling"), after working on plant maintenance shutdowns at DuPont.

In construction, "CPM schedule" means a schedule built this way, usually in Primavera P6 or MS Project. Federal and state contracts that ask for a "CPM schedule" mean a logic-linked network like this, updated every month.

A worked example

Seven activities. Site preparation comes first. Foundations and underground utilities both follow it. Structure follows foundations, paving follows utilities, finishes follow structure, and completion needs both paving and finishes. Durations are in working days; a finish on day 5 means the end of day 5.

• Forward pass: each activity starts when its latest predecessor finishes. Completion can happen on day 47, when finishes are done; paving is done on day 19.
• Backward pass: from day 47, each activity must finish by the earliest late start of its successors. Paving must finish by day 47, so it must start by day 39; utilities must finish by then.
• Critical path: A, B, D, F, G: 5 + 10 + 20 + 12 = 47 days, all with zero float. A week lost on foundations moves completion to day 52.
• Utilities and paving share 28 days of float. If utilities slip 10 days, paving starts 10 days later and completion does not move.

Critical path and longest path in P6

P6 can mark activities critical in two ways (Oracle P6 help):

• Total float less than or equal to a limit, zero by default. Simple, but float misleads when activities use different calendars, carry date constraints or are leveled: an activity can show low float without driving the finish.
• Longest path: "the sequence of driving activities that determine the project end date", traced back through driving links. GAO calls it "the only guaranteed method of identifying the driving sequence of activities when using multiple calendars."

Owners' specifications usually settle it: USACE's UFGS 01 32 01.00 10 says "The critical path is defined as the longest path", NAVFAC's UFGS 01 32 17.00 20 defines critical activities as those on the longest path, and FDOT does the same.

What breaks a critical path

These problems make float, and so the critical path, unreliable. GAO's guide and the DCMA 14-point assessment look for each of them:

• Missing logic. An activity with no predecessor or no successor (an open end) can slip without moving anything, so its float means nothing.
• Hard constraints. A Must Start On or Must Finish On date overrides the logic: "each hard constraint in the schedule generates its own sequence of critical activities, and the purpose of CPM scheduling is defeated" (GAO).
• Lags. "The critical path should be free of lags because they obfuscate the identification and management of critical activities" (GAO). A lag hides work or waiting that nobody can track.
• Level-of-effort and summary activities. They follow other work instead of driving it; "a critical path cannot include LOE activities" (GAO).
• Out-of-sequence progress. Work started before its predecessors finished leaves logic that no longer matches the site, and float is calculated wrong until the links are fixed.
• Long durations. An activity of several months hides which part of it is critical. Specifications commonly cap construction activities at 20 working days.

GAO also warns that "an important activity is not necessarily 'critical'": a long or expensive activity is critical only if it sits on the longest path.

See the critical path in your schedule

Open a P6 .xer or MS Project .xml file below. The page lists the critical activities in start order with their dates and float, as the file marks them (P6's longest-path flag or MS Project's Critical field), and runs the DCMA 14-point check for the problems above. The file is read in your browser and never uploaded. The page does not recalculate the schedule. For MS Project details see critical path in MS Project.