Demystifying self-consolidating concrete

by nithya_caleb | May 17, 2019 12:00 am

by Michael Stanzel

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Self-consolidating concrete (SCC) is designed to flow and consolidate on its own without internal vibration. It can maintain enough cohesivity to fill any form without significant segregation or bleeding. This makes SCC useful where placing conditions are difficult, such as in highly reinforced concrete members or where complex geometries are required. Since its inception in 1988 in Japan, the use of SCC has grown. The development of high-performance admixtures, complex design algorithms, and appropriate test methods have made it possible to advance SCC technology, while maintaining or improving the concrete’s strength and durability.

Benefits of SCC

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Although SCC is not a magic bullet or the optimal solution for every job, there exist a number of applications where its unique characteristics offer significant benefits, such as in architectural members, precast columns, beams, tanks, heavily reinforced concrete elements, and for formed repairs. Some of the advantages for improving constructability by using SCC include:

• ease of placement and flowability;
• faster placement rates with less effort, thereby reducing project costs, time, equipment, labour, and rework;
• more uniform architectural surface finish;
• enhanced strength and durability parameters due to a low water-to-cement (w/c) ratio;
• greater flexibility for innovative structural and architectural design, shapes, and finishes;
• improved consolidation around and bond with reinforcement and other embedded elements;
• enhanced pumpability and reduced wear and tear on equipment; and
• reduced risk of worker strain or injury and noise levels.

Design of SCC

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At its simplest, SCC is a mixture of two different phases. Aggregates form the skeleton of the concrete and are a strong, stable, economical, and inert component. The paste (a mixture of cementitious materials, water, admixtures, and entrained air) is the phase that hardens and binds the aggregate together into one strong, durable mass. Based on mix specifications and requirements, the basic design philosophy is to first generate an optimal combined gradation of aggregates to balance the consistency and economy of the mix. Once the desired gradation is achieved, one wants to fill the leftover void spaces between the particles with cementitious paste designed to perform according to the project’s strength and durability requirements. An optimal quantity of paste creates a thick enough layer around each particle to provide enough lubricity for the required particle mobility (Figure 1).

While conventional concrete has a higher proportion of large aggregates to maximize particle packing and minimize paste requirement, a flowable concrete requires a more uniform distribution of the overall gradation to maintain cohesivity. The trick with SCC is to find the sweet spot balancing both requirements. More information on the mix design process can be found in the American Concrete Institute (ACI) 211.6T, Aggregate Suspension Mixture Proportioning Method (Figure 2).

SCC chemistry

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Consideration of SCC rheology first requires an understanding of the physical and chemical interactions within the paste system. The paste is a highly concentrated suspension of solid particles undergoing a series of chemical reactions. Portland cement-based materials comprise calcium silicates, calcium aluminates/alumino-ferrites, and calcium sulphates. When in contact with water, the hydration of cement proceeds in the following four stages:

1. An initial rapid reaction where the highly soluble species dissolve to produce an aqueous solution of calcium and hydroxyl ions, aluminates, and sulphates. These ions create inter-particle charge differences resulting in particle agglomeration.
2. An amorphous layer of calcium sulphoaluminate forms around each grain, thereby creating a diffusion barrier to provide a short dormancy period during which the concrete can be placed. During this stage, the concentration of the calcium and hydroxyl ions continues to rise until the water is saturated with calcium hydroxide.
3. As the barrier wears off, there is a quick reaction where the calcium silicates in the saturated water precipitate out to form calcium-silicate-hydrate and calcium hydroxide crystals that grow and interlock with each other over a few hours, resulting in the setting of concrete and initial development of its strength.
4. As the hydration products precipitate out of the solution, it allows further dissolution of the cement minerals. The reaction rate decelerates due to the thickening of the layers of hydration products around the cement grains and the filling up of free space within the matrix for the crystals. Supplementary cementitious materials react with, or in the presence of, the calcium hydroxide to form more calcium-silicate-hydrate to densify the product matrix and enhance its durability.

