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Fresh concrete cracks most often because the exposed surface loses moisture faster than the concrete can replace it with bleed water. Wind, low humidity, heat, mix characteristics and delayed setting can raise that risk. If a crack appears while the concrete is still plastic and workable, some plastic-shrinkage cracks may be closed during finishing—without adding water to the surface. Once the concrete has hardened, identify the likely cause and whether the crack is moving or consequential before choosing a repair.
The timing, pattern and site conditions provide useful clues, but they do not guarantee a diagnosis. Early cracks can also result from settlement around reinforcement, movement or deflection of forms and falsework, differential settlement where section depth changes, or thermal effects. Treat the first response as an assessment and risk-control step, not as permission to apply a generic patch.
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What causes cracks in fresh concrete?
The most common early cracking described in new concrete is plastic shrinkage. Before the concrete sets, water can evaporate from its exposed surface faster than bleed water rises to replace it. The surface layer then shrinks while the concrete beneath restrains it. That restraint creates tension in concrete that is still weak and stiffening, and a crack can form.
Plastic shrinkage is therefore a race between moisture leaving the surface and moisture becoming available at the surface. It is not simply a matter of the air temperature being high. A relatively mild day can still be unfavorable when wind and low humidity remove surface water quickly, while a hotter day may present a different risk if the concrete sets promptly and the surface is protected. The condition of the mix and the timing of placement, finishing and protection all matter.
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The risk depends on interacting conditions, not one weather reading. Wind, low relative humidity, and high air or concrete temperatures can accelerate surface drying. Mixes that bleed less may be more susceptible. The National Ready Mixed Concrete Association (NRMCA) identifies factors such as higher cementitious-material or fines content, reduced water content, entrained air, high concrete temperature and thinner sections. These characteristics can reduce the amount or timing of bleed water available to replace evaporation.
Delayed setting can also extend the period when the surface is vulnerable. Cool weather or a cool subgrade, retarders, some water reducers and supplementary cementitious materials are among the factors that can delay setting. A delayed set does not automatically cause cracking, but it can leave an exposed surface subject to drying for longer. Several moderate influences can therefore combine into a high-risk placement.
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A free scan shows the junk files, broken settings and background clutter dragging Windows down - then fixes them in one click.Free scan · Windows 10 & 11Not every crack that appears soon after a pour is plastic shrinkage. Concrete can settle around reinforcing bars; forms can move or deflect; falsework can settle; and differential settlement can occur where section depth changes. Thermal effects are another possible cause. Caltrans’ Concrete Technology Manual discusses construction-related causes such as falsework settlement and form deflection. Because these mechanisms can occur at similar times, a label based only on appearance can be misleading.
What does a plastic-shrinkage crack look like?
NRMCA describes these cracks as occurring mostly on horizontal surfaces. They are often roughly parallel, relatively shallow and generally do not reach the slab perimeter; typical spacing is about 1 to 3 feet. The description is useful for recording what is visible, especially on a new slab, but it is not a diagnostic test, a tolerance or a rule for judging whether a slab is acceptable.
Use the pattern as one clue alongside when the crack appeared, the concrete’s condition, the pour’s weather and construction conditions. Note whether lines are parallel, whether they stop short of the edge, and whether they seem to follow reinforcement or a change in section. Also record whether the crack is continuing to change, whether the surfaces are displaced, and whether water is passing through. These observations help a concrete professional compare possible mechanisms.
Do not assume a crack is harmless because it looks shallow, or assume it is plastic shrinkage because it is parallel to another crack. Settlement cracks can occur around bars, and movement of forms or falsework can create patterns that do not fit a simple drying explanation. NRMCA’s CIP 5: Plastic Shrinkage Cracking describes the typical pattern and the conditions associated with this mechanism.
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How timing narrows the possibilities
Timing is one of the most practical pieces of information to preserve. A line that appears while placement or finishing is still underway may be associated with plastic shrinkage, settlement or movement, but it cannot be assigned to one cause from timing alone. A line first noticed after the concrete has hardened requires a different response because the limited opportunity to work a crack closed has passed.
Record the sequence rather than relying on a later memory. Write down when placement started, when the surface was finished, when protection or curing began, and when the crack was first seen. Note weather changes, wind exposure, shade, and any delay between operations. If the slab was covered or otherwise protected, record that as well. This record does not prove causation, but it gives the supplier, contractor or engineer the context needed to evaluate it.
What should I do when a crack appears?
First establish whether the concrete is still plastic and workable. The appropriate response differs sharply between a crack that can be worked during finishing and one found after the concrete has hardened. A quick visual impression alone is not enough to select a repair.
