Formwork Striking Times: When to Strike, and the Risk of Early Loading

On site, formwork is a material waiting its turn; everyone wants it struck and up to the next floor as soon as possible. That pressure is the source of the quietest errors in reinforced concrete. Formwork struck early rarely results in a noisy collapse; it leaves a few millimetres of permanent deflection in a beam, a fine network of cracks under a slab, a piece of arris broken off a column. None of these can be undone. Striking is not a calendar decision but a strength decision: the question is not how many days have passed, but whether the concrete has reached the strength to carry the load that will come onto it.

When Does Formwork Finish Its Job?

Formwork has three separate tasks, and they do not end at the same time. The first is to give shape: fresh concrete is fluid and cannot hold its own shape. The second is to carry: until the concrete has reached a strength at which it can carry its own weight and the construction loads on it, the formwork and shoring carry that load. The third is to protect: formwork holds the concrete's moisture in and shields it from sun and wind, in other words it provides a form of curing.

The shaping task ends soonest; once the concrete has set and can support itself, the side faces no longer need formwork. The load-carrying task lasts far longer and depends on the element's span, its section and the load on it. The protection task, on the other hand, is independent of strength: if the formwork is struck early the surface dries and cools suddenly, and in thick sections that directly increases the risk of thermal cracking. So the right approach is not “the formwork is off, the job is done” but “the formwork is off, we are continuing the cure by another means”.

Another point to consider in the striking decision is the load the element will meet once the formwork is off. If a slab is only going to carry its own weight, the strength required is far lower than for a slab on which walling materials will immediately be stacked or the formwork for the floor above erected. On site it is usually the second case that applies; in other words the concrete meets a construction load heavier than the service load allowed for in the design, while it is still young. When you set the striking programme, ask “what will this element carry tomorrow?” as well.

Side Forms, Load-bearing Forms and Shoring: Three Separate Timetables

Separating formwork elements according to their task puts an end to the confusion on site.

Non-load-bearing side forms. Column and wall sides, beam side faces, foundation edge forms. These do not carry the weight of the concrete; they only resist the lateral pressure of fresh concrete. They can be struck once the concrete has reached a surface hardness at which the arrises will not break and the face will not be damaged during striking. In mild weather that is usually between one and three days; the deciding factor, though, is not the time but the state of the surface. The signature of a column form struck too early is broken pieces of concrete at the bottom arrises of the column and scrapes that leave marks on the face.

Load-bearing formwork. Slab soffits, beam soffits, the underside of cantilevers. These carry the element's own weight and the construction loads. Before they can be struck, the concrete must have reached the flexural and shear strength needed to carry those loads. Common site practice requires a defined proportion of the target strength to have been reached; for cantilever elements that proportion is markedly higher than for the rest, because a cantilever has no alternative load path and collapse comes without warning. Balcony and cantilever canopy props are the last shoring to be struck on a site.

Shoring (props). Even once the formwork sheeting has been struck, the props can be left in place; in multi-storey construction this is almost the rule. Modern formwork systems use drop-head props, so that certain props stay in place, never lowered, while the slab formwork is struck. When the props come out depends on the slab span, on the work continuing on the floors above, and on strength gain.

A common mistake on site is to lump all three timetables under one heading: “striking day has come”, and the props are lowered along with the slab soffit. In fact striking should be staged: first the side forms, then the load-bearing soffits, and last of all, as a separate decision, the shoring.

One note: striking times are defined in the project specification and in the relevant regulations. If the specification gives a period, that period is a minimum, not a target; where conditions are unfavourable the period lengthens, it never shortens.

Maturity: Not Days, but Temperature Times Time

Concrete gains its strength not from the calendar but from hydration, and the rate of hydration is set by temperature. The concept that combines these two variables into a single number is called maturity. In the most widely used approach, maturity is the sum of the products of the concrete temperature and the time elapsed, expressed in degree-hours. In the calculation, a datum temperature at which hydration is taken to stop is subtracted from the concrete temperature; that datum varies with the mix, but is commonly taken as minus ten degrees Celsius.