Most of the action with chemical admixtures is happening during the first few stages of the hydration process. The benefits of using high-quality admixtures cannot be overstated. It is critical to work with admixture and cement suppliers as some of the mechanisms and effects may compete with each other at various points.

It is important to note polycarboxalyte super-plasticizers absorb on to the cement grain surfaces to impart repelling electrostatic charges and create physical steric repulsion through the long polymer chains to de-agglomerate the flocculated suspension. This results in improved workability, slump retention, and a better water to cement interface for the hydration reaction and strength development of the mixture.

Additionally, accelerators and retarders absorb onto the hydrated surface to promote or impede the dissolution of certain cement ions to speed up or slow down the rate of reaction. Further, air-entraining agents stabilize the bubbles formed during mixing with a calcium hydroxide film.

Viscosity-modifying admixtures, typically based on polysaccharides, cellulose ethers, biopolymers, synthetic anionic polymers, and high molecular weight glycols, act as ‘micro-switches’ where they align and open while the concrete is under motion, and close when the concrete is at rest. This allows flow while under motion but causes a gelling effect when the concrete is still.

Other admixtures undergo various mechanisms at different stages. For example, shrinkage-reducing admixtures decrease the intra-capillary surface tension of the pore solution to bring down internal tensional forces, while shrinkage-compensating admixtures undergo slight expansion to cancel out the natural shrinkage of concrete.

Plastic performance of SCC

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The workability of SCC describes the mixture’s ability to be mixed, transported, placed, consolidated, and finished. It is often described in terms of a mixture’s:

• filling ability, or the ease with which SCC can flow into and completely fill all spaces within the form;
• passing ability, or the ease with which SCC can pass around various obstacles and through narrow spacing without resulting in blockage; and
• stability, or the ability of the mixture to maintain homogeneity during both its flow (dynamic) and while at rest (static).

The robustness of a SCC mixture can be regarded as its ability to maintain its rheological properties though the various production and construction stages. SCC rheology is often quantified through:

• static yield stress, or the minimum shear stress to initiate flow from rest;
• dynamic yield stress, or the minimum shear stress to maintain flow once in motion;
• plastic viscosity, or the change in shear stress as the shear rate changes (important for stickiness and cohesion); and
• thixotropy, or the reversible, time-dependent reduction in viscosity in a material subject to shear forces influencing its ability to gel while at rest (Figure 3).

Consideration of Stoke’s Law provides valuable insight into the segregation potential of a mixture:

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Where w is the settling velocity, r_p and r_f are the particle and fluid densities, r is the radius of the particle, and μ is the dynamic viscosity of the fluid.

Since SCC is a suspension of particles within a fluid, the frictional forces from particle interlock, due to shape and texture, also play a role. As is evidenced by the structure of the equation, having a lesser density difference between the particles and fluid, a smaller particle size, and a higher viscosity reduces the settling velocity and chance of segregation.

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Although a number of test methods are suitable for prequalification and acceptance of SCC, the most practical and easiest to use and interpret onsite is the Robustness Box, a combination of both the slump flow (and visual stability index) and T-50 cm time test information in an easy-to-interpret chart. Essentially, SCC can be quantified by:

• amount of flow, indicative of the yield stress; and
• rate of flow, indicative of the viscosity (Figure 4).

Depending on placement considerations, materials, ambient conditions, and the target slump flow itself, there exists a certain window for both the slump flow and T50 that is advisable to stay within to ensure good flow and minimize segregation. The key strategy when using the Robustness Box is understanding what changes in the mixture proportions will cause the performance to move outside the ideal parameters. Assuming a properly designed combined material gradation, the two main levers available to adjust the mix are:

• the high-range water reducer dosage that has a significant impact on the slump flow and a minor impact on the time; and
• ‘thickness’ of the mix that has a significant impact on the time and minor impact on the slump flow.

