Who this guide is for
- Self-builders about to have a raft, slab or foundation cast
- Owners coordinating a services design against a structural programme
- Anyone running a project where the drawings are still being developed
- Owners assembling a record of what was cast into their building
- Buyers or owners planning to alter a concrete element later
One deadline, several trades, no single owner
The difficulty is not that concrete is unforgiving. It is that the information which has to be complete before a pour is produced by people who do not share a drawing. Drainage falls come from one designer, socket positions from another, reinforcement from a third, and the barrier arrangement from a fourth.
The raft entry states the consequence plainly: barriers, any insulation below the plate, the reinforcement, the service entries and the finish above all have to be agreed before casting, since almost none of them can be revisited afterwards without breaking the structure. A raft concentrates a great deal of decision-making into a single operation, which is an advantage on constrained sites and a risk where information is still changing.
What is actually being closed
It helps to see the list as one object rather than as several packages. Everything below is fixed at the same moment, and each item becomes either invisible or destructive to alter afterwards.
- The barrier plane beneath, and every lap, upstand and closure within it
- Any insulation below the element, which may also be carrying it
- The reinforcement arrangement, and where it stops being continuous
- Sleeves, ducts and openings for drainage and incoming services
- Cast-in fixings, bearings, straps and holding-down assemblies
- The positions of construction and movement joints
- Starter bars projecting into whatever is cast next
Two things cast at the same moment that are not the same thing
A kicker is a short concrete upstand cast with the pour beneath, standing in the line of the wall or column above so the formwork for that element can be set against it. Its purpose is positional: it fixes the line while setting out is easiest to control, and it is the physical record of that set-out.
A starter bar is reinforcement left projecting from the completed pour so the next element can be reinforced continuously with it. The two occur at the same place in the same operation, which is why they are confused, but one positions the mould and the other carries force, and an element may have starters with no kicker at all.
Neither is recoverable by improvisation. A starter cannot be threaded into hardened concrete afterwards, and whether a missing one can be replaced by drilling and fixing a bar is a design decision rather than a like-for-like substitution.
Joints look alike and mean opposite things
A construction joint is where a pour stopped and restarted, and the finished element is meant to behave as though the joint were not there. A movement joint is a deliberate discontinuity provided so parts of a construction can move relative to one another. The physical tell is the reinforcement: a construction joint is normally crossed by continuous reinforcement, while a movement joint is deliberately free of anything restraining the movement it exists to permit.
Confusing them runs both ways. Where a movement joint is treated as a construction joint and continuity is carried across it, the structure cannot make the movement it needed and cracks somewhere it was not intended to. Where a construction joint is treated as a movement joint, the frame has a break where it needed to be whole.
The barrier is the part that suffers most
Barriers beneath a floor are laid early and then walked over, loaded, drilled and covered by other trades. Damage happens before completion and is concealed by the very construction that follows it, which is why verification happens while the plane is still visible and why the protection layer over it belongs to the design rather than to site conduct.
Every rising service pierces the plane at a point fixed by the layout above, so penetration positions are worth fixing before the barrier is laid. A late alteration through a finished floor is not a minor change: a hole cut afterwards damages the structure and the barrier together.
There is a temporary structure holding the permanent one up
While concrete matures, the falsework and the frame are one system. A fresh floor is carried entirely by the temporary support beneath it, and that support is standing on the storey below, which is therefore carrying its own load and part of the floor above. The sequence in which support is struck and moved is a designed arrangement rather than a site preference.
The same theme reaches the smallest details. Fluid concrete finds any gap at the base of a pour, so the seal between a kicker and the formwork above it is where the base of a wall is either well formed or not, exactly where it meets the pour below.
Where the deadline lands earliest
Not every concrete element closes at the same point. An insulating concrete formwork wall is stacked dry and then filled, so the point of no return is the fill, and anything that must be cast in, aligned or braced has to be settled before then.
