Plastic-Sheet Concrete Curing Problems: How to Prevent Tiger Stripes, Trapped Water, and Soft Spots

Plastic-sheet curing is used to retain moisture and moderate temperature at the concrete surface while the paste hydrates. The sheet acts as a simple barrier to evaporation and can help reduce thermal gradients during early strength gain.

It’s commonly specified where rapid moisture loss is a concern or where liquid curing compounds are impractical, but other methods (wet coverings, curing compounds, or internal curing) are chosen based on job conditions and specification requirements. Check product labels or project curing specifications to confirm which method the specifier requires for your slab.

Benefits and common applications

Plastic sheeting is a popular choice for concrete curing due to its effectiveness and versatility. It’s commonly used in situations where moisture retention and temperature control are crucial.

Flatwork, like sidewalks and driveways, benefits greatly from plastic-sheet curing. The sheets help maintain consistent moisture levels across the entire surface, promoting even curing and preventing cracks.

For large slabs, like those used in foundations or warehouse floors, plastic sheeting is often specified. It helps control temperature fluctuations that could otherwise lead to uneven curing and warping.

Plastic sheets are also ideal for temperature-sensitive pours, such as those in cold weather conditions. They trap heat from the hydration process, keeping the concrete warm and promoting proper curing.

How plastic-sheet curing works physically

Plastic sheeting creates a barrier around the freshly poured concrete, trapping moisture and heat within. This process is crucial for proper concrete hydration.

The sheets act as a vapor barrier, preventing water from evaporating too quickly. This helps maintain consistent moisture levels throughout the curing process, ensuring even strength development.

At the same time, plastic sheeting traps heat generated by the exothermic hydration reaction. This trapped heat speeds up the curing process and helps prevent freezing during cold weather pours.

By controlling both temperature and moisture loss, plastic-sheet curing ensures that concrete cures evenly and consistently, leading to a stronger, more durable final product.

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Tiger stripes are surface discoloration or patterning that follows local moisture or finishing differences, trapped water is ponding under the sheet, and soft spots are areas of reduced surface hardness or cohesion. Visually, tiger stripes look like streaks or blotches, trapped water appears as visible bulges or wet patches under the plastic, and soft spots feel weak or powdery when probed.

These issues matter because they affect finish uniformity, long-term durability, and load capacity—cosmetic defects can hide deeper consolidation or curing failures, and soft spots can lead to premature wear or localized failures. When in doubt about severity, document observations and verify against manufacturer guidance or project testing requirements.

What are tiger stripes

Tiger stripes, also known as ‘tiger striping’, is a common issue that occurs during the curing process of concrete when plastic sheets are used. They appear as alternating light and dark bands on the surface of the cured concrete, resembling the stripes of a tiger.

These stripes typically form at the edges of slabs, along joints, or in wheel paths where evaporation rates are higher due to increased exposure. The uneven hydration and evaporation cause the top layer of cement paste to dry out faster than the underlying layers, leading to the formation of these stripes.

The appearance of tiger stripes doesn’t necessarily mean that the concrete is structurally compromised. However, they can affect the finish and aesthetics of the surface, making it less appealing.

How trapped water forms under plastic

Trapped water under plastic sheets during concrete curing can lead to various issues. This water can come from several sources – surface irregularities, ponding (water collecting in low spots), condensation (water vapor turning into liquid), or poor sealing of the plastic sheet.

When water is trapped, it can weaken the surface and finish of the concrete. It may cause delamination, where the top layer of concrete peels off, or efflorescence, a white powdery deposit that forms on the surface as the water evaporates and leaves behind salts.

The trapped water can also lead to increased pressure under the plastic sheet, which can cause blisters or pop-outs on the surface of the concrete. These issues can compromise the durability and strength of the concrete.

Definition and causes of soft spots

Soft spots in concrete refer to areas where the concrete is weaker than it should be due to inadequate curing or other issues. These spots are typically found beneath the surface, hence the term ‘subsurface’ soft spots.

The most common causes of soft spots include inadequate consolidation during placement, isolation of water (due to poor sealing or lack of moisture), and contamination of the concrete mix. Inadequate consolidation can lead to voids in the concrete, which weaken its structure. Isolation of water can cause the concrete to dry out too quickly, leading to reduced strength. Contamination can interfere with the hydration process, preventing the concrete from reaching its full potential strength.

