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Tech Brief: Hot Weather Concrete

SCP Tech Brief: Hot Weather Concrete

Placing concrete in hot weather poses one of the greatest challenges in construction. This is because concrete gains the majority of its properties during the temperature-dependent cement hydration reaction. Temperature of the concrete, surrounding air temperatures, and form temperatures can all play a substantial role in the quality of the final concrete product. Large volumes of concrete are placed during the hottest months of the year, therefore managing and mitigating threats associated with placing concrete in hot weather is crucial in today’s fast-tracked construction industry. Hot weather is defined as job-site conditions that accelerate the rate of moisture loss or rate of cement hydration of freshly mixed concrete, including an ambient temperature of 80F or higher, and an evaporation rate that exceeds 1kg/m2/h or as revised by the Architect/Engineer [1].

Understanding the threats that hot weather poses and how to mitigate those threats in an efficient and effective manner can determine the success of a project.

Some of the main effects of hot weather to concrete are:

  • Workability – slump or workability decreases as temperature increases [Fig 1]
  • Set time – set time decreases as temperature increases [Fig 1]
  • Compressive strength – A high early compressive strength followed by lower long-term compressive strength gains
  • Plastic shrinkage cracking – cracks that occur due to rapid evaporation of bleed water.

Planning during hot weather concrete placements is recommended in ACI 305 “Hot Weather Concreting”. Planning can consist of preplacement conferences, use of admixture and evaporation products, adjusting concrete mix proportions, and others. All of these should be focused on reducing and controlling temperatures and evaporation rates. Wind, relative humidity, ambient temperature, and concrete temperature are the largest factors when considering evaporation rate. The nomograph below has been used throughout the industry for decades and still prevails as the most widely used tool for calculating evaporation rates [Fig. 2].

There are many different ways to mitigate risks that come with hot weather concreting. A combination of measures taken at the ready-mix plant and the jobsite can allow for concrete to be placed at high temperatures. Acceptable production methods to reduce the temperature of concrete at include: shading aggregate stockpiles, sprinkling water on coarse aggregate stockpiles; using chilled water for concrete production; substituting chipped or shaved iced for portions of the mixing water; and cooling concrete materials with liquid nitrogen [1]. Acceptable jobsite measures include timely placement and finishing, control of bleed-water evaporation, curing, and protection. These jobsite measures are all focused on reducing evaporation rates when conditions are threatening. Methods to reduce evaporation rates on site include erecting wind shades and sun shades, using evaporation retarders, using proper curing methods, and others.

The initial curing of concrete strength specimens is often overlooked in hot weather. Because the specimens are small quantities of concrete compared to actual cast in place members, cylinders and beams can be even more severely affected by high temperatures. ASTM C31 Casting of Concrete Strength Specimens in the Field requires that strength samples

be stored at temperatures between 60 and 80 degrees F for normal strength concrete, and between 68 and 78 degrees F for concrete with a specified compressive strength of 6000 psi and higher. Additionally, the specimens should be shielded from direct sunlight and protected from moisture loss [2].

Another consideration is thermal shrinkage of the concrete structure due to rapid drops in concrete temperature of 22C (40F) in the first twenty-four hours. Thermal shrinkage can be reduced by using chilled concrete, using cool pipes in the concrete structure, using supplemental cementitious materials (SCM), using mist water, shading the slab, or using curing methods [3].


[1] Specification for Hot Weather Concreting. ACI 305.1-06. Retrieved December 18, 2018, from http://www.arquitectosrp.com/archivo/download/ACI 305.1-06 Specification for Hot Weather Cncreting.pdf

[2] ASTM C31 / C31M-18b, Standard Practice for Making and Curing Concrete Test Specimens in the Field, ASTM International, West Conshohocken, PA, 2018, www.astm.org

[3] Association, N. R. (2009). CIP 42 – Thermal Cracking of Concrete. Retrieved January 17, 2019, from https://www.nrmca.org/aboutconcrete/cips/42p.pdf

[Fig 1] CIP 12 – Hot Weather Concreting. Concrete in Practice – What, Why, How?Retrieved December 18, 2018, (2000) from https://www.nrmca.org/aboutconcrete/cips/12p.pdf.

