Cold Weather Concreting

Dry cast concrete production - what to know as temperatures fall

Concrete professionals know that weather can have a big impact on concrete production. Here’s a short primer on low and freezing temperature challenges.

Cement hydration basics

Concrete is made up of aggregates (sand and stone), cementitious materials and water. The chemical reaction between cement and water is called hydration. The cement hydration reaction slows down as it gets colder, and speeds up as it gets hotter.

The business model of manufactured concrete products producers is based on efficiency, and typically requires kilns or curing chambers to raise the internal temperature of units to speed up the cement hydration reaction and meet production quotas, particularly in cooler weather. 

In general, for dry cast concrete, cold weather measures should be implemented when concrete temperatures fall below 55° F (13° C) for more than 3 days.

 What happens to fresh concrete when it freezes?

What happens to concrete made below 55° F (13° C)?

Why can slower cement hydration be a good thing for production?

In some ways, concrete production can be easier when temperatures drop: 

What can go wrong with cold weather production?

The general rule of thumb for concrete is that for every 20° F (approx 10.5° C) drop in temperature, set time will double and early strength gain will be much lower. 

What this means is that concrete units that are manufactured and placed in the yard at 40° F (5° C) will essentially stop hydrating until temperatures rise and restart the hydration process. 

An under-appreciated risk of placing under-cured units outside in cold weather is the increased risk of efflorescence when the units warm up in the Spring. Cold temperatures slow down the cement hydration reaction, leading to extended periods spent in hydration phases that are higher calcium hydroxide generators. Of course, calcium hydroxide is a primary contributor to efflorescence production. When warmer weather and moisture occur in the Spring, there may be more calcium hydroxide available in these units for efflorescence production.

What can we do to mitigate the negative effects of cold weather?

When is it too cold to make concrete?

Good question! With proper processes, infrastructure and equipment in place it is possible to produce concrete in very cold ambient conditions. However, there are significant raw material, capital equipment and production issues that must be addressed first. 

For most producers, 55° F (13° C) ambient temperatures for more than 3 days are a commonsense place to start with cold weather concreting practices. 

How can I learn more about controlled curing?

ACM has a Learning Center on our website with many tools to help dry cast producers with daily production needs.

Concrete 101 – Controlled Curing

A 15 minute video on the basics of controlled curing.

Who can I contact if I have questions?

Your Technical Services Representative can assist you with questions about all aspects of concrete technology. Please reach out to them directly, or call our office at 770-417-3490.

Face Mix Production

Best practices and guidelines for production crews.

Face mix is now established in the North American concrete paver market, and many paver producers offer both traditional thru mix and face mix choices. 

In contrast to traditional thru mix pavers that use the same concrete mix throughout the paver, face mix pavers incorporate a top layer, usually less than 3/8” (9 mm). Both types of pavers deliver beauty and durability to hardscapes, and either one can be used with confidence.

Face Mix Production Challenges

Raw Materials & Mix Design

Water Content: As always, it is important to maximize the water content in both face and base mix. 

Cementitious Materials Content: This should increase as aggregate Fineness Modulus (FM) decreases. FM is an index number that represents the mean size of particles in an aggregate sample, and is used to get an idea of how coarse or fine an aggregate is. The higher the FM the coarser the aggregate. A lower FM indicates a finer aggregate. 

Aggregates: When using aggregates with a lower FM (smaller particles) it may be necessary to increase cementitious fines to cover the greater aggregate surface area, and to effectively block the smaller void structure that results from finer aggregates. ACM can show you how to properly balance face mix proportions and aggregate blend curves to avoid overly fine mixes, while still achieving an attractive, smooth-textured surface. You can also find out more about aggregates and how to calculate FM on our ACM Academy Courses: Aggregates Part 1 and Part 2. 

The higher the FM, the coarser the aggregate

A note about unintended consequences: Increased portland cement and other cementitious materials content can deliver many positives for pavers, but there is an important downside to be aware of. Increased cement content may lead to an increased potential for efflorescence. 