Soft spots can compromise the strength and durability of the concrete. They can lead to cracking, delamination, or other structural issues over time, making it important to identify and address them as soon as possible.

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Plastic curing can fail from improper placement (wrinkles, gaps, poor sealing), material incompatibilities (breathable versus non‑breathable film interacting with admixtures), and environmental stresses like wind, sun, or large temperature swings. Any action that allows uneven moisture loss, local cooling, or ponding creates a risk for surface defects and weak zones.

Human factors—timing of sheet placement, how the slab was finished, and traffic on the fresh surface—also contribute, as do mix factors such as water content and cement type. When you suspect a cause, consult the plastic product data sheet, mix design notes, and jobsite records to narrow the root causes.

Placement and workmanship issues

Proper placement of plastic sheets is crucial for effective curing. Laps should be tight and well-sealed to prevent evaporation.

Wrinkles in the sheeting can trap water, leading to trapped moisture problems. Ensure sheets are laid flat and smooth.

Gaps between sheets allow for rapid evaporation and can lead to tiger striping. Make sure sheets overlap properly (shingling) and are anchored securely to prevent wind from lifting them.

Environmental factors (temperature, wind, sun)

Temperature plays a significant role in concrete curing. High temperatures speed up evaporation, while low temperatures slow down hydration.

Wind can drive off surface moisture, causing rapid drying and tiger striping. Use weights or anchors to keep sheets secure.

Solar heating through plastic can create temperature gradients, leading to uneven curing and potential cracking. Avoid placing dark-colored sheets in direct sunlight.

Material incompatibilities and permeability

The choice of plastic sheeting material greatly impacts moisture control during curing.

• Non-breathable film: Prevents water vapor from escaping, leading to trapped moisture and potential soft spots. Use breathable film instead.
• Thin film: Offers little protection against evaporation or temperature fluctuations. Choose a film with adequate thickness for your climate.
• UV-degraded film: Becomes brittle and permeable over time, allowing water to evaporate too quickly. Inspect film regularly and replace if necessary.
• Chemically-resistant film: Some chemicals in concrete can degrade plastic sheets. Ensure your film is rated for use with the specific concrete mix you’re using.
• Film with low moisture vapor transmission rate (MVTR): Slows down evaporation too much, leading to trapped water and potential soft spots. Choose a film with an MVTR suitable for your climate.

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Select film based on job needs: thickness for durability against tears, UV resistance for sun exposure, and opacity if sunlight or heat transfer is an issue; consider slip resistance if walking on the plastic is required. Some jobs call for impermeable film while others use specialized breathable membranes—follow manufacturer instructions and the project curing specification to decide.

Also consider accessory materials: tapes rated for outdoor use, weighted anchors, and seam overlaps sized for your conditions. If you aren’t sure which film property matters most for your site, check product technical data sheets and any applicable standards referenced by the contract.

Film thickness and permeability considerations

The thickness of your plastic film matters. Thicker films resist punctures better but can trap heat, causing excessive curing temperatures that lead to tiger stripes.

Breathable films allow moisture vapor to escape, preventing trapped water issues. But they might be less durable against jobsite hazards.

Check product ratings and recommended applications. Some films are designed for specific slab types or environmental conditions.

Alternatives and supplemental materials

Plastic sheets aren’t always the best choice. Other methods can provide better results depending on your situation.

• Curing compounds: These form a protective membrane over concrete. They’re great for vertical surfaces but can stain if not removed timely.
• Wet coverings (burlap, straw): These retain moisture well and provide insulation. But they can leave imprints on the surface and may harbor mold growth.
• Combination methods: Using plastic sheets with curing compounds or wet coverings can enhance results. But ensure compatibility to avoid bond issues.
• Sealants and admixtures: Some sealants and waterproofing admixtures can reduce the need for plastic sheeting. Check their compatibility with your concrete mix.

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Prepare the slab by removing debris, ensuring the surface is not ponded, and timing placement to avoid disturbing fresh finish; lay the sheet smoothly with controlled wrinkles to prevent water channels. Overlap seams according to manufacturer recommendations and use compatible tape or adhesive to seal edges and laps against wind uplift.

Anchor edges with sand, boards, or weights spaced so the sheet stays flat without creating pressure points that could trap water. If you need to walk on the sheet, use planks or designated walkways to avoid punctures and localized softening.

