Concrete Foundation Master Class

Christchurch -Dene Cook (CPEng)Auckland – Dene Cook (CPEng)

Residential Concrete Foundations

Topics-1.Why construct in concrete2.Components of a concrete foundation3.Performance requirements from NZ Building Code4.Ground conditions5.Acceptable Solutions (NZS3604)6.Alternative Solutions7.Specific Design (Liquefaction)8.Decorative concrete9.Crack control and good trade practice10.On site quality control11.Questions

Why concrete?

• Cost effective• Durable• Low height transition between indoor and outdoor• Colour and finish flexibility• Thermal mass• Speed of construction• Quite (no creaking)• Solid- foot traffic not cause vibration• Insulation

Components of a concrete floor

Global Ground Conditions• Peat• Soft lenses• Liquefaction potential• Land slide

Local ground conditions• Bearing capacity

Components of a concrete floor

Hardfill-Provides capillary breakBearing capacity under slab.

Sand blindingAchieves correct level (fine tuning)Protection to DPM.

DPME2 compliance- prevention of moisture entering structure.

ReinforcementFlexural and tensile strength to foundations, plus crack control

MeshCrack control of slab

ConcreteWearing surfaceDurability and protection of reinforcementThermal massDecorative effects

Sometimes seen in floors

• Fibres

Micro syntheticNon structural, reduces plastic shrinkage cracking.

Macro syntheticNon structural, not used in floors

Steel, structural but needs to be combined with standard reinforcing in foundations to provide ductility.

Performance Requirements

B1 Structures• Objective are-

– Safeguard people from injury caused by structural failure– Safeguard people from loss of amenity caused by structural

behaviour– Protect other property from physical damaged caused by

structural failure

Achieved by– Sufficient reinforcement in foundations– Accurate interpretation of the ground conditions– Using acceptable solutions (NZS3604) or equivalent

performance alternative solutions (eg RibRaft)

Performance Requirements

B2 Durability• Required to last 50 years (with appropriate maintenance)

as foundations provide structural stability to the building.

Achieved by• Increasing concrete strength for marine environments• Appropriate concrete cover to reinforcement• Follow good construction techniques (NZS3109) to

achieve hard wearing surfaces

Performance Requirements

E1 Surface water– Safe guard people from injury or illness and other property

from damage caused by surface water– Protect the outfalls of drainage systems.

Achieved by• Appropriate finished floor level (refer E1/AS1)

Performance Requirements

E2 External Moisture• Objectives

– To safeguard people from illness or injury which could result from external moisture entering the building.

Achieved by• Required use of DPM used to be in

E2/AS1

Clearances above ground to prevent moisture entering building

Rebates in edge of floor when using Masonry walls (E2/AS3)

Performance Requirement

Energy Efficiency -H1 of Building Code

Require R=1.3 for floor

All concrete floors deemed to achieve R 1.3

Performance Requirements

R1.9 for Heated floors

New Developments

Edge insulation options

Near Surface ground conditions

Point (c) has been expanded to include liquefaction

Good ground

NZS 3604 -Concrete Strength

Most common

Must be class E, SE62

75mm cover

50mm cover

Rebate

Alternative Solutions- RibRaft

Technical Information

All technical information is presented in the Firth RibRaft Floor Systems Manual

Codemark means that for TC1 sites, if you fully follow the manual then no Building Control Authority can refuse a Building Consent.

RibRaft® Components

Lower required bearing pressure, can be used on softer sites

Liquefaction Specific Design solutionsTC1 foundation zoneTC 2 foundation zoneTC3 foundation zone

TC3 solutions using RibRaft

Slab raised 150mm in less than 3 hours

Firth special concrete mix RP2519TC3

Decorative Concrete

Decorative Options

• Exposed Aggregate

• Coloured Concrete

• Polished Concrete

• Finishing techniques

Exposed Aggrgate

Typically used for drives and paths not floors

Colour

Colour achieved by adding oxides to the concrete

Concrete has natural variations sometimes get mottled effect

Acid Etch

Texture

Trowel

(sponge)

Power FloatRoller Finish

Coloured Concrete - Finishes

Most common for floors

Polished Concrete

Painted

Stains

Indoor-Outdoor

Crack Control and good trade practice

• Early age cracks tend to be due to construction deficiencies

• Late age cracks are usually due to limitations in the joint design

• Cracks caused by heat associated with hydration are typically design and construction issues.

• First question to ask is - when did the cracks first appear?

Plastic Shrinkage 30 mins - 6 hours

Early Thermal 1 day to 2 weeks

Drying Shrinkage months

Plastic Shrinkage Cracking

Plastic Shrinkage -30 minutes to 6hrs

While the concrete is still plastic.

Not structurally a problem but may open up later with drying shrinkage

Reduced durability

Drying of top surface caused by loss of water causes it to rip apart

Restrained Early Thermal ContractionTypical scenario cracks close to saw cuts or straying of saw cuts.Common in Christchurch, less so in Auckland

Early entry saw

Crack inducers

Drying Shrinkage

Typically saw cuts are placed at a spacing of 40 times the slab thickness. Reduce the spacing if the slab is restrained by significant thickenings

Key points for onsite quality control

Confirm ground conditions assumptions

DPM in place

Reinforcement class E, correct grade (300 vs 500) and correct cover

Correct concrete grade ordered from Plant with current Audit Certificate.

List of Plants at -www.rmcplantaudit.org.nz

No water added to concrete

Early Age care plan in place.

Applying anti- evaporate sprays

Early cutting

Curing

Example of Inspection check list

Questions?