This happens for two reasons: 

  1. Increased Portland cement means more calcium hydroxide is produced during the cement hydration reaction. Calcium hydroxide is an important contributor to efflorescence production. 
  2. The second reason is that cementitious particles are very fine, and increased cementitious content may affect the capillary void structure in the concrete matrix, typically making voids or capillaries smaller. The laws of fluid dynamics dictate that the smaller the capillary the higher the suction, or wicking action, of moisture. Therefore, an unintended consequence of additional finer cement particles in a coarser aggregate blend may be that the unit experiences more frequent wetting and drying cycles as available moisture is more efficiently wicked into the unit through the smaller void structure. As always, the more moisture in the unit, the greater the potential for efflorescence. In summary: face mixes with coarser aggregate blends (higher FMs) should contain relatively lower cementitious contents than face mixes with finer aggregate blends (lower FMs).

Admixtures: Most producers find that using a plasticizer in combination with an efflorescence control admixture delivers best results for pavers. Typically, the same admixtures may be used in the face and base mix, but may need to be used at different dosage rates. 

Consider the use of ProCast™ 710 retarding admixture in warm weather, with white cement, or variegated color blends. Retarders extend workability and allow more time for mixing and molding operations. Consult your ACM Technical Services Representative for optimal admixture addition rates and mix sequencing guidelines

Base Mix


As demonstrated above, narrower openings allow capillary action to pull water through voids more efficiently

The most important requirement for a good face is a good base! 

Face Midx


Using angular fractured or manufactured aggregate in the base can help the base and face to key together

Mix Time: Final mix of up to 60 seconds is recommended.

Batch Size: Right size face mix batches to usage time. Many facilities install smaller face mix mixers to facilitate smaller batch sizes. Minimizing work time for blended colors helps ensure proper color distribution throughout the mix.

Vibration and Feed: Typically, we recommend vibration on the base mix is kept to a minimum. Once the face mix is added, use the main vibe to drive the face mix into the base. Following this process will help produce quality pavers that are homogeneous across the whole mold – front to back and side to side, and that have the optimum physical properties. 

Pay attention to the amount of material in the feed drawer – adding too much material may result in clumps and restricted flow.

Wet Side, Inline Quality Checks

Using wet side, inline quality checks provides real time feedback on the quality of the concrete as it is being manufactured. 


Water beading tests done on the wet side, before curing, can provide instant feedback, and prevent a small problem from becomng a large one.

As noted previously, even if the face surface looks tight, appearances can be deceiving. Finer particles and associated finer void structure in a poorly designed face mix can be very efficient at wicking moisture. A water beading test offers a more reliable and objective measure of the absorptive qualities of the face than a visual inspection.

To perform the Water Beading Test: 

The Thumb Print Test, and Water Wicking Test can also be used for real time quality checks. To find out more, check out Concrete 101 QA/QC Part 2.

Check out the ACM Learning Center
for more information on concrete technology, including aggregates, mix design and QA/QC

Hot Weather Concreting

Dry Cast Concrete - What to know as temperatures rise

Concrete professionals know that weather can have a big impact on concrete performance. Here’s a short primer on high temperature challenges.

Cement hydration basics

Concrete is made up of aggregates (sand and stone), cementitious materials and water. The chemical reaction between cement and water is called hydration. The cement hydration reaction slows down as it gets colder, and speeds up as it gets hotter.

In general, hot weather measures should be implemented when concrete temperatures exceed 90° F (32° C).

What happens to concrete made above 90° F (32° C)?

Hot materials drive off water fast, so more moisture evaporates from aggregates before they are mixed into the concrete.

The rate of cement hydration accelerates so the mix stiffens and loses workability faster. The mix may not feed and compact well in the mold.

Poor compaction leads to more interconnected voids.

More interconnected voids lead to lower density and higher absorption, which negatively affects strength, durability and the general quality of the concrete. 

What does faster cement hydration mean to me?

