precast and cast in situ slab systems for residential buildings

PRECAST AND CAST IN SITU SLAB SYSTEMS FOR RESIDENTIAL BUILDINGS

F. Giussani, Dept. of Structural Engineering, Politecnico di Milano, Italy

F. Mola*, Dept. of Structural Engineering, Politecnico di Milano, Italy

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PRECAST AND CAST IN SITU SLAB SYSTEMS FOR RESIDENTIAL BUILDINGS

F. Giussani, Dept. of Structural Engineering, Politecnico di Milano, Italy F. Mola*, Dept. of Structural Engineering, Politecnico di Milano, Italy

Abstract

The sub-systems nowadays used in Italy to construct the slabs of residential buildings are presented. Starting from the most traditional cast in situ one-way ones, composed by reinforced concrete elements and terra cotta bricks, new types of precast slabs either in reinforced and prestressed concrete are described, pointing out their most significant prerequisites regarding structural efficiency and construction feasibility. Two other interesting types of slabs, the first adopting composite truss beams and the second based on a cast in situ construction process, behaving as a two-way thin plate resting on fixed supports are then discussed. In the second part of the paper, the most important aspects related to the check of structural safety are illustrated, giving the basic references regarding the structural models which can be conveniently applied. keywords: Slab, reinforcement, precasting, prestressing, cracking, punching. 1. Introduction The slabs represent one of the most important sub-systems of the structural arrangement governing the load bearing capacity of buildings. Regarding the static behaviour, many prerequisites are requested to slabs, in particular we remember:

- The load bearing capacity for vertical loads; - The efficiency in redistributing vertical concentrated loads; - The flexibility in allowing the location of holes; - The capability in serving as a plane diaphragm connecting the vertical members; - The high flexural rigidity; - The durability; - The speed of the constructional process.

Besides these prerequisites, other properties have to be taken into consideration, like the acoustic and the thermohygrometric insulation, the resistance against fire and many others. Nevertheless, these last properties do not affect the static behaviour of the slabs, so they will not be taken into consideration in the present work, which is devoted to the discussion of the structural prerequisites of the slab sub-systems. Regarding this point, the most general way to define a slab sub-system refers to the mechanical concept of thin plate subjected to transverse loads and to loads acting in the plane of the slab. According to this definition, a slab can be described as a two-dimensional structural system, variously restrained, able to serve as a plate, subjected to flexural and shear stresses, when equilibrating the external transverse loads, or able to serve as a diaphragm, subjected to in-plane stresses, when redistributing the horizontal loads acting on the building, such as wind and seismic loads, among the various vertical members. Regarding the constructional aspects, these prerequisites can be assured by slabs working as one-way or two-way systems. In the first case, the principal elements of the slabs, represented by the beams, are disposed along a prescribed direction, while the secondary elements, consisting in one-way elements exhibiting various geometrical and mechanical properties and different constructional procedures, are disposed along the orthogonal direction. These elements which are often synthetically called slabs as they strongly characterize the structural sub-system will be discussed in the present work. In the second case, the slab is built by recurring to a cast in situ procedure with the steel rebars disposed along two orthogonal directions. In this way it is possible to construct an efficient two-way system able to behave as a plate or as a diaphragm. One

