Essay: Comparison of 50MPa Powder-type and VMA-type Self-Compacting Concrete Mixes

1. Introduction
A concrete mix initially consisted of a simple mixture of water, cement and aggregates along with the addition of an external vibration to compact and place the concrete. In recent decades however, there has been a remarkable evolution in concrete performance with the inclusion of cement replacement material, mineral and chemical additives.
Self-compacting concrete (SCC) is one such evolution that does not require vibration to compact, it simply consolidates under its own weight. SCC completely fills formwork by flowing under gravity alone, without segregation or causing blockages even in highly reinforced areas.
A traditional vibrate concrete (VC) can be transformed into a SCC with the addition of mineral admixtures, a reduction in coarse aggregate and the inclusion of chemical admixtures.
There are three types of SCC, powder, viscosity-modifying-admixture (VMA) and a hybrid of the two know as a combination SCC.
In this report the powder type and VMA type were developed through trial and error, cured for 30 days and tested in the hardened state. Both were investigated in regards to their mix development, behaviour in the fresh state and mechanical behaviour in the hardened state.
Objectives
Prepare a 50MPa powder type SCC and a 50MPa VMA-type SCC through trial and error that satisfies the requirements of SCC.
Cast 6 cubes (100 x 100mm), 6 cylinders (100 x 200mm) and
4 beams (100 x 100 x 500mm) for both the powder-type and VMA-type SCC.
After 28 days of curing conduct a compressive strength test on the 6 cubes, a tensile splitting strength test on the 6 cylinders, a modulus of elasticity test on one of the cylinders and a flexural strength test on all 4 beams.
Analyse the results and discuss the behaviour of the two SCC mixes and draw a conclusion on the results obtained.
All fresh and hardened concrete state test were conducted in accordance with the British Standards Institute as detailed in section 3.
2. Literature Review
2.1. Overview of SCC
SCC is a mixture that is both fluid and homogenous, which flows and consolidates under its own weight without the need for external vibration or compaction. In its fresh state there are three properties that define SCC; flowing ability, passing ability and segregation resistance.
Flow-ability: The ability of the concrete to completely fill the formwork in which it is placed.
Pass-ability: The ability to pass through and around congested reinforcement without segregation.
Non-segregation: The ability to maintain a homogenous material.
SCC is classified in three types; powder, VMA type and combination type.
Powder type SCC is characterised by a large percentage of powder within its mix. The high powder content provides the segregation resistance and the flow-ability and fill-ability are achieved with the inclusion of a superplasticiser.
VMA type SCC on the other hand has a much lower powder content, between 20 to 60% lower. The segregation resistance is provided by the VMA and the flow-ability and fill-ability are achieved by the superplasticiser. In the case of this report, an all in one VMA – SP solution was used.
The combination type SCC mix consists of a powder content between that of a powder type and VMA type SCC with the inclusion of a VMA and superplasticiser. The combination type is not discussed further in this report.
The origin of SCC dates back to the early 1980’s in Japan. The high seismic activity of this geographical region necessitates that concrete structures are highly reinforced. This posed a difficult challenge in efficiently pouring the concrete around the reinforcement. Conventional methods using vibration to compact the concrete were insufficient, resulting in honeycombing and compromising the structures quality assurance. SCC was born from the desire to make the concrete compacting completely independent of the production context, whether in the technical plan or the manpower. [Amziane et al. pp, 2. 2013]. Goodier (2003) states that one of the main drivers for the development was due to the reduction in the number of skilled labour the construction industry was experiencing. The use of SCC was found to offer economic, social and environmental benefits over traditional VC, therefore interest in SCC picked up outside of Japan.
Its introduction into Europe started in the early 1990’s, with Sweden developing SCC in 1993 and it quickly spread to other Scandinavian countries. The concrete industries in the UK, France, Germany and the Netherlands are all using the material and most other European countries conduct some form of research and development into SCC. [Goodier. 2003]
2.2. Formulation of SCC
Amziane et al. (pp, 7. 2013) states that the principle idea in designing SCC involves considering the mixture as a concentrated suspension in which the paste (cement, GGBS, chemical additives and water) is viscous and dense enough and in which the coarse aggregates is at a low enough concentration to prevent too much interaction.
Simply put, SCC is a compromise between have a highly fluid mix while ensuing an adequate consistency to prevent segregation or bleeding.
In powder type SCC the volumetric proportion of the paste must be sufficiently high enough to limit interactions between the constituents. Increasing the cement content would be one option, an unsatisfactory one due to the detrimental effect cement production has on the environment and the additional cost of production. Mineral admixtures are the preferred choice.
In comparison to a conventional VC, at similar quantities of cement, sand a water, SCC has a much higher paste content through the reduction of coarse aggregate and the addition of fine particles (mineral admixtures), such as limestone powder. The volumetric increase in paste limits the inter-granular contact and reduces the risk of blockages through reinforcement, achieving the pass-ability requirement.
A mix with an increased concentration of fine particles requires the introduction of fluidising chemical admixture, such as a super-plasticiser, to achieve the flow-ability requirement.
In the case of VMA type SCC with an identical composition to that of a conventional VC,
a fluid mix is achieved with the inclusion of a fluidising chemical admixture.