Faster cement hydration means less time to work with concrete before it loses workability and begins to harden.When this happens it can lead to the following: 

  1. The amount of time that concrete can be held without forming product decreases.
  2. There is less time to move concrete through the machine before the mix feed and flow slow down, making it harder to fill the molds and compact the mix properly.
  3. Improper mold filling means that product heights can be inconsistent, and corners may be compromised with greater potential for cracks and damage.
  4. Lack of compaction means more interconnected voids in the mix. This translates to reduced density and increased absorption, which in turn affects product quality and durability in freeze-thaw environments. 
  5. Mixer efficiency may be compromised as hardened concrete builds up faster on mixer paddles and walls. Poor mixing can lead to segregation, or uneven distribution of cement, water and aggregates throughout the mix.
  6. Excessive buildup in the mixer also means a lot more work for the cleaning crew. 

Which units are most affected?

In general, units that are the hardest to make under ideal conditions will often also be negatively affected by hot weather conditions.

  1. Lightweight mixes – due to their already high water demand, lightweight aggregates must be pre-wetted in hot weather to ensure that there is sufficient moisture available to hydrate the cement. Pre-wetting methods can include: spraying the stockpile, spraying on the belt, or adding lightweight aggregates into the mixer with additional water prior to adding cement and other raw materials.
  2. White cement – white cement may be more likely to lose workability as temperatures rise. Use of a retarding admixture, like ProCast™ 710, is strongly recommended during hot weather conditions. 
  3. Face mix – the smaller volume of face mix compared to the base mix means that it is essential to calibrate the batch size and timing of the base and face batches. Smaller face mix batches are often helpful. 
  4. Smaller units - like twinkies. Compromised feed and fill make it harder to fill smaller mold cavities. 
  5. Blends – as mixed materials sit in their individual hoppers they may begin hydrating and stiffening before there is time to place them in the mold cavity.
  6. Mixes with a high percentage of fines, like face mix, have a naturally higher water demand due to the higher aggregate surface area, and may be harder to mix and hydrate properly. 

What can we do to mitigate the negative effects of heat on concrete and equipment?

When is it too hot to make concrete?

Good question! That’s hard to put an exact number on, but if hot weather is preventing you from meeting your QA/QC performance targets, then it is time to consider implementing some of the measures we mentioned above.

How can I learn more about cement hydration?

ACM has a Learning Center on our website with many tools to help dry cast producers with daily production needs. We have two free learning tools, available 24/7 on our Learning Center, that are focused on cement hydration. 

Concrete 101 – Cement - A 15 minute video on the basics of cement

ACM Academy – Cement - A one hour webinar for concrete professionals for a deeper dive into cement chemistry.

Water Repellent Masonry

What is an Integral Water Repellent? 

Integral water repellents are polymer based chemical admixtures added to Concrete Masonry units (CMU) during the manufacturing process. The water repellent is incorporated into the concrete mix at the plant to ensure that each block has water repellent distributed throughout the concrete matrix. 

Untreated masonry units readily absorb water through a process called capillary suction or wicking action. RainBloc anti-wicking action ensures that even if rainwater penetrates past the exterior face of the wall, RainBloc system water repellent properties minimize the amount of water that is absorbed into the concrete, causing any water inside the wall to flow to properly designed flashing and weep holes.

*Integral water repellents do not change the appearance of the units.

Will RainBloc prevent any moisture from entering a masonry building?

RainBloc is typically used in single-wythe masonry construction for additional protection to water ingress. Single-wythe masonry walls can provide the performance of cavity wall construction in a cost-effective manner. 

RainBloc is not a replacement for proper single-wythe wall design and construction.  A complete water repellent system emphasizes proper masonry design, details and implementation. Use of the RainBloc system provides an effective barrier to water intrusion and water wicking into  CMU by allowing any intruded moisture to fall down the interior of the cores and exit at the flashing weeps.  

