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

SCP Tech Brief: Concrete – Drying vs. Curing

The difference between “drying” and “curing” of concrete may not always be readily apparent. These
terms are often incorrectly treated as interchangeable, especially in reference to concrete that will receive
flooring materials. The following definitions of drying and curing are provided to assist in guiding the
discussion on these two very important and distinct processes.

Drying

Drying of concrete is defined as providing the proper conditions to allow the concrete to achieve a
moisture condition appropriate for its intended use, usually a concern with the use of flooring or coatings
over concrete. The moisture condition of a concrete slab is important for the application of moisture
sensitive floor finishes such as vinyl composition tile, linoleum, wood flooring, and non-breathable
coatings like epoxy. The moisture condition is specified as a maximum relative humidity by percent or a
vapor transmission rate in lbs./1000 ft2/24 hr. A typical value specified for relative humidity may be less
than 75 to 80 percent to assure the successful application of the flooring materials, while a commonly
specified value for vapor transmission rate may be 3 lb/1000 ft2/24 hr.i

Curing

Curing of concrete is defined as providing adequate moisture, temperature, and time to allow the concrete
to achieve the desired properties for its intended use. For conventional concrete not treated with Spray-
Lock Concrete Protection (SCP) products, this typically means maintaining a relative humidity in the
concrete of greater than 80 percent, a temperature greater than 50 degrees Fahrenheit for a time typically
ranging from three to 14 days depending on the specific application.ii Without proper curing, concrete
will often not achieve the required performance.

SCP Products’ Effects on Drying and Curing

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. In this way, SCP products serve as an excellent method of curing concrete while providing a slab
surface that can receive resilient flooring, adhesives, and coatings in as little as fourteen days.
For more information, see the following SCP Tech Briefs:

Concrete Curing: Click here for linked article

Is Spray-Lock Concrete Protection a Bond Breaker?: Click here for linked article

i  Kanare, Howard M., Concrete Floors and Moisture, EB119, 2008, 172 pg.

ii  Kosmatka, Steven H., Kerkhoff, Beatrix, and Panarese, William C., Design and Control of Concrete Mixtures, 15th

Edition, EB001.15, 2002, 372 pg.

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.

SCP Products vs. Dry-Shake Surface Hardeners

SCP Products vs. Dry-Shake Surface Hardeners

Abrasion resistance is a property of concrete that is important in structures that are subjected to repeated traffic from vehicles or repeated scouring from the movement of water or other liquids containing particulate matter.  Although not normally a structural concern, abrasion of concrete can cause dusting, and may reduce the expected lifespan of the concrete member.  Floor slabs, exterior pavements, dams, spillways, and other structures can all be affected by abrasion.

In general, abrasion resistance is proportional to the compressive strength of the concrete under consideration.  In other words, as compressive strength increases, the concrete’s ability to resist abrasive forces increases.  Other factors that influence abrasion other than the compressive strength include finishing and curing operations.  If inadequate curing or improper finishing techniques are used, the concrete’s abrasion resistance may be only a small percentage of what is indicated by its compressive strength.

Among other recommendations, ACI 201.2R-16 Guide to Durable Concrete recommends using special dry shakes or topping mixtures when severe wear is anticipated.  Dry shake hardeners rely on hard aggregates combined with portland cement or other binders to provide their resistance to abrasion and are generally divided into metallic aggregate and non-metallic aggregate categories. ACI 302.1R-15 Guide to Concrete Floor and Slab Construction provides recommendations for specialized finishing procedures when using dry shakes.  The ACI 302 document also references that “experience is necessary to determine proper timing for the required procedures.”

While dry shake hardeners have a place in construction, their use requires specialized procedures with trained personnel.  Cements have changed in recent years to a finer grind than in decades past, producing less bleed water.  Bleed water is essential for shake on hardeners to work properly, and in many cases the mix design must be adjusted, requiring coordination between the contractor and the concrete supplier. A project-by-project evaluation of the projected benefits provided by dry shake hardeners versus other methods is advised.

