In Germany, reinforced concrete slabs are the most common type of slab. They are made of concrete and reinforcing steel and are in principle suitable for any type of building. In addition to the particularly flexible but labour-intensive in-situ concrete slabs, prefabricated slabs with quick assembly and no formwork effort have established themselves in many areas. The advantages of both systems are combined in the system of precast slabs, which consist of prefabricated elements on the underside and a concrete layer poured on site. For confined installation locations or in the renovation of old buildings, hollow stone slabs with hollow blocks made of normal or lightweight concrete suspended between narrow reinforced concrete beams are suitable.

More about reinforced concrete floor slabs

In literature and building practice, a wide variety of names for ceiling types can be found, some of which refer to different constructions with the same terms, while identical constructions often have different names. This knowledge page distinguishes between the most common designations of reinforced concrete slabs "as pure form", in practice there are other mixed forms that deviate from this, sometimes also with their own names.

Notes on planning

Slab types in comparison: The reduced construction time of precast slabs can mean an economic advantage over in-situ concrete slabs and, in a reduced form, also over "semi-prefabricated elements", such as precast slabs or hollow stone slabs with a concrete layer. The higher costs for the construction of the precast elements in the concrete plant can thus be economically offset by a faster construction process.

However, additional (cost) factors associated with the individual ceiling types must also be taken into account in the individual decision for a ceiling type. These can include: costs for crane and/or formwork, length of transport routes, post-treatment work of in-situ concrete slabs, space conditions of the construction site and access roads, plastering of the soffit of the ceiling, input of building moisture, etc.:

Construction time: Fully prefabricated slabs in full assembly, such as hollow slab or solid slabs, can be erected in a significantly shorter construction time compared to conventional in-situ concrete slabs. The ceilings will be delivered and installed on the agreed date. When fully assembled, they can be walked on and loaded immediately. Prefabricated slabs with a concrete layer have the advantage of greatly reduced formwork effort, but reinforcement and concreting work is still necessary to a reduced extent for the concrete layer. The construction time of these "semi-prefabricated elements" on site is to be classified between that of in-situ concrete slabs and fully precast slabs.
Formwork: The time-consuming formwork of the slab and installation of reinforcement and recesses on the construction site is completely eliminated with solid slabs and hollow slabs in full assembly. In the case of slabs with a concrete layer, such as element slabs and hollow stone slabs, linear support by beams and columns is necessary.
Surface quality: Hollow slabs, element and solid slabs are characterised by a soffit in exposed concrete quality. Joints between the elements can be filled and painted. The plastering process can be omitted.
Transport, construction equipment: Due to their dimensions and weight, precast elements require heavy transporters and powerful cranes to install the elements. Precast slabs, on the other hand, which are significantly reduced in weight due to the concrete slab only a few centimetres thick, require only a small lifting capacity. Hollow slab ceilings could even be installed by hand. The dimension of the access roads must be suitable for delivery.
Building moisture: While the in-situ concrete slab has the largest input of building moisture in the building, it is already reduced in the element slab due to the smaller layer to be concreted on site. Fully prefabricated elements, i.e. elements that have no or only a small proportion of grouting, do not cause any significant building moisture in the building.

Stability: Stability calculations are carried out by the structural engineer according to Eurocode 2. Precast plants often offer the static calculations together with the production of the ceiling elements. In-situ concrete, element and solid slabs are particularly common as flat slabs, both as a single or two-axis system.

Concrete selection: The compressive strength class (e.g. C25/30) and the consistency class (e.g. F3) must be defined in the planning, possibly also the exposure class (e.g. XF 2), especially if a component comes into contact with the outside air (e.g. as a ceiling soffit in an open car park). These specifications must be made by the structural engineer in consultation with the client and the architect.

Sound insulation: The protection goals in terms of noise protection must be determined in advance between the planner and the client, as several sets of regulations exist side by side. Especially in residential buildings with several units, high requirements apply according to the state of the art, which in particular concern the sound transmission between the units, i.e. also the transmission via false ceilings. DIN 4109 regulates the absolute minimum standard, which is considered outdated today. Planners should use the increased values according to DIN 4109 Supplement 2 or VDI Guideline 4100 as the absolute minimum standard.

The protection against airborne noise increases with the increase in the area-related mass of the ceiling tile, which is determined by the thickness and bulk density of a component. In principle, concrete as a heavy building material provides good conditions for very good insulation of airborne noise.

An improvement in impact sound insulation is hardly achieved by increasing the area-related mass. A double-shell construction is far more effective in this respect. Floating screed is particularly effective as a second shell. It is acoustically decoupled from the ceiling and wall construction by impact sound insulation and edge insulation strips, see construction site screeds Thermal

insulation: Due to its high density, concrete has a very good thermal conductivity, so that the building material has almost no thermal insulating effect. Ceilings made of structure-tight or porous lightweight concrete and reinforced concrete ceilings with cavities offer a comparatively higher insulating effect.

