Hot weather creates some of the most demanding conditions concrete contractors face on the jobsite. High temperatures, low humidity and high winds accelerate the cement hydration reaction, increase water demand and speed up slump loss. These conditions make it significantly harder to properly adjust mixture proportions, deliver, place, consolidate and finish concrete. The consequences include retempering that lowers strength, increased cracking potential and greater variability in concrete quality. Understanding these mechanisms is essential for producing durable concrete in challenging climates. For a broader overview, see our guide on hot weather concreting effect of hot weather on concrete.
Understanding Retempering and Its Effects on Concrete Quality
What Is Retempering in Hot Weather Concreting?
Retempering refers to the addition of mixing water and remixing a cementitious mixture to restore workability lost due to hot weather. While retempering is acceptable as long as the final water-to-cementitious materials ratio (w/cm) does not exceed the mix design limit and the concrete is thoroughly remixed, it often signals that the mix design or placement strategy needs adjustment.
Petrographers can identify retempered concrete by examining polished surfaces under a microscope. Retempered concrete shows dark, low w/cm mortar coatings that form around aggregate particles, contrasting with lighter, higher w/cm paste in the surrounding matrix. This gives retempered concrete a distinct, mottled appearance distinguishable from properly proportioned concrete.
How Retempering Affects Air Content and Strength
One of the most significant consequences of retempering is its effect on air content. Retempering commonly increases air content substantially. In mixtures with air-entraining admixtures, retempering can produce air contents of 15 to 20 percent, far exceeding the typical target of 4 to 8 percent. Even in mixtures without air-entraining admixtures, retempering can generate entrained air if admixtures containing lignins are present, such as those used for water reduction.
Excessive water and excessive air represent the most common link between hot weather effects and low strength concrete identified by petrographers. When w/cm exceeds the design limit and air content rises well above specifications, the concrete cannot achieve its intended compressive strength.
Delamination Risks from Retempering
Delaminated slabs are another undesirable hot weather effect. This problem arises through two distinct mechanisms:
- Retempering-induced air entrainment: Excessive air in slabs receiving a hard trowel finish can be trapped below the closed surface, creating a delamination plane.
- Premature stiffening: Rapid moisture loss from the slab surface can fool finishers into believing concrete is ready for hard trowelling while the underlying material remains plastic.
Understanding the workability of concrete types and effects on concrete strength is critical for anticipating these challenges before they result in defective slabs.
Cracking Mechanisms in Hot Weather Concrete
Cracking associated with hot weather concreting occurs at early ages through several distinct mechanisms. Petrography combined with field observations provides the most reliable approach for distinguishing between crack types and identifying root causes.
Plastic Shrinkage Cracking
Plastic shrinkage cracking occurs when concrete is stiffening but not fully set, behaving in a ductile manner. These cracks have several characteristic features:
- Discontinuous, curved paths through the concrete matrix.
- Irregular fracture walls that cannot be easily matched together.
- Usually linear and appearing on the slab surface.
- Variable depth from the finished surface.
- Cracks cut around aggregate particles rather than through them.
- No secondary deposits, though carbonation may appear along fracture walls over time.
The primary driver is rapid surface evaporation. When evaporation exceeds the rate at which bleed water reaches the surface, the surface concrete shrinks while underlying material remains plastic, creating tensile stresses that cause cracking.
Adhesion Cracking
Adhesion cracks develop within the concrete matrix and wrap around aggregate particles, creating curved gaps between paste and aggregate surfaces. These cracks often result from plastic shrinkage combined with external stresses from finishing or form stripping while concrete is still stiffening. Adhesion microcracks are commonly found just below the finished surface of hot weather concrete, indicating the concrete experienced tensile stress when it lacked sufficient strength to resist it.
Drying Shrinkage Cracking vs. Thermal Cracking
| Characteristic | Plastic Shrinkage Cracks | Drying Shrinkage Cracks |
|---|---|---|
| Timing | Before concrete sets (ductile state) | After concrete hardens (brittle state) |
| Crack path | Discontinuous, curved | Continuous, straighter |
| Fracture walls | Irregular, hard to match | Cleaner, easier to match |
| Depth | Variable, often surficial | May be full-thickness |
| Pattern | Linear, surface-oriented | Map patterns on exposed surfaces |
| Aggregate interaction | Cuts around aggregate | Cuts around aggregate |
Thermal cracks share many attributes with drying shrinkage cracks, developing fairly clean, straight cracks at early ages after concrete hardens. However, diagnosing thermal cracks requires broader project knowledge. Key influencing factors include cement content, concrete temperature at placement, member size, form removal timing, steel reinforcement details, and early age protection measures. This highlights why petrographers must work closely with engineers familiar with the project design and site conditions.