A direct comparison between Spray-Lock Concrete Protection (SCP) product performance and various dry shake hardeners has not been performed, but an analysis of SCP product improvements to abrasion resistance is provided for review.  A general product comparison is provided below:

SCP products may be used to increase abrasion resistance of concrete and may replace some dry shake hardeners at the discretion of the design professional.  SCP products provide many other benefits to concrete that dry shake hardeners cannot.  There are a few applications where shake-on hardeners cannot be replaced by SCP products, including but not limited to:

  • If the dry shake hardener provides a pigmented surface.
  • If the shake on is an iron-based hardener with metal aggregate fines – specific to the intended use.
  • If a high shine polish is required for reflectivity unless the option is given to polish later in the process.

SCP products may provide further advantages over dry shake hardeners including:

  • Less intensive labor demands.
  • Greater depth of reaction and more consistent coverage.
  • Reduced impact on health & environmental concerns.
  • Improved chances of overall success by the finisher by reducing the variables and timing demands of the process.

Improving Freeze/Thaw Performance of Concrete

SCP Tech Brief: Improving Freeze/Thaw Performance of Concrete

Spray-Lock Concrete Protection (SCP) technologies close capillary void space in concrete, significantly reducing permeability to liquids. Because of this action, the ability of liquid water to enter the concrete to then subsequently freeze and cause freezing/thawing damage is mitigated. This generally translates to longer service life of concrete in freeze/thaw exposure conditions.

When specifying SCP products as freeze/thaw improvement measures, consideration should be given to the potential routes of water ingress. SCP products should be applied to any surface of the concrete where water may potentially enter and cause subsequent freeze/thaw damage. To evaluate whether SCP products are the correct product to specify for freeze/thaw mitigation, the following test results from water permeability and freeze/thaw testing are provided for evaluation by the engineer of record.

Water Permeability Testing under Hydrostatic Pressure (EN 12390-8)

SCP technologies, when applied to new or existing concrete correctly, greatly reduce the ability of liquid water to penetrate the matrix. The following test results represent testing in accordance with EN 12390-8, a European norm that introduces water under 5 bar (72 psi) hydrostatic pressure to concrete for 72 hours and is an excellent indicator of concrete permeability.

Table 1: SCP Products Reduction of Water Penetration Under Hydrostatic Pressure

Freeze/Thaw Durability Testing (ASTM C666)

The most recent freeze/thaw durability testing results are presented below.

Figure 1: Freeze/Thaw Testing Performance of SCP-Treated Concrete vs. Control

Conclusion/Discussion

Damage from freezing and thawing is mitigated by reducing concrete’s permeability or the use of an appropriate air void system. By reducing water permeability, SCP products may sufficiently mitigate the potential for freeze/thaw damage of new and existing concrete. When specifying SCP products as a freeze/thaw mitigation measure for existing concrete, SCP recommends that pre- and post- treatment cores should be extracted and subjected to water permeability testing to verify that the treatment meets the requirements for low permeability concrete for freeze/thaw mitigation. For new concrete applications, SCP recommends following ACI guidelines for air entrainment volume percentage in addition to using SCP products for additional freeze/thaw protection.

SCP Compared to Silicate Salts Used as Admixtures

Silicate Salt Admixture Behavior in Concrete – What Does It Do? 

The silicate salt admixtures, generally comprised of sodium, potassium, or lithium silicates work by the formation of Calcium Silicate Hydrate (C-S-H) as the mechanism of closing void structure to decrease permeability of concrete. While a track record of permeability reduction may have been established, permeability reducing admixtures still have the problem of capillary and bleed water channel formation that is only partially addressed. No known technologies completely remove capillary and bleed water channel void structure when used as an admixture.

The silicate admixtures introduce silica chemically bonded to a metal (typically sodium (Na), potassium (K) or lithium (Li)). Because the silica is chemically bonded already, the silicate salt requires a certain amount of energy to be spent from the concrete system to disassociate the cation (Na, K, or Li) from the silica. During this dissociation, other reactants are formed in addition to C-S-H including sodium hydroxide and potassium hydroxide, both of which have been found to be detrimental to concrete.

