CIVL1810 Chap.5 Building a Slab on Ground, Step by Step
Building a Slab on Ground, Step by Step
The most procedural material in the unit, and among the most reliably examined. Eleven steps take a levelled site to a finished, cured slab: excavate and set out, trench, allow for services and recesses, set formwork, screed a sand bed, lay and seal the membrane, install reinforcement on tied chairs, pour, screed and finish, and cure.
One idea governs the whole sequence, which is that from the moment concrete arrives until the final trowelling the concrete controls the time clock, so every earlier step exists so that nothing has to be fixed afterwards. The chapter closes with the structural elements the slab carries and the load path back down to the ground.
What this chapter covers
- 01
Step one: excavate to level and set out, cut and fill a sloping site, and remove all grass, roots and organic matter
- 02
Why cut and fill leaves two founding materials and may require different footings
- 03
Step two: trenches by backhoe then hand cleaned, with edge beams on natural soil or controlled fill
- 04
Controlled fill defined as fill whose engineering properties are controlled
- 05
Step three: a pipe outside the middle third of the beam depth requires increased depth or reinforcement
- 06
Step four: preserving footing thickness under wet area recesses, with the slab top set down 40 to 50 mm
- 07
Step five: formwork pegs outside the building line, vertical, and clear by the formwork thickness
- 08
Step six: a screeded and compacted sand bed for level and to protect the membrane
- 09
Step seven: the damp proofing membrane lapped at least 200 mm and sealed around penetrations
- 10
Step eight: reinforcement to the engineering plans, cover maintained, mesh pre cut, bar chairs tied
- 11
Step nine to eleven: placing without damaging chairs or membrane, screeding between level dots, and curing moist with hessian or water spray
- 12
Structural elements: floor, slab, beam, column and wall, with typical dimensions
- 13
The load path from concentrated, distributed and linear loads down to the ground
- 14
The repeating element cycle of fixing reinforcement, fixing formwork, pouring, curing and striking
A service penetration through an edge beam
- +1Identify the rule. Divide the beam depth into thirds. A pipe or conduit crossing an edge or internal beam outside that central band obliges you to deepen the beam, add reinforcement, or do both.
- +1Locate the middle third. For a 600 mm beam the middle third runs from 200 mm to 400 mm below the top. The pipe centreline at 480 mm lies below that band, so the penetration is outside the middle third.
- +1State the consequence. The beam depth or the reinforcement, or both, must be increased to compensate. The reason is that the middle third is the least stressed zone of the section, so a hole there removes little of the material doing the work, whereas a hole near the tension face removes material where the beam is working hardest. Either move the pipe into the band or redesign the beam.
Key terms
- Cut and fill
- Excavating the high side of a sloping site and placing the material on the low side to produce a level area for the slab. The result is a platform founded partly on natural soil and partly on fill, which may require different footings on the two.
- Controlled fill
- Placed material whose engineering properties have been deliberately managed, put down to carry structures or the pavements that go with them. Edge beams may be founded on natural soil or on controlled fill, but not on uncontrolled fill.
- Middle third
- The central third of a beam depth. A pipe or conduit passing through a beam outside this band requires the beam depth or the reinforcement, or both, to be increased.
- Damp proofing membrane
- The sheet laid under a slab to stop moisture rising into it. Joins overlap by 200 mm as a minimum, the sheet is held down so it cannot creep, and it is sealed wherever a pipe or conduit comes up through the footing.
- Bar chair
- The support that holds reinforcement at its designed position so the specified cover survives the pour. Chairs are tied to the reinforcement, because untied chairs are how cover is lost on a finished slab.
- Span
- The clear distance from one vertical support to the next, which in concrete buildings usually falls between five and twelve metres. Moment and deflection both grow rapidly with span, which is why it limits slab and beam depth.
- Load path
- The route a load takes to the ground. Concentrated, distributed and linear loads arrive on the slab, pass to the beams, then to the columns, then to the spread footing or foundation, and finally into the ground.
Building a Slab on Ground, Step by Step FAQ
Why does the sequence insist on a sand bed under the membrane?
Two reasons, and both earn marks. It lets you bring the ground beneath the slab to an accurate finished level, gives the slab something firm to sit on, and cushions the waterproofing sheet so nothing holes it while concrete is going in. Without it the membrane lies on uneven, stony formation and is holed by a boot or a barrow wheel before the truck arrives.
Is curing the same as drying?
No, and the exam knows the difference. Cement reacts with water and hardens, so water is a reactant rather than something to be removed. Curing lets the concrete gain its intended strength, durability and water tightness, and the instruction is to keep the concrete moist using hessian or a water spray. Losing water early, which is what a hot and windy day does, is exactly what costs those three properties.
Why do hot and windy conditions get flagged twice?
Because they are the same problem twelve hours apart. At the pour they accelerate stiffening, making the concrete very hard to level off and float to a smooth finish; at curing they accelerate water loss, costing strength and durability. Naming both moments in an answer shows you understand the mechanism rather than having memorised a warning.
What is the difference between a column and a wall in this unit?
It is stated as a difference in how the support acts, not in shape. A column is an upright member picking up slab or beam load at one point, usually 200 to 1000 mm across. A wall is an upright member, long and thin in plan, picking that load up continuously along its length, generally 150 to 400 mm thick.
That distinction is what makes converting a column into a wall a legitimate answer when a column will not fit an apartment layout.
Exam move
Learn the eleven steps as six blocks, because a six mark question is marked in six moves and an answer that spends three paragraphs on excavation and one line on curing loses marks it did not need to. Group them as prepare the platform, trench, allow for services and recesses, form and bed and seal, steel and cover, then pour and finish and cure. Attach one reason to each block.
Draw the finished section from memory with every layer labelled, since the section and the sequence are the same information in two forms. Finally, practise the short consequence questions under a strict fifty word limit, giving one ultimate and one serviceability sentence, because the paper asks for both.
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