Fib Sandwich Panels (Draft Oct 2012 No Track Changes)

5/23/2018 Fib Sandwich Panels (Draft Oct 2012 No Track Changes)

D r a f t : O c t o b e r 2 0 1 2 1 |P a g e

Commission on Prefabrication

Task Group TG 6.11

Guide to Good Practice:Precast Insulated

Sandwich Panels



Guide to Good Practice:

Precast Insulated Sandwich Panels

fib - Commission 6

Task Group TG6.11:

Simon Hughes, Hollow Core Concrete, [email protected] (convenor)

Arto Suikka, RTT Finland, [email protected] (co- convenor)

Alessandra Ronchetti, Assobeton, [email protected] (secretary)

Carlos Chastre Rodrigues, Universiade Nova de Lisbona, [email protected]

Antonello Gasperi, [email protected]

George Jones, CDC Ltd., [email protected]

Holger Karutz, CPI Concrete Plant International, [email protected]

Jason Krohn, PCI,[email protected]

Gosta Lindstrom, Stangbetong, [email protected]

Larbi Sennour, CEG, [email protected]

Venka Seshappa, Composite Technologies, [email protected]

Mathias Tillmann, German Association for Precast Concrete Construction, tillmann@fdb-

fertigteilbau.de

Spyros Tsoukantas, [email protected]

Corresponding Members:

Arnold Van Acker, [email protected]

Diane Lalibert, BPDL International, [email protected]

Sthaladipti Saha, Larsen & Toubro Ltd., [email protected]

Draft: October 2012



Index

Chapter 1: Forward

1.1 Introduction1.2 Benefits of Precast Insulated Sandwich Panels

1.3 Understanding Thermal Mass

1.4 Understanding Acoustics

1.5 Global use of Precast Insulated Sandwich Panels

Chapter 2: Definitions and notations

Chapter 3: Energy efficiency, humidity and acoustic

performance

3.1 Introduction

3.2 Panel types and variations of thermal performance

3.3 Energy efficiency demands

3.3.1 Cold climate3.3.2 Hot climate

3.4 Insulation materials

3.5 Panel connectors

3.6 Thermal performance

3.7 Condensation considerations

3.8 Acoustic Performance

Chapter 4: Structural design and detailing

4.1 Introduction

4.2 General Rules

4.3 Structural Behaviour

4.3.1 General

4.3.2 Non Composite Precast Insulated Sandwich Panels

4.3.3 Composite Precast Insulated Sandwich Panels



4.4 Precast Insulated Sandwich Panel Applications

4.5 Wythe Design

4.5.1 General

4.5.2 Loadbearing Precast Insulated Sandwich Panels

4.5.2.2 Connections of load-bearing sandwich panels

4.5.2.3 Mechanisms for dissipation of seismic energy

4.5.3 Non Loadbearing Precast Insulated Sandwich Panels

4.6 Wythe Connectors

4.6.1 General

4.6.2 Shear Connectors

4.6.3 Non Shear Connectors

4.7 Other Considerations4.7.1 Joints

4.7.2 Fire

4.7.3 Durability

Chapter 5: Manufacture of Sandwich Panels

5.1 General

5.2 Typical production process5.3 Production requirements

5.3.1 Preparation of production and erection drawings

5.3.2 Manufacturing facilities

5.3.3 Formwork / Moulds

5.4 Reinforcement

5.2.1 Reinforcement cage assemblies

5.2.2 Prestressing

5.5 Concrete Placement

5.5.1 Transportation

5.5.2 Segregation

5.5.3 Consolidation

5.5.4 Facing Concrete

5.6 Surface Finishes

5.6.1 General Methods

5.6.2 Chemical surface retarders



5.6.3 Abrasive blasting to expose aggregate

5.6.4 Honing and polishing

5.6.5 Acid etching

5.7 Concrete Curing

5.7.1 Curing recommendations5.7.2 Curing techniques

5.8 Storage

Chapter 6: Transport and Installation

6.1 General

6.2 Transport and Delivery

6.3 Planning and Preparation

6.3.1 Coordination

6.3.2 Access

6.3.3 Project meetings

6.3.4 Contract documents

6.3.5 Pre-erection check

6.4 Panel Handling and Site Storage

6.4.1 General6.4.2 Delivery sequence

6.4.3 Lifting devices

6.5 Jobsite Storage

6.5.1 General

6.5.2 Panel support

6.5.3 Storage on delivery vehicle

6.6 Panel Erection

6.6.1 Workmanship

6.6.2 Equipment

6.6.3 Bracing and guying

6.6.4 Alignment

6.6.5 Bolted connections

6.6.6 Welded connections

6.6.7 Post-tensioned connections

6.6.8 Dowels and grouting



Chapter 7: Inspection and Repair

7.1 General

7.2 Inspection at Manufacturing Plant

7.3 Dimensional Tolerances

7.4 Job site inspection, construction damage and repair

7.5 Cleaning

7.5.1 Protection

7.5.2 Stubborn stains

7.5.3 Sandblasting and steam cleaning

7.5.4 Sealers

7.6 Patching and Repair

7.6.1 General

7.6.2 Repair consideration

7.6.3 Chips and spalls

7.6.4 Crack repair

7.7 Joint Sealing (Caulking)

7.7.1 Joint preparation

7.7.2 Sealant installation

7.7.3 Two-stage joints

7.7.4 Fire-resistant joints

7.8 Repair during life-cycle

References.

Appendixes:

Typical connections and details

Design examples: with European Standards

with US Standards

with Australian Standards


D r a f t : O c t o b e r 2 0 1 2

1 Forward

1.1 Introduction

With sustainability and en

precast concrete insulated san

Tighter government contrwithin the construction indus

and construction methods. T

benefits of passive solar desi

tightness.

Improving the environmen

been applied, with many tradi

Recycled materials inc

Supplementary cemenFume and Geopolyme

High Strength concret

However, it is the combin

provide that is seeing the syst

What is a precast insulated

Fig. 1.1 Possible Sandwich

rgy efficiency becoming a major concern

wich panels has increased significantly in re

ls over greenhouse gas emissions have betry to explore more environmentally friend

is exploration has also lead to an increasin

n utilising thermal mass, appropriate solar

al performance of concrete within the const

ional alternatives now widely accepted, incl

luding aggregates and water,

ticious materials such as Fly Ash, Blast F

s, and

used to reduce component section sizes and

ed benefit that Precast Insulated Sandwich

m becoming the material of choice for many

andwich panel?

Precast Insulated Sandwich

comprised of a minimum of th

an outer layer of reinforced p

insulation layer and an inner la

prestressed precast concrete.

mechanically held together usities or connectors. Additional

moisture barrier, air void and/o

may also be incorporated as sh

Refer to Chapter 4 for furt

each layer.

Panel Fig. 1.2 Photo of Sandwich P

7 |P a g e

lobally, interest in

cent years.

n a major catalystly design, material

g awareness of the

orientation and air-

uction industry has

ding the use of:

urnace Slag, Silica

geometry.

Panels are able to

buildings.

anels are typically

ree separate layers;

recast concrete, an

er of reinforced or

The layers are

g various types oflayers such as a

cladding materials

wn in Figure 1.1.

er detail regarding

nels



Precast Insulated Sandwich Panels are able to reduce building energy costs, improve long

term performance and make use of more environmentally friendly concrete.

Life cycle analysis and the long term performance of a building need to be an important

consideration for all modern structures, with CO2 emissions from the HVAC (Heating,

Ventilation and Air Conditioning) being the

greatest contributor to energy usage and

resultant CO2emissions.

Achieving a sustainable building solution

must take into account a variety of

environmental factors, from the embodied

energy in the construction materials, to the

energy consumption during a buildings life - a

holistic sustainable solution.

Precast Insulated Sandwich Panels offer

significant reductions in energy usage for both

heating and cooling, especially when combined

with additional passive and active energy savinginitiatives (such as passive solar design, use of

high thermal mass flooring, appropriate

orientation, systems like (Thermal mass

activating flooring systems) and mechanical

shading). The need for air-conditioning and

heating can be significantly reduced or

eliminated altogether in some climates.

