kevin dare - why do floors fail part 1

Why do floors go wrong and what are we going to do about it ?

Kevin DareManaging Director

Face Consultants

UK

• Mesh reinforced – 1 layer mesh, bottom (50mm)• Jointless Steel fibre (35 Kg) – Ground supported• Jointless Steel Fibre (45 Kg) – Pile supported

• All on 1 layer 300 micron slip membrane• On 150 to 250 road base +0 / - 15mm• 35 – 40 MPa cube concrete (28 – 33 cylinder)

Mainland Europe

• Jointed Steel fibre (25 Kg) – Ground supported• Jointless Steel Fibre (35 Kg) – Ground supported

• On 2 layers 300 micron slip membrane• On 150 to 250 road base +0 / - 15mm• 28 MPa Concrete (cylinder)• Dry shake topping

Middle East

• Jointed mesh, two layers – Ground supported• Jointless Steel Fibre (35 Kg) – Ground supported

• All on 1 layer 300 micron slip membrane• On 150 to 250 road base +0 / - 15mm• 35 – 40 MPa cube concrete (28 – 33 cylinder)

South Africa

• Jointed (approx 3 to 4 metres) No reinforcement.• Rough sub-base• No slip membrane• Thicker slabs

• Jointless Steel Fibre (35 Kg) – Ground supported

• 1 layer 300 micron slip membrane (Jointless)• On 150 to 250 road base +0 / - 15mm• 35 – 40 MPa cube concrete (28 – 33 cylinder)

Asia

• A real mixture.• Invariably poor ground leads to traditional reinforced slab

on piles.• Lack of specialist flooring contractors. Supervised labour• Multi level warehouses with topping slabs.• Mesh reinforced – 1 layer mesh, bottom (50mm)• Jointless Steel Fibre (45 Kg) – Pile supported

• 35 – 40 MPa cube concrete (28 – 33 cylinder)

Australia

• Mesh reinforced – 1 layer mesh, top (50mm)• Jointed steel fibres (20Kg) – Ground supported• Jointless Steel fibre (30 Kg) – Ground supported• PT – Ground supported

• All on 1 layer 200 micron slip membrane• On sand blinding • 40 MPa concrete (cylinder)

Why do floors fail ?

• We give too much importance on designing for imposed loads and not how the floor will be used or change with time. (race to the bottom)

• We fail to grasp that floors crack not because of drying shrinkage but restraint to drying shrinkage.

• We fail to understand the requirements or the impact of MHE that operate on the floor.

• We are driven to reduce price and blindly accept proprietary designs by material suppliers

Why do floors fail ?

• A floor is the table top on which the tenant runs his business.

What goes wrong ?

• Geotechnical reporting (Deep Layer)• Top layer ground or Sub Grade• Sub base / sand blinding• Mesh in the top• Steel fibre (jointed)• Steel fibre (jointless)• Un planned cracking should not be the norm.• Design methods• Concrete strength (shrinkage)• Detailing

Ground (Deep Layer)

• Geotechnical reports are generally carried out.

• Settlement modeling is rarely carried out or considered.

Ground (Deep Layer)

• We do not need this information to determine the slab thickness. Only knowledge of the top metre or so of the ground (plate tests).

• As racking gets higher and materials handling equipment becomes more sophisticated. Settlement and more importantly differential settlement becomes essential.

Ground (Deep Layer)

Ground (Deep Layer)

Top Layer Ground

• Sub Grade• Plate tests• Determines slab thickness

Top Layer Ground

Heave

Top Layer Ground

• Expansive clays• Moisture content• Heave

Sub Base / Sand Blinding

A sub base should:

1. Transmit the floor load to the sub grade thus improving the quality of support from soil under.

2. Provide a level formation

3. To provide a firm working platform for construction (without rutting)

Sub Base / Sand Blinding

Sub Base

Road base graded material, well compacted

Should not rut under load

+0 / -15mm

Sub Base / Sand BlindingSand Blinding:

Moves

Ruts under load

Causes restraint

Variable slab depths

Sub Base / Sand Blinding

Mesh in the top

• Has no structural benefit• Maintaining cover • Bends down dowel sleeves• Risk of being saw cut

Mesh in the top

Mesh in the top

Steel Fibre (Jointed)

Method not used in the UK and is not recommended in TR34 because of issues of dominant joints and loss of load transfer

Steel Fibre (Jointless)

Benefits:• No induced sawn joints.

Disadvantages:• Wider joint openings• Increased risk of cracking• Panel sizes – aspect ratio• Wire guidance issues• Fibres at the surface

Steel Fibre (Jointless)

Recommendations:• 1000 sq metre panels• Aspect 1:1.2 max• Min 35Kg steel fibres• 35 MPa concrete max.• 300 micron slip membrane• No sand blinding• Good isolation• As long as possible before loading• No wire guidance

Un planned cracking should not be the norm.

Design Methods

There are two parts to a floor design:

1. Slab thickness and reinforcement.

2. Detail design:– understanding how the floor will shrink– Understanding the performance requirements of the MHE that

will operate on it.

Design Methods

• TR34 unravels proprietary designs issued by material suppliers and makes it easier for design engineers to do to their job in checking them.

Detailing

Shrinkage

Detailing

Shrinkage

Detailing

Shrinkage

Detailing

Shrinkage

Detailing

Shrinkage

Detailing

Restraint:• Sub-base• Reinforcement• Columns• Services• Door openings• Joints• Racking too early

Concrete Strength

Reduce shrinkage:• Reduce strength. 40 MPa not required for abrasion• Reduce cement• Reduce water• Sands and aggregates???• Water reducing admixtures.

What do we do about it ?

Thank you!

Kevin DareFace Consultants