SCP’s Colloidal Silica Behavior in Concrete

Spray-Lock Concrete Protection (SCP) products contain colloidal silica – that is, a suspension of amorphous silicon dioxide (SiO2) particles that are sufficiently small enough not to be affected by gravity. SCP products are applied after initial set of the concrete – after capillary and bleed water channel formation. SCP products then enter the concrete, reacting with available alkalis. SCP product particles are chemically the same as silica fume, but many times smaller, which provides significantly more surface area to improve the pozzolanic reaction over that of even silica fume or metakaolin. The pozzolanic reaction is the conversion of calcium hydroxide to C-S-H. With SCP products, C-S-H fills the void structure of the concrete within the reaction zone, providing many benefits to concrete.

Comparison of SCP Products to Silicate Salt Admixtures

Because SCP products contain unbonded silicon dioxide particles that are very small, they are ready to combine chemically with calcium hydroxide with no dissociation energy required. The resulting reaction products are even longer-chain, more stable C-S-H than primarily formed by cement hydration.[i] This improved C-S-H structure leads to greater performance of SCP products than that of silicate admixtures, or other waterproofing admixtures. The following table represents SCP’s published test results compared to a leading silicate admixture brand’s published test results.

[i] Kontoleontos, F., Tsakirdis, P., Marinos, A., Kaloidas, V., & Katsioti, M. (2012). Influence of colloidal nanosilica on ultrafine cement hydration: physiochemical and microstructural characterization. Construction and Building Materials 35, 347-360.

Another important concern when evaluating silicate salt admixtures versus colloidal silica is the delivery system. Silicate salt admixtures are introduced into the mixer truck in powder form, usually by the concrete plant or construction personnel at the rate of 20-60 lbs. per cubic yard. On a 10-cubic yard load, this translates to 200 to 600 lbs. of material that must be handled manually and introduced into a turning drum. Safety concerns, from lifting to inhalation hazards, are associated with manual loading of any powder into a mixer truck. Conversely, SCP’s brand of colloidal silicas is spray-applied in liquid form on fresh or existing concrete, and contains zero VOCs.

Conclusion

Concrete permeability reducing admixtures may be effective in some cases, but bleed water channels and capillary structure are likely to still be present in all conventional concrete. SCP products have the advantage of closing capillaries and bleed water channels with reaction products after they have formed. Additionally, the use of SCP products remove the safety hazards associated with plant-dosing of concrete trucks with a powdered product.

SCP Tech Brief: Concrete Curing

ACI 308R-16 Guide to External Curing of Concrete refers to curing as “Curing is an action taken to maintain moisture and temperature conditions in a freshly placed cementitious mixture to allow hydraulic cement hydration and, if pozzolans are used, pozzolanic reactions to occur so that the potential properties of the mixture may develop.” Spray-Lock Concrete Protection (SCP) products work by penetrating into the capillaries and voids left in concrete as bleed water exits.  Once in the concrete, SCP products react with available alkalis to form Calcium Silicate Hydrate (C-S-H), the same reaction product provided by cement hydration that provides most of concrete’s desirable properties.  The reaction products formed fill capillaries and shut down liquid water transmission.  This action makes SCP products an effective waterproofing agent, even under hydrostatic pressure.  The action works in the other direction as well, holding in water that would normally evaporate at early ages.

By making the water that would normally evaporate at early ages available for continued hydration, SCP products meet ACI’s definition of curing.  Evidence of the continued hydration is drawn from the fact that SCP products improve concrete’s compressive strength and reduce drying shrinkage significantly, even when compared to moist-cured samples.

Because SCP products penetrate into the concrete, they do not form a surface membrane.  Therefore, the ASTM test methods and specifications (ASTM C309 and ASTM C1315, respectively) do not apply to SCP products.  An important consideration when evaluating products for use with concrete is that ASTM develops standards to generate performance numbers while ACI writes the guides and specifications that are used to put the numbers generated into practice.  At least two ASTM C09 subcommittees are currently working on standards to better evaluate the performance of colloidal silica.  Until those standards are available, and ACI 308.1 can be updated to reflect those standards, the ACI guidance in ACI 308R-16 on curing describing the desired effects must take precedence.