However, even with the additional protection, designers of masonry walls must still expect and prepare for moisture to penetrate into the walls through defects, cracks or other means and incorporate water collection and draining details into the design. Proper flashing, weeps and movement joints designed and located appropriately to control cracking are essential. Of course it is also important for the mason contractor to follow the design specifications and drawings and apply good construction practices to their work.

View the RainBloc Product Data Sheet.

Concrete Masonry Structure Exposed to Wind-Driven Rain

Untreated Concrete Masonry Unit

  • Moisture can penetrate through concrete surface
  • Untreated concrete matrix has excess voids that moisture can penetrate into
  • Wicking action pulls moisture through excess coils and capillaries deeper into the concrete

RainBloc Treated Concrete Masonry Unit

  • Water repellent blocks moisture from entering at the surface
  • High quality concrete matrix reduces voids that moisture can penetrate into
  • Anti-wicking action reduces water movement in voids and capillaries

Can a post applied sealer or water repellent be applied to RainBloc treated masonry?

Yes, there are no incompatibilities between RainBloc and post applied sealers which are often used to provide additional protection in a “belt and suspenders” approach. 

Topical (surface) treatments are applied to the weather exposed side of the wall after the wall is in service.

Can paint be applied to RainBloc treated masonry?

RainBloc typically does not change the “paintability” characteristics of CMU, and therefore if untreated masonry units can be painted, the same will apply to RainBloc treated units.  Latex, acrylic latex, cementitious coatings and waterborne epoxies may all be used. Note that oil based paints (alkyls) are not recommended for any exterior applications and should be carefully tested before application in any masonry construction.

Is there a RainBloc treatment for mortar?

Yes, RainBloc for Mortar is specifically designed for mortar applications, and contains additives to improve the workability and water repellency of the mortar. RainBloc for Mortar does not affect the bond strength between the CMU and the mortar.

Can RainBloc for CMU and RainBloc for Mortar be interchanged with each other?

No, their formulations are different and they cannot be interchanged.

Can an accelerator be added to RainBloc for Mortar if needed during colder weather?

Non-chloride accelerator integral admixtures may be used during winter months when faster set times are desired.  Chloride accelerators are not recommended, as they can react with steel reinforcement and cause corrosion of the steel. 

To use an accelerator admixture, simply replace a portion of the mix water with the recommended dosage rate of non-chloride accelerator.

View the RainBloc for Mortar Product Data Sheet.

What does it mean to be RainBloc Certified?

RainBloc certified producers go through a rigorous certification process to ensure that performance of their CMU conforms to RainBloc standards for physical properties and water repellency. Certification must be renewed each year to ensure continued compliance. 

Is Masonry treated with RainBloc breathable?

RainBloc is not impervious to moisture and it does not form a film on the masonry surface. RainBloc is not a vapor barrier, and the masonry units remain breathable. 

What is the life expectancy of RainBloc?

RainBloc is designed to perform for the life of the masonry building. 

Contact Us for help finding a RainBloc producer.

Concrete Pavers and Sustainability

Light Color Pavers and the Urban Heat Island Effect.

For hardscape owners and designers color is a very important consideration for aesthetics and function, but heres another factor: color can also impact sustainability. Choosing lighter colors can help mitigate the urban heat island effect. 

Urban Heat Islands

Heat islands can result in cities when built structures and paved surfaces radiate energy from the sun to a greater extend than farmland or natural areas. In this way, cities create their own microclimates that can be up to 7° F (4° C) warmer than the surroundings.

Some examples of how heat islands negatively impact our environment include  more air conditioning  use, increased air pollution and green house gas emissions from power plants meeting air conditioning energy demands, and lower human health and wellbeing from excessive heat.  

 Landscape architects can play a role in cooling cities by specifying increased tree and vegetation cover, adding living or green roofs to structures, and selecting light color surfaces also known as cool pavements. 

Research was done in New York City which found that planting trees and vegetation would be greatly beneficial to cool surfaces. However, they encountered a common urban problem - in many New York City neighborhoods there is no space.

Many large cities just don’t have the space required to plant enough new trees and greenery for an effective heat island reduction strategy. 

