Activity Planning and Network Scheduling

Introduction

Construction projects are delivered through a sequence of interdependent activities. Excavation must normally precede foundations, foundations must reach the required stage before frame construction, and services installation may depend on wall and ceiling work. If these relationships are not planned clearly, labour and equipment may be idle, materials may arrive too early or too late, and completion dates may become unreliable.

Activity planning converts the scope of a construction project into manageable work packages. Network scheduling then represents the logical relationships among those activities and identifies the activities that control project duration. This chapter introduces bar charts, precedence networks, the Critical Path Method (CPM), milestones, and introductory Programme Evaluation and Review Technique (PERT).

Learning Objectives

By the end of this chapter, the learner should be able to:

1. Define activities, events, milestones, dependencies and work packages.
2. Estimate realistic activity durations using construction information.
3. Prepare and interpret bar charts and precedence networks.
4. Calculate early and late schedule dates using CPM.
5. Identify the critical path and total float.
6. Explain the difference between CPM and PERT.
7. Apply scheduling methods to a practical construction situation.
8. Recognise limitations, risks and ethical responsibilities in schedule preparation.

1. Activity Planning in Construction

An activity is a clearly defined item of work that consumes time and usually uses resources. Examples include setting out, excavation, reinforcement fixing, concrete placement and curing. An activity should have a measurable start and finish and should produce an identifiable result.

A work breakdown structure (WBS) divides the project scope into progressively smaller components. For example:

The appropriate level of detail depends on the purpose of the programme. A client summary may show twenty activities, whereas a site-control programme may show several hundred. Excessive detail makes updating difficult; insufficient detail conceals risk and prevents effective control.

Activity information

Each scheduled activity should normally have:

• an identification code;
• a clear description;
• a responsible person or trade;
• a quantity or measurable output;
• a duration;
• required resources;
• predecessors and successors;
• a planned start and finish;
• constraints and assumptions;
• a method of measuring progress.

The activity description should use an action and an object, such as “Install ground-floor drainage,” rather than a vague label such as “Drainage.”

2. Durations and Productivity

An activity duration is the working time required to complete the planned scope under stated conditions. Duration is not the same as effort. Ten labour-days of effort could be completed by one worker in ten days or five workers in two days, although productivity, access and congestion may change.

A basic production relationship is:

where:

• D = activity duration, usually in working days;
• Q = quantity of work;
• R = production rate in quantity per working day.

The formula assumes that the production rate is appropriate, resources are continuously available, and the work is not significantly interrupted. For example, if 240 m² of blockwork is planned at 40 m² per day:

Calendar duration may differ from working duration because of weekends, public holidays, weather stoppages, restricted working hours and planned shutdowns.

Factors affecting duration

Duration estimates should consider:

• quantity and complexity of work;
• crew size, skill and supervision;
• equipment capacity and availability;
• site access and storage;
• weather and ground conditions;
• inspection, testing and approval requirements;
• material lead times;
• working hours and calendar restrictions;
• health and safety controls;
• rework and learning effects.

Increasing crew size does not always reduce duration proportionally. Limited work fronts, shared equipment, congestion and supervision capacity can produce diminishing returns.

Estimating techniques

Common techniques include:

1. Historical comparison: using verified records from similar projects, adjusted for scale and conditions.
2. Productivity-based estimation: dividing quantity by expected production rate.
3. Three-point estimation: using optimistic, most likely and pessimistic estimates.
4. Expert judgement: consulting experienced engineers, supervisors and specialist subcontractors.
5. Supplier information: using manufacturer or specialist installation rates, while checking their assumptions.

A sound estimate records its basis. A duration copied from an earlier project without checking quantities, access or workforce conditions is not a reliable estimate.

3. Dependencies, Logic and Milestones

A dependency is a logical relationship between two activities. The predecessor influences when the successor can begin or finish. The most common relationship is finish-to-start (FS): the predecessor must finish before the successor starts.

A lag is a deliberate waiting or separation period between linked activities. Concrete curing is a physical example. A lead permits overlap, but it should be used carefully because it may conceal incomplete work or create coordination risk.

Dependencies may be:

• mandatory: imposed by physical or technical necessity;
• discretionary: selected as a preferred construction method;
• external: controlled by parties outside the project team, such as utility authorities;
• internal: controlled within the contractor’s operations.

A milestone is a significant event with zero duration, such as “building permit approved,” “foundations accepted,” or “section handed over.” Milestones are useful for reporting and payment, but they do not represent quantities of work.

A good network should show logical relationships, not merely desired dates. Artificial constraints introduced to force a preferred completion date weaken the schedule’s reliability.

4. Bar Charts and Schedule Presentation

A bar chart, often called a Gantt chart, displays activities against a time scale. Each activity is represented by a horizontal bar whose position and length indicate its start, finish and duration.

