Concrete Foundations & Slabs: A Guide for Australian Homes

Introduction

Concrete foundations and slabs carry your whole house. For most homeowners, the language around them is overwhelming.

This guide explains the components, design considerations, reinforcement details and terms you'll come across, so you can follow what your builder and engineer are talking about.

Site classes
Class	Ground movement from moisture changes
A	Little or none (most sand and rock)
S	Slight (slightly reactive clay)
M	Moderate (moderately reactive clay or silt)
H1	High
H2	Very high
E	Extreme

Summarised from AS 2870 Table 2.1. Sites with fill, trees or soft soils can be Class P.

Section 1: Understanding Soil Conditions and Site Classification

1.1 Site Classification Based on Soil Reactivity

Before construction starts, a soil test determines your site's classification, which drives the whole foundation design.

In Australia, soils are classified based on their reactivity—how much they expand and contract with moisture changes.

It matters because reactive clay soils swell when wet and shrink when dry, and that movement can crack a slab that isn't designed for it.

AS 2870 (Residential slabs and footings) classifies sites as:

Class A: Most sand and rock sites with little or no ground movement from moisture changes.
Class S: Slightly reactive clay sites, which may experience only slight ground movement from moisture changes.
Class M: Moderately reactive clay or silt sites, which may experience moderate ground movement from moisture changes.
Class H1: Highly reactive clay sites, which may experience high ground movement from moisture changes.
Class H2: Highly reactive clay sites, which may experience very high ground movement from moisture changes.
Class E: Extremely reactive sites, which may experience extreme ground movement from moisture changes.
Class P: Problem sites, such as soft soils, uncontrolled fill, landslip risk or trees. These need specific engineering design.

Each of these classifications determines the necessary precautions and design adjustments required for the foundation.

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For example, highly reactive soils (Classes H1, H2, and E) require more robust foundation systems, such as deeper footings, more piers (deeper) or heavier reinforced slabs, to mitigate the risk of cracking or shifting.

Site class from expected surface movement (ys)
ys (mm)	Class
up to 20	S
20 to 40	M
40 to 60	H1
60 to 75	H2
over 75	E

From AS 2870 Table 2.3. Your soil report states ys and the class.

1.2 Characteristic Surface Movement

The characteristic surface movement (ys) is how far the ground surface is expected to move up and down with moisture changes, in millimetres. It ranges from 0 (Class A) to more than 75 mm (Class E). The higher the movement, the stiffer the slab and footings need to be.

Simplified map of Australia showing the tropical, arid and temperate climatic zones with a highlighted coastal strip, and a scale showing that harsher exposure needs stronger concrete, more cover to the steel and better curing. — Where your site is located determines foundation inclusions and the specification required

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1.3 Exposure Classifications for Concrete

Exposure classification is about how aggressive the ground is to concrete. Sulfate-rich soils and acidic groundwater attack concrete and the steel inside it. The exposure classification is based on the sulfate content of the soil and groundwater and the pH.

For example:

A1: Sites with low sulfate content and minimal exposure risk.
B2: Sites with higher sulfate levels, requiring more robust concrete and additional protection measures.

The classification sets the minimum concrete strength and the protection needed for the foundation to last.

For instance, higher sulfate levels necessitate the use of concrete with a higher characteristic strength (f’c) and more rigorous curing practices to protect against chemical attack.

Slabs in aggressive soils: class, strength, curing and cover
Exposure class	Min strength	Min curing	Cover, saline soil	Cover, sulfate soil
A1	20 MPa	3 days	see note	40 mm
A2	25 MPa	3 days	45 mm	50 mm
B1	32 MPa	7 days	50 mm	60 mm
B2	40 MPa	7 days	55 mm	65 mm
C1	50 MPa	7 days	n/a	70 mm
C2	50 MPa	7 days	n/a	85 mm

The exposure class comes from the soil test (sulfate, pH, salinity). A damp-proof membrane can reduce saline-soil cover to 30 mm and sulfate-soil cover by 10 mm. Summarised from AS 2870 Tables 5.2 to 5.4.

