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Frequently Asked Questions
Why use a cast-in anchor instead of drilling afterwards?
Because it is stronger, more predictable, and does no damage to the concrete around it.
A cast-in element sits in concrete that has never been drilled. There is no microcracking around the hole, no dust in the bond line and no dependence on friction or on an adhesive's cure.
Design codes reflect that. Cast-in anchors are the baseline case with the best-established capacities, and post-installed anchors carry the additional factors that account for installation variability.
Reinforcement is another reason. Drilling after the pour risks cutting rebar, which is a structural problem far larger than the fixing being made.
The practical case is repetition. Where hundreds of identical fixings are needed - a facade, a pipe rack, a crane rail - casting in is faster and more consistent than drilling each one.
The counter-argument is always coordination: post-installed anchors exist because the position is often not known in time.
What happens if a cast-in anchor ends up in the wrong place?
The options are all expensive, which is why setting out and restraint matter more here than in any other fixing.
A small error may be absorbed by slotting the steelwork, which is the usual and cheapest remedy - and the reason connection plates are often supplied with slots rather than holes.
A larger error means abandoning the anchor and drilling a post-installed one nearby, which needs its own design check for edge distance and spacing from the redundant anchor.
Bending a misplaced bolt into position is a common site practice and a poor one. It yields the steel, changes the load path and is difficult to justify afterwards.
Anchor channels avoid most of this by being adjustable along their length, which is the main reason they are specified for facades where tolerances stack up.
On a designed connection, a remedial fixing is signed off by whoever is responsible for that connection - it is not a site call.
How are cast-in anchors held in position during the pour?
Rigidly, to the formwork or to the reinforcement - and the fixing has to resist vibration, not just weight.
Templates are the reliable method for bolt groups. A plate drilled to the final hole pattern holds every bolt in the group at the correct spacing and projection, and it also proves the pattern before the concrete is placed.
Channels are usually nailed or screwed directly to the formwork face, with their infill strip left in place so concrete cannot enter the slot.
Vibration is what moves anchors. A fixing that holds under static weight can walk several millimetres while a poker vibrator works nearby.
Clashes with reinforcement should be resolved before the pour by moving the anchor within its permitted tolerance or by agreeing a rebar adjustment, never by forcing bars aside.
Checking positions immediately after placing, while the concrete is still workable, leaves a short window to correct anything that has shifted.
Does the concrete strength change what the anchor can carry?
Yes, substantially - concrete strength is one of the main inputs to anchor capacity, along with edge distance and spacing.
Most anchor failures in concrete are failures of the concrete rather than the steel. The material breaks out in a cone around the anchor, so its strength governs the result.
Edge distance matters for the same reason. An anchor close to a free edge has less material to mobilise and a lower capacity, and the reduction can be severe.
Spacing between anchors interacts too. Anchors close together share overlapping cones and cannot each develop their full individual capacity.
Reinforcement can be designed to carry the load out of the anchor zone, which changes the calculation entirely and is common in heavily loaded connections.
The governing rules are set by the design code in the country of construction, and they differ enough that a capacity table from one market should not be applied in another without checking.
Can cast-in anchors be loaded as soon as the concrete is hard?
No - they need the concrete to have reached the strength the design assumed, which takes considerably longer than it takes to become hard to the touch.
Concrete gains strength over days and weeks. A surface that feels solid may be well below the strength the anchor capacity was calculated against.
The relevant figure is the strength at the time of loading, which on a fast programme is often demonstrated by testing rather than assumed from the age of the pour.
Temperature changes the timeline in both directions. Cold weather slows strength gain markedly; hot weather accelerates early strength but can reduce the final figure.
Temporary and construction loads are the usual point of failure here, because they arrive before anyone is thinking about design capacity.
Where early loading is unavoidable, it is a question for the engineer, who may accept a reduced load until a stated strength is confirmed.
How are cast-in anchors protected from corrosion?
By material choice, by cover, and by keeping water away from the interface - the exposed end is the vulnerable part.
The embedded length in sound, alkaline concrete is well protected. The problem is almost always where the anchor breaks the surface and where water can track along it.
Hot-dip galvanising is the common answer for general external work; stainless is specified where the environment is aggressive or the fixing is inaccessible for life.
In facade and coastal work the molybdenum-bearing stainless grades, and in some specifications duplex grades, are used because chlorides attack the standard grade at crevices.
Mixing metals at the joint creates galvanic corrosion, so bolts, channels, washers and the bracket should be a compatible set rather than whatever fits.
Cover to the anchor and detailing that sheds water off the fixing do more for its life than an upgrade in material.
Which cast-in type should be specified when the fixing points are not yet known?
An anchor channel if the line is known but not the position along it, and threaded inserts if even the line is uncertain but the surface is accessible later.
Channels are continuous, so any position along their length is available indefinitely. That is why they suit facades, pipe runs and cable containment, where the supported item is designed long after the frame.
Inserts defer the bolt entirely. They leave a female thread flush with the concrete and nothing projecting, so following trades work over a clean surface.
Bolt groups are the wrong choice for uncertain layouts, because their whole value is in a fixed, templated pattern matched to a specific base plate.
Cost differs in the expected direction - channels are the most expensive per metre and the cheapest per unresolved decision.
Where nothing at all is known, the honest answer is often a post-installed anchor later, accepting the lower capacity in exchange for the flexibility.