LEEMUNSAMSX005553AWDO4D04T4.doc

7/27/2019 LEEMUNSAMSX005553AWDO4D04T4.doc

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CHAPTER IV

COMPOSITE CONSTRUCTION

4.1 Introduction

Composite construction offers many advantages in precast concrete

design, particularly in enhancing the flexural and shear strength of prestressed

beams where greater axial stresses may be generated in the precast unit than in

ordinary non-composite designs. To most minds composite construction mean

adding insitu concrete on top of precast components to form a single unit acting as

though it were one. However, in the context of precast design there are many

ways in which insitu concrete is used compositely in the structure. For example,composite action is used mainly to:

to increase flexural and shear strength of floor slabs

tie floor slabs to beams, thereby ensuring a secure bending, and

increasing the flexural and shear strength of beams

provide the compressive and/or shear transfer between adjacent

precast units, e.g. between walls, shear walls and columns, and at

column foundations

ensure floor diaphragm action, with or without structural screeds

anchor stability tie steel in to precast components

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In all cases insitu concrete surrounds the precast components to form a

monolithic structure. Shear and compressive forces are carried through the insitu

concrete by shear friction, wedging and/or bearing. Tension is effected by fully

(a)

(b)

(c)

(d)

(e)

Figure 4.1 Composite slab and beam sections. (a) Composite hollow core

slab; (b) composite double-tee slab; (c) composite plank floor; (d) composite

beam action with structural screed and; (e) requirements for composite beam

action without structural screed.

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anchored rebars, or other mechanical means, so that the concrete is confined to

prevent lateral splitting. Design values at the interface vary over a wide range

depending on the surface characteristics of the joining faces, the loading, and the

mode of failure, where non-ductile situations attract higher partial safety factors.

The strength of two concretes may be different; usually the precast

concrete is grade C40 to C60 and the insitu concrete is grade C25 or C30 (See

Table 4.1), but this is taken into account in the analysis for both the service and

ultimate limit states. Reference is also made to the design recommendations in BS

8110, Part 1, clause 5.4.

Non-structural finishing screeds may be applied directly on to precast

concrete slabs, but allowances for precamber should be made in calculating

overall floor depth. It is only in the presence of very large line or point loads, or in

cases where the dynamic or acoustic characteristics of the precast slab are judged

to be inadequate that a structural insitu rc screed might be required. Structural

screeds are nearly always necessary where double-tee units are used and are an

obvious prerequisite for flat plank construction. For a screed to act compositely

with the precast slab, in a structural sense, the concrete must be reinforced and

unbroken by service chases, etc.

Table 4.1 Strengths and short-term elastic modulus for typical concrete usedin composite construction.

Type of concrete fcu (N/mm2) fct (N/mm

2) Ec (kN/mm2)

Insitu

Insitu

Precast reinforcedPrestressed

Prestressed

25.0

30.0

40.050.0

60.0

-

-

-3.2

3.5

25

26

2830

32

The two main areas where composite construction is carried out is in floor

slabs and beams.(Figure 4.1(a)-(e)). The structural function of some precast

elements, e.g. precast planks (Figure 4.1(c)) rely implicity on composite action.

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However, composite action in other elements, e.g. hollow core slabs and beams

*Figure 4.1(a) and (e)) is optional and may be used at the discretion of the

designer wishing to increase flexural and shear capacities, stiffness, fire resistance

and vibration characteristics. Composite construction may also be used to create

extended bearings at the ends of units.

The design is carried out in two stages, before and after the insitu concrete

has reached its design strength. The main design criteria are:

flexural and shear strength, serviceability and ultimate states

confinement or reinforcing of insitu concrete to avoid separation,

called delamination, from precast concrete

interface shear transfer

constraint of insitu concrete shrinkage

4.2 Calculation of stresses at the interface

Shear at the interface need only be checked for the ultimate limit state.