Fig. 1.3 Sandwich Panels Building

(Xerox Building, Lisbon Portugal)

Fig. 1.4 IR camera pictures show the energy efficiency and air-tightness of precast concrete sandwichpanels (ENOTHERM Institute of Energy-Optimised Construction, Bochum; Hering Bau GmbH & Co. KG,

Burbach)



With Precast Insulated Sandwich Panels the reduced U-Value (increased R-Value) and

benefits in both the Thermal Mass and resultant Thermal Lag provide a building element that

is able to supply significant benefits to an environmentally conscious building.

Produced in a controlled factory environment, Precast Insulated Sandwich Panels are

similar in appearance to solid precast panels, with the distinct difference being their superior

thermal and acoustic properties.

Unlike many other systems, Precast Insulated Sandwich Panels have the majority of their

thermal mass on the inside of the building. This high thermal storage capacity makes them

capable of storing and releasing heat very slowly, thereby minimising fluctuations of the

internal temperature of the building. They also insulate against noise transmission of both

airborne and impact sound.

Precast Insulated Sandwich Panels can be

used as both the external and internal walls of a

building. They can be used as cladding or as a

structural element to the building and can have

a variety of architectural finishes.

While this document will mainly focus on

vertical precast wall panels incorporating two

concrete wythes separated by an insulation

layer, the same principles may be applied to

other precast elements, such as Hollow Core

Sandwich Floor and Wall Panels, Double Tees,

prestressed and reinforced floor planks, etc.

Fig. 1.5 Hollow Core Sandwich Panel

1.2 Benefits of Precast Insulated Sandwich Panels

Due to the unique manufacturing process precast concrete sandwich panels offer many

benefits to the designer, the builder and most importantly the end user. They are an energy

efficient, sustainable, economical, fire resistant, durable, strong and aesthetically versatile

building solution.

Sustainable

Precast Insulated Sandwich Panels are typically manufactured using local products.

Recycled materials such as reclaimed aggregate, recycled steel and water, along withsupplementary cementicious replacements such as blast furnace slag and flyash can be

included in the concrete mix. Utilising efficient production methods mean a higher quality

product with minimal production waste.

On site the use of Precast Insulated Sandwich Panelscreates less air pollution, noise and

debris, and site waste is reduced as exact elements are delivered from local precast

manufacturers to the construction site. Sites become safer because they are less cluttered with

materials and labour.

The high quality of Precast Insulated Sandwich Panelsmeans that they can be left exposed

internally in order to maximise the benefits of the high thermal mass. Owing to its high

density, precast has the ability to absorb and store large quantities of heat.



Furthermore, the integrity of precast means that maintenance and operating costs are low.

The durability of precast over other materials (including other concrete elements) results in a

longer service life.

Economical

Precast insulated sandwich panels can be used for both wall cladding or as a load-bearing

structural element. They are similar to solid precast concrete panels for erection purposes andare therefore easily erected by an experienced precast erection crew.

When used as structural elements, Precast Insulated Sandwich Panels can act as a beam,

column, external wall, and insulated internal element all in one, which significantly shortens

the construction cycle and reduces the cost of construction.

Fire Resistant

As Precast insulated sandwich panels typically comprise of a concrete layer both externally

and internally, sandwich panels are inherently fire resistant. Depending on the various types

of insulation and connectors adopted, various fire rating levels are achievable.

DurableThe inherent durability of precast concrete structures is well documented. Examples of

precast concrete structures that were built centuries ago still exist today. Using high strength,

high quality precast concrete allows for a long service life to be achieved whilst protecting the

integrity of the insulation layer.

Their long life makes them a cost-effective construction solution. Externally, a variety of

available finishes provide architectural freedom whilst requiring minimal maintenance. This

equates to on-going cost savings over the life of the building.

Multi-hazard resistant

Since the structural load bearing material is concrete, precast insulated sandwich panels areresistant against blast effects, tornados, chemical attack, bullets, etc

Speed of construction

Precast insulated sandwich panels provide the same benefits as other precast systems: They

are easy and fast to erect on site, this results in shortened construction periods and earlier

tenancy.

Aesthetically Pleasing

Designers of precast sandwich panels have the flexibility of custom manufacture for size,

shape, finish and colour. Precast Insulated Sandwich Panels can be produced with a smooth

off-form finish and can be of natural grey colour, or subsequently painted or stained. Falsejoints may be incorporated into the design and mould liners can be used to create an endless

array of patterns. Textures can be achieved by acid washing, sand (or grit) blasting, honing or

polishing, often with an integrated colour pigment, to achieve impressive results.

Brick or ceramic tiles or natural stone plates can also be used as an outer surface of

sandwich panels.



Fig 1.6 Completed projects incorporating sandwich panels (Top Left - Sporting Clube de Portugal,

Lisbon, Portugal; Top Middle - Manuel Gaspar, Sintra, Portugal; Middle Right Auchan-Jumbo,

Castelo Branco, Portugal; Bottom left - Ramos e pereira, Sines, Portugal; Bottom Right - Carrefour,

Loures, Portugal)

Energy Efficient

Due to their high thermal mass and insulating properties, precast sandwich panels are able

to provide a more comfortable living environment with reduced internal temperature

fluctuations. This consistency of internal temperature results in a reduction in energy

consuming artificial heating and cooling systems.

The inner concrete layer provides an insulated thermal mass within the building. The heat

that is absorbed by this inner layer during the day is released into the buildings interior at

night when temperatures drop.

Precast Insulated Sandwich Panels allow the flexibility to allow for varying thermalperformance requirements in different climate zones or for different compliance requirements.

The component geometry can be varied to achieve the desired performance level.



More details on blast resistance, PCI reference ?

Solar reflectance

Sustainability

Air Quality

Site storage less clutterJason to send list re Advantages

1.3 Understanding Thermal Mass

Historically the benefits of thermal mass have been well understood for centuries, however

in todays environment there has been a growing trend towards lighter structures with little

consideration to the amount of energy required to keep them at a comfortable temperature. It

is well accepted that high thermal mass and the resultant thermal lag can have a significant

positive influence on the internal environment of a building. Having a building of high

thermal mass will produce a structure that is able to even out large temperature fluctuationsand significantly reduce peak temperatures that are common in light weight construction.

Fig. 1.7 Thermal Mass Diagram (From the Concrete Centre: Thermal Mass Explained)

The Thermal mass (thermal capacitance or heat capacity) is measured as the amount in

which a material is able to store energy and release it back to the environment as required.

Concrete has the highest volumetric heat capacity of any common building material and as a

result can provide significant benefits in smoothing out a buildings indoor temperature. The

resultant indoor environment is commonly accepted as a more comfortable space without the

need for heated or cooled air to be forced into a space.

The result of this high thermal mass is a significantly reduced need for high energy using

air conditioning, and/or space heating systems. It is commonly accepted that between 40-

50% of a households energy usage is due to the heating and cooling of the indoorenvironment; thus, a significant reduction of the need for this heating or cooling will result in

significant energy saving and resultant CO2reductions.



When considering the Life Cycle Assessment of a structure it is important that it be

considered from cradle to grave or cradle to cradle so that the full environmental impact of

a building is recognised.

It is important to consider the structure as a whole; and while thermal mass and thermal lag

will provide significant advantages, a holistic approach to energy efficient design must be

considered. Consideration must be given to building orientation to maximise the solar

benefits, including shading window sizing, etc. Windows need to be carefully considered with

double or triple glazed options considered, as well as air tightness of the structure.

Insulation is widely accepted as a benefit to assist with internal comfort. The combination

of insulation and the benefits of materials with high thermal mass result in a superior building

material. To gain the most from the system it is vital that the insulation is located behind the

majority of the thermal mass and wherever possible allow the thermal mass to be exposed to

the internal space.

Precast insulated sandwich panels are able to provide the best of both worlds. They provide

the benefits of a structure with high thermal storage capacity and also incorporate insulation

that is able to keep the thermal energy where it is required, indoors. The result is a structure

that is able to provide a very consistent indoor environment that requires very little

mechanical intervention to keep it comfortable.

1.4 Understanding Acoustics

A comfortable living environment is not only the ability of a building to provide a

comfortable internal temperature; it must also consider the buildings acoustics. A quiet living

environment or a building with a high Rw rating is considered to be more desirable and more

comfortable.