Figure 1: Compressive Strength as a Function of Age

for a Variety of Curing Conditions (ACI 308R-16)

According to ACI 308R-16, when determining the duration of curing, consideration should be given to the concrete properties that are desired in addition to compressive strength development.  “For example, if both high compressive strength and low permeability are required concrete performance characteristics, then the curing needs to be long enough to develop both properties to the specified values.  The appropriate duration of curing will depend on the property that is the slowest to develop.”  By providing a permanent barrier to water migration, SCP products maintain curing indefinitely.  In other words, the semi-evaporable and non-evaporable water continues to be made available for hydration until it is consumed or an equilibrium state is achieved when SCP products are used.  While membranes degrade over time, and water ponding methods cannot be continued indefinitely, the use of SCP products provides a continuous curing environment.  Figure 1 above shows the percentage of strength gain associated with various curing methods.  SCP products will allow concrete to generally follow the “moist-cured entire time” trendline with values that are typically 7-16% higher.

Figure 2: SCP Product Performance Versus Air and Water Cured Concrete

Figure 2 shows the performance of concrete treated with SCP products versus air and moist cured concrete.   Compressive strength development is the primary reason curing of concrete is required, and SCP products help achieve even better performance that moist-cured concrete, long held as the gold standard of curing concrete.  Figure 3 below shows the performance of treated concrete tested for drying shrinkage, another important reason curing is required, demonstrating improvements over moist-cured control samples.  Figure 4 shows the performance of treated concrete in water permeability testing in a hydrostatic pressure environment, another reason curing is often called out.

Figure 3: SCP-Treated Drying Shrinkage Performance (ASTM C157)

Figure 4: SCP-Treated Concrete Performance Under Hydrostatic Pressure

Concluding Remarks

ASTM C309 membrane-forming curing compounds were developed as a solution to the need for a time-saving alternative to water ponding.  They are widely recognized as providing “adequate” curing, but are not seen as equal to water ponding or water-saturated coverings.  Additionally, SCP products become a permanent part of the concrete while membranes and water ponding are temporary.  The objective of curing is to allow the concrete to meet performance parameters; SCP products produce those performance parameters at a rate equal to or better than moist curing.

Specification Guidance for Designers

ACI 308.1-11 Specification for Curing Concrete section 1.1.2 Exclusions allows designers to stipulate special curing procedures not covered by the specification.  Instead, refer to SCP’s example specifications for its products and incorporate them as needed.

SCP and Structural Integrity

 

Structural integrity is defined by Dr. Steve Roberts of Oxford University as “the science and technology of the margin between safety and disaster.” When taken in context of the broader idea that structural integrity refers to the ability of a structure to resist loads or other damaging effects from its environment, Dr. Robert’s definition is of particular interest because it concentrates on the idea of safety factor.

 

Concrete is the most widely used building material globally, and many count on its contribution to the structural integrity of our built environment. Design professionals rely upon concrete to perform as anticipated to provide confidence that the factors of safety in the structural designs are reliable. In addition, concrete performance is essential in the longevity for the structures for the intended function. The primary consideration of concrete in a structural system is its capacity to function under designed load conditions.  However, there are many other contributors to the life of a structure other than the load that can impact a concrete structure’s ability to endure.  To fully rely on concrete’s structural integrity, its ability to resist the effects of outside influences other than load – defined as durability – must also be considered in addition to concrete performance.

 

Spray-Lock Concrete Protection (SCP) products contribute to concrete durability by restricting access to the concrete from the outside influences mentioned above.  SCP products close capillary bleedwater channels and pore spaces with reaction products that are more stable forms of the naturally occurring reaction products already in concrete; SCP products effectively fill the voids in concrete with more concrete.  The effects on concrete durability enabled by this simple action are numerous, including improvements to chloride penetration resistance, abrasion resistance, sulfate attack resistance, water permeability, and others.  Additionally, SCP products reduce the drying shrinkage of concrete when used at time of placement.

 

As stated, load considerations are often of greatest concern.  SCP products improve compressive strength, allowing designers additional confidence that the concrete used in their designs will perform properly.  Structural integrity is a subject to be taken seriously.  With SCP, the overall structural integrity and durability of concrete structures can be improved.