(a) What do you understand by a Gantt or Bar chart with respect to construction project management ? Explain it with a neat…

(a) Bar (Gantt) chart in construction project management (20 marks)

A bar or Gantt chart is a horizontal-bar diagram in which the activities of a project are listed down the left and a time scale runs across the top. Each activity is a bar whose start, end and length show the planned start date, finish date and duration. Progress is shown by shading the completed part of each bar against a "today" line. It is the simplest and most widely used scheduling tool on construction sites because it is easy to prepare, read and update.

Limitations.

• It does not show dependencies. The chart does not show which activity must finish before another starts. In the sketch, the bar for brick masonry starts in week 10, but the chart cannot tell whether that is because the slab must be cast first, or merely because of the labour available. A delay in footings would not visibly move the columns and the slab.
• It cannot identify the critical path or float. Nothing shows which activities can slip without delaying the completion and which cannot. In a building project, the delay of an activity such as the slab, on which everything above depends, cannot be told apart from a delay in plastering, which is less critical.
• It is poor for large, complex projects. With hundreds of activities in a bridge or a multi-storey complex, the chart becomes crowded and the links between the trades cannot be seen. Network methods (CPM or PERT) are needed.
• It is hard to update and to re-plan. If one activity is delayed, every later bar must be redrawn by hand, and the consequence for the finish date is not obvious.
• It ignores uncertainty and resources. It gives a single fixed duration for each activity, with no allowance for variation, as in weather-dependent earthwork or concreting. It does not show the resources (labour, cranes, cost) required or the resource clashes when two bars overlap.
• Progress can mislead. A bar that looks 60% shaded does not show the work remaining on a critical activity.

Bar charts are therefore best for small projects and for the summary programme that is issued to the site.

(b) Effect of vibration on strength and the w/c ratio (10 marks)

Effect of compaction. Abrams' law states that strength falls as the water-cement ratio rises, S = K₁/K₂^(w/c). It holds only for concrete that is fully compacted. A lower w/c gives a stiffer, less workable mix, which cannot be compacted by hand. It then holds trapped air and voids, and the strength curve turns down instead of continuing to rise. Every 1% of voids reduces the strength by about 5–6%.

The sketch shows that the fully compacted (vibrated) concrete follows Abrams' curve, rising steadily as w/c is reduced. The hand-compacted concrete follows it only at higher w/c, where the mix is workable, and falls below it at low w/c because of honeycombing. Vibration overcomes the internal friction between the particles and makes a stiff mix flow, so it makes the low w/c practicable and allows the high strength to be achieved.

Advantages of vibrating fresh concrete.

• It removes entrapped air and voids and gives a denser, stronger concrete of higher and more uniform strength.
• It allows a lower water-cement ratio, so the concrete is stronger, less permeable and more durable, for the same cement content, or the same strength with less cement.
• It gives better bond with reinforcement and fills congested formwork completely, without honeycombing.
• It gives better surface finish and reduces bleeding, and permits stiff and lean mixes, so it is economical and faster to place.

Over-vibration must be avoided, since it causes segregation.

(c)(i) Equilibrium cant and permissible speeds on a BG curve (15 marks)

Data: R = 1000 m, equilibrium speed V = 90 km/h, cant deficiency and cant excess limits of 75 mm each. Take G = 1750 mm, the distance between the centres of the rail heads on a broad gauge track.

Equilibrium cant.

C = GV²/127R = 1750 × 90²/(127 × 1000) = 1750 × 8100/127 000 = 111.6 mm

Maximum permissible speed. The maximum speed corresponds to the equilibrium cant plus the cant deficiency:

C + C_d = 111.6 + 75 = 186.6 mm, which is the cant that would be in equilibrium at V_max, so

V_max = √(127R(C + C_d)/G) = √(127 × 1000 × 186.6/1750) = √13 542 = 116.4 km/h

Minimum permissible speed. The minimum speed corresponds to the cant less the cant excess:

C − C_e = 111.6 − 75 = 36.6 mm, so

V_min = √(127 × 1000 × 36.6/1750) = √2656 = 51.5 km/h

So the speed on the curve must lie between about 52 km/h and 116 km/h. Below the minimum speed the inner rail is overloaded, and above the maximum speed the outer rail is overloaded and passenger comfort suffers.

(c)(ii) Remote sensing and photogrammetry (5 marks)

Remote sensing is the science of obtaining information about the Earth's surface and objects without contact, by recording reflected or emitted electromagnetic energy from satellites, aircraft or drones. Photogrammetry derives accurate measurements and maps from overlapping photographs, mostly in the visible band, using metric cameras, and gives dimensions and topographic maps. Remote sensing uses multispectral, thermal and microwave sensors and gives digital thematic data on land cover, water and vegetation.