Introduction

Construction scheduling is an important part of project management in construction projects. A scheduler shall follow international standards to develop a robust construction schedule. The baseline construction schedule is developed at the initial stage of the project to plan the workflow sequence and to complete the project within the agreed time.  The quality of the baseline schedule is significant for monitoring the progress to ensure the timely completion of the project.  Moreover, a validated construction schedule is required for delay analysis if the project is delayed by events that are beyond the control of the contractor.

Developing a Baseline Construction Schedule

Developing a baseline construction schedule is the procedure of converting the project scope into milestones, activities, work sequences, resource assignments, and cost loading. The development of a baseline construction schedule includes continually improving the outputs of lessons learned from similar projects, planned duration, and sequence of works in a realistic way.

Popular Techniques Used for Schedule Development

The project contract documents, scope of work, contracts, bill of quantities, and other project attributes such as drawings, and soil reports are to be incorporated with the schedule to accomplish an effective construction schedule. The most common technique used for the construction schedule development process is the Critical Path Method (CPM).

Critical Path Method (CPM)

The critical path is the longest in a project schedule. The activities in the critical path are called critical activities. Any delay in critical activities will delay the overall project finish date.

The Critical Path Method (CPM) is one of the Activity-On-Arrow (AOA) project management network techniques used in project scheduling. This is used to conclude the sequence of activities of the longest path and shortest possible duration to complete the project.

Figure 1: CPM Network 

Gantt Chart

Gantt Chart is used in the Critical Path Method to graphically illustrate the project activities, duration of the activity, relationships between activities, start date, and finish date.

Figure 2: Gantt Chart

International Standards for Developing Construction Schedule

 A well-developed schedule contains a proper level of detail to facilitate effective project management. The international standards and quality metrics (DCMA 14 Point Assessment) for a construction schedule as per the recommended practices by the American Association of Civil Engineers (AACE) are as follows.

1- Logic

The work sequence of the tasks shall comply with appropriate logic. The tasks are to be linked with appropriate predecessors and successors to maintain the correct logic.

2- Leads

The lead is the negative lag in the assigned relationship between activities, that is to be avoided in a construction schedule.

3- Lags

A lag is a lapse of time between the start/finish of overlapped activities in a construction schedule.  The use of lags in relationships between activities shall be minimized to 5% of total activities in a construction schedule. Unnecessary lags and high lags in relationships are to be avoided.

4- Relationship Type

The Finish-to-Start relationship between activities shall be used for at least 90 % of the total activities in a construction schedule. 

5- Hard Constraints

The hard constraints listed below shall be reduced to a maximum of 5% of the construction schedule. The hard constraints shall only be assigned for important milestones in the construction schedule.

1. Must-Finish-On (MFO)

2. Must-Start-On (MSO)

3. Start-No-Later-Than (SNLT)

4. Finish-No-Later-Than (FNLT)

5. High Float

The total float for the activities in the construction schedule shall be more than 44 days.

6. Negative Float

The total float with a negative value is termed a negative float.  The negative float is not acceptable for the activities in a baseline construction schedule.

7. High duration

If the duration of any construction activity in a construction schedule is greater than 44 days, it is reckoned as a high duration. The durations for construction activities shall be desirably less than 21 days.  The duration for procurement activities shall be planned as per the requirements.

8. Invalid dates

The forecast/actual dates in a construction schedule which are earlier than the data date, are invalid. The construction schedule is to be developed without any invalid dates.

9. Resources

All the construction activities in a schedule with a duration of 1 day or more, are to be assigned with required resources.

10. Missed tasks

When the total number of tasks in an updated schedule is compared with the baseline schedule it shall be the same without any missing tasks. 

11. Critical path test

If any slippage in the rate of progress is observed in the updated schedule concerning the original baseline schedule, the total floats for the respective task and the milestone for project finish are to be checked.   If the negative total float of the project finish milestone is less than the negative total float of the delayed task, the critical path is incorrect.  To correct this error the actual dates, actual % of works, and the relationships between the critical activities are to be revisited and corrected.

12. Critical Path Length Index (CPLI)

The Critical Path Index is used to measure the efficiency required on the remaining tasks to accomplish the project completion. The critical path index is determined using the below formula.

The value of the Critical Path Index should be 1 or above the construction schedule is deemed as efficient. If the value is less than 1, the efficiency is low, and the sequence of the project schedule is to be revised to obtain the required value of the critical path index.

13. Baseline Execution Index

The Baseline Execution Index is used to assess the efficiency of actual progress in an updated schedule.  It is calculated by computing the ratio of an actual number of tasks completed versus the number of tasks that should have been completed as per the baseline schedule on a particular data date. If the calculated value of the baseline execution index is 1.00 the progress is on track. If the calculated value of the baseline execution index is above 1 the progress of the project is ahead.  If the calculated value of the index is less than 1, the project is delayed, and necessary mitigation measures are to be taken to recover the delay.

Conclusion

The quality of the construction schedule shall be improved by following the international standards discussed above.   The construction schedule developed with high quality will be useful to create an impacted schedule with a reasonable impact.  The impacted schedule is one of the prerequisites for delay analysis to claim an extension of time.  Given the above, it is wise to adopt international standards in construction scheduling.