1.4 Minimum Design and Curing Requirements

Depending on the exposure classification, different concrete strengths and curing requirements are specified to ensure durability. For instance:

A1 Classification: Requires concrete with a minimum strength of 20 MPa and continuous curing for at least three days.
B2 Classification: Requires concrete with a minimum strength of 40 MPa and continuous curing for at least seven days.

Curing (keeping the concrete moist while it gains strength) is critical. Poor curing leaves a weaker slab that's less able to resist cracking and chemical attack.

Section through a slab edge beam built over fill, with a concrete pier under the beam going down through the fill into firm natural ground. — Piers resist downward and upward forces. The type of soil, exposure and wind category (uplift) and house design effects the pier design. the 500min bored pier is called "socketing" into natural ground - to tie the pier in.

Section 2: Pier and Slab Construction Elements

2.1 Pier Construction

Piers transfer the load of the house down to firm natural soil or rock.

Two common types on house sites are screw piles (also called screw piers or helical piles) and bored piers (drilled holes filled with reinforced concrete).

A teal excavator dumping soil into a yellow dump truck at a construction site — Photo by Troy Mortier on Unsplash

2.1.1 Screw Piles

Description: Steel shafts with helical plates welded on, screwed into the ground by an excavator attachment. They're quick to install and work in a range of soils, from soft clay and sand to some rocky ground.
Installation Process: Fast, with little site disruption and no spoil to remove. The pile is screwed down to the required depth and carries load straight away, so work can continue without waiting for concrete to cure.
Bearing Capacity: Each pile is screwed in until it reaches the installation torque the engineer has specified, because torque correlates with how much load the pile can carry. The required torque and capacity (in kN) are set by the engineer for your site and house.
Uplift Resistance: The helical plates bear against the soil above them, which gives good resistance to uplift from wind and soil movement.
Applications: Houses, extensions and underpinning, as well as retaining walls, solar arrays and towers. They suit sites with tight access or variable soil.
One potential issue: in reactive clay, the soil can shrink away from the shaft in a dry season, leaving a gap around the pier. That reduces the lateral support and grip, and with it the pier's ability to resist load and uplift.
- To mitigate this risk, the screw piers must be installed to a suitable depth where they remain anchored in stable soil layers, unaffected by surface moisture fluctuations. Additionally, proper site preparation and moisture control around the foundation can help minimise the likelihood of soil shrinkage and void formation.

2.1.2 Bored Piers

(they need more excitement in their life - don't we all!)

black pug puppy on brown wooden chair — Photo by Priscilla Du Preez 🇨🇦

concrete bore piers — Those things that look like grey "cow pats" are bored piers

Description: Holes drilled into the ground and filled with reinforced concrete. They suit sites that need high load capacity or footings taken down to firm ground.
Installation Process: A drilling rig bores the holes, which are then reinforced and filled with concrete. It takes longer than screw piles, needs bigger machinery and generates spoil to remove.
Bearing Capacity: High. The engineer sets the depth and diameter to suit the loads and soil.
Uplift Resistance: Good, particularly in cohesive soils like clay and silt, because the concrete bonds with the surrounding ground along its full depth.
Applications: On house sites, bored piers are used where the engineer needs footings taken down to firm natural ground or rock, such as through fill or on sloping or problem sites.

2.1.3 Screw Piles vs. Bored Piers: Which is better and when?

Installation Process:
- Screw Piles: Quick and efficient installation with minimal site disruption. Suitable for sites with limited access and overhead clearance.
- Bored Piers: Requires specialised drilling equipment and is more time-consuming. Best suited for sites with ample space and access.
Load Capacity:
- Screw Piles: Provides immediate load-bearing capacity and supports both axial and lateral loads.
- Bored Piers: High load-bearing capacity suitable for large-scale structures and heavy loads.
Site Constraints:
- Screw Piles: Ideal for tight spaces and sites with challenging access conditions.
- Bored Piers: Requires larger equipment and more open space.
Cost Considerations:
- Screw Piles: Generally more cost-effective due to faster installation and less site disruption.
- Bored Piers: Typically more expensive due to the need for specialised equipment and longer installation time.
Time Efficiency:
- Screw Piles: Offers significant time savings, allowing construction to proceed quickly.
- Bored Piers: Installation takes longer, potentially impacting project timelines.
Environmental Impact:
- Screw Piles: Minimal environmental disturbance with a smaller ecological footprint.
- Bored Piers: More extensive site disruption and a greater environmental impact.
Flexibility and Adaptability:
- Screw Piles: Highly adaptable, with the ability to be removed and reused if needed.
- Bored Piers: Flexible design options but not easily removed once installed.
Suitability for Different Soil Types:
- Screw Piles: Versatile across various soil conditions, including rocky terrains.
- Bored Piers: Best suited for cohesive soils and stable rock formations.