Reinforced concrete ceilings that separate heated interiors from outdoor spaces (e.g. passageways, flat roofs) must be insulated in accordance with the Energy Saving Ordinance. Ceilings against rooms that are included in the calculation as unheated or are not within the system boundary of the heated building volume (e.g. unheated basement or attic space) must also be insulated at least to a small extent, otherwise condensation cannot be ruled out. Thermal insulation can only be completely dispensed with in well-ventilated rooms above and below the ceiling (e.g. open underground car park) and in ceilings that only separate heated rooms from each other.

Prefabricated ceilings offered by manufacturers with a permanently attached thermal insulation layer on the underside offer the advantage that the insulation no longer has to be applied on site.

In the case of brick monolithic exterior walls, an edge insulation strip is often inserted on the front sides of reinforced concrete ceilings to compensate for the reduced cross-section of the outer wall in this area.

Fire protection: Reinforced concrete ceilings are considered non-combustible components of class A1 according to DIN EN 13501-1. A reinforced concrete ceiling therefore offers the best conditions for fire protection if it is sufficiently dimensioned and concrete covered , see the encyclopedia article on fire behaviour, class according to DIN EN 13501-1.

Formwork: When installing in-situ concrete slabs, conventional slab formwork is predominantly used, consisting of formwork panels or formwork lining, wooden beams and tubular steel columns with a fork-shaped attachment to insert the beams. The structure of the formwork lining or the formwork panels determines the surface quality and appearance of the ceiling soffit. In order to be able to transfer the loads of the fresh concrete, the construction of the formwork must be stable and sufficiently rigid to avoid bulging and to ensure the required dimensional accuracy.

Installations, recesses, slab openings: Electrical installations can be nailed into the formwork of in-situ concrete slabs as empty pipes and cans before concreting. Recesses and ceiling openings are nailed directly to the formwork in the form of wooden partitions. In the case of prefabricated ceilings, the conduits etc. are already installed in the elements in the factory and the recesses are taken into account accordingly.

Special shapes: In the case of in-situ concrete slabs, especially on large construction sites, so-called roller reinforcement can be economically interesting, as the process of rebarring on site is rationalized and minimized with this technology. These are parallel, interconnected round bars that are supplied in rolls and installed by unwinding in several layers and directions.

Some manufacturers of prefabricated ceilings have also developed heating, cooling or ventilation ceilings that can be part of efficient energy concepts. The hollow plate in particular is suitable for these special functions, as its long cavities are suitable for cable routing.

Prefabricated ceilings with a permanently attached thermal insulation layer on the underside are installed, for example, over cold cellars or over outdoor areas.

Notes on construction Installation

of prefabricated elements: Prefabricated elements are manufactured in the factory according to execution plans and statics. When delivered to the construction site, a laying plan and assembly instructions are usually included. Until installation, all ceiling supports must be constructed at the exact height and fully load-bearing. The flatness and cleanliness of the supports is also important here in order to enable the elements to be supported over the entire surface. In the case of precast slabs and hollow stone slabs, linear assembly support with beams and assembly yokes is necessary. The elements are lifted onto the prepared supports with a crane or truck-mounted crane, usually without intermediate storage. Ceilings without a concrete layer can be walked on and loaded immediately, and the grouting of the individual elements can be started immediately. In the case of slabs with a concrete layer, the upper reinforcement must first be installed before concreting the concrete layer.

Concrete paving: Concrete delivered or produced on site for an in-situ concrete slab or the concrete layer of a precast slab must be installed as quickly as possible. This must prevent cavities from forming in the component. This is prevented by shaking, stomping or poking. However, if this is done for too long, there is a risk of segregation. This is shown by the formation of an aqueous sludge layer on the surface. Concrete must always be placed in layers and should not be brought in from a drop height of more than two metres. Concrete is affected by external conditions during setting. In extreme climatic conditions such as heat (over 30°C) or frost (below -5°C), concrete may not be poured without suitable additional measures.

Reinforcement: When installing the reinforcement on site, care must be taken to ensure compliance with the required concrete covers. Otherwise, the reinforcement can corrode over the years and, in extreme cases, the structure can no longer meet its static requirements. In the case of precast slabs, the reinforcement is already installed and monitored in the precast plant. In the case of precast slabs that require a concrete layer, the upper reinforcement is placed on site before concreting.

In the case of load-bearing structures made of prestressed concrete, special attention must be paid to the fact that subsequent drilling of the slab can lead to injury to the prestressing steels, which can reduce the load-bearing effect of the entire slab and, in extreme cases, lead to collapse. Subsequent drilling or cutting of recesses in these slabs may therefore only be carried out with precise knowledge of the position of the prestressing steels, it is advisable to consult the structural engineer or the manufacturer in advance. In many prestressed concrete slabs, the position of the prestressing steels is marked on the underside, to which sufficient distances must then be maintained, depending on the drilling diameter. This is important, for example, if a suspended sub-ceiling is to be attached to a prestressed concrete ceiling, but also for luminaire fixtures.