Therefore, the most attainable approach is often to redevelop the large areas of dark, paved surfaces with lighter surface materials. 

LEED® Credits for Heat Island Reduction

Heat Island Reduction - Non-Roof: LEED v4.1

LEED points can be awarded for paving materials with an initial solar reflectance (SR) value of at least 0.33. 

https://www.usgbc.org/credits/new-construction-core-and-shell-schools-new-construction-retail-new-construction-data-cent-5

Solar Reflectance Index (SRI)

The Solar Reflectance Index (SRI) is a criterion used by US Green Building Council (USGBC)  that measures values of sunlight and radiation bouncing from built surfaces.

SRI is used to indicate how hot a material is likely to become when its surface comes into contact with solar radiation. On a scale of 0 to 100, standard black is 0, and standard white is 100. According to this scale, testing indicates that absorbent materials have lower numbers while reflective materials have higher numbers. 

Applying this to the hardscape environment, it follows that dark pavements have low SRI values, whereas light pavements typically have higher SRI values. 

In other words, light colored surfaces absorb less heat and make the immediate area more comfortable – think playgrounds or pools where bathers have bare feet. Lighter surfaces also reduce the need for nighttime lighting and make areas safer.

Combining light colors with permeable pavers can provide even more cooling benefits because permeable interlocking concrete pavers (PICP) are designed and constructed to lower surface temperatures through evaporative cooling as well. The Interlocking Concrete Paver Institute (ICPI) has many useful resources for designers at www.icpi.org.

Light Color Paver Protection

New surface treatments from ACM Chemistries protect paver surfaces from fading and stains. Lighter colors no longer have to appear washed out or marred by food or dirt stains. Colors and patterns stay vibrant and stains and dirt can be easily removed. 

For more information on surface treatments, check out https://www.acmchem.com/dry-cast-paver-surface-treatments/

Resources

Interlocking Concrete Paver Institute (ICPI) 

https://icpi.org/benefits-fact-sheets

US EPA

https://www.epa.gov/heatislands/learn-about-heat-islands

https://www.epa.gov/heatislands/heat-island-compendium

Face Mix and Thru-Mix Concrete Pavers

One of the more recent developments in dry cast or zero slump concrete paver production is “face mix”. The difference between face mix and conventional thru-mix pavers is that thru-mix paver mix design and color is the same throughout, whereas with face mix pavers there is a pigmented surface layer with a finer aggregate blend. Face mix pavers concentrate expensive pigment, white cement and finer aggregate in a surface layer where they have the most impact. The base contains larger aggregates for higher compressive and flexural strength, and improved durability. This method is well established in the US and has been used in Europe for decades.

Face mix pavers are also well suited for inline surface treatments to enhance color and resistance to stains. See https://www.acmchem.com/dry-cast-paver-surface-treatments/ for more information on our factory applied inline surface treatment systems.

In the picture we see examples of face mix pavers with concentrated color on the surface layer.

Face Mix Analogy

How are Dry Cast Concrete Pavers Made? 

Most traditional dry cast or zero slump pavers will follow the steps below. Bear in mind that producers are coming up with new and interesting production methods and paver finishes every day!

  1. Mixing raw materials until a homogenous mix is obtained
  2. Feeding mix into a mold
  3. Compression into mold
  4. Inline surface treatment (if applicable)
  5. Curing 
  6. Packaging 

Dry cast or zero slump concrete holds its shape immediately after a mold is removed, similar to packing sea sand into a bucket (mold) to make a sand castle on your beach vacation. 

In dry cast concrete production the raw materials are fed into a mixer which combines them until the mix is homogenous. The mixing is important to get the cementitious materials and water in contact so that a chemical reaction called cement hydration can occur. During the hydration reaction, cement and water interact to form cement paste which hardens and becomes the “glue” that holds the aggregates together.

Dry cast paver production is a highly automated process. The business model of dry cast manufactured concrete products depends on highly efficient, mass production of concrete units that are also efficient to install: at a minimum concrete paver units must be sufficiently strong, dimensionally correct and dimensionally stable when they are installed.