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Increasing the thickness can be achieved by reducing the water content and coarser aggregate, increasing the cementitious content and fine aggregate proportions, or through use of a viscosity-modifying admixture. This admixture increases viscosity at rest but allows a lower viscosity when flowing (Figure 5).

The rheology can thus be optimized for a particular project with the appropriate:

• low dynamic yield stress and adequate plastic viscosity and thixotropy in order to ensure self-consolidation;
• increased static yield stress due to thixotropy to ensure reduced formwork pressures and sufficient viscosity for segregation resistance; and
• appropriate plastic viscosity and stability for pumping applications.

Production of SCC

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All the materials need to be thoroughly mixed to become a homogenous mass. If there is one thing to stress, it is that consistency is key, as SCC mixtures are very sensitive to fluctuation. A comprehensive total quality plan is essential to ensuring stable performance. Aggregate moistures and drum water can be particularly aggravating if uncontrolled.

The concrete must not be over or under mixed. The required mixing time depends on the volume of concrete in the mixer, the type of mixer, and the mix design itself. With proper mixing, all the materials will have intimate contact with each other and admixtures will be properly activated to achieve the needed slump and air-void parameters. Over mixing of concrete can cause a decrease to the slump, air, and strength. Generally, there exists about a two-hour window from the time of batching to when the concrete must be placed (which is affected by ambient conditions and use of accelerators or retarders). Past this time, the concrete is beginning to set and further manipulation will break the newly forming bonds, resulting in lower strength and durability.

Specifying and testing of SCC

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The specification, performance, and conformity requirements for SCC can be found in Section 8, “Concrete with special performance or material requirements,” of the Canadian Standards Association (CSA) A23.1-14, Concrete materials and methods of concrete construction / Test methods and standard practices for concrete, and should follow the performance alternative illustrated in the standard’s Table 5, “Alternative methods for specifying concrete,” where the owner specifies the performance requirements for the concrete mix instead of prescribing material sources and quantities. The workability of SCC is described in Table 22 of CSA A23.1-14, where each property should be independently evaluated and the requirements also vary based on the particular application (Figure 6).

The workability requirements are best addressed by the designer, contractor, and producer discussing the project requirements together. Characteristics should be carefully selected and controlled based on experience with the mix or pre-placement trials. Specific requirements for SCC in the fresh state depend on the type of application, and especially on:

• confinement conditions, concrete element geometry, and congestion of reinforcement;
• placing equipment and methods;
• finishing methods and architectural needs; and
• ambient conditions.

Generally, as the reinforcement level, length or depth of the element, or element-shape intricacy increase, or the space between walls decreases, higher slump flows and viscosity and segregation resistance are required. For information on selecting the appropriate plastic properties, it is recommended to refer to ACI 237, Self-Consolidating Concrete, or the European Guidelines for Self-compacting Concrete.

Certification

As the usage of SCC continues to evolve, there is a need to ensure technicians are equipped with the necessary tools to properly test the performance characteristics of the concrete. To help provide these tools, ACI has recently announced a SCC testing technician certification course featuring the following five test methods:

• ASTM C1610, Standard Test Method for Static Segregation of SCC Using Column Technique;
• ASTM C1611, Standard Test Method for Slump Flow of SCC;
• ASTM C 1621, Standard Test Method for Passing Ability of SCC by J-Ring;
• ASTM C1712, Standard Test Method for Rapid Assessment of Static Segregation Resistance of SCC Using Penetration Test; and
• ASTM C1758, Standard Test Method for Fabrication of a Test Specimen with SCC.

Similar to other ACI courses, the certification consists of written and performance exams and is valid for five years. The certification is offered through ACI’s global network of more than 120 sponsoring groups, including the ACI Ontario Chapter and Concrete Ontario[11].

