This document provides an overview of construction project scheduling and time estimation techniques. It discusses defining work tasks and precedence relationships, estimating activity durations using deterministic and probabilistic methods, developing network diagrams, and calculating activity start/finish times using the Critical Path Method (CPM). CPM involves forward and backward passes to determine earliest and latest times in order to identify the critical path and activities. The document also compares CPM to the Program Evaluation and Review Technique (PERT), which uses optimistic, most likely, and pessimistic estimates to compute expected durations for uncertain activities.

1. NAME OF THE INSTITUTE, PARUL UNIVERSITY
DBU – Construction Technology and
Management Department
Construction Management (CEng5194)
Lec 3: Construction Project Scheduling - Time Estimation
Amanuel G.
2014 E.C.

2. – Project Scheduling
– Time estimation procedure
– The fundamental project scheduling techniques
– CPM (Critical Path Method)
– PERT (Program Evaluation and Review
Technique)
– PERT vs. CPM
– Benefits of CPM /PERT network
2
Lecture Outline

3. 3
Construction Project Scheduling

4. Time estimation procedure
A. Defining work tasks /project work scope
B. Defining precedence relationships among activities
C. Estimating activity duration
D. Develop the project network.
E. Calculate early and late start/ finish times
F. Compute float values and identify the critical path
G. Schedule activity start/ finish times
4

5. Time estimation procedure
A. Defining work tasks:
This defines the scope of work, method statement, and
sequence of work for determining activities involved in
construction works.
B. Defining precedence relationships among activities
• Once work activities have been defined, the relationships
among the activities can be specified.
• Precedence relationships can be illustrated by a network or
graph in which the activities are represented by arrows
(AOA /CPM) or by node (AON /PDM /PNA).
5

6. C. Estimating activity duration
 Duration of an activity is the expected economical transaction time
or the expected length time required to carry out an activity (i,j) from the
beginning to its end.
 Activity durations are directly depending on:
1. The quantity of work involved in the activity.
2. The resources deployed to them (men, materials, and machinery). e.g., crew
size and equipment
3. The productivity of these resources.
 For example, the time taken to paint 100 m2 using 2 painters and
assuming each painter can do 5m2 in an hour, is simply 10
hours.

7. Duration Estimation methods
1. One time estimate (deterministic) [CPM]: this estimation of duration
is used when the exact duration of an activity, is likely or certain.
It is based on one of the following;
 Planning data
 Making educated guesses based on experience and knowledge
 Estimating based on previous (similar) project data
 Estimating based on industry standards and average time
assessed by a group of executives
2. Three time estimate (probabilistic) [PERT]: is used when the exact
duration of an activity, like research and development, is not certain,
to compute its expected duration.
Optimistic, most likely and pessimistic time estimate are used to compute
the expected time.
 Application in construction projects:
 Planning of the projects especially, at the feasibility stage.
 Where time is the main criterion/consideration for management
and the resources employed are of secondary consideration;
 For complex structures, where the exact duration estimate is difficult to
assess.

8. 8
Where;
• To, is the optimistic time in which everything goes extremely
right,
• Tm, is the most likely time in which everything goes under
normal prevailing condition and,
• Tp, is the pessimistic time in which every thing goes wrong,
• Te, is the expected time to complete an activity.
cont’d…
Debre Berhan University

9. 3. Trapezoidal distribution estimate
 In practice, the profile of most activities takes the shape of
trapezoidal distribution.
 Trapezoidal distribution estimate assumes the build-up and run-
down phase which can be expressed in terms of total activity
duration:
The build-up 20% of the total duration &
Run-down time 10%
Note that the productivity (p) is modified p’=0.85*p, P’<p,
Activity duration = total man days / 0.85*P

10. D. Developing the Network diagram
Network representation
For construction planning two kinds of networks can be used:
– activity on arrow /arc (AOA) and
– activity on node (AON).
1. Activity-on-arc /arrow (AOA)
 Each activity is represented by an arc /arrow.
 A node is used to separate an activity (can be looked as either the
starting or the end point (event) of an activity).
 Here we can describe / define the activity in two ways:
1. By its activity title (A)
2. By its starting and finishing event nodes 1–2.
10 30 50 30
EW BW PW
7 2
3

11. 2. Activity-on-node (AON)
 Where each activity is represented by a node.
 The arcs are used just to show the precedence relationships
between the activities.
EW
3
BW
7
PW
2

12. Basic Rules/ laws of network diagrams
1. The starting node of an activity is the finishing node of one or more other
predecessor activities,
2. Each activity must have a different set of starting and finishing node numbers
This poses a problem when two activities start and finish at the same event node.