- Note when it appeared. Record whether the concrete was still being placed or finished, was stiffening, or had already hardened. Timing helps narrow possibilities but does not prove the cause.
- Describe what you can see. Note the crack’s location, direction, approximate length and whether it appears shallow or reaches an edge. Record approximate spacing when several cracks are present, whether they are parallel, and whether they seem related to reinforcement or a change in section. Also note any visible displacement, continued change or water passing through. Do not treat a surface appearance as proof of depth or structural safety.
- Preserve the conditions. Record wind, humidity and air or concrete temperature information that is available, along with the mix and any delay in placement, finishing or protection. These factors interact, so no single reading settles the diagnosis.
- If the concrete is still plastic, contact the finishing lead or concrete supplier promptly. Some plastic-shrinkage cracks may be closed by working the concrete during finishing. The U.S. Bureau of Reclamation advises doing this without adding water to the surface. This narrow step is not a universal fix for cracks caused by settlement, form movement or other conditions.
- Do not turn a finishing action into a patching operation. Working a crack closed applies only while the concrete remains workable and when the responsible project personnel judge that action appropriate. Adding water to make the surface easier to work can change the concrete at the surface and is specifically contrary to the cited guidance.
- If it has hardened, pause before patching. Determine the likely cause, depth, movement and consequences first. A repair that hides the surface may not address the reason the crack formed.
- Escalate consequential cracks. Seek evaluation by a qualified concrete professional or engineer if a crack is wide, deep, growing, leaking, associated with displacement, or in a structural element. The guidance does not provide a case-specific diagnosis or a single repair method for every crack.
The Bureau of Reclamation’s Guide to Concrete Repair describes repair as a process, not a one-material answer. The Federal Highway Administration also lists many possible causes of cracking in hardened concrete, including shrinkage, thermal stresses, chemical reactions, weathering, reinforcement corrosion, poor construction practices, design errors and applied loads. That range is why a hardened crack should be assessed before a repair is selected.
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How can I reduce the chance of cracks during a pour?
Plan for both surface evaporation and setting behavior before placing concrete. The goal is to limit rapid moisture loss and avoid extending the time the surface remains vulnerable. The appropriate precautions depend on the mix, project requirements and conditions at the site.
Before placement
- Review the forecast and the actual exposure. Consider wind, relative humidity, air temperature, concrete temperature and direct sun together. A forecast is a planning aid, not a guarantee of what the slab surface will experience.
- Discuss the mix with the supplier. Bleeding and setting behavior affect susceptibility. Ask for guidance specific to the concrete being supplied, including the effects of cementitious-material or fines content, reduced water content, entrained air, supplementary cementitious materials and any admixtures that may alter setting.
- Plan around delays. Coordinate delivery, placement, finishing and protective curing so the surface is not left exposed longer than necessary. A delay can matter more when setting is already slow or the surface is drying quickly.
- Check the project requirements. Use precautions compatible with the specifications, the concrete and the site. A measure suitable for one mix or placement is not automatically suitable for another.
During placement and finishing
- Limit wind and sun exposure where practical. Arrange the work to reduce direct drying of the exposed surface. The objective is moisture-loss control, not a promise that cracking can be eliminated.
- Keep the operation coordinated. Avoid unnecessary gaps between placing, finishing and protective curing. The longer the surface remains exposed before protection, the more opportunity there is for evaporation to outrun bleed-water replacement.
- Watch the concrete, not only the forecast. Observe whether the surface is drying while the concrete below remains workable and whether setting is delayed. Report changing conditions to the finishing lead and supplier so the response is based on the actual placement.
- Use only specified or approved moisture-loss precautions. An evaporation retarder may be considered as a prevention-oriented product path, but no brand or comparative performance has been verified here. Confirm compatibility, method and timing with the project specification and concrete supplier rather than treating a product as a repair for hardened or structural cracks.
Evaporation estimates can help flag risk, but NRMCA cautions that conditions interact and estimates are not infallible. A single air-temperature or humidity value cannot settle the risk for every mix and placement. Preparation should account for the combination of wind, humidity, temperature, section thickness, bleeding and setting—not just the forecast.
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What mistakes make the situation worse?
Calling every new crack “shrinkage.” Settlement, form movement, falsework settlement, differential settlement and thermal effects remain possible. A wrong label can lead to the wrong response.
Adding water to the surface. The Bureau of Reclamation’s limited guidance for closing some cracks during finishing specifically says not to add water. Water is not a general-purpose crack repair.