way systems are traditionally employed in Italy for the construction of slab sub-systems for residential buildings or for more complex arrangements, like commercial and infrastructural buildings. Only during the last decade the use of two-way systems has become more popular, even though the number of buildings constructed adopting this technology is still restricted. Regarding one-way systems, many constructional techniques have been developed, allowing to significantly diversify the various elements offering different options to the designers. Generally speaking and referring to the construction procedures, a first subdivision can be made between cast in situ and precast elements and among these two categories we can distinguish elements with ordinary or prestressed reinforcement. Another very important aspect regards the lightness of the structural systems. This goal has been achieved by optimizing the quantity of concrete that guarantees the prescribed static efficiency of the transverse sections. As the prominent load effect affecting the slab is of flexural type, the most efficient technique in order to reduce the structural weight maintaining the static efficiency consists in eliminating the material from the central zone of the transverse section, where the flexural stresses are quite low. Operating in this way, the lightening of the slab systems has been achieved by introducing blocks of light materials or recurring to special techniques able to construct hollow core elements. The first construction system of this type adopted terra cotta bricks as lightening elements and ordinary rebars as reinforcement. Successively, the system has been improved by using in some cases prestressed strands. This system, which allows to build slabs with maximum span of about 7m, is very popular in Italy and is almost universally adopted in residential buildings. Another improvement has been introduced by using concrete precast thin plates incorporating steel joists and polystyrene blocks which have allowed to construct more homogeneous and reliable slabs, eliminating the risk of local brittle failures which can affect the terra cotta bricks. The development of the precast industry has allowed to produce lightened prestressed elements, self-bearing during the construction phase. The load bearing capacity and the flexural rigidity of such elements can be strongly increased by casting in situ a layer of collaborating structural concrete, incorporating ordinary reinforcement. The one-way slab elements here briefly introduced will be discussed in detail in the next points, together with two other systems which are becoming more and more popular in Italy. These two systems respectively regard one-way systems with hybrid beams, consisting in robust steel joists able to guarantee the self bearing capacity of the slab systems during the construction phase and two-way cast in situ slabs with solid section with ordinary or post-tensioned reinforcement. 2. Cast in situ slabs 2.1. Slabs lightened by introducing terra cotta bricks The typical configuration of this system is illustrated in Fig. 1, 2. The bearing elements are small precast beams incorporating a steel joist and having the inferior part covered by a terra cotta bottom. The beams have 50cm inter-space and their span does not exceed 7m. The terra cotta bricks can serve only as lightening elements or can also collaborate with concrete in equilibrating compressive stresses. In the first case, the cast in situ superior layer is mandatory, while in the second case it can be omitted. As we can observe from Fig. 2a, b, the collaborating bricks are thicker and they have to satisfy appropriate geometrical and mechanical limitations. During the construction phase the bearing elements are conveniently propped and are completed by means of cast in situ concrete. The final static scheme of these slabs is that of a continuous beam and the upper bars necessary to equilibrate the bending moments arising along the internal supports are disposed during the construction phase. In order to assure a good redistribution of concentrated loads, an albeit partial plate behaviour has to be provided. This goal is achieved by inserting transverse small beams, disposed at mid span of the slab when the span exceeds 4m. This operation is quite simple as the producers provide special terra cotta pieces for the bottom of the transverse beams.

(a)

(b)

Fig. 1 One-way slab with terra cotta bricks Fig. 2 Transverse sections: (a) Collaborating bricks; (b) Non collaborating bricks

When larger spans up to 10m are involved, this technology can be still adopted by using prestressed elements. These elements are disposed at an inter-space of 50cm. The terra cotta bricks and the special pieces forming the bottom of the transverse beams are disposed between them. The slab is then completed by cast in situ concrete. The most significant features of the slab are illustrated in Fig. 3, 4, 5. The prestressed slab does not exhibit a homogeneous intrados, as the load bearing elements are not covered by a terra cotta bottom. This fact can generate some disadvantages from the aesthetical point of view as the different thermal conductivity existing between the beams and the bricks increases the deposit of the dust on the bottom of the beams so that annoying dark strips become evident at the slab intrados.

Fig. 4 Transverse section

Fig. 3 Slab with prestressed elements and terra cotta bricks


An interesting alternate configuration of this type of slabs is represented in Fig.6, 7. The slab is formed by precast panels 120cm large, exhibiting a homogeneous intrados, consisting in a thin layer of

concrete in which the prestressing strands are disposed. This slab, which has the same static prerequisites of the preceding one, exhibits better performances as a consequence of the homogeneity of the intrados surface.

2.2. Slabs with inferior precast concrete layer As it will be discussed in the next points, the terra cotta slabs can be subjected to local brittle failures of the bricks. Even though these failures do not affect the load bearing capacity of the slabs, they can generate situations of significant risk for people and can reduce the structural durability. For

this reason, a new type of slab has become very popular. It consists of a precast concrete layer (4-6)cm thick, incorporating steel joists with 50cm inter-space. The layer has a width of 120cm and polystyrene blocks are connected to it so that the total structural weight is conveniently reduced. This type of slab, which

Fig. 6 Precast panels with terra cotta bricks


Fig. 8 Precast concrete layer with steel joist


can be employed for spans up to (10-12)m are completed by means of cast in situ concrete, obtaining the structural system of Fig.8, 9.