Precast Insulated Sandwich Panels provide outstanding acoustic performance. This isachieved by the density and high mass of concrete, making it good for controlling

reverberation. This, along with the inclusion of the insulation layer acting as an internal

buffer, result in Precast Insulated Sandwich Panels being efficient at blocking out noise.

Unwanted noise such as airborne noise and impact noise can effectively be blocked by

Precast Insulated Sandwich Panels, providing that joints and openings are detailed and sealed

correctly. Poorly sealed joints and openings can lead to unwanted noise entering a space,

reducing the buildings sound attenuation properties. One of the benefits of precast concrete

buildings is that they normally have less joints or voids than other forms of construction,

improving their acoustic qualities.

As a result precast sandwich panels are ideal for buildings particularly in built upresidential areas, or for buildings with significant external noise, such as near major arterial

roads to block out traffic noise, in industrial settings, etc. In apartment buildings, residents can

also benefit from the acoustic sound attenuation properties of Precast Insulated Sandwich

Panels acting as the common wall or internal partition.

1.5 Global use of Precast Insulated Sandwich Panels

Precast Insulated Sandwich Panels are used for many purposes in many countries around

the world. Besides being used for residential, office, commercial and industrial buildings,

precast insulated sandwich panels can also be used for schools, hospitals, cold stores,controlled atmospheres, prison walls, etc.



A recent survey showed that in some countries Precast Insulated Sandwich Panels have

been used for more than 50 years. Nowadays, mainly due to increasing awareness of energy

efficiency aspects and energy saving housing design concepts, more and more attention is

paid to the use of Precast Insulated Sandwich Panels.

Fig. 1.8 Buildings with Precast Insulated Sandwich Panels in Russia [Ebawe], Dubai [Ebawe],

Iceland [Nuspl], France [Vollert]

Throughout the world, a tendency to reduce U-values (Increase R-Values) for outer walls

is widely accepted as a future trend in the construction industry; with Precast Insulated

Sandwich Panels this can be achieved. Different climates in different countries lead to

different requirements worldwide. As can be seen following, in general, the requirements are

not as onerous in warmer climates; Reference to survey?

In Spain in the future U-values of 0.74 W/mK are considered to be sufficient (andvary throughout the country)

The United States will demand U-values of 0.34 W/mK for certain buildings

France will require U-values of 0.22 W/mK from 2012

Ireland and UK are becoming more restrictive, with a target requirement in the orderof U = 0.18 W/mK and 0.20 W/mK, respectively

In cold climates, such as in Finland, U-values of less than 0.10 W/mK for outer wallsof certain buildings will be required from 2015. With the development of sandwich

panels these types of figures are achievable.

Australia prescribes different R-values for outer walls (Note: Australia uses R=1/Uvalues). Converted to U-values, the future range of required U-values will be between



0.26 W/mK and 0.56 W/mK. Australia will also allow an adjustment in U-value

depending on surface density and shade angle.

In Germany, most likely future requirements for outer walls will not exceed U=0.28W/mK, however the overall building energy efficiency needs to be guaranteed.

From this we can see that different countries with diverse climates and varying

requirements are following many concepts for the common goal: The reduction of the carbon

footprint, energy savings and getting closer to a totally sustainable environment.

Fig. 1.9 High rise buildings with Precast Insulated Sandwich Panels are realized as well.

Examples from Finland (16 and 26 stories high), and Sporting Clube de Portugal, Lisbon

Portugal

In many countries Extruded or Expanded Polystyrene (EPS/XPS) or Polyurethane

(PUR/PIR) as insulation material with thicknesses of 50-75mm are commonly used; France

and Germany are producing Precast Insulated Sandwich Panels with up to 200mm of

insulation material. While in Scandinavian countries, mineral wool is often used as the

insulation material with thicknesses of up to 240mm. Development of the Precast Insulated

Sandwich Panels system will continue with producers developing products such as a vacuum

insulated panels, providing excellent U-values with significantly reduced thickness, as well as

new connection systems continuing to develop.

Sandwich panels are made of two concrete wythes separated by at least an insulation layer;the two concrete wythes are connected by connection systems.

Non composite panels are sandwich panels in which the two wythes act structurally

independently(for the full life of the structure). In each non composite panel one of the two

wythes supports the other wythe; the supporting wythe, which most commonly is the inner

layer, bears all vertical and horizontal loads.

Composite panels are sandwich panels in which the two wythes act as a fully composite

unit (for the full life of the structure). Composite panel connection systems provide for

composite behaviour.

Partially composite sandwich panels are panels in which the two wythes do not act

independently nor as a fully composite unit, but rather somewhere in between. Partially

composite panels are panels in which the connection system provides horizontal shear



transfer between the wythes, but the composite moment capacity is limited to that obtained

from the horizontal shear capacity of the connection system. This will always be less than the

full composite potential of the panel. Generally partial composite action may be used for the

temporary conditions of lifting and handling, but should carefully considered before being

utilised in-service conditions.

We point out that between non-composite action and full composite action there are a

number of solutions (with varying continuity) which may be adopted in the design.

Fig. 1.10 Precast insulated sandwich panels with mineral wool insulation [Weckenmann]

In most countries, non-ventilated precast insulated sandwich panels, or sandwich panels

without air gaps, are the most common solution.

To connect both wythes of precast insulated sandwich panels, different types of connectorscan be used. Connectors are typically made of steel, stainless steel, fibreglass, carbon fibre or

fibre reinforced polymer (FRP) composites. In addition to the connector capability to transfer

shear flow and connect the two wythes, their thermal conductivity, or thermal bridging, must

be considered.



Precast insulated sandwich panels can be designed as non-composite, partially composite

or totally composite. Details for the structural design concepts of sandwich panels will be

covered in more detail in Chapter 4 of this publication.

With the continued use and development of sandwich panels, more building designers will

realise the potential of this type of building element.

Fig. 1.11 Precast insulated sandwich panel with integrated installation for heating and

electricity in Austria [Ratec]

The typical panel size varies from country to country. In Finland and France, Precast

Insulated Sandwich Panels have typical dimensions of 4-6m x 3-4m. In Spain, Germany,

Australia, UK and Ireland the average measurements range from 8-12m x 2.5-4.0m. All of the

above are typically produced in precast plants, either on stationary production facilities or on

circulating pallet plants. In The United States, Precast Insulated Sandwich Panels are also

produced on-site with tilt-up technology up to 27m length and more.

These panels can also be produced on long line beds such as with hollow core slab

production. While typical Precast Insulated Sandwich Panels measurements of approx. 15-40m are used for residential, public or office buildings, large panels with 40+m are mostly

used for industrial and commercial buildings.



Fig. 1.12 Sandwich panel with EPS insulation, produced in Sweden [SommerAnlagenbau]

Precast insulated sandwich panels are very functional in reducing energy demand and

creating a more comfortable environment, however they are increasingly being used for

architectural purposes. Coloured concrete, polished or differently treated concrete surfaces

even graphics and pictures may be applied on the outer wythe of the panel. In Finland,

sandwich panels are often pre plastered inside the production plant, and on site additional

layers of coloured plaster guarantee a perfect surface finish, without any visible joints.

Fig. 1.13 Photo of a Pre-Plastered Precast Insulated Sandwich Panel as used in Finish Apartment

Buildings

Plastering is completed on-site to avoid any visible joints



Fig. 1.14 Plastering for apartment building in Finland in Helsinki city [Parma Oy]

In most countries where precast insulated sandwich panels are produced, they are oftenused for low-rise buildings, up to 5 levels. However, in countries more experienced in using

sandwich panels, we are seeing projects commonly used for mid rise (5-10 levels) and even

high rise (10+levels) buildings, showing that precast insulated sandwich panels are suitable

for many applications.

Fig. 1.15 Some architects wish to hide the joints between the panels. This example from

Finland shows an ability to hide joints very effectively.



Fig. 1.16 Sandwich panels with a variety of architecturally treated surfaces in Russia

[Weckenmann]

Fig. 1.17 Maria Mughal (an artist) has designed the graphic concrete figure (Helen ofTroy). Left panel as produced, right as installed (University Apartment Building, Joensuu,

Finland)



2 Definitions and Notations

Descriptive terminology is used throughout this document. Some of the common terms

relevant to structural design and detailing of Precast Insulated Sandwich Panels are explained

as follows:

Loadbearing Precast Insulated Sandwich Panel: A Precast Insulated Sandwich Panel that

supports vertical loads applied by other construction elements.