Bar charts are effective for:

• communicating the overall plan;
• coordinating trades;
• showing planned and actual progress;
• presenting milestones;
• reporting to clients and site teams.

Their main weakness is that complex logical relationships can be difficult to see. A bar chart may show that two activities overlap without explaining whether the overlap is physically possible. Therefore, bar charts should normally be supported by a logic-based network or schedule database.

A useful bar chart includes activity codes, descriptions, time units, planned bars, actual-progress bars, milestones, a data date and a legend. The time scale should match the management purpose: weeks for a project overview and days for short-term site control.

5. Precedence Networks

In an activity-on-node precedence network, each activity is shown in a node or box, and arrows show logical relationships. A simplified activity box may contain the activity code, duration, early dates, late dates and float.

Figure: An activity-on-node network in which activities B and C both depend on A, and D depends on both B and C.

The network makes a key planning rule visible: activity D cannot begin until both B and C are complete. A network should be logically complete, free from circular relationships, and connected from project start to project finish.

6. Critical Path Method

CPM is a deterministic scheduling technique. Each activity is assigned one duration, and the network is analysed in two stages: a forward pass and a backward pass.

Forward pass

For an activity with duration d_i:

where ES_i is the early start and EF_i is the early finish. For an activity with predecessors, its early start is:

where the maximum is taken over all immediate predecessors j. This maximum is essential: a successor must wait for the latest-finishing predecessor.

Backward pass

For the final activity, the late finish is normally set equal to the project duration. For other activities:

where LS_k represents the late start of each immediate successor k. Then:

Float and critical path

Total float is:

Float is measured in the same time unit as duration. Under the conventional CPM definition, activities with zero total float form a critical path. A project can have more than one critical path. Critical does not mean that the activity is most expensive or most difficult; it means that delay to the activity, without schedule recovery elsewhere, delays the project completion date.

Worked CPM example

Consider the following activities, all measured in working days.

Forward pass:

• A: ES = 0, EF = 0 + 3 = 3.
• B: ES = 3, EF = 3 + 4 = 7.
• C: ES = 7, EF = 7 + 2 = 9.
• D: ES = 7, EF = 7 + 5 = 12.
• E: ES = max(9, 12) = 12, EF = 12 + 2 = 14.
• F: ES = 14, EF = 14 + 3 = 17.

The project duration is therefore 17 working days. The backward pass gives:

• F: LF = 17, LS = 17 − 3 = 14.
• E: LF = 14, LS = 14 − 2 = 12.
• D: LF = 12, LS = 12 − 5 = 7.
• C: LF = 12, LS = 12 − 2 = 10.
• B: LF = min(10, 7) = 7, LS = 7 − 4 = 3.
• A: LF = 3, LS = 3 − 3 = 0.

The critical path is A–B–D–E–F, with a duration of 17 working days. Activity C has three days of total float. It may start as late as day 10 without delaying project completion, provided its successor and other constraints remain unchanged.

The calculation uses a zero-based time convention. In a date-based schedule, the software may display calendar dates and inclusive day counts differently. The planner must apply one consistent convention and account for non-working calendars.

7. PERT and Uncertainty

CPM is appropriate when activity durations are sufficiently predictable. PERT is an introductory probabilistic approach used when durations are uncertain. It uses three estimates:

• optimistic time a: favourable but realistic conditions;
• most likely time m: normal expected conditions;
• pessimistic time b: adverse but credible conditions.

The traditional beta-weighted expected duration is:

An approximate activity variance is:

The standard deviation is σ, and variance has squared time units. These formulas assume that the three estimates reasonably represent a beta-like distribution and that the estimates are not biased or impossibly extreme.

For example, if excavation has a = 4 days, m = 6 days and b = 10 days:

Thus, the estimated duration is approximately 6.33 days and the standard deviation is 1 day. PERT estimates should not be treated as guaranteed completion times. Correlated risks, resource competition and changing critical paths can make simple probability calculations unreliable.

8. Case Application: Small Community Health Centre

This is an illustrative case. A contractor is planning a single-storey community health centre. The client requires a reliable handover date, but the site has restricted access and the local authority requires inspections before concrete and services are covered.

Background and problem

The initial programme was a bar chart with broad activities: foundations, structure, finishes and services. It showed completion in 20 weeks, but it did not identify inspection hold points or relationships between services and finishes. The project manager therefore could not explain which delays threatened handover.

Analysis

The team developed a WBS and identified activities for setting out, excavation, reinforcement, foundation inspection, concrete, backfilling, wall construction, first-fix services, roofing, plastering, finishes, testing and handover. Durations were estimated from quantities, crew productivity and subcontractor input. The network showed that:

• foundation inspection was a mandatory milestone;
• drainage testing had to precede backfilling;
• first-fix electrical and plumbing work had to precede plastering;
• procurement of roof materials was an external risk;
• finishes had little float and were close to the critical path.