13. 3. When two chains of activities are inter-related, this can be shown
by joining the two chains either by a linking activity or a ‘dummy’.
4. Each activity (except the last) must run into another activity.
Failure to do so creates a loose end or ‘dangle/hang’.
• Dangles create premature ‘ends’ of a part of a project, so that the
relationship between this end and the actual final completion node
cannot be seen.

14. 5. No chain of activities must be permitted to form a loop, i.e. such a
sequence that the last activity in the chain has an influence on
the first.
Creating the network is an iterative process and may
involve a number of revisions before an optimum
solution is found.

15. Activity Description Predecessor
A
B
C
D
E
F
G
H
I
Site clearing
Removal of trees
General excavation
Grading general area
Excavation for utility trenches
Placing formwork & reinforcement for concrete
Installing sewer lines
Installing other utilities
Pouring concrete
---
---
A
A
B,C
B,C
D,E
D,E
F,G
15
Example: Precedence r/p for site preparation & foundation work
Suppose that a site preparation and concrete slab foundation
construction project consists of nine different activities:

16. • The following diagrams show the representation of all
the precedence relationships among the activities on
arrow.
16

17. Project constraints (logical r/p constraints)
 A constraint is something that limits or controls what you can do.
 Without constraints on a project, all activities can theoretically begin
on first day of construction
Types of constraints:
 Physical constraints: related to the method of construction
 Resource constraints: having limited amount of resources
 Safety constraints: safety requirements may dictate two or more activities not
to occur simultaneously
 Financial constraints: staggering of high cost activities, necessity of securing
loans before undertaking some portion of the project etc
 Management constraints: related to supervisory time, cash flow needs,
demands of other projects, and generally managerial decisions
 Regulatory constraints: rules and regulations such us environmental
protection agency, and land use restrictions
 Contractual constraints, Environmental constraints….

18. Activity relationships /dependencies:
 A) Finish to start (FS): The successor activity can begin only when
the current (predecessor) activity completes.
 CPM and PERT uses only this type of relationships.
 B) Finish to finish (FF); The finish of the successor activity depends
on the finish of the current activity.
It can be used where activities can overlap to a certain limit.

19.  C) Start to start (SS): The start of the successor activity depends on the
start of the current activity.
 D) Start to finish (SF): The successor activity cannot finish until the
current activity starts (illogical).
It is typically used with delay time or Lag
Eg. Let Duty of Evening Guard be E, Duty of Morning Guard be M;
M cannot Finish her/his duty till E Starts. Simple speaking M cannot abandon the post
even if E gets delayed.

20. NAME OF THE INSTITUTE, PARUL UNIVERSITY
By Amanuel G., COTM, Debre Berhan University 20
• Earliest Start Time of an activity (i,j); [EST(i,j)]:
– This is the earliest time that the activity (i,j) can be started, i.e., all the
necessary preconditions are met.
• Earliest Finish Time of an activity (i,j); [EFT(i,j)]:
– This is the earliest time that an activity can be completed.
– Mathematically, the relationship can be expressed as:
• EFT(i,j) = EST(i,j) + D(i,j)
• Latest Finish Time of an activity (i,j); [LFT(i,j)]:
– Is the latest time that an activity needs to be completed in order to finish
the project without delay.
– LFT(i,j) = LST(i,j) + D(i,j)
• Latest Start Time of an activity (i,j); [LST(i,j)]:
– Is the latest time when an activity must be started, in order that there is
no delay in the project completion.
• LST(i,j) = LFT(i,j) – D(i,j)
E. Activity Timings: Start and finish times:
Debre Berhan University

21. NAME OF THE INSTITUTE, PARUL UNIVERSITY
By Amanuel G., COTM, Debre Berhan University 21
Float or Slack Time
• It is difference between the time available to do a job
and the time required to do a job.
• Is the additional time available to complete a non-
critical activity (the amount of time an activity can be
delayed/extended) to give freedom of flexibility to float.
• Floats are used to determine the criticality of an activity
and the “critical path /CP”.
• The critical path are made of activities that cannot be
delayed without delaying the final completion date of the
project.
• The activities on the critical path has zero total floats.
Debre Berhan University
F. Float computation to identify CP.