The Loadbearing Precast Insulated Sandwich Panels are also subjected to other actions as

their own self weight, loads applied to their surface area, seismic actions, thermal actions

and actions due to the shrinkage of the concrete.

The Loadbearing Precast Insulated Sandwich Panels are commonly used to support vertical

loads applied by floor or roof.

We note that according to the above mentioned definition shear walls used to resist horizontal

forces are not loadbearing .

Non loadbearing Precast Insulated Sandwich Panel: A Precast Insulated Sandwich Panel

that does not support any vertical loads applied by other construction elements.

The non loadbearing precast insulated sandwich panels are subjected to their own self

weight, to loads applied to their surface area and to other action as seismic actions, thermal

actions and actions due to the shrinkage of the concrete.

Non loadbearing Precast Insulated Sandwich Panel are commonly used for cladding.

Composite Precast Insulated Sandwich Panel: A Precast Insulated Sandwich Panel in

which the inner and the outer concrete wythes, during the design life of the structure, acttogether as a fully composite unit, to resist applied loads and actions.

In particular a composite precast insulated sandwich panel acts as a single unit in bending;

full shear transfer is provided between the inner and the outer white by means of rigid ties or

connectors, or other tested devices that connect the two whites.

We point out that in many cases Composite Precast Insulated Sandwich Panel are Partially

Composite Precast Insulated Sandwich Panel according to the definition below.

Non Composite Precast Insulated Sandwich Panel: A Precast Insulated Sandwich Panel in

which the inner and the outer concrete wythes, during the design life of the structure, actindependently to resist applied loads and actions.

Generally the inner concrete white is the supporting one and the outer concrete white is the

supported one. However the outer white may be the supporting white as, for example, in the

case of vertically spanning double tees with external ribs, used in industrial applications.

The supporting white may be ribbed.

All loads and actions applied to the supported white are finally transferred by means of

specific devices to the supporting white.

Partially Composite Precast Insulated Sandwich Panel: A Precast Insulated SandwichPanel in which the inner and the outer concrete wythes, during the design life of the structure,



do not act independently nor as a fully composite unit, but rather somewhere in between, to

resist applied load and actions.

The inner and the outer white are connected by means of ties which do not provide a fully

composite action.

The degree of composite actions should be declared in the design documents.

U-Value: is the thermal transmittance coefficient, which is defined as the rate of heat

transfer, in watts, through one square metre of a structure when the combined radiant and air

temperatures on each side of the structure differ by 1 kelvin (i.e. 1C). This is stated in watts

per square metre of fabric per Kelvin (W/m2K). It should be noted when calculating U-values,

that certain elements of the construction, such as timber joists, structural and other types of

framing, mortar joints and window frames act as thermal bridges and should be allowed for in

U-value calculations.

R- Value: (Include US)

Thermal Conductivity:is the amount of heat per unit area, conducted in unit time through a

slab of material of unit thickness, per degree of temperature difference. It is expressed is watts

per metre of thickness of material per degree Kelvin (W/mK) and is denoted as .

Certified test results of thermal conductivities (i.e. -values in W/mK) and thermaltransmittances (i.e. U-values in W/m

2K) for particular products should be obtained from

individual manufactures. If this proves to be difficult the values contained in the tables in AD

L (which are reproduced below) may be used instead. U-values may be calculated provided

that suitable allowances have been made for the effects of thermal bridging as is the case with

the tables mentioned above.

Editorial note: the degree of composite action should be obtained by computation and may be

validated by some experimental test, particularly for partially composite panels.

The degree of composite action should be discussed in chapter 4.

Admittance values?

What is a wall / panel

What is a layer/wythe

What is thermal mass/thermal lag

Thermal bowing

Building fabric

Deadmen, Screw Anchors, kentledge (Ballast Block)

Define: Length, Width, thickness of panels

EPS, XPS, PUR, Types of insulation

Slenderness Ratio: Effective height between restraints divided by wythe thickness

Refer Georges Book.

movement accommodation factor, MAF



3 Energy efficiency, humidity and acoustic performance

3.1 Introduction

Look at sustainability (George to review)

Include a Rule of thumb table to include:

Layer thickness, insulation thickness, insulation type, u-value (maybe

review a graph) Larbi to review

3.2 Panel types and variations of thermal performance

May move as introduction review (review repetition)

Precast Insulated Sandwich Panels are typically formed by 3 layers, the outer and inner

layers made of concrete and a thermal insulation layer between them. In some countries, the

outer layer can also be made of plaster. The inner and outer layers are typically joined by

mechanical connectors. Mechanical connectors are used sparingly, using them only as

required structurally, because of the thermal (cold) bridges they may create.In some Precast Insulated Sandwich Panels the thermal insulation and the structure as a

whole can be ventilated to reduce possible moisture in the structure.

To ensure building efficiency, the joints between panels must be made airtight. In critical

environments the inner concrete layers may be connected with an in-situ cast joint or elastic

sealant, an airtight internal thermal insulation layer and the outer concrete layer finally sealed

with an elastic sealant.

All such details or connections creating thermal bridges must be avoided.

Fig. 3.1. All the connections must be properly detailed to provide a continuous insulation

layer and to avoid thermal bridging.



Fig. 3.2. Sandwich panels may be ventilated. On the right hand side, glasswool sheets with

vertical ventilation channels are provided to the outer surface.

While the panel type and materials used in sandwich panels can change, the thermal massof the total wall assembly is one of the elements best assets when considering thermal

performance. In the case of a building in a hot, dry climate for example, sandwich panels

need to function differently than in cooler climates. During the summer months, their mass

effectively minimizes temperature fluctuations between the interior and exterior

environments. Whereas, during cooler months the sandwich panels primarily insulate the

interior environment from exterior conditions.

3.2 Energy efficiency demands

3.2.1 Cold Climate

Precast Insulated Sandwich Panels are commonly used in both cold and warm climates.

In cold climates, such as Northern Europe and Canada, where winter involves long periods

of temperatures dropping below zero degrees centigrade, the external temperature along with

the extent of heating are important considerations in the panel design.

The panels must have adequate thermal insulation and the building being adequately

sealed. Air leakage can compromise the ventilation system, which in turn increases heating

energy consumption considerably.

Cooling is also required in cold climates, particularly in offices and residential buildings.

As shown previously, due to environmental requirements and energy saving initiativesbuilding codes worldwide have changed, or are expected to change in the near future. With

these changes thicker and more efficient insulation materials will be required.

More and more low, passive and zero energy buildings are being constructed. Precast

Insulated Sandwich Panels are ideal for these buildings because of their high thermal mass,

air tightness, durability and flexibility in thermal performance.

3.2.2 Hot Climate

It is in hot climate conditions, the thermal performance versatility of high mass walls

becomes apparent. In these environments, daily outside air temperatures may not fall below

40 degrees Celsius and can rise above 50 degrees Celsius regularly. Hot outdoor airtemperatures and direct sunlight heat the outer layers through the day while night time



temperatures may only fall to 30 degrees Celsius. In this situation, a walls thermal mass and

insulation layer provide a valuable thermal delay, or thermal lag, for the interior spaces.

When exterior temperatures increase during the day, the interior mass of a sandwich wall

heats up slowly. It may take as long as 6 hours or more of constant exterior heating for the

interior face of the mass wall to reach temperatures above comfort levels. That time

differential translates into time the building is cooling less or a cooling load lag time called

the Thermal Time Constant or TTC. A larger TTC indicates a longer delay for heat

transmission through a wall assembly. It is important to cool the interior mass at night

quickly to dissipate any radiant heat from the interior mass of the panel, and to allow the

largest lag time possible the next day.

Even in hot climates a cooler winter season is normal. Sandwich walls will benefit a

buildings efficiency during these seasonal periods by storing heat during the day and slowly

releasing it at night to help provide a constant, comfortable interior temperature with minimal

mechanical system aid.

For these reasons, sandwich walls in hot climates can provide a versatile and flexible

solution to summer cooling inefficiencies and still provide efficient winter insulation.