Approach and solution

The project team prepared a logic-linked schedule, a weekly two-week look-ahead plan and a procurement schedule. The master programme remained at activity level, while the look-ahead plan converted activities into measurable tasks such as “complete 80 m of drainage” and “obtain inspection approval for grid lines A–D.” A risk register was linked to activities, and progress was measured by installed quantities rather than by subjective percentage estimates.

The team also divided the building into work zones. This permitted services installation to begin in released areas without assuming that the entire building was complete. However, the overlap was accepted only after confirming access, safety separation and inspection requirements.

Outcome

The revised schedule identified roof-material procurement as an early action rather than a late construction concern. It also showed three days of float in part of the internal plastering work, allowing labour to be moved temporarily to a constrained zone. The client received a clearer completion forecast and could see the effect of approving inspections promptly.

Learning points

1. A bar chart is more useful when supported by verified network logic.
2. Milestones should represent genuine approvals, releases or handovers.
3. Quantity-based progress is more defensible than unsupported percentages.
4. Float is a management resource, not permission to delay work carelessly.
5. Construction planning must include procurement, inspections and external approvals.

9. Schedule Updating and Control

A schedule is a management model, not a one-time document. At each update, the planner should record actual starts, actual finishes, remaining durations, progress quantities, revised logic and approved changes. The data date separates historical information from forecast information.

Good updating practice includes:

• preserving the original baseline;
• recording reasons for variance;
• distinguishing delay from approved scope change;
• checking whether the critical path has changed;
• validating remaining durations with site personnel;
• reviewing near-critical activities;
• reporting forecast completion dates honestly.

Software can calculate dates quickly, but it cannot determine whether a dependency is physically valid or whether a duration is credible. A schedule with thousands of activities can still be poor if it contains open ends, excessive constraints, circular logic or unrealistic resource assumptions.

Resource and risk considerations

CPM traditionally assumes that resources are available when required. Construction projects rarely satisfy this assumption. If two critical activities require the same crane or specialist crew, the schedule may need resource levelling, resequencing, additional resources or a revised completion date.

Resource levelling can extend project duration but produce a feasible plan. Resource smoothing attempts to adjust activities within available float without changing the completion date. These decisions should be documented because they alter the relationship between the theoretical network and the buildable programme.

10. Professional, Ethical and Technological Issues

A planner has a professional duty not to manipulate dates to create a misleading impression. Artificially shortening durations, hiding constraints or omitting known approval periods can cause unsafe acceleration, disputes and financial loss. Schedule information should be traceable to quantities, assumptions, records and responsible contributors.

Digital scheduling systems and building information modelling can connect activities to locations, quantities and models. Four-dimensional BIM links construction sequence to time and can reveal access conflicts. Mobile reporting can capture photographs, quantities and inspection records. AI-assisted tools may identify unusual progress patterns or suggest forecast dates, but their outputs require human verification. AI should not replace engineering judgement, contractual interpretation, safety review or communication with the construction team.

Contemporary practice increasingly integrates schedule risk analysis, location-based planning, lean look-ahead planning and digital progress measurement. These methods complement rather than eliminate fundamental CPM principles: clear scope, credible durations, valid logic and disciplined updating.

Challenges and Limitations

CPM and PERT have important limitations:

• activity durations may be inaccurate;
• networks may omit informal or external dependencies;
• resource limitations may invalidate calculated dates;
• weather and ground risks may be correlated;
• multiple critical paths can make control difficult;
• changes can invalidate the baseline;
• PERT assumptions may not represent actual distributions;
• software output can create false confidence.

The solution is not to abandon formal scheduling. It is to combine network analysis with field verification, risk management, procurement control, short-term planning and transparent reporting.

Best-Practice Checklist

Before issuing a construction schedule, verify that:

• the scope is represented by a suitable WBS;
• every activity has a clear deliverable;
• durations state their basis and calendar;
• mandatory inspections and approvals are included;
• dependencies reflect construction reality;
• the network has a logical start and finish;
• critical and near-critical paths have been reviewed;
• resources and work fronts are feasible;
• procurement and external interfaces are represented;
• milestones have zero duration and clear acceptance criteria;
• baseline and update rules are defined;
• assumptions, risks and constraints are documented.

Chapter Summary

Activity planning translates project scope into measurable work packages with responsible resources, durations and deliverables. Duration estimates should be based on quantities, productivity, working conditions and explicit assumptions. Dependencies describe the order and overlap of construction work, while milestones identify significant zero-duration events.

Bar charts communicate timing effectively but do not always reveal logic. Precedence networks show relationships among activities and provide the foundation for CPM. The forward pass calculates early dates, the backward pass calculates late dates, and total float identifies scheduling flexibility. Activities with zero total float form a critical path under conventional CPM assumptions.