[Fig 2] Specification for Hot Weather Concreting. ACI 305.1-06. Retrieved December 18, 2018, from http://www.arquitectosrp.com/archivo/download/ACI 305.1-06 Specification for Hot Weather Cncreting.pdf

 

Tech Brief: Cold Weather Concrete

SCP Tech Brief: Cold Weather Concrete

Portland cement concrete relies on chemical reactions between cementitious materials and water, known collectively as hydration, to harden and develop strength. Like most chemical reactions, the rate of early-age hydration is significantly temperature-dependent. Generally, as concrete temperature rises, the time for hydration to occur shortens and vice-versa. The American Concrete Institute produces two key documents that discuss concrete operations in cold weather: ACI 306.1-90 Standard Specification for Cold Weather Concreting and ACI 306R-16 Guide to Cold Weather Concreting. The information provided in both the specification and guide should be considered any time concrete will be placed during cold weather, defined by the Guide as “…when the air temperature has fallen to, or is expected to fall below, 40° F (4° C) during the protection period.” For most concrete mixes, this protection period lasts until the concrete has achieved at least 500 psi.¹

Generally, protective measures include providing sufficient concrete temperatures as mixed, not placing concrete on frozen base material or subgrade, and not allowing concrete to freeze before reaching 500 psi, generally at least 48 hours from time of placement. Heated aggregates and/or hot mixing water are often used by ready mixed concrete companies to achieve required minimum as-delivered concrete temperatures. Heated enclosures and curing blankets are often used to maintain concrete above freezing temperatures in the field once placed. Care should always be taken to vent the exhaust of hydrocarbon-burning heaters outdoors if they are used due to the risk of death of workers and carbonation of the concrete surface.

The damage from early-age freezing of concrete is often readily identifiable upon visual inspection due to the formation of ice “lenses” in the concrete surface. Concrete that has frozen during its early age (during the protection period) often will never achieve required compressive strength or durability characteristics.

Concrete hydration is an exothermic reaction. However, the rate at which the heat is lost to the atmosphere can overcome concrete’s ability to replenish the heat in the absence of insulating materials. ACI 306R gives recommendations on minimum temperatures to resist damage from early-age freezing based on cement content, concrete member thickness, and insulation R-value employed.

One obvious result of cold weather is that concrete takes longer to reach initial set. Generally, chemical reactions double in time required for every 10 degrees C variation. So, for concrete without accelerator and with normally encountered cementitious contents that sets in 6 hours at 70 degrees F (21 C) the following is a general guide for what to expect:

(*) Concrete below 50 F (10 C) may have extended set times beyond what is expected.

Cold Weather and Spray-Lock Concrete Protection Products

Various techniques are used by concrete companies to offset the effects of low concrete temperature, including heating aggregates, heating water, and the use of chemical accelerators. Spray-Lock Concrete Protection (SCP) products should be kept from freezing during storage and should not be applied to concrete with temperatures less than 40° F (4.4° C). When chemical accelerators are used in the concrete, care should be taken when using SCP products due to early setting of the product on the surface after application. If this occurs, delay SCP product application until the product penetrates without reacting on the surface. Otherwise, SCP products can be expected to perform normally as long as cold weather concreting procedures are in place.

Conclusion

Quality concrete construction is achievable in cold weather, but takes careful preparation, planning, and execution. Generally, producers and contractors that operate in cold weather often are accustomed to the requirements of cold weather construction. Problems typically occur more often in warmer regions that rarely have cold weather for extended periods where cold weather concreting procedures are rarely used.


¹ ACI 306R-16 Guide to Cold Weather Concreting. American Concrete Institute, Farmington Hills, MI.

 

Tech Brief: Concrete Cracking

SCP Tech Brief: Concrete Cracking

An unfortunate truth in concrete construction is that concrete cracks. There is no magic technology that solves all forms of concrete cracking. When properly used, Spray-Lock Concrete Protection (SCP) products may help reduce the chances of some forms of cracking, but not all. In this technical brief, you will learn how to distinguish between different types of common cracks and SCP’s products effects, if any, on those types of cracks.

Cracking is caused by many different reasons, but all cracks in concrete are caused by movement of the concrete in a way that exceeds the concrete’s ability to resist the forces generated. Concrete is strong in compression, but relatively weak in tension, flexure, and torsion. Some cracking mechanisms cause the concrete to move differently from the top to the bottom, for instance, generating stress at the interface between the two movement planes. In general, concrete cracks due to external loading, thermal gradients, moisture gradients, or in response to a chemical reaction (internal or external).