The design method used, based on experimental evidence, willensure that

serviceability conditions are satisfied.

The average ultimate shear stresses at the interface may be calculated

using Equation (3.1). The design values in Table 3.2 are based on the use of this

formula, and allowance has been made for the small errors in defining the shear

stress that occur in the equation:

v ave

Fv

b L:=

(4.1)

where

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vave = the average shear stress at the cross section of the interface

considered at ultimate limit state

Fv = the design force in the concrete to one side of the interface

b = the transverse width of the interface

Lz = distance between the points of minimum and maximum bending

moment

If the interface is in a compression zone, thenFv is equal to the

compression force in the insitu concrete only, i.e. above the interface. If the

interface is in a tension zone, thenFvis equal to the total compression or tension

calculated from the ultimate loads. The force is distributed evenly over the contact

interface breadth and over the length of the beam between points of maximum

and zero moment, thus giving the average interface shear stress vave. The average

stress is then distributed in accordance with the magnitude of the vertical shear at

any section, to give the design shear stress vh. Thus, for uniformly distributed

stress vh = 2 vave. For a pint load at mid-span vh = vave and so on.

Horizontal interface shear stresses vh are checked for the uncracked section

(BS 8110, Part 1, clause 5.4.7.2) against values in Table 3.2 (reproduced from BS

8110, Part 1, Table 5.5). Ifvh is greater than the ultimate stress in Table 4.2 then

reinforcement (per 1 m run) is provided (according to Equation (62) in BS 8110),

as follows:

Af

1000b v

0.87 f:=

(4.2)

The reinforcement should be adequately anchored on both sides of the

interface. If loops are used, as shown in Figure 4.2, the clear space beneath the

bend should be at least 5 mm + size of aggregate. It is found that the bend radius

need not comply with bursting requirements, only the minimum of 3 is required.

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Table 4.2 Design ultimate horizontal shear stress at interface (N/mm2)

Precast unit Surface type

Grade of insitu concrete

25 30 40+Without links

Nominal links

projecting intoinsitu concrete

As cast or as extruded

Brushed, screeded or roughtamped

Washed to remove laitance, or

treated with retarding agentand cleaned

As cast or as extruded

Brushed, screeded or roughtamped

Washed to remove laitance ortreated with retarding agentand cleaned

0.40

0.60

0.70

1.20

1.80

2.1

0.55

0.65

0.75

1.80

2.00

2.2

0.65

0.75

0.80

2.00

2.20

2.5

The bars are uniformly distributed along the length of the interface,

although the spacing could in fact be reduced towards the point of zero shear.

Nominal links should be at least equal to 0.15 per cent of the contact area. The

spacing of links should not be too large, with 1.2 m being typical for hollow core

slabs. Where links are provided in ribs of T-beams the spacing should not exceed

four times the minimum thickness of the insitu concrete, nor 600 mm.

The permissible interface shear stress for hollow core and double-tee units

is therefore 0.4 N/mm2 and 0.6 N/mm2, respectively, for normally produced units.

In short spans where the shear is large (compacted with flexural requirements)

interface links can be left projecting in the longitudinal joints between hollow

core units using loops (T10 at 1.2 m centres for example) as shown in Figure 4.2.

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4.3 Losses and differential shrinkage effects

4.3.1 Losses in prestressed composite sections

It is difficult to make an assessment of the losses of prestress that occur in

a composite section, as it obviously depends on when the insitu concrete is added

to the prestressed component, normally between one and four months. Although it

may be assumed that most of the losses occur before this addition, in many cases

this will not be stricken correct and the effects of differential shrinkage and creep

should at least taken into account.

If little or no losses have taken place in the prestressed component before

the screed is placed (i.e. in less than seven days), then the shortening of the

precast unit will be restrained to some extent by the added concrete, though the

amount of the actual restraint will depend on the shape of the section and the

quantity of the insitu concrete used.

Figure 4.2 Projecting loops placed in the longitudinal joints between hollow

core units.