Large, sophisticated plants can cost millions of dollars with new options for molds and finishes becoming available every year.

Dry cast concrete units are mixed and molded into shape in minutes. The freshly compacted units are able to hold their shape immediately after mold is removed including during transportation to the curing station, which is often a kiln. A kiln is a controlled environment where temperature and humidity are optimized to maximize cement hydration, strength gain and color development by the concrete. Dry cast concrete must be cured, usually for at least a few days, so that it can gain sufficient strength to withstand handling, installation, traffic loads and weathering over time.

Inline surface treatments are spray applied to the paver surface before the units are cured, and are bonded to the paver surface during curing. Inline treatments are used to enrich color and protect paver surfaces from staining and fading.

For comparison purposes, see the dry cast vs wet cast production and output summaries below:

Dry Cast vs. Wet Cast

To find out more about the benefits of interlocking concrete pavers from ICPI, click https://icpi.org/benefits-fact-sheets

Example of dry cast step units and segmental retaining wall

Example of wet cast step units and segmental retaining wall

Concrete Pavers - a Brief Explainer 

Concrete pavers are made out of … concrete. Sounds obvious, but there are actually at least two different types of concrete used to make pavers and slabs – dry cast (zero slump) and wet cast concrete. Dry cast concrete is the most common, so lets deal with that first.

How does cement work?

Dry Cast Concrete paver proportions:

Introduction to Dry Cast Concrete Pavers

We tend to think of public transportation as a modern invention – but the Romans used segmented paving stones for public highways that sped troops, trade goods, tax collectors and administrators that ran and funded the empire.

Roman roads were paved with locally available stone materials, but the Romans knew and applied sound, common engineering practices:

Today, 2500 years later, we still follow the same principles.

In modern times, concrete pavers took off as a building material in Europe after World War II.

Rebuilding efforts after World War II faced major shortages of building materials. German and Dutch road designers and contractors were forced to find replacements for clay brick which was needed for houses They developed cost effective, uniform size concrete-based pavers made from readily available materials. The new concrete pavers were tolerant of unstable sub-base and traffic loads, and could be installed by relatively unskilled labor.

Pavers accommodate cars, bicycles, pedestrians and trees! Amsterdam, NL

Number of paver sq. ft. per person

Today, Germany is still the recognized leader in the paver market with over 1 billion sq.ft of pavers installed per year.

Efflorescence - What is it and what can I do about it?

What is Efflorescence?

Efflorescence in concrete units is usually observed as a white deposit on the surface. This deposit can occur as a slight haze up to a crusty layer. Efflorescence is not a structural problem, it does not affect concrete strength or durability. But it is unsightly and lowers the perceived value of the concrete. 

Efflorescence is usually composed of salts that are deposited on a concrete surface. It’s worth noting that any concrete product that contains cement has the potential to produce efflorescence. There are four factors that combine to produce efflorescence, and they can occur during production, or once the units are installed on the jobsite. In this article, we will focus on what concrete producers can do to limit efflorescence. 

The Four Factors

  1. A source of soluble salts - Portland Cement
  2. Sufficient water to dissolve the salts
  3. Path for the salt solution to migrate to the surface
  4. Driving force to move the salt solution through the pathway

1. A Source of Soluble Salts

As you know, there’s a lot of chemistry involved in concrete, so we have to get into some chemical reactions to understand what’s going on. When water is added to Portland cement in the mixer, a chemical reaction called hydration occurs. Cement hydration is what causes concrete to harden and gain strength, so it’s very important we understand hydration and maximize it.

The hydration reaction between cement and water forms calcium silicate hydrate (CSH) or “good” gel. This is the glue that hardens and holds concrete together. Unfortunately, the reaction is not very efficient, and a by-product called calcium hydroxide, a “bad” gel, is also formed.  