A significant improvement can be achieved by using prestressed layers as prestressing markedly enhances the static and the functional prerequisites of the slab. In particular, a lower deformability and a higher durability can be guaranteed by eliminating cracking phenomena (Fig. 10, 11). The layers have to be propped during the construction phase and the inter-space of the props has to be maintained

conveniently small as in this phase the resisting section of the slab is represented by a truss whose inferior and upper chords are respectively constituted by the concrete layer and its reinforcement and by the top bar of the joist. A too large inter-space of the props increases the compression force in the top bar which can collapse for instability, producing the sudden failure of the slab. Even though the producers supply detailed documents regarding the type, the inter-space and the preparation of the propping system, a certain number of accidents has taken place in the past and still now, especially in small worksites, where not sufficiently skilled personnel is employed, sudden failures in the construction phases have not been completely eliminated. An important fact, affecting the aesthetical aspect of this slab, regards the configuration of the slab intrados, where the separation joints along two adjacent layers are clearly visible. For this reason, this slab is largely employed in car parking or commercial centres where the presence of joints does not represent an aesthetical inconvenient. Furthermore, the slab is widely used in buildings where its intrados is not visible for the presence of ceilings. On the contrary, in residential buildings, where the intrados is visible, this type of slab has been used in a very small number of cases. 2.3. Slab with precast elements The first generation of precast slabs consisted in prestressed elements with webs emerging from a bottom layer. The slabs were completed by cast in situ concrete and were lightened by means of terra cotta bricks or polystyrene blocks. Typical slab configurations are illustrated in Fig. 12, 13, 14, 15.

Fig. 12 Precast prestressed lightened slab Fig. 14 Prestressed slab with steel connectors

Fig. 13 Transverse section Fig. 15 Transverse section

Fig. 10 Prestressed concrete layer with steel joist


The slabs are not self-bearing, so they need props during the construction phase and their maximum height is about (50-55)cm. In their most efficient static configuration, characterised by the presence of steel connectors between the precast elements and the cast in situ concrete, the slab can reach a span of 15m. The use of these slabs is advisable for special buildings, where large spans have to be covered in presence of heavy variable loads. An alternate system to the ones now discussed is illustrated in Fig.16, 17, 18. The system includes precast elements with double webs and an upper thin slab completed by cast in situ concrete. The elements are generally supported by precast beams according to the scheme of Fig.19.

Fig. 16 Double T precast element Fig. 18 Precast frame system with double T slabs

Fig. 17 Double T precast element with cast in situ slab Fig. 19 Connection between precast slab and beam

Such scheme does not allow to assure the flexural continuity of two successive elements which have to be considered as simply supported. Anyhow, in order to maintain the continuity of the upper surface of the slab, the cast in situ layer is continuous along the joint between the precast element and the beam. This fact requires to carefully define the structural details in order to avoid large cracks which can reduce the functional behaviour of the slab, making necessary complex and expensive works for their elimination. A very interesting system, self-bearing in the construction phase and easy to connect to every type of beam, is the prestressed hollow core element sketched in Fig. 20, 21. The width of the

element is 120cm, while the inter-space and the shape of the holes are defined in order to guarantee the static prerequisites of the slab. In the construction phase the elements behave as simply supported, while in the final configuration they can be made continuous by

means of a cast in situ cover incorporating additional upper reinforcement. The variability of the static scheme and the different behaviour of the system between the span, where the prestressing is active, and the continuity supports, where only the added ordinary reinforcement is present, require to perform a refined structural analysis as it will be discussed in the next points.

Fig. 20 Hollow core element


2.4. Slab with truss steel beams The typical element of this slab is represented by the principal beams. The secondary ones can be of any type among those previously discussed, excluding the precast elements with double web. The beam element of this system consists of a self-bearing steel truss including an inferior steel plate to which longitudinal and diagonal steel bars are welded. These bars, together with a longitudinal chord, composed by steel bars connected to the diagonal ones, form a spatial steel truss. The beams are completed by cast in situ concrete including steel rebars, allowing to obtain a one-way slab with continuous principal composite steel concrete beams.