Post-treatment: The drying process of the concrete is called hydration. This leads to drying out and hardening of the concrete component. Concrete components must be treated by suitable measures during the setting period. Otherwise, the concrete will set unevenly quickly as a result of solar radiation or wind, so that cracks can occur. After 28 days, the concrete component is usually fully hardened and hydration is complete. Thorough and careful follow-up treatment is expressly required in DIN 1045-2. The following measures are available for the post-treatment of in-situ concrete components:

Leaving the concrete components in the formwork
Covering the concrete components with foils or mats
Covering them with water-storing covers
Spraying/watering the concrete surface
Application of post-treatment agents with proven suitability

The type and duration of the post-treatment are regulated in DIN 1045-3. The Cement Leaflet B8, published by the Association of German Cement Works (see Standards and Literature), is also helpful in this context.

Stripping: According to DIN 1045-3, cast-in-place concrete components may be formed if the concrete has hardened sufficiently. However, the standard does not define any guideline values for the dismantling period. This depends on the individual cement strength class, span, weather and loads. Large loads on the fresh concrete components should be avoided as far as possible. In particular, loads that only occur during the construction phase, e.g. a ceiling above that has not yet hardened, which in turn is supported on the ceiling, or the setting up of a crane, must be carefully included in the decision.

In any case, the stripping time must be considered and estimated individually. As a rule, an in-situ concrete slab is stripped after 14 days and secured with emergency supports for another 14 days. The stripping period is not exclusively based on static concerns, the longer stay in the formwork can also be a measure of post-treatment (see sub-section After-treatment).

Standards and literature

Note: DIN 1045-1 was withdrawn on 31.12.2010 because this standard contradicts the European design standard (DIN EN 1992-1-1:2011-01 and DIN EN 1992-1-1/NA:2011-01).

DIN 1045-2, Structures made of concrete, reinforced concrete and prestressed concrete - Part 2: Concrete - Specification, properties, production and conformity - Rules of application for DIN EN 206-1

DIN 1045-3, Structures made of concrete, reinforced concrete and prestressed concrete - Part 3: Construction - Rules of application for DIN EN 13670

DIN 1045-4, Structures made of concrete, reinforced concrete and prestressed concrete - Part 4: Supplementary rules for the production and conformity of precast elements

DIN 4109, Sound insulation in building construction; Requirements and verifications

DIN 4109 Supplement 2, Sound insulation in building construction; Instructions for planning and execution; proposals for increased sound insulation; Recommendations for sound insulation in one's own living or working area

DIN 4213, Application of prefabricated reinforced components made of porous lightweight concrete in buildings

DIN EN 206, Concrete - Specification, properties, production and conformity

DIN EN 1168, Precast concrete elements - hollow panels

DIN EN 1520, Prefabricated components made of lightweight concrete and with structurally chargeable or non-chargeable reinforcement

DIN EN 1992-1-1, Eurocode 2: Design and construction of reinforced concrete and prestressed concrete structures - Part 1-1: General design rules and rules for building construction

DIN EN 1992-1-1/NA, National Annex - Nationally defined parameters - Eurocode 2: Design and construction of reinforced concrete and prestressed concrete structures - Part 1-1: General design rules and rules for building construction

DIN EN 1992-1-1/NA/A1, National Annex - Nationally defined parameters - Eurocode 2: Design and construction of reinforced concrete and prestressed concrete structures - Part 1-1: General design rules and rules for the construction of reinforced concrete and prestressed concrete structures Building construction, Amendment A1

DIN EN 1992-1-2, Eurocode 2: Design and construction of reinforced concrete and prestressed concrete structures - Part 1-2: General rules - Structural design for fire

DIN EN 1992-1-2/NA, National Annex - Nationally defined parameters - Eurocode 2: Design and construction of reinforced concrete and prestressed concrete structures - Part 1-2: General rules - Structural design for fire in the event of fire

DIN EN 13369, General rules for precast concrete

elements DIN EN 13501-1, Classification of construction products and construction methods with regard to their fire behaviour Part 1: Classification with the results of the tests on the fire behaviour of construction products

DIN EN 13670, Design of concrete structures

DIN EN 15037-2, Precast concrete elements - Beam slabs with intermediate components - Part 2: Intermediate concrete components

DIN Technical Report 159, General rules for precast concrete elements - Compilation of DIN EN 13369, General rules for precast concrete elements and DIN V 20000-120, Application of construction products in buildings - Part 120: Rules of application for DIN EN 13369

►DBV leaflet "Concrete formwork and stripping deadlines"

►Cement Leaflet B8, Technical Notes on the Aftertreatment of Concrete Components, Publisher: Verein Deutscher Zementwerke

►InformationsZentrum Beton

►Bundesverband Spannbeton-Fertigdecken e.V.

Source: bauwion