The choice depends on site conditions, loads, access, time and budget.

Screw piles are often preferred for their speed, cost-effectiveness, and minimal spoil removal from site, while bored piers are chosen for their high load capacity.

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Take a look at typical residential house engineering general notes and house design details in the above Docsend embed.

2.2 Slab Construction

The slab is the floor and foundation in one. Its strength and durability come down to a few key components.

2.2.1 Components of Slab Construction

Edge Beams: Deeper, reinforced sections around the perimeter of the slab that carry the external walls and stiffen the slab against ground movement.
Internal Beams: Like edge beams but inside the slab, adding stiffness and strength, particularly under load-bearing walls.
Slab Thickening: Locally deeper areas of slab under concentrated loads, such as columns or heavy point loads.

Concrete strength by location
Where the concrete is	Exposure class	Min strength	Curing
Outside, within 1 km of the coast	B2	40 MPa	7 days
Outside, 1 to 50 km from the coast	B1	32 MPa	7 days
Outside, over 50 km inland, temperate zone	A2	25 MPa	3 days
Outside, over 50 km inland, arid zone	A1	20 MPa	3 days
Outside, near industry or in the tropics	B1	32 MPa	7 days
Inside a home	A1	20 MPa	3 days
On non-aggressive ground, homes	A1	20 MPa	3 days

Summarised from AS 3600 exposure classes as set out in CCAA guidance. Your engineer sets the class on the drawings.

2.2.2 Concrete Types for Slabs

Strength/Slump: Concrete strength is measured in megapascals (MPa). Residential slabs commonly use 25 to 32 MPa concrete. Slump measures how wet and workable the mix is, and is specified on the engineering drawings; adding water on site to make it easier to place weakens the concrete.
Subgrade Preparation: The ground under the slab is compacted and often topped with a layer of crushed rock or sand. It must be level and stable so the slab doesn't settle unevenly.

Concrete strength for wear
Use	Min strength
Footpaths and home driveways	20 MPa
Car parks, light vehicles only	25 MPa
Driveways for trucks over 3 t	32 MPa

Summarised from AS 3600 abrasion requirements.

2.2.3 Damp-Proof Membranes (Vapour Barriers)

Purpose and Placement: A damp-proof membrane (usually 0.2 mm polyethylene) is laid under the slab to stop ground moisture rising through the concrete into your floors and walls. It controls moisture; it doesn't stop subsidence or soil movement. That's the job of the footing design and good site drainage.
Edge Beams and Slab Edges: The membrane should run under the edge beams and up the outside face of the slab edge to finished ground level, so moisture can't wick in through the edge. This Cement Concrete & Aggregates Australia (CCAA) data sheet explains slab edge dampness and how to prevent it:

Concrete and Steel: A Strong Partnership for Construction

Section 3: Reinforcement Details in Concrete Slabs

Concrete is strong in compression but weak in tension. Steel reinforcement takes the tension, so the slab can carry the house and resist wind, temperature changes and ground movement.

Notes you will see on slab drawings
Note	Why it matters
Crack-sensitive floor coverings: go one mesh size heavier (e.g. SL82 instead of SL72), or delay tiling 3 to 6 months	Lets most shrinkage happen before tiles go down
Flexible tile adhesive on all projects	Takes up small slab movement
0.2 mm vapour barrier under habitable slabs	Stops ground moisture rising into the slab
Footings and slab built to AS 2870	The design and inspections follow the residential slab Standard

Typical engineering general notes, in our words.