22. NAME OF THE INSTITUTE, PARUL UNIVERSITY
By Amanuel G., COTM, Debre Berhan University 22
Total Float (TF(i,j)):
• Total float is the amount of time by which the start of an activity can be delayed
without causing a delay in the completion time of the project.
• The amount of float available for a given project (path).
• This is calculated as;
TF(i,j) = LFTj – ESTi – D or,
TF(i,j) = LFT(i,j) – (EST(i,j) + D) = LFT(i,j) – EFT(i,j)
TF(i,j) = (LFT(i,j) – D) - EFT(i,j) = LST(i,j) – EST(i,j)
Free float (FF(i,j)):
• FF is the amount of time by which the start of an activity can be delayed
without delaying the early start date of a following / succeeding activity.
• The amount of float available for a particular activity.
– Free Float = (Earliest start time of the following activity – Earliest start time
of the activity – Duration of the Activity)
– Free Float = ESTj - ESTi – Di
– FF = ESTj – (ESTi + Di) or FF = ESTj - EFTi
Debre Berhan University

23. Floats

24. Fundamental Project Scheduling (Network Analysis)
Techniques
 Critical path method (CPM)
 Program evaluation and review technique (PERT)
 Bar charts
 Precedence network analysis
 Line of balance technique, and
 Time scale network
24

25. Critical Path Method
NAME OF THE INSTITUTE, PARUL UNIVERSITY
By Amanuel G., COTM, Debre Berhan University 25
• CPM is a network diagraming method/technique used to predict total
project duration.
• Critical path method (CPM) is a technique where you identify tasks that
are necessary for project completion & determine scheduling flexibilities.
• CPM revolves around discovering the most important tasks in the project
timeline, identifying task dependencies, and calculating task durations.
• It is an analysis method with the following main purposes:
• To calculate the projects finish date.
• To identify to what extent each activity in the schedule can slip/float
without delaying the project completion date.
• To identify the activities with the highest risk (CA) that cannot slip
without changing the project finish date.
• Facilitates effective resource management: helps project managers
prioritize tasks, giving them a better idea of how /where to deploy
resources.
• Improves future planning: used to compare expectations with actual
progress. Debre Berhan University

26. CPM Network Terminologies/ Elements
• Activity: An item of work that consumes time and resources to produce
some result/event
• Dummy activity: is used to define interdependence between activities
and included in a network for logical reasons. This activity does not
consume any resources and time.
• Event/Milestone: the start or completion of one or more activities.
Unlike an activity, does not consume time or resources and hence, it
only expresses a state of being a system/product/result.
• Path: Any series of activities connecting the starting point to the
finishing point. In a project having several activities can have several
such ‘paths’.
• Critical path: the path that gives the longest time (duration) of the
project in a network, which in turn defines the shortest possible
project time.
• Critical path identifies the minimum time to complete project
• In a critical path, all the series of activities must finish on time
for/through the whole project to finish the project on time, and a critical
path has zero float.

27. NAME OF THE INSTITUTE, PARUL UNIVERSITY
By Amanuel G., COTM, Debre Berhan University 27
To calculate the projects overall duration, the critical path method uses
two method of calculation:
• Forward pass: moves from the ‘start’ node towards the ‘finish/end’ node to
determine the earliest occurrence time of all events.
• Backward pass: moves from the ‘end’ node towards the ‘start’ node to
determine the latest occurrence time of all events and floats & is also used
to determine the float.
1. Calculate the early start & early finish time for each activity:
• The earliest occurrence time for any node can be estimated from the
(maximum) time taken to reach that node from the different
incoming arrows.
2. Calculate the late start & finish time for each activity:
• The late occurrence time for any node can be estimated from the
(minimum) time taken to reach that node from the different
outcoming arrows.
3. Calculate the floats for each activity
4. Identify the critical activities and find the critical path
Steps to follow in CPM computation
Debre Berhan University

28. Activity Predecessor Duration(in days)
A
B
C
D
E
F
---
A
A
B
C
D,E
3
5
7
10
5
4
28
Example 1:
Draw the network and conduct CPM computation.
Identify the Critical path, critical activities, and project
duration.

