Why Even Small Concrete Jobs Need Attention to Detail
Concrete seems simple – just pour and finish, right? Not quite. Even small concrete projects like shed slabs, footpaths, and garden pads require careful planning and execution. A single mistake can lead to premature cracking, drainage problems, shortened lifespan, or costly repairs.
We've seen plenty of both DIY and poorly-executed professional jobs. This article covers the most common mistakes we encounter and how to avoid them.
Mistake 1: Skipping Proper Site Preparation
What goes wrong: Rushing to pour concrete without properly preparing the ground. This includes not clearing vegetation, not removing existing material, not compacting the soil, or not establishing proper levels.
Why it matters: The quality of a concrete slab depends heavily on what's underneath. Poor site prep leads to uneven settling, differential movement, and cracking. Soft, uncompacted soil can shift, causing the slab to break unevenly. Organic matter under a slab can decompose, leaving voids that cause sections to sag.
How to avoid it: Properly clear the site – remove vegetation, roots, debris, and any old concrete. The ground should be compacted and level before pouring. Gravel base layers should be properly laid and compacted. This preparation is labour-intensive but essential. It's worth paying for proper preparation or doing it thoroughly yourself.
Mistake 2: Wrong Slab Thickness for the Purpose
What goes wrong: Using 75mm concrete for a heavy-duty shed slab, or 100mm for a light garden pad. The slab either overbears its load or fails prematurely under loads it was designed for.
Why it matters: Thickness directly impacts durability and load capacity. A shed slab with stored tools and equipment needs 100mm minimum. A light garden path might be fine with 75mm. Too-thin concrete will crack under load. Too-thick is wasteful and expensive.
How to avoid it: Match thickness to intended use. Consult with your concreter about the right thickness. For most residential shed slabs, 100mm is standard. Footpaths can be 75–100mm. Heavy-duty areas or vehicle-accessible surfaces need 125mm or more.
Mistake 3: Poor Drainage Planning
What goes wrong: Not considering water drainage around or under the slab. Water pools around the concrete, creating wet areas that encourage mold, decay, and structural issues.
Why it matters: Brisbane's high rainfall means drainage is critical. Water pooling around a slab can seep under it, causing the subgrade to become waterlogged and unstable. Poor drainage accelerates concrete deterioration and creates slippery, dangerous surfaces. Concrete under sheds can trap moisture, promoting rot in wooden structures above.
How to avoid it: Ensure the slab is slightly crowned (curved higher in the middle) to shed water away from the edges. Grade the surrounding area so water flows away from the slab. For footpaths, consider French drains or gravel channels if the area is naturally wet. Proper drainage is part of professional slab installation and is worth the extra investment.
Mistake 4: Not Using Reinforcement or Using the Wrong Type
What goes wrong: Pouring concrete without any reinforcement (mesh or rebar), or using improper reinforcement that doesn't prevent cracking effectively.
Why it matters: Reinforcement prevents cracks from spreading and holds concrete together if stress occurs. Without it, small cracks easily expand into major structural issues. The reinforcement needs to be positioned correctly in the middle of the slab to be effective – reinforcement sitting on the ground doesn't do much good.
How to avoid it: Use standard SL62 or SL72 wire mesh for most small slabs. Ensure the mesh is properly supported in the middle of the pour, not sitting on the ground. For larger or heavier-duty slabs, rebar might be specified. This is a standard part of professional slab installation and shouldn't be omitted to save money.
Mistake 5: Pouring in Wrong Weather Conditions
What goes wrong: Pouring concrete during extreme heat, heavy rain, frost, or high winds. The concrete sets too fast (or too slowly), doesn't cure properly, or is damaged before it hardens.
Why it matters: Concrete needs specific conditions to cure properly. Extreme heat causes rapid surface drying and cracking. Heavy rain washes away the cement paste and causes weak surface zones. Frost can damage freshly set concrete. High winds accelerate surface drying unevenly, causing cracking patterns.
How to avoid it: Plan concrete work for ideal conditions – mild temperatures (15–25°C), no rain forecast, light winds. Autumn and early winter (April–June) are ideal in Brisbane. If you must pour in less-than-ideal conditions, the concreter needs to take special precautions, which increases cost and complexity. It's often better to wait for better weather.
Mistake 6: Not Allowing for Expansion and Control Joints
What goes wrong: Pouring large slabs without expansion joints or control joints. The concrete cracks randomly and extensively as it cures and as temperature changes.
Why it matters: Concrete expands and contracts with temperature and moisture. Large slabs need expansion joints to control cracking. Without them, random cracks develop and spread. Control joints create stress relief points where controlled, narrow cracks form instead of random, wide cracks across the surface.