In practice this means that concrete left for three days at 20 °C and concrete left for six days at 10 °C have roughly the same maturity, and therefore roughly the same strength. In the same way, the maturity of concrete left for seven days at 5 °C in winter can be lower than the maturity reached in two days at 25 °C in summer. A site that goes by the calendar strikes formwork early in winter and does not notice.

Using the maturity method on site is straightforward. First, in the laboratory, a calibration curve is established for the mix to be poured: the strength of samples is measured at different maturity values and the maturity-strength relationship is derived. Then, on site, a temperature sensor is embedded in the element poured; the sensor records the concrete temperature continuously and the instrument calculates maturity. When the target maturity is reached, the decision to strike is taken. The greatest benefit of the method is that the decision moves from estimate to measurement; it also shortens the period safely in favourable weather and so speeds up the formwork cycle.

If there is no maturity meter there are alternatives. The first is field samples: some of the cubes taken for strength testing are left not in the laboratory cure but beside the element poured, in the same weather conditions. These samples represent the conditions the element has experienced, and crushing them at an early age gives a basis for the striking decision. The second is at least to record the daily maximum and minimum air temperatures and extend the striking time in line with that record. A 28-day sample under standard curing, on the other hand, is not a suitable criterion for a striking decision; that sample represents the conditions of the laboratory, not those of the element on site. Non-destructive measurements such as a rebound hammer give supporting information, but are not relied on alone.

The Effect of Air Temperature: Same Concrete, Different Timetable

Strength gain varies markedly with temperature. As a rough rule, when the concrete temperature falls by 10 °C the rate of hydration roughly halves. The effect on the striking programme is direct: a load-bearing form struck in three days in summer may require more than a week in a cold winter. Winters in Bodrum are mild; but in spells when the night temperature drops and the wind does not let up, an element's temperature can fall below the air temperature, particularly in thin slabs. We set out the precautions to take when pouring in cold weather and the concept of critical strength in detail in our article on pouring concrete in winter.

Hot weather has a double effect. Early strength comes quickly and the formwork can be struck sooner; but hot concrete tends to reach a lower ultimate strength in the long term, and the risk of shrinkage grows. Striking early creates a second problem here: formwork also retains moisture, and in hot, windy weather a struck surface dries rapidly. If curing is not continued after striking, a network of hairline cracks and dusting appear on the surface.

In thick sections a separate mechanism comes into play. In raft foundations, thick walls and large-section columns the heat of hydration builds up inside; the internal temperature rises well above the external one. Formwork acts as a layer of insulation in this situation. Striking it early exposes a hot surface to sudden cold air; the surface shrinks, the hot core does not allow it to, and a thermal crack opens. In thick sections the decision to strike looks not only at strength but at the difference between core and surface temperature; if the difference is above the safe limit, the formwork is left in place, or the exposed surface is covered immediately with an insulating blanket. For detail on this type of crack, see our article on concrete cracks.

The mix itself also changes the timetable. Concretes made with slag or fly ash cements gain early strength more slowly and long-term strength generally better; with such mixes the striking time is longer. Accelerating admixtures raise early strength. You cannot set a striking programme without knowing which mix you are using; the strength class stated on the order is not sufficient information on its own. For the relationship between the amount of water in the concrete and strength gain, see our article on the water/cement ratio and concrete strength; water added on site lengthens the striking programme as well.

The Price of Striking Early: Deflection, Cracks, Surface Damage

Striking formwork early usually results not in a sudden event but in a permanent defect. Four main outcomes are seen.

Permanent deflection. Young concrete has a low modulus of elasticity; under the same load it deforms more than hardened concrete. Creep is added to this: concrete continues to deform under sustained load over time, and creep is far greater in concrete loaded at an early age. The deflection of a beam or slab loaded early does not recover, even after the concrete later reaches its full strength. The result is doors and windows that stick, a visible dip in the slab and, in advanced cases, cracks in finishes and partition walls.

Flexural cracks opening early. Concrete cracks in the tension zone; that is in the nature of reinforced concrete. But if the crack opens while the concrete is still weak and bond with the reinforcement is not fully developed, the crack width increases permanently. A wide crack opens the door to water and chlorides; in an area exposed to the sea, that is the beginning of reinforcement corrosion.