Special consideration should be given to the design and construction of Precast Insulated

Sandwich Panels in hot climates. High temperatures and extreme climate conditions can

require usual sandwich wall construction and design techniques to be refined in an effort to

create panels and buildings suited to perform at the highest level. The areas highlighted in

figure 1 are regions where heating degree days totals are minimal or non-existent for ambient

interior temperature buildings. Energy consumption used by a buildings heating and cooling

systems is almost entirely from cooling requirements in these areas.

A well-sealed envelope is very important in these climates as any hot air infiltration into a

buildings interior can drive cooling costs upward. Air infiltration can cause condensation on

sandwich panel faces due to interior and exterior differences in humidity and temperature. Awall with properly sealed joints and well-designed panel configurations will perform without

any air infiltration or condensation issues. See section 4 for detail information and

suggestions in sandwich wall designing.

The durability of concrete as a building material coupled with the insulative and thermal

mass performance of sandwich wall panels make them an ideal choice for projects in hot

climates.



Figure 3.3: Hot climate zones according to the Kppen classification system.Af = Tropical Rainforest-No dry Season,

Am = Tropical Monsoonal-Short Dry Season

BWh = Subtropical Arid Desert-Low Latitude Desert,

BSh = Subtropical Arid Steppe.Hot

(Images from the Updated world map of the Kppen-Geiger climate classification

3.3 Insulation materials

Various thermal insulation materials have different resistance against heat, fire, sound,

compression and moisture. The thermal capacity is typically designated as design or

nominated as a K value.

Some commonly used insulation materials include mineral wool, such as rockwool and

glasswool, expanded or extruded polystyrene (EPS, XPS) and polyurethane (PUR, PIR).

However, the types of insulation are constantly changing and developing with various

vacuum systems and gel materials currently under development.



Fig 3.4. Sandwich panels with polyurethane

insulation (is this correct rockwool?).Fig 3.5. Sandwich panels with mineral-wool

insulation.

Fig 3.6. Sandwich panels with EPS- graphite

insulation.

Fig 3.7. Sandwich panels with EPS insulation.

Table 3.1. Measured properties of different insulating materials. Compression strength is measured

with 10% deformation /1/.

Insulation

type

Thermal

conductivity

Compression

strength

Moisture

penetration

Fire safety

classification

Max.

temperature

designW/mK kPa x10- kg/msPa C

Rockwool 0,034 - 0,041 5 - 30 150 A1 1000

Glasswool 0,033 - 0,039 10 - 30 150 A1 or A2-s1d0 200 - 600

EPS 0,030 - 0,040 60 - 100 3 - 7 D or E 80 - 110

XPS 0,030 - 0,037 150 - 250 1,5 E 70 - 100

PUR 0,023 - 0,027 100 - 250 0,1 - 1,2 D or E 100 - 250



Fig 3.8. New vacuum insulation type with aluminium laminate surface

A relatively new opportunity for sandwich panels is the possibility to make use of vacuum

insulation panels (VIP) for the insulation layer. With integrated vacuum insulation, concrete wall

elements having an insulation thickness of 150mm can reach U values of 0.06 W/m2K.

Alternatively, to reach the same U value with EPS insulation (l = 0.03), a 500mm insulation layer

would be needed. (Check)

However, vacuum insulation panels quite fragile, and are to be handled very carefully to ensure thatthe internal vacuum is not compromised. As such, special attention has to be paid for the anchors

connecting inner and outer concrete layers. Notwithstanding, solutions to integrate VIPs into precast

sandwich panels already exist. The below figures (Fig. 3.9/3.10) show some examples of how VIPs

can be integrated into precast insulated sandwich panels.

Fig. 3.9 Integration of vacuum insulation panels into sandwich wall elements (Variotec)


D r a f t : O c t o b e r 2 0 1 2

Fig. 3.10 Building with vacuum i

3.4 Panel connectors

The primary purpose of the

connectors can also create parti

previously noted. Within the san

with vertical loads need to be

deformations also need to be takthe stiffness.

Connectors are typically ma

fibre reinforced polymer (FRP)

spacing, with additional connect

Every connector type has its

3.8 has a tensile capacity in the

This results in an approximate ca

Update figures to latest (from Cha

Fig 3.11. A steel truss connector

Peikko).

Fig 3.12. A stainless steel tie (co

sulated sandwich wall panels (Variotec)

connectors is to tie the outer layer to t

l or total composite action between the out

wich structure forces from wind and other

considered. Restraints from temperature

n into account, with the forces within the co

ufactured out of steel, stainless steel, fibre

composites. They are commonly located at

rs provided around openings within the pane

wn load bearing capacity. For Example: th

inclined direction approximately 5.6 kN for

pacity of 4kN per bar in the vertical directio

ter 4)

(Courtesy of

rtesy of Halfen-Deha)

29 |P a g e

e inner layer. These

er and inner layers as

orizontal loads, along

and other long term

nnectors dependent on

glass, carbon fibre or

between 600-900mm

l.

PD-Connector in fig.

a 5 mm diagonal bar.

.


D r a f t : O c t o b e r 2 0 1 2

Fig 3.13.A stainless steel tie (co

Fig 3.14A fibre polymer tie (cou

some new connector types add

3.5 Thermal performanU-value

The thermal performance, or

[m2K/W], thickness (d) and ther

1-dimensional calculation

U = 1/ (R1+R2+R3+Ro+Ri

Where:

R1= d1/1,

Ro+Ri = thermal resistance of

rtesy of SemtuOy, Finland).

tesy of Thermomass, USA)

d as a picture

e

U-Value, for the structure is calculated by

al conductivity ()[W/mK] of the separate l

)

the outer and inner surfaces

30 |P a g e

thermal resistance (R)

ayers:



2- dimensional calculation

U= q/ T

where

q = density of heat stream [W/m2]

T= temperature difference over the structure [C].

design values are nationally certified thermal conductivity values. The following table shows

some calculated U-Values for Precast Insulated Sandwich Panels.

Table 3.2. Example of Precast Insulated Sandwich Panels for U-values 0,24, 0,17, 0,14 and 0,09

W/m2

K. Connectors are 600 mm c/c steel trusses or approx. 750 mm c/c pin ties. Inner and outerconcrete layers are 80 mm thick /4/.

Insulation designW/mK

U= 0,24 U=0,17 U= 0,14 U= 0,09

Thickness of insulation (mm)

Mineral wool 0,037 160 230 280 430

EPS 0,036 150 220 260 410

0,031 130 190 230 360

XPS 0,037 160 220 270 420

PUR/ PIR 0,026 110 160 190 300

0,023 100 140 170 270

Include Vacuum Sealed Insulation?

Thermal mass

When detailing energy calculations for the whole building, U-values and the air tightness of the

wall, roof and floor are critical. However it is also important that the thermal mass of the concrete

be used to absorb solar gains. This will reduce the need for heating or cooling energy and balance

the inside temperature.

The thermal mass should be located on the interior of the wall panel and the thermal insulation

layer should be located outside this thermal mass. Except in very hot climates, the internal surfaces

of heavyweight walls, floors and ceilings should be left exposed where possible to aid heat

absorption. Internal finishes such as plasterboard and carpet, will to some extent, act as a barrier to

the heat flow.

In climates where outdoor air temperatures continually exceed 50 degrees Celsius during the

day, it may be advisable to minimize heat absorption into building components. In these hot

environments heat radiating from interior masses can significantly increase building cooling loads.

Thermal bridges

Thermal bridges, or cold bridges, are weaknesses in the thermal insulation layer. These are

caused by a connection or bridge between the inner and outer layer, and may be caused by items

such as the layer connecters, lifting hooks and protruding steel ties for connections for balcony or



foundations. Thermal bridges can also be caused when the concrete is cast as a solid section through

the insulation layer.

Thermal bridges may significantly affect the overall U-value of a sandwich wall panel,

especially in situations where significant differences are present between exterior and interior

temperatures. Special detailing eliminating the use of thermal bridges in sandwich wall panels is

preferable. Several examples of these specialized details are shown in chapter 4.

Fig 3.15. Lifting hooks can form a thermal bridge in a sandwich panel. Protruding steel parts

should be cut after assembly.