How to avoid it: For slabs larger than 20m², expansion and control joints should be planned. Joints are typically spaced every 2–3 metres. They're simple to include when pouring but impossible to fix properly afterward. Any professional slab pour should include properly planned and executed joints.
Mistake 7: Rushing the Curing Process
What goes wrong: Walking on the slab, building on it, or exposing it to heavy use before it's fully cured. The immature concrete is damaged, cracked, or weakened.
Why it matters: Concrete reaches initial set (hardness) in 24–48 hours but continues curing for 28 days. Full strength isn't achieved until then. Loading immature concrete damages it permanently. Even foot traffic on fresh concrete can leave footprints and cause issues.
How to avoid it: Keep people off freshly poured concrete for at least 3–7 days. Don't build on it or apply heavy loads for at least 7–14 days. Wait 28 days for full strength. The concreter will provide specific curing guidance based on conditions. Patience during curing is essential to final quality.
Mistake 8: DIY Work Without Proper Knowledge or Tools
What goes wrong: Attempting to pour concrete without proper experience, equipment, or understanding of technique. The result is uneven, poorly finished, or structurally compromised concrete.
Why it matters: Concrete work requires specific knowledge and tools. Proper compaction, levelling, finishing, and curing all demand skill. A poorly executed DIY pour might look okay initially but fail within months. Fixing a bad slab is expensive – often more than proper work would have cost initially.
How to avoid it: Small concrete jobs are generally more cost-effective when professionally installed than when DIY attempts fail and need reworking. If you're determined to DIY, get proper training and hire equipment. For most people, hiring a professional concreter is the sensible choice and provides a warranty on the work.
Mistake 9: Not Getting Proper Quotes (Cheapest Isn't Best)
What goes wrong: Choosing a concreter based purely on lowest price. The cheap option cuts corners – poor site prep, inadequate reinforcement, rushed finishing, or lack of proper curing care.
Why it matters: You get what you pay for. A significantly cheaper quote often reflects shortcuts, not greater efficiency. You might save $200 upfront but end up with a slab that cracks and fails within a few years, requiring expensive repairs or replacement. The cost difference between poor and good work is usually small compared to the cost of fixing a failed slab.
How to avoid it: Get 2–3 quotes and compare what's included, not just the bottom-line price. Ask questions about site prep, reinforcement, drainage, joints, and curing practices. Choose a concreter with experience in Brisbane conditions, transparent pricing, and positive references. A slightly higher initial cost for quality work is always the better investment.
Important Disclaimer
This article covers common concrete work best practices. For specific project advice, consult with qualified professionals who can assess your site and requirements.
Not a licensed builder. We specialize in small concrete jobs only – shed slabs, garage slabs, concrete footpaths, and small pads. We do not construct house slabs or provide structural engineering. Always consult with appropriate professionals for your specific project needs.
Common Questions About Concrete Mistakes
Minor surface cracks (hairline, less than 3mm wide) are often cosmetic and don't affect the slab's integrity. They can be sealed to prevent water seeping in. Larger cracks indicate structural issues and may require concrete repair specialists to inject epoxy or polyurethane into the crack, or in severe cases, slab replacement. Prevention through proper installation is far better than dealing with repairs later.
Expansion joints are gaps between sections of concrete that allow room for the material to expand and contract without causing stress. Control joints are intentional cracks (or grooves) cut into the concrete that direct where cracks will occur – they break the slab into manageable sections. Both are necessary in larger slabs. Spacing, depth, and positioning should be planned before pouring by the concreter.
A properly finished slab should have: (1) a smooth, even surface free of ridges or hollows, (2) proper slope for drainage (slightly crowned or pitched away from structures), (3) clean, uniform joints evenly spaced, (4) broom finish or other specified texture applied consistently, and (5) no major surface cracks. Minor hairline cracks are normal. Ask your concreter about their finishing standards before work begins.
Sealing is optional but beneficial, especially in Brisbane's climate. A penetrating sealer helps protect concrete from water penetration, weathering, and staining. Sealers should be applied after the slab is fully cured (at least 28 days). Some people choose to seal footpaths and decorative slabs for easier cleaning and longer life. Shed slabs under cover don't necessarily need sealing, but it doesn't hurt. Discuss sealing options with your concreter.
Concrete requires minimal maintenance. Keep it clean by sweeping or hosing off debris and dirt. Remove stains promptly to prevent permanent discolouration. Monitor for new cracks and seal them if they develop (this prevents water seeping in and expanding the crack). Avoid pouring de-icing chemicals on slabs in winter. If sealed, reapply sealer every 2–3 years. Overall, a quality concrete slab is very low-maintenance and can last decades with minimal care.