The Root Causes of Pool Cracks: Understanding the Problem and Preventing It

In this article, we will explore the root cause of the most common pool (structural) cracks.  These visible cracks in swimming pools have become an increasing concern among pool owners. These issues are often attributed to concrete shrinkage or structural deficiencies, and in some cases, the problems arise even before the project is completed. What’s causing these persistent and costly issues?

At J Designs Pool & Spa, we prioritize educating our clients on the root causes of pool cracks, the common missteps that lead to them, and the steps we take to avoid such problems altogether. The solutions are often more straightforward than people think. With proper material selection, adequate supervision, adherence to industry codes and regulations, skilled, trained personnel, and a healthy dose of common sense, pool installations can be done right the first time.

This article examines the causes of pool cracks and outlines industry standards, providing practical advice for both homeowners and contractors to ensure a durable and long-lasting pool structure.

1. Soil Conditions and Their Role in Cracking

Soil stability is essential for the success of any pool installation, and selecting the right site to build your dream pool is key. The chosen location may have inadequate soil compaction or an old-fill site, which you or your contractor may not be aware of.  While this article primarily focuses on concrete shrinkage and construction-related issues, soil conditions must not be ignored as the most disregarded contributing factor in pool failures. The most common soil-related cracks are differential settlement cracks, which are visually vertically oriented. They are often wider at the top of the pool wall and narrower as they move toward the floor. Another cause of failure related to the soil is slope creep. This term describes the movement of the land downward. You can detect slope creep when trees or other vertical elements on the hillside point outward rather than straight up.

Even if not required by municipalities, a soil engineer field visit is highly recommended to determine whether the soil is suitable for construction. A professional assessment from a licensed engineer can identify potential issues like expansive soil/clay, poorly compacted soil, or high groundwater levels. Additional steps such as soil stabilization, over-excavation, or drainage systems can help ensure the pool installation’s success if the soil is unsuitable.

The following are red flags that need to be observed before any installation: When two or more different soil types beneath the pool are visible, avoid standing water inside the pool, soil cave-ins (pool wall collapse), and adverse weather conditions such as rain, wind, cold, or high temperatures, which can also increase the risk of failures. Soil engineers widely accept that pool structures should be built on consistent, undisturbed native soil.

2. Common Causes of Pool Cracks

When reflective cracks of significant size and length appear in the pool walls or floor, either isolated or multiple cracks that follow a pattern, even on steps and benches where there is a lack of steel or inappropriate concrete coverage, they are considered a sign of shrinkage. Shrinkage cracks are smaller and occur as the concrete cures and loses moisture. Improper concrete mix design, inadequate curing practices, and low reinforcement ratios contribute to this issue. While some degree of shrinkage/movement is expected, poor planning and improper installations can exacerbate cracking and lead to long-term damage.

How to Minimize Shrinkage:

• Use a low water-to-cement (W/C) ratio to reduce shrinkage during curing.
• Incorporate admixtures designed to control shrinkage.
• Ensure proper curing practices, such as covering the surface with plastic sheeting and maintaining moisture for at least 14 days.

Structural Deficiencies

Improper reinforcement placement, low-quality shotcrete application, and poor construction supervision are common culprits behind structural cracks. Many engineers and contractors rely on American Concrete Institute, ACI 318 and ACI 350 standards, considered bare minimums. These standards recommend a steel-to-concrete ratio of 0.18-0.20%. However, ACI 224R-01 suggests exceeding 0.60% — three times the minimum for better crack control.

primary weaknesses

• Insufficient reinforcement ratios: Failing to exceed minimum standards increases the likelihood of cracking. Maintaining the minimum compressive strength of 4,000 psi for the shotcrete design mix.
• Poor steel placement: Displaced, inadequately spaced, or poorly supported steel undermines the pool’s structural integrity.
•  Insufficient steel reinforcement: Steel grade and thickness (diameter bars) are often disregarded and contribute to failures.
• Over-thickened walls and floors: Excess thickness can reduce the steel-to-concrete ratio, leading to uneven stress distribution. The Pool and Recreational Shotcrete committee must request a certified Nozzellman from the ASA (American Shotcrete Association) to Place Shotcrete in compliance with ACI 318, which will prevent placement issues.
• Improper shotcrete practices: Excess rebound material, high W/C ratios, and uneven application create weak points in the concrete shell.