Cracking Due to External Loading 

When loads exceed concrete’s ability to resist the forces applied cracks may occur. An example is negative-moment cracks above beams in an elevated deck caused by flexural forces between the beams moving in a downward direction. The concrete remains stationary above the beams while the concrete between the beams moves, sometimes resulting in cracking. Another example is cracking in a slab on grade that is exposed to traffic loads along an edge, breaking the concrete in a half-moon shape. SCP products do not help resist cracks due to external loading. Proper design to resist forces applied, including the strength of concrete and reinforcement sizing and positioning are the keys to limiting cracking due to external loading.

Plastic Shrinkage Cracking 

Sometimes the surface of a freshly placed concrete slab dries out before the remainder, forming a “crust” that tends to crack. The cracks that form are typically shallow, run parallel to each other, and do not intersect the edges of the slab. The cracking typically appears in the time period just before finishing operations begin up to final troweling. These cracks are known as plastic shrinkage cracks because they happen when the concrete is still relatively plastic – it has not yet reached final set.

Plastic shrinkage cracks can occur when weather conditions cause rapid evaporation of bleed water before it can be replaced naturally by the subsurface concrete. Low relative humidity, high winds, and high concrete temperature can all contribute to plastic shrinkage cracking.

The good news is that plastic shrinkage cracks are not usually structural problems. The bad news is that they are cosmetically unappealing in instances where the concrete is exposed. Weather conditions conducive to plastic shrinkage cracking of concrete can be readily predicted by using several weather-related websites and/or commercially available instruments. There are several recommendations that help prevent plastic shrinkage cracks from ACI, NRMCA, and other sources, including erecting wind breaks, misting or fogging the slab with water, placing concrete in the early morning hours, lowering concrete temperatures, and using micro fibers. SCP products have little to no effect on plastic shrinkage cracking since most plastic shrinkage cracks occur before it is the proper time for SCP product application.

Plastic Settlement Cracking 

Also known as subsidence cracking, plastic settlement cracks appear over embedded items such as reinforcing steel as concrete settles or subsides. Plastic settlement cracking is caused by insufficient consolidation, high slumps (overly wet concrete where aggregate segregation is occurring), or lack of adequate cover over embedded items. These types of cracks are recognizable by their resemblance in number and spacing to the reinforcing steel pattern below in the slab. SCP products have no effect on plastic settlement cracks. Plastic settlement occurs due to the lack of proper consolidation, high slumps, or inadequate cover over rebar, all of which are outside the

parameters that SCP technology affects. Plastic settlement can be of particular concern on deep (> 12”) reinforced concrete slabs.

Crazing 

Crazing is the development of a network of fine random cracks on the surface of concrete caused by differential

shrinkage of the surface layer. These cracks are rarely more than 1/8 in. (3 mm) deep and are more noticeable on steel-troweled surfaces and when concrete is wet. Crazing is most often caused by a higher water to cement ratio at the surface of the concrete as a result of over-t

roweling, sprinkling water on the surface of the concrete during finishing operations, or finishing concrete while bleed water is still present.Because the shrinkage that occurs in the top surface differs from the substrate concrete, SCP products can do little to mask or alleviate that difference. Although SCP technology can reduce drying shrinkage as a whole, crazing is caused by the differential between the top surface and the rest of the concrete. For instance, if the top surface of untreated concrete is expected to shrink 0.08% and the bottom of the concrete is expected to shrink 0.04%, SCP technology may reduce both values, but a difference will still occur, and therefore crazing may occur if the appropriate conditions exist.

Settlement Cracking 

Settlement cracking is caused by the loss of base or subgrade support of the slab. Concrete is strong in compression, but relatively weak in tension or flexure. When support is lost, concrete may “settle” along with the base or subgrade material. These cracks often indicate a significant structural issue that should be addressed and are recognizable by the vertical displacement from one side of the crack to the other. Settlement cracking occurs due to loss of support beneath the slab. SCP products do not provide sufficient additional flexural strength to counteract this type of failure.

