Self-Mending Concrete
Introduction
A question was raised as to what is Self-Consolidating Concrete (SCC) and what it can do to the building industry. The SCC is actually an advancement and new breakthrough in the field of cement and concrete technology. This SCC has been gathering enough attention and interesting impression to construction companies.
The SCC is highly flowable, non-segregating concrete that can spread into place, fill the formwork and encapsulate the reinforcement without any mechanical consolidation.
Self-Consolidating Concrete (SCC), also referred to as self-mending, self-compacting, self-leveling, or self-placing concrete, is a high-performance concrete that can flow easily into tight and constricted spaces without segregating and without requiring vibration. The key to creating the SCC is a mixture that is fluid but stable in order to prevent segregation.
How does SCC work?
To achieve the desired flowability a new generation of superplasticizers based on polycarboxylate ethers works best. Developed in the 1990s, they produce better water reduction and slower slump loss than traditional superplasticizers. The required level of fluidity is greatly influenced by the particular application under consideration. Obviously the most congested structural members demand the highest fluidity. However, element shape, desired surface finish, and travel distance can also determine the required fluidity.
Generally, the higher the required flowability of the SCC mix, the higher the amount of fine material needed to produce a stable mixture. However, in some cases, a viscosity-modifying admixture (VMA) can be used instead of, or in combination with, an increased fine content to stabilize the concrete mixture.
The flowability of SCC is measured in terms of spread when using a modified version of the slump test. The spread (slump flow) of SCC typically ranges from 18 to 32 inches depending on the requirements for the project. On one hand, the viscosity, as visually observed by the rate at which concrete spreads, is an important characteristic of plastic SCC and can be controlled when designing the mix to suit the type of application being constructed.
Advantages
General Advantages:
· SCC places quickly and easily with little or even no vibration to give a smooth surface finish.
· SCC can save money by reducing the wear and tear of equipment and improve the working environment for employees.
· SCC achieves very high early stripping strengths that will yield a quicker turnaround on you forms.
· The smooth surface will minimize or eliminate the need for time consuming cosmetic repairs.
· More rapid strength development than conventional concrete
· Fluid consistency measured not by slump but spread
· Tendency to produce finished surfaces of superior uniformity and smoothness
Specific Advantages:
· Can be placed at a faster rate with no mechanical vibration and less screeding, resulting in savings in placement costs
· Improved and more uniform architectural surface finish with little to no remedial surface work
· Ease of filling restricted sections and hard-to-reach areas. Opportunities to create structural and architectural shapes and surface finishes not achievable with conventional concrete
· Improved consolidation around reinforcement and bond with reinforcement
· Improved pumpability
· Shorter construction periods and resulting cost savings
· Quicker concrete truck turn-around times enabling the producer to service the project more efficiently
· Reduction or elimination of vibrator noise potentially increasing construction hours in urban areas
· Minimizes movement of ready mixed trucks and pumps during placement
· Increased jobsite safety by eliminating the need for consolidation
Placement efficiencies of the SCC can increase by 300% and labor costs can be reduced by 70%. It is the architects and engineers that will benefit from increased design flexibility without sacrificing performance or increasing placement costs.
Composition
When compared to other conventional concrete, the SCC looks very different. Usually, concrete that had the fluidity of SCC had a very high water to cement ratio that lowered compressive strengths and compromised durability. With this, a properly designed SCC can save time and labor without sacrificing performance.
There are two important properties specific to SCC in its plastic state, the flowability and stability. The high flowability of SCC is generally attained by using high-range-water-reducing (HRWR) admixtures and not by adding extra mixing water. The stability or resistance to segregation of the plastic concrete mixture is attained by increasing the total quantity of fines in the concrete and/or by using admixtures that modify the viscosity of the mixture. Increased fines contents can be achieved by increasing the content of cementitious materials or by incorporating mineral fines. Admixtures that affect the viscosity of the mixture are especially helpful when grading of available aggregate sources cannot be optimized for cohesive mixtures or with large source variations (2004).
A well distributed aggregate grading helps achieve SCC at reduced cementitious materials content and/or reduced admixture dosage. While SCC mixtures have been successfully produced with 1 ½ inch aggregate, it is easier to design and control with smaller size aggregate to producing a good mixture. SCC mixtures typically have a higher paste volume, less coarse aggregate and higher sand-coarse aggregate ratio that typical concrete mixtures.
Cautions and Disadvantages
Despite the conventional idea of using SCC in producing many aesthetically critical projects, it has been also found that it has inappropriateness. Even though mixes of various levels of strength and durability can be designed to generate smooth, defect-free surfaces, this does not ensure that the finished structure will be unflawed ( 2005).
(2005) added that there are two outside influences greatly affect the appearance of concrete. The first is forming materials – the most popular are steel and plywood. The second factor is release agents. Different types and brands of release agents (form oils) give varying degrees of surface defect. The method of application of these agents also plays a part in the final product appearance.
Another negative aspect of SCC is the shrinkage. Given that generally a large amount of fine materials is used in the mixtures and the nominal maximum size is limited, the concrete typically has higher shrinkage. As shrinkage increase, there will be more cracks in the restrained concrete elements which can accelerate the deterioration of the concrete and the reinforcement (2004).
Assessment of SCC
It would be easier to evaluate SCC through comparison with the conventional concrete. There are several aspects that make SCC different and more advantageous to use than the conventional concrete. Conventional concrete meets its setback on the adequacy of consolidation in thin sections or areas of congested reinforcement. Consequently, this would lead to a large volume of entrapped air voids and compromises the strength and durability of the concrete. However, the solution to this problem has been answered through the use of SCC. SCC can minimize the problem in view of the fact that it was designed to consolidate under its own weight.
SCC differs from that of a conventional concrete in the sense that it has a lower viscosity resulting into a greater flow rate when pumped. Correspondingly, during the pumping, the pumping pressure is lower thus reducing wear and tear on pumps.
After tested and found feasible, SCC was then used in a field application in an arch bridge in Fredericksburg, Virginia. SCC proved to be appropriate for the bridge since arches were heavily reinforced, thin, curved sections that would be difficult to construct with conventional concrete. During the construction and application of the SCC, it has been found that the percentages for the individual materials were similar in the top and the bottom which means that no segregation had occurred. For the reason that there was no segregation, SCC was proven to be possible and more effective in a commercial process and on a larger production scale.
The made several conclusions after the SCC test. They concluded that (1) SCC that flows into formwork and through reinforcement under the influence of its own weight can be made such that no external vibration is required. (2) With careful adjustments to the mixture proportions and careful batching, SCC can be produced successfully with local materials. (3) SCC can have high compressive strength and low permeability for use in bridge structures. (4) The use of SCC has the potential to provide initial savings because of the reduction in labor required to place the concrete and the speed of construction.
There were actually a number of conclusions made by the after the test but they also come up with recommendations. (1) Use rheometers to provide data on yield stress and viscosity to describe flow characteristics while mixes are being developed. (2) Use SCC in transportation structures that can benefit from concretes with high workability, particularly in thin sections and areas with dense reinforcement. (3) Conduct further research to determine the relationship between viscosity and segregation.
Conclusion
SCC brings about advancement in the field of construction and concrete technology. It can never be denied that SCC took a step higher than its antecedent however SCC has several cautions, as stated earlier, that should not be taken for granted for the purpose of finding where and when SCC will be appropriate.
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