If you’ve modeled your concrete cure time as a task in Primavera P6 Professional having a unique 7-day workweek calendar you may have found that your critical path in certain instances becomes discontinuous or disjointed.
There is more than one approach to modeling lag, (e.g. concrete cure time) in Primavera P6. It is possible to define a lag and associate a 24-hour calendar in the schedule options dialog. This works great because your concrete cure time measures across every day, including weekends, which is what you want.
However, doing this will mandate that all your lag in your schedule have the 24-hour schedule options calendar. Most likely this is not what you want. You prefer some activities to measure lag over weekends and others to measure lag only on standard workweek days. So reserve your lag for only standard workweek measurement, and find another approach for modeling concrete cure time.
The popular alternative cure time approach is to model the concrete cure as a separate task and assign it a 7-day work week calendar. This is a good solution as this cure time task measures continuous progress across both the workweek and weekends.
However, this technique is not without its issues: the critical path in certain situations may become intermittent. Minor tweaks to the schedule address this issue, so that your schedule will still be helpful for ‘longest path’ optimization efforts.
This article discusses how to tweak the Primavera P6 Professional cure time task approach to retain a continuous ‘longest path’ in support of schedule optimization.
Those of you not familiar with the process of modeling concrete cure time as a task should refer to the following blog Primavera P6: A Simplified Procedure For Scheduling Cure Time.
In this blog I detail the procedure for modeling cure time as a task. My goal today is to simply complement this cure time procedure with minor tweaks to support schedule optimization efforts. Let’s proceed.
We have in Figure 1 our concrete installation project schedule.
Note, in particular, task ‘E- Cure Concrete’. Note on the Gantt chart that this task has two red stripes that indicates that it is a critical activity along the critical path. You will also observe an additional light blue stripe that identifies the activity as having a ‘7-Day Week Cure Time’ calendar, as per our bars definition.
If you consider the 6-day duration of this activity you will notice that the activity span measures through the weekend. That’s great! That is exactly what we intended.
Everything looks in line for our concrete cure activity. However, let’s adjust this task original duration so that the concrete finishes curing on Saturday, Figure 2.
Okay, now we see something is amiss; we have lost our longest path. What happened? Well, the cure process completed on Saturday, which means we have 1-day total float. This means we can delay the cure process 1-day and still not affect our schedule end date. Well, that may be convenient, but delaying the cure process is unrealistic; the concrete cure process always begins directly after the pour concrete activity. So we have a total float value that does not truly represent the schedule narrative, and we lost our longest path. Not good.
Okay, we know we have an issue. There are two ways we can tweak our schedule to address this longest path discontinuity, both involve adjusting the definition of a critical activity. Currently, in the schedule options dialog we define critical activities as having 0-hours total float, Figure 3.
We can change the threshold of critical activity total float or toggle to make critical tasks simply all activities along the longest path.
In the first approach, we change the total float critical activity threshold from zero to 2-days to include weekends, Figure 4.
Now activities that have 2-days or less total float become critical activities. This may not be what you prefer. But it’s not unprecedented; activities that have 2-days total float are near critical and do require special attention in good scheduling practice. Including weekend days in the critical activity threshold may be a small tradeoff, but it keeps our critical path intact for cure processes that conclude either on Friday or Saturday. Figure 5 demonstrates this with activity ‘E – Cure Concrete’ on Saturday.
Here we have 1-day total float, but our critical path remains in place, which is good for schedule optimization among other reasons.
In the second method, we modify the definition of critical activities to all ‘longest path’ tasks, Figure 6.
Now regardless of a tasks total float, if it is along the schedule’s ‘longest path’ it is flagged and appears as critical. In Figure 7 our concrete cure process concludes on Friday, so numerous activities have 2-days total float.
We find in Figure 7 that we still have one continuous ‘longest path’. This may be less than ideal if you have activity constraints creating multiple float critical paths. However, we retain, again, one continuous ‘longest path’ through the network that will provide support for schedule optimization efforts, which is a plus.
Modeling concrete cure time as a task is preferable because, again, it will not use up your available lag definition; you may still reserve your lag definition for a standard workweek calendar.
Not all scheduling software supports the Gant chart graphical features required to highlight tasks assigned a unique 7-day workweek calendar. So this may become your limiting factor.
But if you do use the task approach to model concrete cure you will want to either include weekend time in your critical activity total float threshold or limit your critical activity definition to solely tasks along the ‘longest path’. Both these schedule option tweaks retain the ‘longest path’ and provide support for schedule optimization.