Surface and edge preparation

Before laying the plastic sheet, ensure your concrete surface is clean. Remove any dirt, debris, or form oil that could prevent intimate contact with the plastic.

Level the surface as much as possible to avoid wrinkles and gaps. Use a straightedge to check for low spots where water can pool.

Pay special attention to the perimeter. Ensure it’s level and free of obstructions that could cause trapped water paths or gaps in the plastic.

Lapping, taping, and anchoring best practices

Overlap sheets by at least 12 inches to create a continuous barrier. Use a straightedge to ensure the overlap is even.

Seal the overlap with a suitable tape designed for concrete curing. Avoid using duct tape or other household tapes as they can’t withstand the conditions.

Anchor the plastic at intervals no greater than 3 feet apart. Use stakes, weights, or sandbags to keep it in place and prevent wind from lifting it. Be careful not to overstress the film by pulling it too tight.

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Prevent ponding by sloping the slab surface or providing temporary drainage routes before laying the plastic, and avoid large continuous low spots where water can collect under the sheet. Add venting details or raised spacers in long runs to allow trapped air and condensation to escape during temperature swings.

If condensation appears, open a vent or lift a corner to dry and inspect the concrete quickly; do not leave heavy ponded water sitting under the film. When unsure how to handle a condensation event, follow the plastic maker’s guidance and job curing specs for corrective steps.

Designing temporary drainage and venting

To prevent water from pooling under your plastic sheet, you need to design a simple drainage system. Here’s how:

Perimeter Drainage: Create a small trench around the perimeter of your slab, about 2-4 inches deep. This will allow water to flow off and away from the slab.

Venting: Poke a few small holes in the plastic sheet, about 1 inch in diameter, near the top edge. This allows any trapped air or moisture to escape as temperatures rise during the day. Cover these vents with a piece of fabric or mesh to keep insects out.

Detecting and safely removing trapped water

Trapped water under your plastic sheet can cause serious issues. Here’s how to detect and remove it safely:

Detection: Look for signs of water buildup, such as bulging or sagging of the plastic sheet. You might also see condensation forming on the underside of the sheet.

To confirm, carefully lift a corner of the sheet and look for standing water. If you find any, act quickly to remove it before it causes damage.

Removal: Use a squeegee or a piece of flat wood to gently push the water towards one end of the slab. Once there, carefully lift the plastic sheet at that corner and allow the water to drain off. Be sure not to let the water sit on the surface for too long, as this can cause rapid drying and create cracks.

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Create a simple inspection schedule focused on early visual cues: look for streaks or discoloration, bulging under the film, and areas that feel soft when gently probed. Record observations with locations and times so trends can be tracked and correlated to weather or job activities.

Include checks after major temperature swings, storms, or high winds and after any on-sheet traffic or repairs. If measurements such as surface hardness or moisture readings are required by the specification, note the instrument and method and compare to the project criteria.

For cosmetic tiger stripes, light surface repair or planned finishing after the curing period may be sufficient; for trapped water, lift the sheet, remove the water, dry the area, and re‑seal the plastic properly. Soft spots that affect function require more invasive assessment—probe depth, pull test, or cores may be needed to determine if repair or removal and replacement is necessary.

While temporary stabilization (pads, plates, or shoring) can allow safe use, follow the engineer’s direction for structural repairs and document the repair method and materials used. If you’re unsure what repair route to take, check product instructions and consult the project engineer or testing lab for acceptance criteria.

Fixing tiger stripes and surface defects

Tiger stripes are cosmetic issues, not structural. Here’s how to fix them:

Light Grinding: Use a concrete grinder to remove the affected layer. Be careful not to go too deep.

Patching: If grinding doesn’t help, use a concrete patching compound. Apply it, let it cure, then refinish the surface.

If tiger stripes keep reappearing, there might be an underlying issue with your plastic sheeting or curing process. Refer back to Managing Water and Condensation Risks for help.

Addressing subsurface soft spots and structural issues

Soft spots indicate a problem beneath the surface. Here’s how to address them:

Inspection: Core test and perform load checks to determine the extent of the damage.

Temporary Propping: If the structure is unstable, temporarily prop it up to prevent further damage.

If soft spots are extensive or deep, partial removal and re-pour might be necessary. For smaller issues, consider injection repair. Refer back to Proper Placement and Sealing Techniques for tips on preventing these issues in the future.

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