Precast – Cracked Product – Diagnosis and Prevention
The Problem – Although cracks in cast-in-place concrete are inevitable due to the natural tensile properties of concrete, for any Precaster, a cracked product can be a major headache, potentially resulting in costly repairs, scrapped product, re-fabrication and re-shipment depending on the severity of the crack and the ruling of the inspector. Any rework is usually rushed to maintain schedules on-site or to minimize the impact of the delay. A cracked product can be a very frustrating problem in precast industry, since there are many different scenarios and causes of cracks in precast elements. Addressing the root cause swiftly is key, but as the cause is not always apparent, it is easy to go down the wrong path to solve the problem, leading to frustration and lost productivity. Furthermore, cracks in prestressed elements can be especially difficult to diagnose because of the dynamic nature of prestressing forces; more so because these forces are designed to keep the concrete in a state of compression to prevent cracks, not cause them.
Are supplementary materials a good solution?
There are many commercially available supplementary materials for preventing cracks, such as synthetic micro and macro fibers, which can vastly improve the tensile properties of concrete and the resistance to cracking. However these materials generally have a practical limit to their capabilities, can be expensive, and/or are not suited to all products/manufacturing methods. Their use is arguably representative of a symptomatic approach to the underlying problem. Where such materials are not an option, the appropriate course of action is usually an in-depth investigation into the value chain.
Investigating the cause
Investigating the cause(s) of cracks should always begin in collaboration with the Engineering, QC and Production departments to evaluate each stage of the production process in detail leading up to the discovery of the crack. This is where it is essential to have an established QC process and supporting documentation, including (at least) qualified mix design, relevant material test data, engineered shop drawings and pre and post-pour inspection logs. These days, some producers are supporting inspection logs right through to final delivery onsite with photographs at strategic QC checkpoints which can add tremendous value during a process audit. Good data is crucial as it enables the QC manager to investigate the cause and implement changes to reduce or eliminate the source of cracking.
Based on the available data, the task force can narrow down the point in the process at which the crack occurred.
According to NPCA, some of the most common sources of crack repair are:
- Engineering (Omissions, dafting errors or incomplete drawings, inappropriate design)
- Production (Material deficiency, concrete mix design, placement and consolidation, improper form preparation, curing)
- Post-pour (stripping strength inadequate, incorrect detensioning process, improper storage)
- Shipping (poor handling, damage, bearing or bindings)
- Onsite (poor handling and installation practice)
An in-depth presentation is available from NPCA at the link below which demonstrates the root cause analysis, right through to appropriate repair methods:
Cause Analysis & Appropriate Repair Methods
Who is responsible? (Producer vs Installer)
Depending on the type of contract, whether supply-install or supply-only, the point at which the crack occurred could be a deciding factor in determining liability for replacement or repair costs (Producer vs Installer).
Fortunately, for difficult-to-diagnose cracks, there are established resources that can assist in troubleshooting cracks in prestressed elements. The most useful of these are the following manuals available from PCI:
Although these manuals have been available for many years, they are surprisingly underutilized in the industry. These resources have been proven to be very valuable in solving cracking issues for both established and new precast businesses across the globe. The diagnostic principles presented therein can be applied to various other products outside of their scope (within reason), and can form the basis for root cause analysis protocol within any quality control program.