 

Drying Shrinkage Cracking 

Drying shrinkage cracks are caused by the change in volume of concrete associated with the loss of some of the water in the concrete due to evaporation. When concrete is first placed, it is typically at its greatest volume. Only a fraction of the water used in concrete is consumed by the cement hydration process. Much of the remaining water leaves the concrete, causing the concrete to shrink. When concrete is restrained by the ground, embeds, re-entrant corners, etc., tensile forces develop that can exceed the concrete’s ability to withstand them, and cracks form. Contraction joints are generally introduced to concrete to provide vertical planes of weakness that allow the concrete to form cracks in predetermined straight lines.

Drying shrinkage cracks are most likely to form at or near sources of restraint such as turn-down footings, depth changes, plumbing, and other penetrations. SCP products can reduce drying shrinkage of the concrete significantly by trapping some of the water inside the concrete and filling capillary voids, but drying shrinkage cracks will still occur near these sources of restraint.

SCP technology can significantly reduce drying shrinkage, but sources of restraint and improper jointing can still cause cracking.

Conclusion 

Please note that there are many other types of concrete cracks that may occur. The preceding descriptions are the most commonly encountered early-age cracks in concrete construction. The use of SCP products cannot guarantee crack-free concrete. While proper slab design and installation methods can help reduce the chances for random cracking, ACI states that cracking can still be expected in at least 2% of all concrete panels, even where all work is executed properly.

For further information, many sources exist on concrete cracking and the mechanisms involved. SCP recommends information from ACI, PCA, and NRMCA as sources for further reading. Please contact SCP Tech with any questions at SCPTech@SprayLock.com.

SCP Products and Special Purpose Aggregates

SCP Tech Brief: SCP Products and Special Purpose Aggregates

The choice of aggregates for a concrete mix design is a critical decision that affects fresh and hardened performance. When the need arises to use a special purpose aggregate for reasons such as making concrete that is heavier, lighter, or different in appearance, the considerations for properly blending sizes and shapes still apply. Changes in aggregate type, shape, size, and density can all affect fresh and hardened properties such as workability and strength and must be considered in the mix design process.

Spray-Lock Concrete Protection (SCP) products interact with the paste fraction of concrete. Entering through the bleed water channels and other capillary structures, SCP products react with available alkalis to primarily form calcium silicate hydrate (C-S-H) to fill void space. Because SCP products interact with the paste, the type of coarse aggregate used does not affect SCP product performance. There are a few things to consider when evaluating SCP product use with some special purpose aggregates.

Lightweight Aggregates

Lightweight concrete aggregates typically absorb, retain, and release significantly more water than normal weight aggregates. In fact, lightweight aggregates have been shown to contribute to the curing process by slowly releasing water over time as shown by Ben Byard and others.[i] Although a desirable behavior in many types of concrete, this slow release of water over time can be problematic for elevated floors that will be receiving flooring materials. Suprenant and Malisch found that lightweight concrete took nearly four-times as many days to reach a moisture vapor emission rate (MVER) of 3.0 lbs as normal weight concrete in the same environmentally controlled conditions (183 days compared to 46 days).[ii] This behavior has led many contractors and specifiers to the conclusion that lightweight concrete slabs must receive moisture mitigation before flooring materials are applied.

When used at time of placement, SCP products allow application of flooring materials in as little as fourteen (14) days from the date of application to lightweight Portland cement concrete slabs. No moisture mitigation or testing of slab moisture is required, but joints, penetrations, and any cracking must be treated normally with materials and methods specified by the designer.

Heavyweight Aggregates

Heavyweight aggregates consist of naturally-occurring materials that are more dense than normal aggregates or man-made materials such as iron or steel. They are most often specified in radiation shielding concrete projects such as x-ray rooms in hospitals and a number of places in nuclear power plants and spent-fuel storage facilities. Segregation (separation in the fresh state) of heavyweight coarse aggregate from the mortar fraction of the mix may occur and may need to be compensated for with the addition of fines to the mix.[iii]

SCP products may be applied to Portland cement concrete containing heavyweight aggregates using standard application methods. Heavyweight concrete treated with SCP products is available to receive adhesives, coatings or paint in as little as fourteen days after product application.

Recycled Concrete Aggregate

The use of recycled concrete as aggregate has seen a growth in popularity in recent years due at least in part to the idea that using recycled materials is an environmentally-friendly initiative. ACI 555 provides specific and detailed guidelines for the removal and reuse of concrete as an aggregate. Even if all concerns outlined in ACI 555 are satisfactorily met, the consensus is that the absorption rate of recycled concrete aggregate is significantly higher than that of normal weight virgin aggregate.[iv] This higher absorption rate may cause issues with MVER performance in similar ways to lightweight aggregate depending on the percentage of recycled aggregate used.

