Cracks Beneath the Surface: Lessons from a Patio Slab Inspection in Calgary

Concrete patios are often seen as durable, low-maintenance additions to homes. But what happens when cracks begin to form just days after pouring? This was the case for AERZ civil structural consulting’s client, a homeowner in Calgary, Alberta, who noticed troubling signs in his newly installed patio slab. As a structural engineer, I was called in to investigate the issue and provide a professional assessment. What I found was a textbook example of how small oversights during construction can lead to long-term structural concerns.

The First Signs of Trouble

The patio slab in question was poured on July 22, 2024, during a heat wave with temperatures soaring to 32–35°C. Within just four days, the first crack—later labeled Crack I—appeared. Over the following months, additional cracks emerged, some stretching over 20 feet in length. By the time I conducted my inspection in May and June 2025, six distinct cracks had formed, with widths ranging from 0.45 mm to 0.65 mm.

These cracks weren’t just cosmetic. They hinted at deeper issues related to the slab’s design, construction, and environmental exposure.

What Went Wrong?

1. Improper Curing in Hot Weather

Concrete needs time to cure properly, especially in hot conditions. In this case, the contractor failed to initiate curing immediately after finishing the slab. Without protective measures like water spraying or plastic sheeting, the concrete lost moisture too quickly, leading to shrinkage and early cracking. Uneven shrinkage through the slab thickness plays a big factor where top surface shrinkage more than the bottom surface of concrete slab.

2. Misplaced Reinforcement

The steel reinforcement (rebars) was placed too low in the slab—closer to the bottom than the top. According to CSA A23.1 standards, rebars should be positioned within the top third of the slab to resist shrinkage and temperature-induced tension. In this patio, the rebars were found at depths of 3.8” to 4.5” in a slab that was only 4” to 6” thick. This misplacement left the top surface vulnerable to cracking. the amount of reinforcement was not enough quantity to control the cracks according to ACI 360R report.

3. Frost-Susceptible Base Material

Perhaps the most critical issue was the use of recycled concrete aggregate as the base layer beneath the slab. While recycled materials are environmentally friendly, they are not always suitable for structural applications. In this case, the aggregate contained fine particles and clay, making it highly susceptible to frost heave—a phenomenon where moisture in the soil freezes and expands, lifting the slab and causing cracks.

4. Inadequate Contraction Joints

Contraction joints are essential for controlling where cracks occur. The patio slab had joints that were only 1/8” deep, whereas the recommended depth is at least 1/4” or one-third of the slab thickness. Additionally, the spacing between joints exceeded the recommended limits, allowing uncontrolled cracking to develop.

Environmental Factors at Play

Calgary’s climate presents unique challenges for concrete structures. Freeze-thaw cycles, temperature swings, and moisture variations all contribute to concrete movement. In AERZ civil structural consulting’s client case, Crack II and III appeared during winter, likely due to heaving at the slab edges where water had accumulated under the house’s overhang. Snowmelt, rain, drying and wetting, hot, and cold weather further exacerbated the issue, leading to expansion and contraction that the slab was ill-equipped to handle.

The Cost of Poor Repairs

Crack I was initially treated by the contractor using a product called “Magic Crack Sealer.” Unfortunately, the application did not follow manufacturer guidelines. The contractor used a dry-setting method and mixed the sealer with coloring agents without moisturizing, which compromised its effectiveness. AERZ civil structural consulting’s client later attempted to repair the crack with epoxy, but the damage continued to worsen due to ongoing shrinkage and environmental stress. Sealing and water proofing the cracks prevent deterioration of slab and prevent water from penetration under the slab and does not restore the strength of the slab or return it to original condition.

What Can Homeowners Learn?

This case underscores the importance of proper planning, materials, and execution in concrete work. Here are a few takeaways:

• Cure concrete properly, especially in hot weather. Begin curing immediately after finishing.
• Ensure correct placement of reinforcement to resist shrinkage and heaving forces.
• Avoid frost-susceptible base materials like recycled concrete aggregate in cold climates.
• Design contraction joints correctly—both in depth and spacing.
• Follow manufacturer instructions when applying crack sealers or repair products.

Looking Ahead

Without intervention, the patio slab may continue to deteriorate over the next 3–5 winters. Water infiltration through cracks can lead to further frost heave, widening existing cracks and potentially compromising the slab’s integrity. Remediation options include reinforcing the slab with properly placed rebars or replacing it entirely.

For homeowners, this case serves as a reminder that concrete may be strong, but it’s not invincible. Attention to detail during construction—and timely maintenance—can make the difference between a long-lasting patio and one that crumbles under pressure.

 How Can AERZ Civil Structural Consulting Add Value to Your Project?

If you’re planning a home addition, renovation, or basement suite, and need expert support with:

• Structural engineering and drafting
• City of Calgary permit applications
• Construction support and field reviews
• Framing inspections
• Retaining and foundation walls as-built field reviews
• Assessment of reinforced concrete cracks

Reach out to us at Ashraf@AERZcivStrConsulting.ca or call (403) 708-3275.
Our team at AERZ Civil Structural Consulting Inc. is ready to help bring your next project to life with precision and care.

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