3. Plumbing Placement and Its Impact on Cracks

Plumbing placement is a critical yet often overlooked factor in pool construction. When reinforcing steel is properly elevated off the soil (at least 3 inches), placing a 2-inch pipe parallel between the steel and the earth can displace a significant quantity of concrete, creating weak points and changing the dimensions of the structure without the Engineer's knowledge or approval.  

Engineers guide best practices in concrete work, but many overlook the plumbing aspects contractors include. It's advisable to discuss these considerations with the project engineer.

Common Plumbing Mistakes to Avoid:

• Improper pipe placement: A 2-inch pipe displaces two-thirds of the concrete behind the steel, reducing encapsulation and protection.
• Shallow trenching: Plumbing lines placed too close to the pool shell can compromise the structure.
• Rebound-filled trenches: Using rebound material or trimmings in plumbing trenches creates voids and obstructs future repairs.
• Avoid stacking: Plumbing lines should not be placed on top of each other vertically or horizontally on walls or the pool floor. Special care of the spas is often overseen as the increased amount of plumbing lines in those restricted areas will not permit proper concrete encapsulation of the steel, much less large-diameter plumbing lines of fixtures.

Best Practices for Plumbing Installation:

• Trench below the pool floor or behind the pool walls under decks: Ensure plumbing is placed safely beneath the frost zone and away from critical structural areas.
• Backfill carefully: Avoid using waste material to fill plumbing trenches, and compact the soil adequately to prevent future movement.

4. Diagnosing and Testing Pool Cracks

When cracks appear, contractors and homeowners often use destructive testing methods to determine the root cause. Coring the shotcrete is commonly used to validate compressive strength, investigate cracks, or evaluate the concrete matrix.

What Happens During Testing:

• Core sampling: A concrete sample is removed for analysis by a certified laboratory.
• Compression testing: Determines whether the concrete meets strength requirements.
• Petrographic analysis: Examines the composition of the concrete to identify deficiencies, such as insufficient cement content.

Note: Even if a core sample looks solid, it may test poorly due to low cement content or improper application. Dry mix shotcrete, in particular, is prone to this issue.

5. Preventative Measures During Construction

Preventing cracks starts with adopting best practices at every stage of construction. The following measures can help pool contractors and homeowners avoid costly structural issues:

Key Steps to Take:

1. Exceed ACI minimums: Design with reinforcement ratios of 0.60% or higher for better crack control. ACI recommends a minimum of 4,000 psi and 5,000 psi for salt environments or in the presence of high sulfates.
2. Ensure proper steel placement: Use chairs or spacers to elevate steel off the soil and maintain spacing.
3. Apply shotcrete correctly: Avoid rebound material, ensure even application, and maintain a low W/C ratio.
4. Invest in supervision: Adequate oversight ensures compliance with design specifications and quality standards. Engineers and most municipalities often require a continuous inspection for the duration of the Shotcrete installation in compliance with ACI 318, Section 26.13.3.2
5. Consult professionals: Hiring qualified engineers and soil experts to assess site conditions and approve designs is not uncommon as an additional layer of protection and is also part of most building codes in the nation.

The Role of Educating Clients and Contractors

A lack of education among contractors and unrealistic expectations from homeowners or designers unfamiliar with these rules often fuel misunderstandings about pool cracks. Homeowners assume cracks should never occur, while contractors struggle to explain the limitations of industry standards.

For Homeowners:

Ask your contractor these questions to ensure quality work:

• What reinforcement ratio will you use for my pool?
• How will you ensure proper curing?
• Do you consult a soil engineer before construction?

• Hire only licensed and industry-certified pool professionals.

For Contractors:

Work with engineers to exceed ACI standards and educate clients on realistic expectations for pool construction. With proper training and supervision, avoid defects and get ahead of the construction process. Using unlicensed or inexperienced subcontractors is a growing problem in our industry, and they are often unaware of these harmful practices.

Conclusion: Building Better Pools Together

Cracked pools are more than an eyesore; they represent a growing challenge in the pool construction industry. By exceeding minimum standards, using proper materials, and adhering to best practices, contractors can prevent cracks and protect their reputations.

At J Designs Pool & Spa, we’re committed to quality and transparency. Contact us today for expert guidance if you’re planning a pool project or dealing with structural issues. Let’s work together to build a pool that stands the test of time.

Check out our blog article that explains weepers common issue in pool construction.