As long as the performance of the recycled concrete aggregate meets the requirements outlined in ACI 555 and the concrete is Portland cement-based, SCP product performance can be expected to be similar to when used in conjunction with conventional concrete.

Architectural Aggregates

Sometimes aggregates are chosen for their inherent aesthetics. They may be exposed to view by grinding, polishing, or the use of surface retarders. SCP products will not interfere with the appearance of architectural aggregates, but because the methods used to expose aggregates in architectural concrete differ, attention to the timing of SCP product application is a key concern. Mock-ups may be required to evaluate the timing of both SCP product application and aggregate exposure methods.

Conclusion

SCP products work with almost all concrete that contains Portland cement. Because SCP products react in and become part of the paste fraction of the mix, little interaction between aggregates and SCP products occurs. Architectural applications require attention to the timing of SCP product application and aggregate-exposure operations.

[i] Byard, Benjamin (2011) “Early-Age Behavior of Lightweight Aggregate Concrete,” Auburn University. Retrieved 10/4/18 from: https://etd.auburn.edu/handle/10415/2876

[ii] Martin, David; Zimmer, Alec; Bolduc, Michael; Hopps, Emily (2013) “Is Lightweight Concrete All Wet?” Structures Magazine retrieved 10/4/18 from: https://www.structuremag.org/wp-content/uploads/C-BuildingBlocks-Martin-Jan131.pdf

[iii] ACI International. ACI 221R-96 Guide for Use of Normal Weight and Heavyweight Aggregates in Concrete. American Concrete Institute Manual of Concrete Practice, Farmington Hills, MI.

[iv] ACI International. ACI 555R01 Removal and Reuse of Hardened Concrete. American Concrete Institute Manual of Concrete Practice, Farmington Hills, MI.

SCP and Vapor Retarders

SCP Tech Brief: SCP Products and Vapor Retarders/Barriers

ACI 302R-15 Guide to Floor and Slab Constructioni discusses vapor retarders/barriers at length. Vapor retarders/barriers are meant to minimize the transmission of water vapour through a concrete slab from sources located beneath the slab but have no effect on water vapor that comes from the concrete itself. There is no industry-recognized dividing line between what constitutes a vapor retarder and a vapor barrier, but both must not exceed 0.1 perms according to ASTM E1745.

Location of the vapor barrier has been a topic of debate within the concrete construction community. Placing the vapor barrier within direct contact of the bottom of the slab has been proven to cause problems with curling and shrinkage ii. An alternative recommended practice is to place a layer of sand between the vapor barrier and the bottom of the slab to offset those associated problems. This layer of sand can provide access to moisture from the outside to the bottom of the slab and cause problems with water vapor transmission. ACI 302R-15 provides guidance in the form of a flow chart summarized in Figure 1.

When used at time of placement, Spray-Lock Concrete Protection (SCP) products reduce water vapor transmission to the point where the performance of moisture-sensitive flooring, adhesives, and coatings are not affected after fourteen (14) days post-treatment.

Because vapor barriers are sometimes required by code or for other reasons other than water vapor, SCP does not state that we replace vapor barriers outright. An effective plan to address moisture transmission through cracks and joints should always be part of the design of any slab that is moisture-sensitive. When a plan is in place and executed to prevent moisture transmission through cracks and joints, SCP treatments are effective whether or not a vapor barrier is present.

Figure 1: Summary of Flow Chart from ACI 302.1R-15 to Determine Vapor Barrier Use and Location in Slab Section

SCP products have been used successfully in hundreds of projects where vapour barriers were not used, providing water vapor protection to flooring, adhesives, and coatings. SCP recommends that project design teams consider their local codes and intended performance of the proposed vapor barrier before deciding to replace the vapor barrier with SCP technology.

 

i American Concrete Institute (ACI) ACI 302R-15 “Guide to Floor and Slab Construction” ACI Manual of Concrete

Practice. Farmington Hills, MI, USA.

ii National Ready Mixed Concrete Association (NRMCA), n.d. “CIP 29 – Vapor Retarders Under Slabs on Grade”

NRMCA Concrete in Practice Series, Retrieved 8/17/18 from: https://www.nrmca.org/aboutconcrete/cips/29p.pdf