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bt

t

b

bb

bt

bb

Any bending moment that is induced in the composite section as a result

of differential shrinkage affects only the elastic stress conditions, and does not

affect the ultimate behaviour (in the same way that the level of prestress has a

small influence on the ultimate strength). Because it is difficult (and probably

unnecessary) to make a 100 per cent correct assessment of the differential

deformations due to shrinkage and creep, it seems sensible to first compute the

shrinkage and creep movement taking place in the precast beam, and then to add

the effects of the relative movement in the topping concrete. To proceed in this

manner will usually lead to slightly conservative results, as the relative shrinkage

strain in the interface will be greater than if all the shrinkages are assumed to

occur simultaneously.

4.3.2 Design method for differential shrinkage

(a)

(b)

Figure 4.3 Theoretical approach to shrinkage-induced deflections in

composite construction. (a) Definitions; (b) shrinkage effect in reinforced

sections;

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(c)

(d)

Figure 4.3 (continued) Theoretical approach to shrinkage-induced

deflections in composite construction. (c) Effective eccentric force due to

differential shrinkage and (d) shrinkage-induced deflection.

The forces due to differential shrinkage may be calculated by the

following method. In this analysis, the term differential shrinkage is used to

describe the difference in the free strains due to the shrinkage of the insitu

concrete and the combined shrinkage and creep of the precast concrete. At the end

of the span, the stress in the precast concrete due to dead load and prestress is

small and the differential strain should be normally be taken as the difference in

the free shrinkage values of the two concretes.

If the beam or slab reinforcement is placed non-symmetrically, a bending

moment is induced by shrinkage due to non-uniform restraint by the

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reinforcement and, as a result, shrinkage increases the curvature and consequently

the flexural deflection of the components.

In the following equivalent tensile force method for estimating shrinkage-

induced stresses, applied loads and shrinkage forces are resisted by an uncracked,

cracked or partially cracked member. In the case of the cracked or partially

cracked member, the assumption is made that the shrinkage occurring prior to

cracking is insignificant. It is therefore possible to treat shrinkage-induced stress

in a similar way to load-induced stress at the serviceability limit state.

Consider a unit length of the composite precast beam of precast depth h,

and topping screed depth hs, shown in Figure 4.3(a) in which, after any interval of

time following the casting of the insitu concrete flange, the free shrinkage of the

flange is f, and the combined free shrinkage and creep of the beam is b at the

centroid, with values ofbt and bb at the top and bottom fibres, respectively. Refer

also to the deign guidance given in BS 8110, Part 2, clause 7.4. The analysis

considers that the concrete member is free to shrink, and when this happens the

compression and tension steels are compressed by fictitious force bAsEs and bAs

Es respectively, whereAsandAs are the areas of the compression steel and tension

steel, respectively, andEs is the Youngs modulus of steel bars or tendons. When

these loads are released, it is equivalent to eccentric tension loads bAsEs and bAs

Es applied at the steel level to the entire transformed area of the member, as shown

in Figure 4.3(b). These forces produce a bending moment and consequently a

curvature and flexural deflection of the concrete member. IfAs >Asthen the

deflection is downwards. These forces also produce an interface shear stress as

follows.

The bending momentMb produced by the effects of the beam shrinkage is

given by:

Mb = bEs [As (d-x) - As(x d)] (4.3)

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where dand dare the depths of the tension and compression reinforcement

respectively, andx is the distance from the top of the screed to the neutral axis of

the transformed composite section. In multi-layered reinforcement use the sum of

each layer in the termAs (d x).