Key Factors That Influence Thermal Cracking
- Cement content: Higher cement generates more heat of hydration.
- Placement temperature: Warmer concrete accelerates hydration heat generation.
- Member size: Thicker sections trap more heat.
- Form removal timing: Early removal exposes surfaces to temperature differentials.
- Steel reinforcement: Location and size affect stress distribution.
- Early age protection: Insulation and curing moderate temperature gradients.
Finishing Problems Caused by Hot Weather Conditions
Crusting and Its Consequences
Crusting occurs when the concrete surface dries rapidly while the main slab body remains plastic. This differential drying creates a stiff surface layer over a workable interior. On pavements, crusting produces highly irregular surface textures. On warehouse floors and large slabs receiving a hard trowel finish, crusting leads to rough, irregular surfaces that fail flatness specifications. Petrographic examination reveals large voids and abundant adhesion cracks just below the finished surface, indicating tensile stress and delamination during finishing.
The Problem of Surface Blessing
Hot weather can lead contractors to bless a slab by sprinkling additional water on the surface to aid finishing. While this temporarily restores workability, it creates lasting defects:
- A band of lighter paste indicative of elevated w/cm at the surface.
- Contrast with the densified paste produced by early finishing operations.
- Surfaces that are relatively soft and prone to excessive, uneven wear.
- The softened zone may delaminate under traffic or surface loads.
For practical strategies to avoid finishing issues, see our guide on placing concrete in hot weather essential techniques for quality results in high temperatures.
Curing Challenges in Hot Weather and How to Address Them
Why Curing Fails in Hot Weather
Curing is one of the most challenging aspects of hot weather concreting. Inadequate curing produces several distress indicators: significant crazing, shrinkage micro-cracking invisible to the naked eye, and poor hydration of cementitious materials at the finished surface. Petrographers use three main assessment methods:
- Acetone application: Reveals crazing cracks and micro-cracks by changing surface refractive properties.
- Thin section analysis: Compares cement grain concentration at the surface versus depth to assess whether curing sustained hydration.
- Phenolphthalein staining: Detects carbonation depth, which correlates inversely with curing quality.
Surface Strength Loss from Inadequate Curing
Inadequate curing inhibits cement hydration at the exposed surface, creating a strength gradient that leads to:
- Increased wear rates on floors and pavements from soft surface layers.
- Surface dusting under light traffic.
- Accelerated freeze-thaw deterioration as the weakened surface absorbs more moisture.
- Reduced abrasion resistance in industrial applications.
Petrographic investigation determines whether problems are limited to the surface or extend deeper, guiding repair decisions. Surface-limited defects may be addressed by grinding or hardeners, while deeper problems require more extensive remediation.
Best Practices for Hot Weather Curing
To mitigate curing problems, contractors should implement these measures:
- Apply curing compound immediately after finishing, before the surface dries.
- Use wet curing methods such as ponding, continuous fog spraying or saturated burlap for a minimum of 7 days.
- Schedule placements to avoid the hottest part of the day (10 a.m. to 4 p.m.).
- Use evaporative retarders to reduce moisture loss during finishing.
- Monitor ambient conditions with a jobsite weather station.
- Use ice in mixing water or liquid nitrogen cooling to reduce concrete temperature at placement.
For additional guidance, see placing concrete in the heat essential tips for hot weather concreting.
Conclusion
Adverse effects from hot weather conditions may show up at early ages as excessive cracking and insufficient strength, or may not become apparent until almost a year after project completion. Petrographers play a vital role in identifying how hot weather affected a placement and whether problems are limited to the surface or penetrate deeper.
The key to successful hot weather concreting lies in understanding the interconnected effects of temperature on hydration rate, workability loss, evaporation and early-age strength development. By recognizing signs of retempering, understanding different cracking mechanisms, anticipating finishing difficulties and implementing rigorous curing practices, concrete professionals can produce durable concrete even under challenging summer conditions. When problems arise, petrographic investigation provides the diagnostic precision needed to develop targeted, cost-effective solutions that restore concrete performance to its intended level.