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When ceramic floor tiling is laid - your build is supposed to upgrade the slab mesh to these areas by one full grade from the engineer design, that is, SL82 goes to SL92 - have you seen this done before? (see above)

3.1 Types of Reinforcement

The most common types of reinforcement in house slabs are:

Mesh: A grid of steel bars welded together (slab fabric). It controls cracking and provides tensile strength across the slab. Common types are SL62, SL72 and SL82; the first number relates to the bar diameter (roughly in mm) and the second to the spacing (2 = 200 mm).
Bars: Steel bars (rebar) add strength where it's needed, such as in beams, at edges and around openings. N12 bars are common in house slabs.
Trench Mesh: Narrow strips of mesh used as the main reinforcement in edge and internal beams and strip footings.
Corner Bars: These bars are placed at corners or where the direction of reinforcement changes. They help to maintain the continuity of reinforcement and provide additional strength in areas that are prone to cracking.
Ligatures: Ligatures, or stirrups, are closed loops of steel that are placed around the main reinforcement bars in beams or columns to hold them in place. They help to prevent the main bars from buckling and provide additional shear strength.
Z Bars: Z bars are used in stepped or sloping slabs to maintain reinforcement continuity across the steps. They are bent into a Z shape to fit into the transition between different slab levels, ensuring that the reinforcement is properly anchored and that the slab remains structurally sound.

3.2 Slab Reinforcement Techniques

Reinforcement must be placed and lapped exactly as the engineer designed, and inspected (by the engineer or certifier) before the pour.

Additionally, we recommend conducting an extra pre-pour slab inspection alongside the engineer's inspection.

Plan of two reinforcement mesh sheets lapped so the last two cross wires of each sheet sit in the overlap, with butted sheets and a short lap shown as not acceptable. — Has the slab mesh been lapped correctly? Engineers check this at the pre-pour slab inspection

3.2.1 Lapping of Mesh

Importance of Proper Lapping: Lapping is overlapping sheets of mesh or bars so the reinforcement acts as one continuous network. Too short a lap creates a weak point where cracks form.
Lapping Trench Mesh: There are several acceptable methods, shown in the figure above. The right one depends on the design and the mesh and bars used. Get it wrong and it's a structural weakness, and non-compliant with the engineer's design.

3.2.2 Placement of Reinforcement

Positioning within the Slab: Reinforcement only works if it's in the right place. Check the engineer's drawings for where the mesh must sit and the cover required (see Spacing and Cover below).

Section through a slab before the pour showing reinforcing mesh held up on bar chairs with base plates standing on a lapped and taped vapour membrane over sand blinding. — the humble bar chair - a concretor's greatest nemesis?

Bar Chairs: Small supports that hold the mesh and bars at the right height during the pour. Without them, the steel gets walked into the bottom of the slab and does little good.
- In driveway concrete pours, the use of bar chairs is particularly important due to the high loads that driveways must support. Vehicles exert significant pressure on the surface, and without adequate reinforcement placement, the slab may be prone to cracking and failure.
- Proper use of bar chairs in driveway slabs enhances durability, distributes loads more effectively, and contributes to the overall longevity of the concrete.

Bar chairs help maintain the proper clearance between the reinforcement and the formwork, ensuring consistent concrete cover and reducing the risk of corrosion due to exposure.

Spacing and Cover: Bars and mesh must be spaced as designed, with enough cover (the depth of concrete between the steel and the surface) to protect the steel from rust. For slabs on ground, AS 2870 sets minimum cover such as 40 mm to unprotected ground, 30 mm to a membrane in contact with the ground, and 20 mm to internal surfaces. Aggressive exposure classes need more under AS 3600 (for example around 65 mm for B2 concrete cast against ground).

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The docsend embed above features the CCA Data sheet on Concrete Curing

3.3 Curing Techniques and Times

Curing lets the concrete develop its full strength and durability, and reduces cracking and shrinkage.

Curing Methods: There are several methods used to cure concrete slabs, including:
- Wet Curing: Keeping the slab moist by applying water to the surface or covering it with wet hessian or plastic sheeting.
- Curing Compounds: Applying a liquid membrane-forming compound that slows the evaporation of moisture from the concrete surface.
- Curing Blankets: Using insulated blankets to retain heat and moisture, accelerating the curing process in cold weather conditions.
- Pond Curing: This method involves creating a dam around the slab edges and then flooding the slab with water, which is retained within the dam.
Curing Times: The minimum curing time depends on the exposure classification and concrete strength. For example:
- A1 Classification: Requires a minimum of three days of continuous curing.
- B2 Classification: Requires a minimum of seven days of continuous curing.