Fig 3.16. Example of simulated thermal imaging show how ties act as thermal bridges. The lighter

blue and yellow colour shows higher temperature in the outer panel surface /1/.

In situations in which conductive ties are used, such as steel ties, trusses, pins, etc that are

commonly used throughout Europe, they cause a thermal bridge between the two concrete wythes.

It is estimated that this thermal bridge will negatively influence the U-value of the component by

between 0.005 and 0.015 W/m2K. Sandwich wall ties designed to minimize thermal bridging are

available through specialty product manufacturers. These non-conductive connectors are usuallycomposed of vinylester resins reinforced with glass or carbon fibre. These connectors provide both

a low thermal conductivity and the required structural strength to connect exterior and interior

wythes of concrete.



Table 3.3. Additional point conductance of one steel tie in a sandwich panel with structure:

inner layer 80 mm, insulation EPS 200 mm (design= 0,031W/mK), outer layer 70 mm.

(what is Uf? Should this just be U-Value) Can we include 8 to 10mm)

What is the U-Value for solid Concrete.

Diameter of a tie

(mm)

4 5 6 7

Uf (W/m2K) 0,0008 0,0012 0,0018 0,0032

Fig 3.17. Example of heat flow rate around a steel tie. In this model the internal temperature is

+21C and the outside temperature is -15C, a difference of 36C. /1/.

Jason has additional information availableThe following details and tables show by comparison the detrimental effects highly conductive

wythe connectors and solid panel sections can have on sandwich panel R or U-values. The first

example is a fully insulated panel, note the high R-value and low U-value in the table to the right.

These values drop significantly in samples two (panel with steel connectors) and three (panel with

steel connectors and solid border) as analysed.



Fig 3.18. Examples showing the adverse effects steel connectors and solid sections can have on

sandwich wall panel performance. (Images and tables from ACI committee 122R-02 report: Guide

to Thermal Properties of Concrete and Masonry Systems, June 21, 2002)

Larbi to review if a metric version is available -

Should these values be converted to metric units (or both) ?

Creating an Air Tight Building (Leakage)

Concrete is a very dense, air tight material. When constructing a precast structure the sealing of

the panel joints is critical to ensure the structures air tightness. Within the structure the most

important connections are the wall to window, wall to wall, wall to roof and wall to foundation.

To ensure an air tight building, all the joints should be sealed with an air tight material. Thismaterial can be concrete grout, an insulation material, such as PUR foam or an elastic caulking

material.



Fig 3.19. Typical detail showing a Wall-

Window connection. (Change text to English)Fig 3.20. A typical corner connection between

a loadbearing and non-loadbearing sandwich

panel.



The leakage or air tightness of a building envelope is normally measured by pressure

testing (blower door test). A typical test involves pressurising the building to 50 Pascal and

measuring the air leakage value. With concrete sandwich wall panels in multi-storey

residential buildings air-leakage values like 0,5 1/hour can be achieved. This figure (?)

indicates how many times the internal air of the space changes in 1 hour.

Also q50 ( m3/h, m2) value maybe used. This figure indicates the air leakage through the

envelope in an hour and per m2. Where is the figure?

Blower Door Test

The Blower Door Test is used to evaluate the airtightness of the building envelope of small to

medium size buildings. The blower door test uses a special temporary door panel that has an

integrated calibrated fan. This test is able to monitor and measure the differences in the

pressure that the fan generates from the inside to the outside of the building. The tighter a

building, the less fan power is needed inside the building to generate a nominated pressuredifference to the outside.

Fig 3.21 Photo of a typical Blower Door Test

While all interior doors should be opened, all exterior openings have to be closed, even

fireplaces or mechanical exhaust outlets. Mostly, the blower door is used for depressurization

testing of the inside of the building. If the tightness is not sufficient, artificial smoke can be

used inside the building to detect the leaks.



Energy Pass

In Germany, Energy Passes have been developed to enable people, who are not technically

trained, to evaluate and compare the energy efficiency of different buildings. This system has

been in use since 1995, and is obligatory for all new buildings. Since 2007 they are also

mandatory for all existing buildings. These Energy Passes are valid for 10 years but can berenewed earlier, such as for cases in which modernization of the building may lead to better

results in the Energy Pass.

Fig 3.22 An example of an Energy Pass from Germany.

The bottom right of the Energy pass provides values for comparison, these values enable

everybody to evaluate the efficiency of the nominated building. The total energy

consumption of a building is marked by the arrow on top of the colour bar. The building

evaluated with the Energy Pass in Fig. 3.22 is a state-of-the art for condominium buildings in

Germany in 2012 and ranges at 62 kWh/m2a. While, an average building in Germany, that

has not been modernized, has a total energy consumption of about 250 kWh/m2a.

3.6 Condensation considerations



Historically, very little attention was given to the formation of condensation within a

building except for situations, such as Cold Stores. However this situation has changed as

buildings become better sealed and building moisture analysis is often required

Condensation is the process in which water vapour becomes a liquid. Within a building

this can occur either on the surface of a wall or within the elements of the wall structure.

Precast sandwich panels can effectively eliminate both these forms of condensation.

The most common design method to evaluate this performance is the dew-point method.

The dew point temperature corresponds to 100 % relative humidity for a given absolute

humidity at constant pressure. This method compares the moisture vapour pressures within

the building element with results detailed as either vapour pressures or more commonly as

temperatures through the building element. Should the actual temperature drop below the

dew point temperature, the air is cooled to a point in which it cannot hold any more water

vapour, at this point condensation may occur.

A recent European study (Can we reference this?) investigated how the thickness of theinsulation layer will affect the hydrotechnical performance of a sandwich panel. The results

concluded that an increase in the insulation thickness will not significantly affect the

hydrotechnical performance of the concrete sandwich structure. Whereas, changing the

insulation type had more of an impact on its performance.

When mineral wool or an open cell insulation was used within the sandwich structure the

insulation is able to provide a path to both the internal and external wythe. However, when

using more air tight materials, or closed cell materials, such as EPS or PUR, the insulation

acts as a vapour barrier.

This needs to be taken into consideration at the surface coating stage, particularly if an air

tight coating is to be used.

For all faades the most critical source of moisture is wind driven rain. This can keep the

outer concrete layer of a sandwich panel humid all the year round.

Larbi to review Dew Point calculation

Time in hours

Watercontent

(kg/m3)

P! 240

"P# 180

"P# 240



Fig 3.16. Moisture content of the inner layer with different thermal insulations. The

calculation period was 5 years. In the beginning humidity of the inner layer was 90 %RH

(100 kg/m3) (better image?)

/5/.

3.7 Acoustic Performance

The acoustic performance or sound insulation properties of external walls are important,

particularly when external traffic noise is significant. All precast insulated sandwich panels

offer very good sound insulation properties, however, using insulation materials, such as

mineral wool, the sound insulation is somewhat better than with the more dense insulation

materials like, EPS or PUR, refer to the table 3.4. below.

Need to review impact sound?

Table 3.4. Sound insulation of some wall structures against traffic noise /6/.

Inner concrete

layer

Thermal insulation Outer layer

(concrete,

plaster)

Rw

(dB)

Rw,Ctr against traffic

noise (dB)

100 mm

150 mm

200 mm

nil nil 50

57

61

46

52

57

80 mm

150 mm

240 mm mineral wool 70 mm 54

60

50

56

150 mm 240 mm mineral wool 25 mm 58 53

150 mm 240 mm EPS 10 mm25 mm 5254 4445

Window

MSE, 3 glass

frame 170 mm

frame 210 mm

46

47

40

42

As with thermal insulation, it is the sealing of the joints that is critical to ensure excellent

acoustic performance. Noise paths created through gaps and around edges of building

elements are commonly referred to as flanking. Properly sealed junctions between building

elements result in a significantly reduced occurrence of this flanking phenomena. This

reduction in the transfer of sound from space to space is important in multi storey residential

buildings.


D r a f t : O c t o b e r 2 0 1 2

4 Structural de

4.1 Introduction

Precast insulated sandwich

and design methods and detaiexpanding appeal for this pro

Most precast concrete prod

of a sandwich panel, and cons

typical precast concrete eleme

required to the relative interac

In order to start the design

the makeup of the overall pan

below:

Look at using in introduc

4.5.6.

ign and detailing

panels have been used throughout the world

s have been developed over that time to keeuct as mentioned earlier.

ucts used for wall construction can also be i

equently the design of a sandwich panel is si

nts, both prestressed and reinforced, but wit

tion of the components of the sandwich pane

of a precast insulated sandwich panel one m

l. A diagram of the possible component par

tion

Figure 4.1.1

1.2.3.