Surface and arris damage. With column and wall forms struck early, arrises break away, the surface skin peels, and pieces of concrete stuck to the form face damage both the element and the formwork. Cosmetic though such damage may look, it reduces the concrete cover and calls for repair.

Structural inadequacy and collapse. This is the most severe outcome, and it usually arrives when two mistakes coincide: striking the formwork early, and loading the element with a heavy construction load immediately afterwards. Stacking a pallet of bricks on a slab the day it is struck, or starting to erect the formwork for the floor above, means the concrete meeting a load greater than the design service load while it is still young. This combination is at its most dangerous on cantilevers and long-span slabs.

Striking is itself a form of loading. The shoring must be released gradually and in sequence; releasing the load suddenly at one point creates a concentration of stress there. Dropping the formwork down both impacts the element and damages the formwork material. Striking proceeds from the middle towards the edges, under control, watching how the element deflects. An unexpected deflection or a new crack seen during striking is a reason to stop.

Reshoring and Multi-storey Pouring

In multi-storey construction the formwork for the floor above is erected before the concrete below has reached its full strength. The load of the new floor cannot then be placed on a single young slab; it must be distributed over several floors. Reshoring does this job.

Two different practices are confused; it is important to know the difference:

• Continuous shoring: The formwork sheeting is struck, but the props are removed one at a time, in sequence, and put straight back. At no moment does the slab carry its own weight alone. This is preferred for young slabs and long spans.
• Reshoring: The formwork and props are removed completely; the slab deflects once under its own weight. The props are then re-erected. From that point on the props no longer share the slab's own weight, only the new loads that come from above.

The most critical point in practice is that the props must not be preloaded. A prop is set so that it only just touches the underside of the slab; if its screw or wedge is forced tight, load is applied to the floor below and unwanted stresses arise in the young slab beneath. The “the tighter, the better” logic does harm here.

How many floors are kept shored depends on the slab span, the rate of construction, the rate at which the concrete gains strength, and the construction loads. Two mistakes recur on site. The first is taking the shoring down early to speed up the formwork cycle; the second is failing to line the props up with those on the floor below when they are erected. If the props are not on the same vertical axis floor by floor, the load does not follow the expected path and bends the young slab. It matters just as much that the surface the props stand on is sound and even, with a load-spreading pad where necessary.

Another overlooked source of load is the concrete being poured itself. While the floor above is being poured, the fresh concrete, the crew, the pump line and materials all load the floor below at the same time. These loads differ from the design service loads and need separate assessment at the construction stage. On large, repetitive projects, having the shoring arrangement planned by a structural designer is far safer than leaving it to the site's own judgement.

What Should the Striking Decision Be Based On?

The order of decisions should be as follows.

1. Design and specification. If the structural designer or the specification defines a strength threshold or a period, that is the starting point. The periods given are minimums; where conditions are unfavourable they are extended.
2. Evidence of strength. The decision must be supported by measurement, not by guesswork. In order of preference: maturity monitoring with a sensor embedded on site; early testing of field samples cured beside the element under the same conditions; and, failing both, a conservative extension of the period based on the record of air temperatures.
3. The type of element and its load. Is it a side form, a load-bearing soffit, or shoring? A cantilever or a simple span? What will be placed on this element once the formwork is struck? If heavy stacked material or the formwork for the floor above is coming the same day, the strength threshold required rises.
4. Weather and section. In cold weather the period lengthens. In thick sections the difference between core and surface temperature is checked. In hot, windy weather the curing plan for after striking must be ready.
5. Responsibility. The decision to strike formwork is not given verbally. The measurement, date and time on which the decision was based are recorded, and the site manager or an authorised technical member of staff signs it off. That record both protects you if a problem arises later and shows what was done and how.

It bears repeating that the job is not finished when the formwork comes off: curing of the struck surface is continued according to the age of the concrete and the weather. A damp covering, a curing compound or water curing fills the protective gap the formwork leaves. Otherwise you will lose surface quality even though you did not strike early.

The striking programme can be planned together with the mix design: a mix with high early strength safely speeds up the formwork cycle and shortens the site programme. To choose the concrete class and mix that suit your project's formwork cycle, talk to the Bodrum Beton technical team before the pour; you can see the concrete classes available on our products page.

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