Section 4: Common Slab Terminology Explained

Here are the terms you'll hear most often on site and in your engineering drawings.

In this section we will explain some of the key terms that are frequently encountered in the context of concrete foundations and slabs.

4.1 Understanding Key Terms

4.1.1 Cover to Reinforcement

Definition: "Cover" refers to the distance between the outer surface of the concrete and the embedded reinforcement. This layer of concrete protects the steel from environmental factors, particularly moisture and corrosive agents, which can cause the steel to rust and weaken over time.
Importance: Too little cover and the steel rusts, swells and cracks the concrete. The cover required increases with harsher exposure (see 3.2.2 above).

4.1.2 Slab Thickening

Definition: A locally deeper area of slab where higher loads are expected, such as under walls or columns.
Purpose: The extra depth, usually with extra reinforcement, spreads the load and reduces the risk of cracking or settlement.

Section through a concrete slab at a set-down recess, with the slab thickened underneath and the mesh stepping down and lapping past the step. — Typical detail showing two methods of reinforcement placement at a slab recess

4.1.3 Slab Set Downs

Definition: A step down in the slab level, commonly in bathrooms, laundries and showers, to allow for waterproofing, falls to the waste and tiles.
Construction Considerations: Set downs need correct reinforcement detailing so the step doesn't become a weak point.

Two sections comparing a concrete path beside a house on a class A, S or M site, with a minimum crossfall of 30 mm, and on a class H or E reactive clay site, with a minimum crossfall of 50 mm, both falling away from the house. — 900mm, 1000mm long levels are really handy to measure pathway falls

4.2 Slab Edge Dampness Protection

As covered in 2.2.3, the membrane should run under the edge beam and up the slab edge to ground level. See the detail below:

Section through a slab edge beam under brick veneer showing the membrane running under the whole slab, under the edge beam and up the outside face to finished ground, with lapped and taped joints. The cavity flashing runs from the stud frame across the cavity on top of the first brick course to the outside face of the brickwork, with weep holes in the course above. — Knowing this, check out your local new housing estates and see which builders actually comply and which do not! You will be surprised.

4.3 Engineering Design Requirements

Two sections at a slab edge under brick veneer: a standard shallow rebate with the flashing across the cavity to the outer face of the brickwork, and a deeper edge rebate where the flashing runs down the rebate face and out through the bed joint to the outer brick face. In both, the weepholes are in the course above the flashing and stay above the finished ground. — The minimum edge rebate is 20mm. Barrier extends up the edge beam toe to ground level

4.3.2 Edge Rebates and Slab Recesses

Edge Rebates: A step formed in the top edge of the slab where the brickwork sits, so water can't run across the slab under the bottom course.
Slab Recesses: Lower areas formed within the slab, such as for wet rooms. They need proper detailing and reinforcement.

4.4 Specific Joint Types

4.4.1 Control Joints

Purpose: Control joints are intentionally placed weak spots in the slab that allow for controlled cracking. As concrete cures and shrinks, it is prone to cracking. Control joints help to manage where these cracks occur, minimising their impact on the overall structure.
Placement: Usually saw-cut at regular intervals. They're common in driveways and paths; house slabs are usually designed without them. Always follow the engineer's design.

4.4.2 Isolation Joints

Definition: Isolation joints are used to separate the slab from other structural elements such as walls or columns. This allows for independent movement between the slab and the adjacent structure, preventing cracks that could result from differential movement.
Application: Isolation joints are essential in areas where different parts of the structure are expected to move independently, such as around columns or at the interface between a concrete slab and masonry walls.

4.4.3 Expansion Joints

Purpose: Expansion joints accommodate the expansion and contraction of concrete due to temperature changes. These joints prevent the slab from cracking as it expands and contracts with changes in temperature.
Construction: Expansion joints are typically filled with a compressible material that allows for movement while maintaining a seal against moisture and debris. They are commonly used in larger slabs or where the slab is exposed to significant temperature fluctuations.