Waiting without documenting change. If a crack is growing, leaking or developing displacement, the change is important information. Record it and seek evaluation rather than assuming that a surface line will remain stable.
Applying a patch before identifying the cause. Hardened concrete can crack because of shrinkage, thermal stresses, chemical reactions, weathering, corrosion, poor construction, design errors or applied loads. A material that conceals a line does not necessarily address any of those conditions.
Relying on a spacing rule. The 1-to-3-foot observation describes a typical plastic-shrinkage pattern. It is not a pass/fail limit, code requirement or structural-safety measurement.
How should a hardened crack be evaluated?
Once the concrete has hardened, begin with a description and a decision about urgency. Identify where the crack is, how long it is, whether it intersects an edge, whether surfaces are displaced, and whether it is changing. Consider whether it is in a slab, wall, beam, column or another structural element. Note leakage and any relationship to reinforcement, joints or a change in section. These observations help define the question a professional must answer: what caused the crack, and what consequence does it have?
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The next step is to determine what information is available about construction. The mix, weather, timing of operations, protection and any known movement of forms or falsework may help distinguish plastic shrinkage from settlement or construction-related movement. The Federal Highway Administration’s discussion of hardened concrete emphasizes the breadth of possible causes, including chemical and weather-related processes as well as loads and reinforcement corrosion. Because the same visible symptom can have different origins, the repair decision should follow the assessment.
There is no verified universal price for repairing a concrete crack. The cited technical guidance does not provide a price list, and cost cannot be responsibly assigned without knowing the cause, depth, movement, consequences, access and project requirements. Obtain a site-specific evaluation and scope before comparing repair quotes. A finishing float is relevant only to the narrow situation in which a crack is worked closed while concrete is still plastic; it is not a repair for hardened or structural cracking.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Why is there no one repair for every crack?
A crack is a visible symptom, not a diagnosis. Plastic shrinkage, settlement, movement of forms or falsework, thermal effects and other mechanisms can produce cracks at an early stage. Hardened concrete has an even broader set of possible causes, including loads, reinforcement corrosion, chemical reactions and construction or design issues. A surface repair chosen without understanding the cause may conceal the crack without resolving the condition behind it.
The supportable immediate action is narrow: some cracks can be worked closed during finishing while concrete remains plastic, without adding water. It does not establish a universal patching material or method for hardened cracks. For those, the decision should follow an assessment of cause, depth, movement and consequence; structural or actively changing cracks warrant qualified evaluation.
A practical record to keep for the project file
For a new crack, keep the date and approximate time, location, photographs showing the surrounding surface, the stage of placement or finishing, and whether the concrete was plastic or hardened. Add weather and exposure notes, mix or delivery information available from the supplier, delays between operations, and the actions taken. If the line changes, add dated observations rather than overwriting the original description. This record supports communication with the contractor, supplier or engineer and helps prevent a later assumption from replacing what was actually observed.
Frequently Asked Questions
Does a 1-to-3-foot crack spacing mean the slab has failed?
No. That spacing is NRMCA’s typical description of plastic-shrinkage cracks, not a pass/fail limit, building-code requirement or measure of structural safety. The pattern alone cannot diagnose a crack or determine whether a slab is acceptable.
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Can a crack that appears during finishing be caused by something other than drying?
Yes. Plastic shrinkage is a common early mechanism, but settlement, form movement, falsework settlement, differential settlement and thermal effects are also possible. The crack’s timing and appearance are clues rather than a definitive diagnosis.
Should I add water to close a crack in fresh concrete?
No. The Bureau of Reclamation’s guidance for closing some cracks during finishing specifically says not to add water to the surface. If the concrete is no longer workable or the cause is unclear, get project-specific advice instead of treating that step as a general repair.
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Not reliably. A crack that looks shallow at the surface may need further assessment, especially if it is growing, leaking, displaced or in a structural element. Those signs are reasons to seek qualified evaluation before selecting a repair.
What weather conditions increase the risk of plastic shrinkage?
Wind, low relative humidity, and high air or concrete temperatures can accelerate surface moisture loss. Risk also depends on the mix, section thickness, bleeding and setting behavior, so one temperature or humidity reading cannot determine the outcome by itself.
The Bottom Line
Fresh-concrete cracks often begin when surface evaporation outruns bleed-water replacement, but settlement, form movement, falsework settlement and thermal effects can look similar. Document the timing and pattern, avoid adding water, and have workable plastic cracks addressed during finishing only when the responsible professionals judge that appropriate. For hardened, changing or consequential cracks, diagnose the cause before selecting a repair.