Fig. 22 Structural system including truss steel beams Fig. 23 Detail of a joint

The main features of the beams are represented in Fig. 22, 23, 24a, where we can observe that the inferior steel plate remains at the exterior of the beam. This reduces the convenience of this slab system as the plate, representing the main element in equilibrating the tensile stresses due to flexure, has to be protected against fire. This produces an increase of the costs and of the construction time. In order to avoid this shortcoming, recently a new generation of beams has been produced, having the bottom part formed only by steel bars covered by a concrete thick layer, supporting the slab elements (Fig. 24b). In this way we succeed in guaranteeing the self-bearing capacity of the beam in the construction phase and the resistance against fire. Nevertheless, it is noteworthy to observe that in this phase the steel percentage, computed referring only to the transverse section of the concrete cover, is very high, making it very sensitive to cracking phenomena. The results derived from the observation of a large number of elements used in different types of buildings have shown that the disposition of a light system of props during the construction phase is very feasible in order to strongly reduce the number and the opening of cracks. This way of operating, even though not requested by the producers, is highly recommendable in order to safeguard structural durability and to provide a reliable behaviour in the service stage.

(a)

(b)

Fig. 24 Transverse sections

2.5. Two-way cast in situ concrete slabs Even though this type of slab system is well known and widely adopted in many countries, it has been only recently employed in Italy for a restricted category of buildings. As it is illustrated in Fig.25, the

use of the cast in situ technique allows to construct a two way slab, working as a continuous thin plate resting on rigid supports.

Fig. 25 Cast in situ two-way slab (from ref. [7])

Owing to the two-dimensional behaviour, the height of the slab can be reduced. The weight of the slab remains in any case quite high as no lightening elements are provided. However, the reduction of the slab height cannot be too high in order to avoid the punching capacity becoming insufficient.

Fig. 26 Model for the evaluation of punching capacity in presence of a capital (from ref. [5])

The punching capacity can be increased by introducing capitals at the interconnection between the slab and the columns as reported in Fig. 26. This possibility is rarely exploited as it precludes the advantages provided by a plane intrados, in particular regarding the plant dislocation. The construction of a two-way slab requires to employ a large number of formworks. In order to maximise their performances, it is necessary to adopt rapid hardening concrete, able to develop high values of compressive and tensile strength and of elastic modulus at early age. In this way the formworks can be removed after three or four days, reducing the costs connected to their hire. Another important aspect, connected to the possibility of reducing the construction time, regards the type and the way of placing the reinforcement. If ordinary reinforcement is adopted, the possibility of reducing the construction time and the structural costs depends on the level of industrialization governing the preparation of the steel layers and on the procedures followed in placing them. Regarding this point, in Fig. 27, 28 a recent technique based on the use of pre-assembled rolls of reinforcement which can be easily placed is illustrated.

Fig. 27 Pre-assembled rolls Fig. 28 Placing of a pre-assembled roll

Adopting an alternate way, it is possible to proceed by introducing post-tensioned unbonded cables, disposed with variable eccentricity, in order to equilibrate the variable bending moments acting along the slab. A typical prestressed slab is represented in Fig. 29, 30.

3. Some aspects regarding structural safety The various types of slab systems discussed in the preceding points require to be carefully checked in the construction phase, in the service stage and at the ultimate limit states. In the present point we will discuss some of the most interesting aspects regarding the structural safety of the slab systems. 3.1. Slabs containing terra cotta bricks In these slabs the terracotta bricks can serve exclusively as lightening elements or they can also collaborate with concrete in equilibrating compressive stresses. Referring to these two types of bricks, sketched in Fig. 31,a,b, the characteristic strengths of the terra cotta non collaborating elements have to satisfy the limitations expressed in Tab.1, while specific limitations for collaborating bricks are reported in Tab.2 [1].