A post-tensioned slab closes hardest of all. Every opening, sleeve and cast-in item belongs to the pre-pour coordination, because after the pour the slab contains no area that can be assumed to be free of a tendon, and a penetration formed afterwards is not a hole in a slab but a cut tendon whose force is released along its whole length. Filling the anchorage recesses is the moment the assembly stops being inspectable.
Turning the list into a programme
The practical move is to treat each pour as a gate with a named list rather than as a date in a schedule. Every item above has a producer, and the question worth asking early is who supplies each one and when their information has to be complete.
What survives afterwards is the record. As-built information on service positions, joints and barrier arrangement is what makes later alteration possible at all, and cutting into an element without it risks damaging reinforcement and the continuity beneath in the same operation.
Before each pour
- 1List every drainage and service crossing, and confirm each position is fixed rather than indicative.
- 2Confirm who supplies each item on the list and when their information is due.
- 3Confirm which items have to be cast in, including bearings, straps, sleeves and holding-down assemblies.
- 4Confirm where the barrier runs beneath, and how it is closed around every penetration.
- 5Confirm how the barrier is protected between being laid and being covered.
- 6Confirm which joints are construction joints and which are movement joints on the drawings.
- 7Confirm where starter bars are required and that they are positioned as designed.
- 8Check that projecting bars have not been bent, cut or displaced since casting.
- 9Confirm the set-out of any kicker before the pour beneath it is placed.
- 10Ask what temporary support is required and what the striking sequence depends on.
- 11Photograph the reinforcement, the barrier and every penetration before concrete arrives.
- 12Record the as-built positions of services, joints and cast-in items.
- 13Collect the pour records and drawings into the handover pack.
Common mistakes to avoid
- Treating a pour date as a programme milestone rather than as the deadline for six separate information packages.
- Fixing socket and pipe positions after the reinforcement has been set out.
- Assuming a hole can be cut later because the element looks thick.
- Reading a construction joint and a movement joint as the same line because they look alike on a drawing.
- Bending projecting bars out of the way as a matter of routine rather than asking whether it is acceptable.
- Drilling and fixing a bar into hardened concrete as a substitute for a missing starter.
- Leaving a barrier unprotected under site traffic and discovering the damage only by its consequences.
- Finishing a floor with no record of where the services, joints and cast-in items are.
When to involve a professional
- Where a construction joint may be placed in a particular structure, and what it must carry, is determined by the designer of that structure.
- What reinforcement is required at a junction, and whether any deviation on site is acceptable, is determined by the engineer responsible for the structure.
- Striking and back-propping sequences are designed arrangements, and the temporary works designer owns them.
- Cutting, coring, drilling or chasing a post-tensioned slab is not comparable to the same operation in a conventionally reinforced one and should never be undertaken on the basis of a general reference.
- Cutting or coring an existing element without knowing what is inside it can sever reinforcement that the structure depends on.
Frequently asked questions
Questions readers ask about this topic
Why can services not be added after the slab is cast?
They can be, but at a different cost. A hole cut afterwards passes through a structural element and through the continuous barrier beneath it at the same point, so it damages both together, and in some elements there is no area that can be assumed clear of reinforcement or tendons.
What is the difference between a kicker and a starter bar?
A kicker is a short concrete upstand cast with the pour beneath that locates the formwork for the element above. A starter bar is reinforcement left projecting so the next element can be reinforced continuously with it. One positions the mould, the other carries force.
Are construction joints a weakness?
Every element larger than a single pour has them. What matters is that they fall where the design allows and are formed as the design assumed, with the reinforcement crossing them that gives the element the continuity the concrete alone does not have.
Can projecting bars be bent out of the way?
Whether that is acceptable for a given bar is a question for the designer rather than a site convenience. Projecting steel obstructs access and following trades, which is exactly why it gets moved, and its condition when it is finally cast in is worth establishing.
What record should exist after a pour?
As-built positions of every service and penetration, the locations of construction and movement joints, what was cast in and where, and photographs of the reinforcement and the barrier before covering. That record is what makes later alteration assessable rather than exploratory.
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