One way to limit calcium hydroxide is to use pozzolans such as fly ash or slag to replace some of the cement. Fly ash and slag are able to react with calcium hydroxide, and this reaction produces more CSH gel, which is what we want. Efflorescence can also result from other soluble salts, called alkalies, usually found in the aggregates.

How Efflorescence Happens

Calcium Hydroxide (free lime salts) particles dissolve into water in voids within the concrete matrix
Over time the water and dissolved salts migrate back to the concrete surface
The water evaporates leaving white calcium carbonate CaCo3  deposits on the surface

2. Sufficient Water to Dissolve the Salts

During mixing: It is very important for concrete strength and durability that there is sufficient water available during mixing to hydrate the cement particles properly. Therefore, maximizing water during mixing is highly desirable. Adding as much water as possible short of pulling, picking or slumping during mixing will lead to products with higher strength and lower absorption. 

After mixing: Concern over excessive water in the units comes into play after mixing, when the concrete products reach the curing stage - in the kiln or out in the yard.

Once the units are in the yard, rain and condensation can penetrate into the units, creating excess moisture conditions. Units that are exposed to repeated cycles of wetting and drying, such as during a wet spring or fall, may have increased potential for efflorescence.

3. Pathways for Salt Migration

All manufactured concrete products have a network of interconnected voids.  Connected voids serve as pathways for moisture migration into and out of the concrete unit. Voids can be minimized but not eliminated. Even if you have the best mix design, materials, equipment and compaction possible, there will still be voids and pathways for salt solutions to migrate to the surface. 

These pathways can be reduced by making the concrete more dense, and thus less able to absorb water. Admixtures, including water repellents and densifiers, can aid in limiting water penetration. 

4. Driving Forces to Move the Salt Solution

When two things occur in different conditions (for example one hot and one cold), the environment will try to find balance between the two situations, and will react to reach the middle or equilibrium. This is a driving force. Driving forces will try to balance humidity and temperature in a concrete unit with ambient conditions. This may cause moisture in the unit that contains the calcium hydroxide salt solution to move to the surface of the concrete.

During curing and storage of concrete units, many common practices can cause a driving force to occur. These include: 

  1. Exposing units to differences in humidity and temperature between the units and their environment. For example,  kilns with very high humidity will cause units to absorb excess moisture.  
  2. When units are moved from a heated kiln environment to a cold yard without allowing the units time for their internal temperature to lower and match that of the outside environment. 

Factors that create driving forces that can affect concrete products

What producers can do to help reduce driving forces and limit efflorescence

  • Units made too cold: concrete internal temperatures below 50° F (10° C) that may occur when kilns are not heated or not full.
  • Reduce pozzolan use and consider using an accelerator admixture to speed hydration. 
  • Units made too hot: Concrete temperatures above 90° F (30° C). Hot materials drive out water and the mix does not compact well, leading to more voids.
  • Lower the temperature of the concrete, use cold water or retarding admixture.
  • Very high humidity in the kiln. For example, water dripping from the ceiling or racks, may be a sign of too high humidity.
  • Monitor and control humidity levels in the kiln.
  • Moving products from kiln to yard too quickly. For example, placing steaming hot units outside in cooler weather.
  • Open and vent the kiln to allow temperature and humidity to come closer to ambient conditions. Allow units time to adjust to ambient temperature before moving into the yard.
  • In the yard, under certain conditions, evaporation from unit surfaces leads to a difference in moisture between the surface and the interior of a unit. For example, evaporation will increase in sunny, windy conditions.
  • Use topsheets on pallets to limit evaporation.
  • Cube efflorescence: Typically appears as streaks, or a ring or halo on the top layer of the cube. Direct sunlight on stretch-wrapped cubes may create a “greenhouse effect”, when trapped warm air and water vapor rise in the cube, and moisture condenses on the interior of the plastic sheeting. Water then pools on the top layer of the CMU or blotches in areas where water is slow to evaporate. Repeated wetting and drying of the top layer raises the potential for efflorescence.
  • Replace stretch wrap with topsheets that allow excess moisture to escape.