Characteristic strength (a) Compression Tension Punching

Rlk R’lk fltk P 15.0 5.0 7.0 1.50

N/mm2 kN on a (5x5)cm2

area

Tab. 1 Characteristic strengths of non collaborating bricks

Fig. 29 Longitudinal eccentricity of prestressing cables (from ref. [6])

Fig. 30 Reinforcing rebars and prestressing cables of a slab (from ref. [6])

(a) (b)

Fig. 31 Non collaborating (a) and collaborating (b) bricks

Characteristic strength (a) Compression Tension Punching

Rlk R’lk fltk P 30.0 15.0 10.0 1.50

N/mm2 kN on a (5x5)cm2

area

Tab. 2 Characteristic strengths of collaborating bricks

Minimum thickness of the external and internal diaphragms [mm] for both non collaborating and collaborating elements.

s = height of the strengthened zone

Limit of the hole ratio in the reinforced area, to be guaranteed in each section orthogonal to the stress direction F/A = (Total area of the holes in the strengthened zone) / (Area included in the perimeter of the strengthened zone) 0.5

Thickness of the slab Minimum height in the strengthened area in collaborating elements [cm] 5 cm for h 25 cm s h/5 cm for h < 25 cm

Limit of the ratio between the total area of the holes and the gross area included in the perimeter of the section Hole area / Gross area 0.6 + 0.625 h 0.75 m

Fig. 32 Minimum thickness of the bricks

The minimum thickness of the bricks is illustrated in Fig. 32. From the static point of view, the sectional behaviour of the slabs including collaborating bricks can be assumed similar to the one of a composite reinforced or prestressed concrete section subjected to a positive moment. The static behaviour of reinforced concrete section along the continuity support, where negative moments are present, governs the structural bearing capacity. Restraining or loading conditions changing the one-way static behaviour of the slab have to be carefully investigated as they can induce local brittle failures in the terra cotta bricks.

(a) (b)

Fig. 33 Introduction of a restraint along a span (a) and presence of cantilevered parts (b)

Two typical cases are sketched in Fig. 33a,b. The first regards the introduction of an additional restraint represented by a partition disposed along the span of a slab element preventing deflections, while the second is related to the presence of cantilevered parts acting orthogonally with respect to the bearing elements. In these two cases, the slab behaves locally as an orthotropic plate, subjected to

transverse negative bending moments generating severe compressive stresses at the bottom of the terracotta bricks, able to induce local failures. In order to avoid this dangerous effects, it is necessary to substitute in the disturbed zones the bricks with concrete parts, able to assure an efficient and reliable structural response under the compressive stresses induced by the local behaviour of orthotropic plate. 3.2. Hollow core slabs The main aspects connected to structural safety regard the various configurations assumed by the elements which have to be considered as isolated or collaborating with the cast in situ layer of concrete or interacting between them after being made continuous. Considering the bare element, a delicate aspect, decisively important for structural safety, regards the evaluation of the stresses produced by the prestressing transferring, briefly sketched in Fig. 34a, b, c. At this point, specific indications are reported in [2]. When considering the elements in their final configuration particular care has to be devoted in computing the long term deformations due to creep which significantly modify the moment distributions as the change in the static scheme operated by introducing the flexural continuity generates redundant bending moments associated to the permanent structural loads applied before the variation of the static scheme. This particular problem has been studied in [3], while a general approach for the analysis of non homogeneous beams has been stated in [4]. We have finally to remember that in order to guarantee the local strength for counteracting the stresses induced by the simultaneous presence of the prestressing force and of the shear force, the following limitations have to be satisfied in the transverse sections

Web thickness

i,min

g

2h mm

b 20 mm

d 5 mm

Horizontal layer thickness f,min

g

2h mm

h 17 mm

d 5 mm

Upper layer hf,sup bc /4 with dg maximum aggregate diameter. Furthermore, to assure an adequate flexural rigidity of the slab able to avoid too large transverse displacements, the following limitations regarding the span/height ratio (l/h), have to be satisfied a) Slabs without cast in situ collaborating layers Simply supported (l/h) < 35 Continuous (l/h) < 42 b) Slabs with cast in situ collaborating layers Simply supported (l/h) < 30 Continuous (l/h) < 36 3.3. Two-way cast in situ slabs For slabs with ordinary reinforcement the most significant aspects regard the analysis in the service stage, in particular the evaluation of the crack opening while at the ultimate limit state the evaluation of the punching capacity is of paramount importance. As illustrated in Fig. 35, this has to be performed