4.4.4 Articulation Joints and "Expanda" Joints

Articulation Joints: These joints are designed to accommodate movement in reactive soils, which can expand and contract significantly with moisture changes. Articulation joints allow different sections of the slab or structure to move independently without causing damage.
"Expanda" Joints: Specifically used in plumbing and stormwater drainage systems, "Expanda" joints provide flexibility and movement in pipes that pass through or are embedded in concrete slabs. They prevent damage to the pipes and the slab caused by soil movement or temperature changes.

Check out our article on understanding the different types of concrete joints (below)

Section 5: Slab Preparation and Installation

Here's what's involved in preparing the site, placing the concrete and getting the slab right.

5.1 Subgrade and Soil Considerations

5.1.1 Importance of Subgrade Preparation

Definition: The subgrade is the soil or fill directly under the slab. It supports the slab and spreads the load.
Preparation Process: Proper subgrade preparation involves several key steps:
- Clearing and Grading: The site must be cleared of any vegetation, debris, and large rocks. The ground should then be graded to ensure a level and stable base.
- Compaction: The soil must be compacted to increase its density and stability. This reduces the likelihood of settlement or movement under the slab, which could lead to cracking or structural issues.
- Addition of Granular Material: In many cases, a layer of granular material such as sand, crushed rock (crusher dust) or gravel is added on top of the compacted soil. This layer, known as the base course, helps to improve drainage and further stabilise the subgrade.

Three sections of the low side of a slab on a sloping site: slab on compacted fill with a batter, an edge beam deepened through fill on both sides, and a deep edge beam holding the fill back with natural ground outside. — Different approaches for sloped site slab construction

5.1.2 Dealing with Sloped Sites

Challenges of Sloped Sites: Constructing a slab on a sloped site presents unique challenges, such as the need to accommodate varying soil levels and ensure that the slab remains level and stable.
Techniques for Sloped Sites (See Figure 6.1 above)
- Stepped Footings: One common method is to use stepped footings, where the foundation is built in a series of horizontal steps that follow the slope of the land. This helps to maintain a consistent slab thickness and prevent uneven settling.
- Deepened Edge Beams: On sloped sites, edge beams may need to be deepened to provide additional support and prevent the slab from sliding or shifting. These beams act as retaining walls, holding the slab in place and distributing the load more effectively.
- Fill and Slab Edge Options: Depending on the slope and soil conditions, it may be necessary to use controlled fill to create a level platform for the slab. The fill must be compacted properly to prevent future settlement (reference to "Fill and Slab Edge Options for Sloped Sites" from the provided images).

Plan and section of a house cut into a sloping block, with a cut-off drain on the high side carrying uphill water around the house to a pit and the legal point of discharge. — sites with falls cutting (site works) details to allow for proper drainage

5.2 Damp-Proof Membrane (Vapour Barrier) Installation

vapour barrier in slab pre concrete pour — The black plastic is the vapour barrier

5.2.2 Installation Best Practices

Placement: Laid directly on the prepared subgrade before the pour, covering the whole slab area and running under and up the edge beams (see the slab edge detail above).
Integration with Edge Beams: The membrane should run down into the edge beam trenches and up the outside of the slab edge, so it forms one continuous barrier.
Sealing Joints: Seal joints and repair any punctures or tears with tape before the pour. Check for damage after the steel fixers have walked all over it.

5.3 Concrete Pouring and Finishing

5.3.1 Concrete Pouring Process

Preparation: Before the pour, formwork must be secure and the reinforcement correctly placed and supported. The subgrade, membrane and reinforcement should have been inspected and passed.
Pouring: Concrete should be placed continuously and as close to its final position as possible. Dragging it around causes the aggregate to separate, which weakens the slab.
Vibration: The concrete is vibrated with a mechanical vibrator to remove air pockets and make sure it fully surrounds the reinforcement.