29. • Path A, B, D, F:
3+5+10+4 = 22 Days
• Path A,C,E,F:
3+7+5+4 = 19 Days
 Compare the times for the two paths.
 Maximum of {22,19} = 22 (project duration)
 Path A,B,D,F is Critical path. B/c 22 is greater than 19.
 Activity A,B,D&F are critical activates
29

30. PERT (Program Evaluation and Review Technique)
• [PERT]: is used when the exact duration of an activity,
like complex construction works are not certain, to
compute its expected duration.
• Optimistic, most likely and pessimistic time estimate are used
to compute the expected time.
• For computation of critical path, the PERT three time duration
estimation is converted to a single time deterministic CPM
model.
Expected activity time: Te = (To + 4Tm + Tp)/6
• Optimistic time /To : This is the shortest possible time in which the
activity can be completed, and assumes that everything has to go
perfect.
• Most-likely time /Tm: This is the most likely time in which the
activity can be completed under normal circumstances.
• Pessimistic time / Tp : This is the longest possible time the activity
might need, and assumes a worst-case scenario. 30

31. 31
Example 3: Draw the network, identify the Critical path,
critical activities and compute project duration.

32. 32

33. 33
• Using Te = (To + 4Tm + Tp)
6
 Three time estimate are changed into one time estimate
(Expected time)
1 2
3
4
8
7
6
5
C(10)
B(6)
A(4)
D(12)
E(9)
F(21) H(5)
G(6) I (3)

34. 34
• Path A,B,D,F,H 4+6+12+21+5=48 days
• Path A,B,D,G,I 4+6+12+6+3=31 days
• Path A,C,E,F,H 4+10+9+21+5=49 days
• Path A,C,E,G,I 4+10+9+6+3=32 days
 Compare the times for the four paths.
 Maximum of {48, 31, 49, 32} = 49 days
 Therefore the critical path is along path A,C,E,F,H.
 The critical activities are A, C, E, F and H.

35. CPM PERT
Uses network, calculate float time,
guides to monitor and controlling
project
Same as CPM
Focus on cost estimation Focus on time control
Uses one time estimating method
– deterministic treatment of time
• CPM assumes that activity
durations are known with
certainty.
• Used where time can be
estimated with confidence, for
familiar activities
Requires three time estimating method to
get the expected time - probabilistic
treatment of time
• While PERT assumes that activity
durations are random variables (i.e.,
probabilistic).
• Used where time cannot be estimated
with confidence, for unfamiliar or new
activities
Can distinguish critical and non
critical activities
• Which enables crashing
Does not provide information to
distinguish between critical and non
critical activities
• Crashing does not apply
Used for repetitive and non
complex projects.
Used for non-repetitive and complex
projects. 35

36. Benefits of CPM /PERT network
• Shows interdependence between activities, work packages and work
units.
• Determines expected project completion date.
• Identifies critical activities, which can delay the project completion
time.
• Identifies non critical activities with slacks that can be delayed for
specified period.
• Determine the dates on which tasks may be started or must be started
if the project is to stay in schedule
• Shows which tasks must be coordinated to avoid resource or timing
conflicts 36

37. 37
Any Queries?
Thank you 

38. Ind. Assignment 2
1. Discuss about the progress of Construction Industry, the role of
construction in national and International economy, and Its
Managemental Requirements.
2. Discuss and differentiates the construction industry from other
industries sector.
3. A construction project has the following activities along with their
relationships. Develop an Activity on Arrow (AOA) network.
– A is the first activity.
– B, C and D follow A and can be done concurrently.
– E and G cannot begin until C is completed, and can be done
concurrently.
– F is the immediate successor to activities B and E.
– H and K run in parallel, and both succeed G.
– L succeeds F and H.
– J and I are immediate successor activities to D.
– M and N are immediate successor to K and I. However, both M and N
can be performed concurrently. 38