(a)

(b)

(c)

Fig. 34 Stresses produced by the prestressing transferring: bursting (a), splitting (b), spalling (c)

hf 1.2 hf 1.2 hf

bc

h

taking into account the interaction effects arising between the slab and the perimetric columns, in particular the ones disposed at the corners, for which the magnification factors of the symmetrical punching force recommended in [5] have to be introduced. Regarding post-tensioned slabs a prominent aspect consists in the evaluation of the redundant effects due to prestressing which significantly modify the stresses directly applied by the tendons in the transverse sections. The analysis of the redundant effects introduced by prestressing has to be performed in the viscoelastic stage adopting refined models. Particular care has also to be devoted in evaluating the load bearing capacity of the slab at ultimate [6]. In fact, an increase of the external load does not generate an analogous increase of the forces acting in the prestressing unbonded tendons, which do not exhibit the same deformation of concrete [7]. For this the effects of prestressing at the ultimate limit state are not significantly different from the ones existing in the service stage. It is so necessary to introduce in the transverse sections adequate amounts of ordinary reinforcements in order to guarantee the required load bearing capacity of the slab.

Fig. 35 Evaluation of punching bearing capacity (from ref. [5])

4. Conclusion Slab is one of the basic components of the structural arrangement of residential buildings. Referring only to structural prerequisites, the different requirements connected to the large variability of residential buildings has forced the construction industry to produce many and diversified slab systems. Among the reasons that can convince the designer in choosing one or another type of slab systems, the economical one is generally very important. This makes justifiable the large number of slab systems actually available as among them, when considering the specific prerequisites requested by the various types of residential buildings, the number of sub-systems able to conveniently accomplish them generally becomes not so high. Apart from the economical convenience, the structural efficiency, which is the basic prerequisite discussed in this paper, has to be refinedly checked in order to guarantee a reliable behaviour of the slab in the service stage and an adequate level of safety at ultimate. Regarding the service stage, cracking and deformation limit states have to be carefully considered as too large cracks can affect structural durability, while a pronounced deformability can cause damages in the not bearing elements. Particular problems like the ones connected to the non homogeneous rheological behaviour arising in precast elements collaborating with cast in situ concrete and made continuous after the application of the permanent structural loads, have to be correctly solved in order to avoid uncontrolled displacements or cracks in the continuity sections. In the same way, the evaluation of punching capacity in two-way slabs, especially when lateral loads are applied, forcing the slab systems and the columns to behave as a spatial frame, represents a prominent aspect in evaluating the structural safety of these systems. Even though for the solution of the problems discussed in the present paper efficient structural models are available and the Codes give detailed recommendations for evaluating the structural efficiency of slabs, a correct conceptual approach is in any case required when designing these basic structural systems. 5. References [1] Italian Building Code. Ministry of Public Works, Jan. 9, 1996. [2] Van Acker, A. et al. (2000), Special Design Considerations for Precast Prestressed Hollow Core

Floors, fib guide to good practice, Lausanne, CH. [3] Mola, F., Gatti, M. (1995), Delayed and Non-linear Aspects of the Service Stage of Prestressed

Hollow Core Slabs with Static Scheme Variable in Time, Proc. of the Symposium AICAP (Italian Reinforced Concrete and Prestressed Concrete Association), pp. 83-96 (in Italian).

[4] Mola, F., Giussani, F. (2003), New Materials in Structural Repairing and Strengthening, Proc. of the Int. Workshop on New Technologies and Materials in Civil Engineering, Milan, Italy, pp. 39-53.

[5] EN-1992-1-1 (2004), Eurocode 2: Design of Concrete Structures – Part 1-1: General Rules and Rules for Buildings.

[6] Lin, T. Y., Burns, N. H. (1982), Design of Prestressed Concrete Structures, Wiley and Sons, New York, USA.

[7] Nawy, E. G. (2003), Prestressed Concrete: a Fundamental Approach, Pearson Education Inc., Upper Saddle River, N. J., USA.

precast and cast in situ slab systems for residential buildings

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