5.3.2 Finishing Techniques

Screeding: A straight edge is dragged across the fresh concrete to level it and remove the excess.
Floating: Once the concrete has started to set, it's floated to smooth the surface and push the aggregate just below the surface.
Troweling: Troweling (by hand or with a power trowel) gives a hard, dense, smooth finish.
Control Joint Cutting: Where joints are specified (more common in driveways than house slabs), they're cut to between a quarter and a third of the slab thickness, at the spacing in the design.

6: Challenges and Best Practices

The quality of the foundation directly affects how long your house lasts. Here are the common problems and how to avoid them.

Three sections of a service trench beside a slab edge beam: a trench too deep and close that undermines the footing, a trench kept above the zone below the footing, and a footing deepened with a mass concrete pier. — The zone of influence (which, for the record, has nothing to do with getting into someone's personal space) is very important for the performance of your footing. 😑

6.1 Common Challenges in Concrete Slab Construction

6.1.1 Subsidence and Soil Movement

Definition: Subsidence refers to the gradual sinking or settling of the ground beneath the foundation, which can cause the slab to crack or shift. Soil movement, particularly in reactive soils, can also lead to foundation issues as the soil expands and contracts with moisture changes.
Causes: Poor subgrade preparation, inadequate compaction, uncontrolled fill or unexpected soil conditions. In reactive soils, moisture changes cause the ground to swell and shrink.
Impact on Slab: Subsidence and soil movement can cause cracking, uneven settling, and structural damage to the slab. This can lead to costly repairs and even compromise the safety of the building.

Read more about subsidence and how to mitigate it during construction in the post below:

6.1.2 Cracking and Shrinkage

Definition: Cracking is a common issue in concrete slabs, often caused by shrinkage during the curing process, thermal expansion and contraction, or structural loads exceeding the slab’s capacity.
Types of Cracks: Cracks can be categorised into several types, including shrinkage cracks, settlement cracks, and structural cracks. Each type has different causes and implications for the slab’s integrity.
Causes: Inadequate curing, improper joint placement, overloading, or insufficient reinforcement can lead to cracks. Environmental factors, such as temperature changes or moisture fluctuations, can also contribute to cracking.
Impact on Slab: While minor cracks may be cosmetic, significant cracking can compromise the slab’s structural integrity, leading to water ingress, reduced load-bearing capacity, and potential failure.

This CCAA data sheet has excellent information on drying shrinkage of cement and concrete. There are separate data sheets on plastic shrinkage cracking, settlement cracking and early-age cracking:

6.1.3 Water Ingress

Definition: Water getting into or under the slab through cracks, joints, the slab edge or from below.
Causes: Poor joint sealing, inadequate damp-proof membranes, and improper slab detailing can allow water to enter the slab. In areas with high water tables or poor drainage, water ingress is a significant concern.
Impact on Slab: Water can rust the reinforcement and weaken the slab. Water collecting around footings in reactive clay also causes the ground to heave, cracking the slab and walls.

Section from the house out to the yard: sealed path falling away, light watering near the house, garden beds further out and trees well clear of the house, with stormwater to a drain. — Trees are great for shading, when they are placed to not affect your foundations

Why Standards Get Ignored

Concrete work in houses is governed by Australian Standards, including AS 3600 and AS 2870. In my experience, some contractors work from habit ("we've always done it this way") rather than the standard, so it pays to check. Shortcuts save time on the day and cost you in the long run.

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The CCA (Concrete Cement Aggregate) Association offers fantastic data sheets that distil all elements of concrete down, incorporating and referencing the relevant Australian standards, providing invaluable resources for anyone involved in concrete construction.

Once you know the requirements, you're in a much better position to manage the quality of your slab, alongside the engineer's inspection. The information is readily available, and contractors can easily turn it into simple quality checklists.

Inspection Test Plans (ITPs) for concrete work are widely available to buy online for around $90, and are a good starting point for any contractor serious about quality. For homeowners, our C5 Pre Pour Slab Inspection Checklist covers what to check before the pour.

Better quality needs both carrot and stick: incentives for doing it right and enforcement when it isn't.

Currently, there is little motivation beyond the contractor's personal morals and ethics to adhere to proper standards.

A Quick Note on Driveways & Paths

If I had to pick one document from this post, it's the CCAA guide on Residential Driveways and Concrete Paths. It shows how concretors should be doing the work, compared with what often happens.