Construction with SCC

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From the contractor’s perspective, the keys to ensuring proper placement of SCC include:

• confirming the proposed mix design is appropriate to the actual project prior to ordering;
• selecting an appropriate delivery rate and time with the concrete supplier;
• ensuring placement methods do not result in harmful segregation due to excessive free-fall heights, poor pump configuration, etc. (SCC should be deposited continuously and in layers);
• documenting and clearly communicating jobsite acceptance test methods and results as well as change requests;
• ensuring the placement crew is familiar with the material and that the formwork is properly designed and constructed;
• levelling (screeding or low-level external vibration) and finishing operations, if necessary, are proper; and
• providing adequate protection and curing for the concrete mix.

Due to the relatively low w/c ratio, it is imperative SCC is properly cured to allow the hydration reaction to continue and avoid self-desiccation of the concrete.

Additionally, the formwork must:

• support the weight of the material (considering placement rates and both full liquid head conditions and thixotropic effects);
• be clean and free of debris and imperfection with a properly applied, high-quality form release agent to provide the required surface finish;
• properly secure the embedded items; and
• be sufficiently tight to avoid leakage of mortar or paste (Figure 7).

For information on best practices, one can refer to the “Best Practices Guidelines for Self-Consolidating Concrete” from Concrete Ontario and ACI 237.

Sustainability of SCC

Concrete is a sustainable and resilient building material. SCC offers a high-performance product with many benefits. Use of high-performance concrete for structural elements can result in smaller members that are better able to resist stresses with long-term durability and thus less overall material consumption. Additionally, due to the excellent placement properties of SCC, most projects realize reductions to noise, truck turnaround time, vehicular emissions, risk of injury or strain, and project completion time due to faster construction. Finally, with advancements in admixture technology, Portland limestone cement, and use of supplementary cementitious materials and mineral fillers, SCC mix designs can easily be optimized by the producer to achieve reductions in the material’s carbon footprint, quarried material consumption, and embodied energy.

Ottawa National Holocaust Monument

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The National Holocaust Monument in Ottawa honours and commemorates the victims and survivors of the Holocaust. The monument stands on a 0.4-ha (1-acre) site in the city’s downtown and is comprised of six triangular concrete elements configured to create the points of a star.

Poured in 2017 and winner of the Concrete Award for Architectural Hardscape, the project incorporated more than 8361 m² (90,000 sf) of custom-engineered formwork for walls with complex geometries, reaching heights varying from 3 to 20 m (10 to 66 ft). Over 3000 m³ (105,944 cf) of concrete was used, of which more than 1000 m³ (35,315 cf) was 35 MPa (5 ksi)- C1 exposure class SCC employing Portland limestone cement and 60 per cent slag replacement, resulting in a lighter colour, and approximately a 64-per cent reduction in the carbon footprint of the cementitious component as compared to a straight Portland cement mix. The extensive use of SCC was chosen due to its high performance, durability, and flexibility, allowing for smooth and even flow into the forms and a superior exposed architectural finish.

Conclusion

SCC offers a flexible, high performing, and sustainable product that can be easily placed. However, it is important to have a solid understanding of what the product is doing and respect and treat it accordingly. A little extra planning, forethought, and communication between all parties involved is critical to a successful project. The quality of a project also depends on the whole concreting system from the specification, raw material selection, and mix design to batching, delivery, proper construction practices, and curing. The keys to success with SCC are understanding, consistency, and a willingness to manage with a hands-on approach.

[14]Michael Stanzel has been the technical services manager for Lehigh Cement in Ontario since 2010. He is knowledgeable about cement quality and operations as he has 18 years of experience in the industry. Stanzel holds a bachelor’s degree in chemical engineering from Queen’s University. Stanzel is a member on the Canadian Standards Association (CSA) A3000, Cementitious Materials Compendium, Committee and the American Concrete Institute (ACI) and an associate member on CSA A23.1, Concrete materials and methods of concrete construction/Test methods and standard practices for concrete. Stanzel can be reached via e-mail at michael.stanzel@lehighhanson.com[15].

 

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