40 |P a g e

for over fifty years

pace with the

corporated as part

milar to that of

careful attention

l.

st firstly consider

s are illustrated

1. Inner Wythe

2. Vapour Barrier

(optional)

3. Insulation Layer

4. Air Cavity Former

(optional)

5. Outer Wythe

6. Special External

Finish (optional)



A Precast Insulated Sandwich Panel may consist of up to six layers as shown in Figure

4.1.1, but often has only three layers:

Layer 1 The rear face or inner wythe, generally the concrete loadbearing element.

Layer 2 A vapour barrier separating the inner wythe from the insulation (Optional

location to be specified).

Layer 3 The insulation layer.

Layer 4 An air cavity (Optional).

Layer 5 The front face or outer wythe, generally the concrete non-loadbearing

element. Can be cementitious render material in place of a concrete layer.

Layer 6 The front face cladding of the outer wythe, .Can be materials such as stone,

brick or terracotta. Various external rendering systems are also a possibility

in residential applications. (Optional)

A sandwich panel would always include Layers 1, 3 and 5. Use of Layers 2, 4 and 6 are

dependent on the application and specification. The internal and external concrete wythes

(Layers 1 and 5) are joined by structural connectors that are normally proprietary systems

manufactured from materials such as steel, stainless steel, fibreglass, carbon fibre or fibre

reinforced polymer (FRP) composites.

sandwich panels have been popular in Northern Europe and North America for many

years, but their appeal has widened due to increased demand for energy conservation and

prefabrication. National requirements for reductions in energy loss through the building

fabric (U Values) has generally involved increasing overall panel thickness. U-values in

excess of 1.0 W/mK forty years ago had been reduced to 0.4 by the mid nineties and

requirements are now between 0.30 and 0.15 for many countries. These changes have

typically resulted in greater thicknesses of insulation with resulting increases in design forces

acting on the concrete wythes. While thermal bridging of the insulation layer may have been

ignored, many years ago, it now must be considered as part of the structural design process,

particularly in the choice of material for the wythe structural connectors.

There have been comparisons between different types of insulation and U value

requirements in Chapter 3. If we take an example of an internal wythe of 150mm thickness

paired with an external wythe of 65mm and consider expanded polystyrene (EPS) as the

insulation layer to achieve a U-value of 0.27; a thickness of 100mm is required. However to

achieve a U value of 0.10 an insulation layer thickness of 335mm would be required.

Therefore, due to the additional insulation layer thickness the eccentricity of the vertical loadin the outer wythe relative to the inner wythe has increased by 235%. It can be appreciated

that in instances where low U values are required then the designer has to consider at an early

stage whether to use a stronger connector system between the wythes to resist the eccentric

forces or a thinner more thermally efficient (and probably more expensive) insulation to

reduce these eccentric effects.



4.2 General Rules

Throughout the world the design of structural concrete is now generally based on limit

state theory. The ultimate limit state of collapse (ULS) has to be considered in all designs,

and sandwich panels are no different to all other structural concrete members in this regard.

Serviceability limit states (SLS) vary between elements depending on the application. Forsandwich panels special attention has to be paid to thermal effects and in particular the

differential temperature relationship between the constant environment of the inner wythe

and the climatic fluctuations of the outer wythe. Panel sizes, layer makeup and chosen

connection system are typically governed by these service effects.

There are many national building codes that can be used for the structural design of a

sandwich panel. Example calculations have been prepared in the Appendix to this document

that use a common design case and provide solutions to European, North American and

Australian building codes.

4.3 Structural Behaviour

4.3.1 General

Precast sandwich panels can generally be categorised as either composite or non

composite members as defined in Chapter 2. There may also be intermediate categories of

semi or partially composite elements. For the purposes of describing structural behaviour,



composite and non composite cases shall be considered; these descriptions can be applied to

the partially composite case depending on the degree of composite action that can be verified.

Figure 4.3.1 shows typical flexural stress and strain distributions for non composite

sections (a) and composite sections (b). There is no shear transfer across the insulation in (a),

but in (b) shear transfer is required to distribute both stress and strain across the section.

Types of connectors are indicated in this example that would be suitable for composite and

non composite sections. There are, however, many types of connectors available. These are

covered more extensively later in this Chapter. It is often the connector type and arrangement

chosen that governs the maximum panel size.

Figure 4.3.1 label composite and non-composite

It can be seen that the capability to provide composite action depends on the capacity of

the connectors to transfer shear across the insulation. The connectors are therefore often

categorised as either shear or non shear connectors. A composite panel would predominantlyhave shear connectors whilst non composite panel would have mainly non shear connectors.

However, the self weight of the non loadbearing wythe of a non composite panel would still

require shear connectors for transfer to the structural wythe. The connectors transferring self

weight shear in the non composite case are ideally located at the centroid of the sandwich

panel or along orthogonal axes through the centroid to minimise thermal restraint and

development of eccentric moments.

4.3.2 Non Composite Precast Insulated Sandwich Panels

The design of a non composite sandwich panel is similar to a solid panel if only a single

wythe is considered structural and the other wythe is non structural. This simplifies the

design approach and is often the case for non composite sandwich panels where the external

wythe is made as thin as possible to minimise the self weight of the panel carried by the

connectors. The limitations on external wythe thickness are:

1. Achievement of reinforcement covers to satisfy durability and fire requirements.2. Sufficient thickness for anchorage of wythe connectors.3. Thickness based on the maximum aggregate size.4. Architectural requirements.5. Accommodation of false joints.



Subject to architectural constraints the preferred minimum thickness of the external wythe

is typically 65mm. Some panels are produced with an external wythe of as little as 50mm but

careful attention to detail is required to satisfy the above parameters.

The thickness of the structural wythe is determined by structural analysis and the need to

satisfy both structural and architectural details. Similar limitations as applied to the non

structural wythe also apply. Minimising the thickness of the structural wythe is often not

economic as the amount of reinforcement required to limit deflection and cracking becomes

excessive. Also, where the structural wythe supports floors the reinforcement connection and

bearing details may dictate the inner wythe thickness as in Figure 4.3.2 :

Figure 4.3.2(Should it be HC unit or PC Deck Slab?)

In applications where there is a structural internal wythe and non structural external wytheand the internal wythe supports the floor at each level the internal wythe thickness is typically



between 150 and 200mm subject to the limitations described earlier. Where the sandwich

panel is non loadbearing then the inner wythe thickness is typically 150mm thick, but can be

reduced provided design requirements are satisfied. In countries, such as Finland, inner

wythes of 80mm thickness have been used.

In flexural situations where the non composite panel has two structural wythes the loads

can be distributed between them based on their relative structural stiffness and then each

wythe individually designed as a solid panel. Deflections are calculated based on the sum of

the wythe stiffnesses. If wythe thicknesses are typically as suggested earlier then the

contribution from the outer wythe is relatively small and is often ignored, for example:

Example 4.3.2

Outer Wythe

Insulation

Inner Wythe

65 100 150

315

PISP thickness

Outer wythe thickness, layer 1 = 65mm

Inner wythe thickness, layer 3 = 150mm

Insulation thickness, layer 2 = 100mm

Total panel thickness = 315mm

Second moment of area, layer 1, I= 100*65/12 = 2.289 x 106mm/m

Second moment of area, layer 3, I= 100*15/12 = 28.125 x 106mm/m

Proportion of load transferred to layer 1 = I/ (I+I) = 0.075

Sum of wythe stiffnesses, I+I= 30.414 x 106mm/m = 1.08 I

4.3.3 Composite Precast Insulated Sandwich Panels

Composite sandwich panels are popular in North America and are typically used in single

storey buildings with large headroom requirements, such as warehouses and production

facilities. The full section properties of the sandwich panel have to be mobilised to resist

slenderness effects that would develop if treated as non composite spanning vertically

between ground and roof. The use of composite sandwich panels in Europe is limited.