Home Owner Maintenance

Looking after the ground around your house reduces the risk of foundation movement. Here's what to do:

Manage Soil Moisture Levels: Clay soils swell when wet and shrink when dry. Keep moisture around the house as even as you can: avoid overwatering, and keep garden beds against the house well drained.
Control Tree and Shrub Growth: Tree roots draw moisture out of the soil and can cause it to shrink under your footings. Plant trees well away from the house. If roots are already a problem, talk to an arborist or engineer about root barriers.
Proper Drainage and Surface Water Management: Make sure water drains away from the house. Paving around the perimeter should fall away from the walls, and water shouldn't pond against the slab. Fix leaking pipes, gutters, downpipes and air-conditioning condensate lines that discharge onto the ground instead of into the drainage system.
Address Erosion: Sandy soils can wash away and undermine footings. Prevent surface runoff and use paving or ground covers to hold the soil in place.
Monitor for Cracks and Structural Changes: Watch for cracks in walls, uneven floors, and doors or windows that start sticking. Catching movement early prevents bigger damage.
Professional Inspections: If you see significant cracking or movement, get a structural engineer to inspect and advise.

Conclusion

Your slab and footings are the one part of the house you can't easily fix later. The Standards (AS 2870 and AS 3600) and the CCAA data sheets set out how the work should be done; the gap is whether anyone checks. Get informed, make sure the engineer's inspection happens, check the pre-pour stage yourself with a checklist, and hold your builder to the design.

Test yourself

Five quick questions on this topic. Click a question to see the answer.

What is the primary purpose of steel mesh in slab construction?

To control cracking and provide tensile strength.

Why are bar chairs important in concrete slab construction?

They ensure reinforcement is positioned correctly within the slab.

Why is adequate cover to reinforcement important in concrete slabs?

To protect the steel from environmental factors and ensure durability.

Where should a damp-proof membrane be placed during concrete slab construction?

On top of the prepared subgrade before the concrete is poured.

What is the purpose of control joints in concrete slabs?

To allow for controlled cracking as the concrete cures and shrinks.

FAQs

What is the role of steel reinforcement in concrete slabs?
Steel reinforcement, such as mesh and bar, gives the concrete tensile strength, helping it resist cracking and failure under load and ground movement.

How do I know if my soil type is suitable for a concrete slab foundation?
A soil test classifies your site (A, S, M, H1, H2, E or P) by its reactivity. Your engineer then designs a slab and footings to suit that class.

What is the importance of curing, and how long should it last?
Curing keeps moisture in the concrete so it reaches its full strength and durability. It typically lasts 3 to 7 days, depending on the exposure classification and the weather.

Can a concrete slab be used on a sloped site?
Yes, using techniques like stepped footings, deepened edge beams and controlled (engineered) fill.

What are the signs of a poorly constructed concrete slab?
Significant cracking, uneven settlement, dampness at the slab edge, and exposed or rusting reinforcement. These usually come from poor subgrade preparation, inadequate cover or curing, or badly placed joints.

What is a damp-proof membrane, and why is it necessary?
It's a polyethylene sheet (vapour barrier) laid under the slab to stop ground moisture rising through the concrete into your floors and walls. It controls moisture; it doesn't stop subsidence or soil movement. That's the job of the footing design and good site drainage.

Do house slabs need joints?
Most house slabs are designed without sawn joints and rely on the reinforcement to control cracking. Driveways and paths use control joints so shrinkage cracks happen in a straight line where you want them. Always follow the engineer's design.

What are the benefits of bored piers over screw piers?
Bored piers are cast into the ground, so they bond with the surrounding soil or rock and resist uplift well. Screw piers can lose grip if reactive clay shrinks away from the shaft in dry seasons, so they need to be installed deep enough to reach stable ground.

What is a slab set down, and when is it used?
A recessed area in the slab, typically in bathrooms and laundries, to allow for waterproofing, drainage falls and tiles while keeping floors level.

How do I maintain my concrete slab after construction?
Keep water draining away from the slab, keep garden beds and trees well back, check the slab edge and walls for cracks, and fix leaks or signs of movement promptly.