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

How do SCP Products Reduce Drying Shrinkage?

SCP Tech Brief: How Do SCP Products Reduce Drying Shrinkage of Concrete?

One of the issues facing conventional concrete is that concrete shrinks. This mechanism of volume change can lead to cracking of the concrete structure. Concrete shrinkage is in response to the loss of water that is held in the pore space of the concrete. This process is known as drying shrinkage. “Pore space” or “pores” refer to the voids that are formed as concrete is made and hardens. This space is made up of bleed water channels, capillaries, entrapped air voids, and other naturally-occurring voids. When pores are interconnected, they provide a pathway for water and water vapor to pass through the concrete to then evaporate once they make it to the air above the concrete. This process, though most noticeable during the first twenty-four (24) hours or so, continues for several days, weeks, months, or in the case of mass concrete, maybe even years. In fact, some of the water, known as non-evaporable water, never leaves concrete because it is trapped within the structure. Though often referred to simply as “water”, the liquid within the pore structure of concrete is more accurately referred to as “pore solution” because of the presence of various ions and salts dissolved or suspended in it. Given a constant composition of pore solution the rate of drying shrinkage is most dependent on the evaporation rate. Evaporation rate is determined by ambient temperature, concrete temperature, relative humidity, and wind speed. In general, greater winds and temperatures and lower humidity lead to faster moisture loss in concrete. When drying occurs more quickly than the tensile capacity (the ability of the concrete to withstand “pulling apart” forces) of the concrete develops it can lead to drying shrinkage cracks. Maintaining moisture levels for an extended period is vitally important to concrete and is the basis of the idea of “curing” along with maintaining proper temperatures. The goal of curing is to slow the rate of drying and provide a continued supply of water for the hydration of cementitious materials to continue.

An important consideration in concrete technology is the difference between “drying” and “curing”. These terms are often incorrectly treated as interchangeable, especially in reference to concrete that will receive flooring materials. Drying refers to the loss of moisture over time from the concrete, while curing refers to the intentional act of holding moisture in the concrete to facilitate continuation of the cement hydration process. While most flooring and adhesives manufacturers require the concrete to be below a certain percent moisture or relative humidity to function properly, they are really concerned about the moisture present at the interface between the concrete and the adhesive and/or flooring. In concrete not treated with Spray-Lock Concrete Protection (SCP) products, a general moisture condition of the concrete determination is sufficient to describe the entire system. SCP products react with available alkalis to close bleed water channels and capillaries with reaction products. This action holds moisture in the concrete to facilitate curing. The surface of the concrete is able to achieve a “dry” condition and able to receive adhesives and flooring while the concrete’s internal structure is kept at a high level of moisture conducive to continued curing. A description of the entire system based on a total percent moisture or relative humidity is no longer appropriate when using SCP products because the water and water vapor transport mechanisms are significantly affected, allowing the surface of the concrete to act independently of the total slab moisture content.

Figure 1: Drying Shrinkage in Concrete

 

The reason that concrete shrinks seems to be obvious – volume loss primarily from loss of water. Similar behavior can be observed in nature when a creek bed or mud puddle dries and cracking in the soil occurs. However, the mechanics of how concrete shrinks when drying has been studied extensively. According to ACI 231R-10 Report on Early Age Cracking: Causes, Measurement, and Mitigation, the loss of pore water results in the development of a meniscus (see fig. 1) and capillary pressure in pores. The radius of the meniscus may be directly related to the extent of capillary pressure that is formed using the Young-Laplace equation (eq. 1)

σcap = 2γ • cosθ
___________
r

where σcap is pore pressure, γ is the surface tension of the pore fluid in lb/in. (N/m), θ(rad) is the contact angle between pore fluid and solids, and r is the radius of the menisci in inches (m). From the equation, it can be observed that the pore pressure increases as pore diameter (meniscus radius) decreases. This pore pressure is identified as the prevailing force by which drying shrinkage occurs – the pressure pulls on the pore walls. By reducing the surface tension of the pore fluid (γ) and the corresponding reductions that occur in the contact angle (θ), it is possible to reduce the pressure that is generated. SCP products reduce drying shrinkage in two main ways. First, the SCP product is applied to the surface of the concrete shortly after finishing operations are completed. SCP products are primarily made of colloidal silica consisting of very small silicon dioxide particles suspended in water. The colloidal silica enters the concrete through bleed water channels and capillaries, penetrating deeply and combining with some of the existing pore solution. Introducing a solute or suspension to water is the most straightforward way of changing the water’s surface tension. By introducing suspended silica particles into the pore solution, treatment with SCP products should change the pore solution’s surface tension and contact angle, thus reducing the pore pressure.

Secondly, the SCP particles then react with alkalis in the concrete to close pores off by primarily forming calcium silicate hydrate (C-S-H). This pozzolanic reaction results in making the pore structure less continuous and significantly reducing the transport of water through the concrete. This action holds in water that normally would be allowed to evaporate without SCP treatment, slowing the rate of drying significantly. This combination of changing the chemistry of the pore solution and then closing off liquid transport combines to significantly reduce drying shrinkage of concrete. Testing has demonstrated a typical decrease of 40% to 60% of drying shrinkage at 28 days of SCP-treated concrete compared to controls.