The strains in the top and bottom surfaces of the beam are given by

Equations (4.4) and (4.5) (assuming b > bb):

bt b =

Mb

Ec

I (4.4)

whereEc is the modulus of elasticity of beam concrete, andIc is the second

moment of area of the transformed composite section. Also, the strain at the

interface between the precast and screed:

bt

Mb x h s( )

Ec Ic+:=

(4.5)

The relative shrinkage strain between the flanged screed (bt) is given by:

s = f- bt (4.6)

The modification factor for the restraining effect of the mesh in the screed (1 +

K) (where = 0.13 to 0.24 per cent, andK= 15 to 25, typically) is not

significant in screeds.

A tensile forceFis applied at the centroid of the flange to overcome the

strain differential between the flange and the beam (Figure 4.3 (c)) and is given

by:

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F= sEcm Af (4.7)

where m =Ec/Ec is the modulus ratio, andAf is the area of flange section. In

composite beams the effective width of the flange is the same as for any T section

according to BS 8110, Part 1, clause 3.4.1.5 [3.1].

The two concretes may now be joined together (theoretically) and the

external actions released if a compressive forceFand a balancingMc are applied

to the composite section, such that:

Mc =F e + Mb (4.8)

where e is the distance between the centroid of the flange and the centroid of the

composite section. The contribution ofMb is usually not more than about five per

cent of the total momentMc. The resultant longitudinal forceFv (of these two

actions) to one side of the interface (tension in the flange and compression in the

beam) is given by:

Fv

F Mc

S

Ic

+:=

(4.9)

where Sc is the first moment of area to one side of the interface about the centroid

of the transformed composite section andIc is the second moment of area of the

transformed composite section. This is the value forFv that must be used in

Equation (4.1).

The resulting induced bending momentMc causes a sagging deflection s

(Figure 4.3(d)) over a simply supported spanL of:

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s

Mc L2

8 E:=

E (4.10)

and so on depending on the beam geometry and loading conditions. Note that in

the unlikely event off< bt, thenFwill be negative. In this case useF= 0 and so

Mc =Mb andFv =MbSc/Ic.

Shear lag effects in the insitu concrete will reduce the relative shrinkage

strains. This will be noticeable in depths of screed greater that about 100 mm.

However, in most applications in precast work where thickness of insitu concrete

are usually less than 75 mm, the simplified version given above is adequate for

design purposes.

A further complication in the analysis is the changing thickness of insitu

screeds on precambered prestressed beams and slabs. In a typical situation the

depth of creed used on hollow core or double-tee floor slab is 50 mm at the crown

(mid-span) of the floor slab and up to 80 mm at the supports. Thus the composite

section properties calculated at the crown of the floor are not appropriate to the

situation at the supports where the effects of differential shrinkage are the

greatest. For the same reason neither are the section properties at the support

correct. The usual practise is to calculate the interface stresses based on the

composite section properties at the position of the mean depth of the screed.

4.3.3 Cracking in the precast and insitu concrete

The question often arises as to whether the insitu concrete at the interface

cracks when the unaxial cracking strain is exceeded, or whether the precast

concrete unit retrains the insitu concrete. Some restraint is given to weaker in

these situations. If the bond at the interface is sufficient for the two concretes to

act monolithically then a linear strain distribution may be assumed throughout the

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entire section. Also flexural cracking at the interface propagates simultaneously in

the precast and insitu concrete.

The precast unit, which covers the entire bottom surface of the insitu

concrete, behaves in a similar manner to reinforcing bars, in that it eliminates any

concentration of strain at any section where the concrete quality is below average

for the specimen; thus the average strain before cracking is greater than that for an

unrestrained plain concrete having the same overall properties.

It has been seen that the assumption of linear strain distribution depends in

the fact that the connection between the two concretes that make up the composite

section is strong enough to ensure that the longitudinal shearing forces cause no

relative movement at the interface. A rough tamped top surface of the precast unit

will be sufficient to ensure that a horizontal shear failure does not occur, though it

may take place as a secondary effect after some other factor has caused the

primary failure. As far as cracking is concerned, a rough surface by itself will also

prove to be a better interlocking medium than the corresponding smooth surface

that also has either castellations or projecting steel stirrups.

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