Figure 4.3.3BETTER PHOTO TO BE PROVIDED?

Composite sandwich panels are those where inner and outer wythes are interconnected

through the insulation by rigid ties or regions of solid concrete that restrict relative movement

between the wythes. However, as energy conservation regulations are tightened and U values

for the building fabric are reduced, then the use of structural concrete thermal bridges through

the insulation layer of sandwich panels will be limited. In North America composite

sandwich panels are often prestressed to further reduce section thicknesses and control

bowing.Because many composite sandwich panels are tall narrow elements spanning a single

storey, flexure tends to dominate axial loading. Flexural design of a composite sandwich

panel is similar to that of solid panels that have the same cross sectional thickness but with

the following differences:

1. A system for shear transfer between the wythes has to be provided and analysed.2. Sandwich panel section properties have to take account of individual wythe

thicknesses, the location of the composite centroid and the lack of concrete between

the wythes.

The transferred shear force for connector design is calculated from the flexural tensile

force and distributed between the points of zero and maximum moment.



Figure 4.3.4 below shows how composite section properties are derived:

A = b (t + t)

c = [ *bt + b t(h t) ]/A

I = bt/12 + bty + b t/12 + b ty

S = I/c, S = I/c

r = [I/A]

Figure 4.3.4

If we take our earlier example 4.3.2:

t = 65mm, t= 150mm, h = 315mm, b = 1000mm

A = 2150cm, c = 17.7cm c= 13.8cm y = 14.5cm y

= 6.3cm

I = 226,612cm

S = 12,803cm

S= 16,421cm

r = 10.3cm

If we compare the above section properties with a non composite sandwich panel with a

single structural wythe then the wythe thickness required for equivalent stiffness is 301mm.

The sandwich panels thickness would then become 466mm, i.e. an increase of 151mm in theoverall panel thickness. The benefits of composite action for these applications can be seen.



ADD PHOTO OF COMPOSITE PANEL SHOWING CONNECTORS.

THERMAL BOWING Figure by Jason

A recognised characteristic of composite sandwich panels is the tendency of longer panels

to bow outwards under prolonged exposure to heat. Bowing is a deflection caused by

differential wythe shrinkage, thermal gradients through the sandwich panels section,

differential modulus of elasticity between the wythes and creep from horizontal storage of the

sandwich panels in a deflected profile. These actions cause one wythe to lengthen or shorten

relative to the other. Compared to non composite sandwich panels the bowing of composite

panels is more pronounced due to the structural rigidity of the wythe connectors required for

flexural strength and shear transfer.

Accurate estimation of the amount of bowing is difficult due to the following reasons:

Variance in shrinkage, creep and elastic modulus of the concrete.

Prediction of thermal gradients and their shape through the sandwich panels.

The degree of restraint provided by external connections. Difficulties in developing precise mathematical models for each of items 1, 2 and 3.

Whilst there are some difficulties, the amount of bow can be accommodated in the design

in a similar manner to the methods used to control precamber in prestressed beams and slabs.

It is important to understand that bowing will occur and to establish a reasonable value for its

magnitude, which may often be based on experience. Consideration must be given to external

connections so that distress is not experienced from forces that may arise.

Some useful observations have been made in relation to bowing of composite sandwich

panels, and should be considered:

1. Sandwich panels generally bow outwards.2. Panels heated by the afternoon sun will bow more than those that are not.3. Sandwich panels bow daily due to transient thermal gradients.4. Sandwich panels experience a greater thermal gradient than equivalent solid panels

due to their superior insulation properties.

5. Panels stored with a deflected horizontal profile will remain with that profile afterinstallation.

6. Differential shrinkage can occur between the wythes due to relative humiditydifferences between external and internal exposures.

7. Sandwich panels with wythes of different elastic moduli due to different concretestrengths but with equal levels of prestress may bow due to differential shortening and

creep after prestress transfer.

In addition to individual panel effects consideration must be given to differential bowing

between adjacent panels. This can arise because of differences in structural stiffness, different

connection levels or lengths, different shrinkage or creep characteristics due to being cast at

different times, different storage conditions and particularly at corners where the adjoining

panels will be bowed about orthogonal planes.

In comparison to composite sandwich panels there is a lower tendency for a non

composite panels to develop thermal bowing. The extent of thermal bowing is chiefly

influenced by; panel size, the rigidity of the wythe connectors, the degree of composite action

and daily temperature variations on the external face of the panel. In a non compositesandwich panel the internal wythe is typically kept at a constant temperature in a controlled



building environment, whilst the external wythe experiences extremes of temperature the

thermal gradient through its section is minimal. The external wythe is free to expand and

contract with variations in temperature.

In order to further minimise the effects of bowing bondbreakers are often used at the

interface of the insulation and the internal wythe, polyethylene sheeting can also act as a

vapour barrier as well as bondbreaker at this location. In addition, air gaps between the

insulation and the outer wythe virtually eliminate bond between the wythes and allow for

ventilation of the outer wythe, control of water ingress and pressure equalisation.

4.4 Precast Insulated Sandwich Panel Applications

Sandwich panels can be either loadbearing or non loadbearing members, and have a range

of composite behaviour, as shown in Figure 4.4.1:

Figure 4.4.1 Where PISP = Precast Insulated Sandwich Panel

Loadbearing members can have the full range of composite actions from fully composite

to non composite, and non loadbearing members can have a similar range.

Loadbearing sandwich panels support loads from other parts of the structure, typically

floors and roofs:



Figure 4.4.2

Installation of Loadbearing Sandwich Panels(Maybe better Photo?)

Loadbearing non composite panels are frequently used for multi storey buildings withstorey

heights typically between 3.0 and 3.5m. The connection detail with the floor often dictates

the thickness of the inner wythe and consequently the panel design is not governed by flexurebut by its axial capacity. Provided the effective height of the panel between floor restraints is

less than about fifteen times the panel thickness for reinforced elements then slenderness

effects due to axial load are minimal. The outer wythe is generally non structural and is

supported at each level by the inner wythe.

An example of this type of application is at Old Hall Street, Liverpool, where a tower of

twenty seven storeys, 95m high, was constructed in 2003. The external wythe of the

Sandwich Panels incorporated white architectural concrete:

Figure 4.4.3

Old Hall Street, Liverpool



Multi storey residential buildings are ideally suited for sandwich panels, particularly

where cellular construction is a prerequisite, such as in apartment blocks and hotel

construction. Grey precast concrete internal walling systems can be used in conjunction with

sandwich panels, where these panels may have an architectural treatment.

Figure 4.4.4

Internal Grey Walling System Ideal for Use with Architectural Sandwich Panels

Various architectural treatments are possible to the outer wythe. Air cavities may be

considered when using non concrete or architectural concrete finishes as below in Figure

4.4.5:

Figure 4.4.5 Close up image of the architectural off-white external layer, an air gap

former, the layer of insulation and the internal structural layer

Non loadbearing sandwich panels typically provide the external building shell and can be

used with all types of supporting structural framing systems; precast and insitu concrete, and

structural steelwork.

4.5 Wythe Design



4.5.1 General

Wythe section thickness and reinforcement/prestress requirements are based on structural

analysis to resist the various design actions, finish, corrosion protection, handling

considerations and past experience.

Section 4.3 of this chapter examined the structural behaviour of composite and noncomposite sandwich panels. This section reviews external actions applied to panels, how they

affect loadbearing and non loadbearing elements and the corresponding design approach.

4.5.2 Loadbearing Precast Insulated Sandwich Panels

Actions to be considered in structural design include:

Self weight.

Dead and imposed loads from supported parts of the structure, particularly roofand floor loads, and supported sandwich panels above.

Wind

Lateral earth pressure Local Climate

Differential shrinkage between wythes.

Accidental

Seismic

Demoulding, handling and installation.

Prestressing that may be used for transport and handling

Loadbearing elements in multi storey residential and commercial construction, such as

apartments, hotels, offices and some retail developments, have effective heights that are

governed by the storey height of the building. Because the slenderness ratio is generallyrelatively low then there are no major benefits in prestressing the panels as slenderness

effects and flexure are not critical, unless suited to the producers plant. The panels are

normally

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Fib Sandwich Panels (Draft Oct 2012 No Track Changes)