Cast In-Situ Reinforced Concrete Frame
Concrete is not a material that arrives on site: it is a material born on site, and born only once. Everything decided before the pour is design; everything discovered after it is demolition. Which is why the most important phase of this process is not the pour, but the two hours before it.
Overview
TAV. 00The cast in-situ frame is still the most widespread building technique, and the reason is monolithic behaviour: columns, beams and slabs are not parts connected to one another, they are a single continuous body in which the joints are born already made. That removes the most delicate part of any assembled structure, its connections, but introduces a condition the other techniques do not share: the quality of the result cannot be checked on a finished component, it has to be governed through a process that happens once and is not reversible. The concrete arriving in the truck mixer complies with its delivery note; the concrete that ends up in the works depends instead on how it was placed, compacted and protected in its first days. Hence the order of the phases below: first you build the void, then you build the steel inside that void, then you fill it without separating the constituents of the mix, and finally you protect the chemical reaction until it is far enough along to look after itself. Striking the formwork, which in the popular imagination is the moment of truth, is in fact only the moment when you find out what was done.
The site score
TAV. HThe detail of the craftthe pour pushing on the formwork
Formwork: building the void
Panels, edge beams and props are erected, levels, plumb and squareness are checked, joints are sealed and release agent is applied. Formwork should be conceived not as a container but as a temporary structure that must hold a heavy, vibrated liquid and stay put while doing so.
«Formwork does not contain the pour: it fights it. And when it leaks at a corner, what escapes is not water, it is the fine part of the mix.»site notebook — editorial synthesis
click a beat · arrow keys ← → walk the site · dashed pauses are the chemistry at work
Where service life is decided
Cover is not a tolerance: it is durability in millimetres
Inside concrete, steel is protected by a chemical condition, not a physical barrier: the strongly alkaline environment keeps the iron passive. Over time, though, carbon dioxide from the air penetrates the concrete and lowers its alkalinity, advancing from the outside inwards as a slow, regular front. As long as that front stays within the cover thickness nothing happens; when it crosses it and reaches the reinforcement, the steel begins to oxidise. And that triggers the mechanism that ruins the work: rust occupies a far greater volume than the metal it came from, so it pushes from within and blows the cover off, exposing more steel and accelerating the whole process.
A qualitative diagram of the mechanism; the rate of advance depends on the density of the concrete, on humidity and on the environment, and where chlorides are present the deterioration follows a different and faster logic.
Cracks without load
Why concrete cracks even when it carries nothing
Most of the cracks appearing on a new structure have nothing to do with loads: they arise because concrete changes volume as it matures, and because that change is nearly always restrained by something. In the first hours plastic shrinkage acts, depending on how fast the surface loses water, and it is fought only with protection. Over the following months drying shrinkage acts, slow and widespread, governed by the mix design and by the joints. In parallel, in massive pours, the heat generated by the reaction itself acts, expanding the core while the surface is already cooling. Recognising which of the three is at work is what distinguishes a crack to be managed from a crack to be investigated.
The three families are distinguished above all by when they appear and by the geometry of the crack pattern; a crack that opens years later and changes width over time belongs to another category altogether and must be investigated structurally.
How it ages (and what betrays it)
TAV. PQuestions from the site
TAV. QWhen can the formwork be struck?
The right question is not "how many days" but "how much strength", because maturing does not follow the calendar, it follows temperature. The same concrete that is ready in a week at twenty degrees can take more than twice as long at five, and below freezing the reaction effectively stops. The practical distinction is between side forms, which carry no load and can come off very early, and soffit forms, which support the structure's own weight and should be removed only when the concrete can carry itself. Even afterwards, in many situations back-props stay, which are not left-over formwork but deliberate support to limit deformation while maturing continues. In practice striking is decided on data, that is, on tests of samples cured in the same conditions as the works and not in a laboratory, which is exactly why those samples must be left on site.
Why water the concrete, if it already contains water?
Because the water in the mix has two different fates and only one of them is useful. Part of it reacts chemically with the cement and becomes part of the hardened material; the rest is there only to make the mix workable and then leaves. The problem is that evaporation does not distinguish between the two: if the environment is dry, windy or hot, it takes away the water the reaction needed as well, and the reaction stops where the water ran out, that is, at the surface. The result is counter-intuitive: an element perfectly sound at its core and porous in its skin, precisely in the layer whose job is to protect the reinforcement and resist wear. Watering, misting or covering does not add quality: it prevents quality being lost. And it cannot be recovered afterwards, because once the reaction has stopped in that zone, wetting it days later does not restart it.
What does "exposure class" mean, and why does it change the mix?
The exposure class describes the environment the element will live in, and it is how the design tells the concrete what it will have to endure beyond the loads. A heated internal column, a retaining wall, a structure exposed to rain and frost and a work in sight of the sea do not suffer the same attacks: carbonation advances differently, frost acts only where water freezes in the pores, chlorides arrive only in certain settings. From this follow very concrete practical consequences: the water-to-cement ratio changes, the minimum binder content changes, the required cover changes and, in frost conditions, air entrainment is added. The typical misunderstanding is to think it is enough to raise the strength class. Strength answers the question "how much does it carry"; the exposure class answers the question "how long does it last", and they are two different properties.
Can recycled concrete be used for columns and beams?
Yes, but with a distinction worth making straight away because it clears up most of the confusion: what is at issue is not structural use, it is the percentage and origin of the recycled aggregate. Recycled aggregates come from crushing demolished concrete and carry with them a share of old mortar, which makes them more porous and more absorbent than natural aggregates. There are three practical consequences and all are manageable if anticipated: the mix absorbs more water, and the dosage must be corrected rather than water added on site; the elastic modulus tends to be lower, so the structure deflects somewhat more under the same load; and shrinkage tends to increase, which demands more attention to curing and to joints. The non-negotiable condition is consistency of supply: a heterogeneous recycled aggregate produces concrete that varies from load to load, and it is that variability, not recycled content as such, that is the real reason it was long kept out of load-bearing structures.