PERT introduces uncertainty through optimistic, most likely and pessimistic estimates. Its weighted expected duration and variance formulas are useful for introductory risk thinking, but they do not remove the need for professional judgement. Effective schedule control requires regular updating, resource and risk review, honest reporting and integration with procurement, inspections, safety and site operations. Digital tools and AI can improve analysis, but they cannot substitute for sound construction logic and field validation.

Key Terms

Activity: A defined item of work that consumes time and resources.

Bar chart: A time-scaled chart using horizontal bars to show activity timing.

Baseline: The approved reference schedule used for comparison.

Critical path: The longest controlling path through a network under stated assumptions.

Dependency: A logical relationship between activities.

Duration: The working or calendar time required to complete an activity.

Early finish: The earliest calculated completion time of an activity.

Early start: The earliest calculated start time of an activity.

Float: Time by which an activity may move without violating a specified schedule condition.

Lag: A planned waiting or separation period between linked activities.

Late finish: The latest allowable completion time consistent with the schedule objective.

Late start: The latest allowable start time consistent with the schedule objective.

Milestone: A significant zero-duration event.

Network schedule: A logic-based representation of activities and relationships.

PERT: A probabilistic scheduling technique using three duration estimates.

Predecessor: An activity that logically influences a successor.

Production rate: Quantity of work completed per unit of time.

Successor: An activity that follows another activity in the network.

Total float: Difference between late and early dates for an activity.

Work breakdown structure: Hierarchical decomposition of project scope.

Review Questions

Short Answer Questions

1. What is the difference between an activity and a milestone?
2. State two factors that can affect construction activity duration.
3. What does a finish-to-start relationship mean?
4. Why is the maximum predecessor early finish used in a forward pass?
5. What is total float?

Long Answer Questions

1. Explain how a work breakdown structure supports construction scheduling.
2. Compare the communication strengths and logical limitations of bar charts.
3. Describe the forward-pass and backward-pass procedures in CPM.
4. Explain CPM and PERT, including the situations in which each is useful.
5. Discuss how resource availability and procurement can affect a network schedule.

Analytical / Application Questions

1. For activities A–F in the worked example, explain why activity C is not critical even though it precedes foundation concrete.
2. A subcontractor proposes reducing a six-day activity to three days by doubling the crew. Identify the technical and management questions that should be answered before accepting the proposal.
3. Prepare a small precedence network for a room renovation containing demolition, inspection, rough-in services, plastering, painting and final testing. State reasonable durations, dependencies and the likely critical path.

Multiple Choice Questions with Answers

Question 1

Which item normally has zero duration in a construction schedule?

A. Excavation

B. Concrete placement

C. A milestone

D. Reinforcement fixing

Correct Answer: C. A milestone

Question 2

If 240 m² of work is completed at 40 m² per day, what is the estimated duration?

A. 4 days

B. 6 days

C. 8 days

D. 10 days

Correct Answer: B. 6 days

Question 3

In a finish-to-start relationship, the successor activity:

A. Must start before the predecessor finishes

B. Starts after the predecessor finishes

C. Must finish before the predecessor starts

D. Always has zero float

Correct Answer: B. Starts after the predecessor finishes

Question 4

During a CPM forward pass, the early start of an activity with several predecessors is normally based on the:

A. Minimum predecessor early finish

B. Average predecessor duration

C. Maximum predecessor early finish

D. Latest project finish

Correct Answer: C. Maximum predecessor early finish

Question 5

An activity has ES = 5 days and LS = 8 days. Its total float is:

A. 3 days

B. 5 days

C. 8 days

D. 13 days

Correct Answer: A. 3 days

Question 6

Using t_e = (a + 4m + b) ÷ 6, what is the PERT expected duration when a = 4, m = 6 and b = 10 days?

A. 5.00 days

B. 6.00 days

C. 6.33 days

D. 10.00 days

Correct Answer: C. 6.33 days

Question 7

Which statement about a critical path is correct?

A. It is always the most expensive path

B. It is the path with the greatest controlling duration

C. It contains every activity in the project

D. It consists only of milestones

Correct Answer: B. It is the path with the greatest controlling duration

Question 8

Which document most directly shows activities against a time scale using horizontal bars?

A. Bar chart

B. Risk register

C. Quantity take-off

D. Inspection request

Correct Answer: A. Bar chart

Question 9

If two activities must both finish before a successor can begin, the successor’s early start is determined by:

A. The earliest of the two finishes

B. The latest of the two finishes

C. The shorter activity duration

D. The activity with the greater cost

Correct Answer: B. The latest of the two finishes

Question 10

What is a major limitation of relying only on scheduling software?

A. It cannot display dates

B. It cannot calculate durations

C. It cannot judge whether construction logic is physically valid

D. It cannot show milestones

Correct Answer: C. It cannot judge whether construction logic is physically valid