However, unlike the powder type SCC, the paste content is low, so in order to prevent segregation of the large aggregates a thickening chemical admixture is also include to increase the paste viscosity. [Amziane et al. pp, 7-8, 11. 2013]
In general SCC mixes, compared to conventional VC contain,
Lower coarse aggregate content to increase the passing ability of the mix
Increased paste content
A higher powder content to increase the cohesiveness of the mix
Lower water/powder ratio
High super-plasticiser dosage to increase flow-ability
In some cases, contain a VMA
2.2.1. Ground Granulated Blast Furnace Slag (GGBS)
Slag is produced as a waste product during the manufacture of pig iron, about 300kg of slag is produced per 1 tonne of pig iron. The slag is quenched so that it solidifies as glass. The rapid cooling by water results in fragmentation of the material into granulated form. [Neville. 2011] It is granulated down to a fineness of greater than that of cement, usually 350m2/kg but on occasion in excess of 500m2/kg is used [Neville. 2011 cited Sakai et al. 1992].
There are many benefits to incorporating GGBS into the concrete mix which include, improved workability in the fresh state, reduced heat of hydration and a denser microstructure of cement paste.
GGBS is a pozzolan, meaning it possesses no cementitious value on its own. However, in the presence of cement and water, the subsequent reaction between the two releases calcium hydroxide with which the GGBS reacts to [Karihaloo. 2015]. Therefore, the initial hydration of a concrete mix containing GGBS is very slow as it is dependent upon the release of calcium hydroxide. The progressive reaction and release of calcium hydroxide results in a continuing reaction of GGBS over a longer period. Thus, there is a long term strength gain
[Neville. 2011 cited Hogan and Meusel. 1981]. Neville (2011) goes on to state that the greater the fineness of GGBS leads to an improved strength development but only at later stages, after the activation of GGBS has taken place.
This slower rate of hydration reduces the overall peak temperature of the concrete mix.
Kolani et al. (2012) states that the experimental investigation by Sakai et al. (1992) shows that the heat of hydration decreases as slag content increases. This is very beneficial when a large mass of concrete is to be place as the heat of hydration can be reduced with the inclusion of GGBS. The heat of hydration rises the temperature of the concrete and thermal stresses occur within the element due to expansion and contraction.
When the core of the concrete element cools the contraction is restrained by the already cool exterior and cracking of the interior may occur [Neville. 2011].
The replacement level of GGBS is dependent upon the required compressive strength. Previous studies by Dinakar et al. (2013) cited Babu and Kumar (2000) have shown a 10% replacement achieving a 100MPa strength after 28 days and 80% replacement achieving a maximum strength of 30MPa.
2.2.2. Aggregate
Neville (2011) states that in conventional concrete at-least three quarters of the volume is occupied by aggregate. It is dispersed within the cement paste for largely economic reasons as it is cheaper than cement but it also offers higher volume stability and better durability than hydrated cement paste alone.
However, in the case of SCC the coarse aggregate (CA) has to be sufficiently low to allow individual aggregate particles to be lubricated by a layer of paste increasing the fluidity of the mix and hence increasing the passing ability. Between 29% to 35% of CA by volume of concrete is the most common proportion with the maximum aggregate size most often in the 16 to 20mm range. [Domone 2006].
The strength of the SCC is provided by the binding between the aggregate and paste at the hardened state, while the workability is provided by the binding during the fresh state
[Nan Su et al. 2001].
Most aggregates exceed in excess of 100MPa compressive strength (Neville 2011, table 3.6, pg. 121) so why is it that the concrete element will failure well below the strength of the aggregate. The influence of aggregate on the strength of the concrete is not limited to the mechanical strength but also the bond between the aggregates and its absorption. The surface texture of the aggregate influences the bond to the cement paste and the water demand of the mix. The bond is due to the interlocking of the aggregate with the hydrated cement paste.
A rougher shaped, that of crushed aggregate will result in a stronger mechanical interlock.
A good bond is exhibited when a crushed specimen of normal strength contains aggregate particles that are broken right through. [Neville 2011]. The interface between the paste and aggregate is the limiting factor in regards to the strength of the hardened concrete.
2.2.3. Limestone Powder
During the quarrying process of carbonated rocks, limestone powder (LP) is generated as a
by-product. Unlike GGBS, LP does not possess any pozzalanic properties and has no cementitious value although it does offer many technical benefits when included within a concrete mix. These include, bleeding control, increased early age strength, improved workability of the mix, improved viscosity and densification of the concrete microstructure. These positive contributions on the properties of SCC as well as the economic benefit of using a waste by-product accounts for the fact it is included within most SCC mix designs. [Adekunle et al. 2015]
Experimental data conducted by Ye et al. (2006) shows that the presence of LP influences the hydration rate in SCC. One of the hypothesis proposed suggests that LP is not inert but an active partner in the hydration reactions. Karihaloo (2015) states that there is small evidence LP adds strength due to the nano-particle reactions. The high surface area to volume ratio results in the particle been more reactive as atoms on the surface of a material are more reactive than those in the centre. Further evidence to back this up is shown by Bentz et al. (2011), concluding that “particle sizes of LP are a key variable influencing performance as accelerators of reaction and setting; while a nano-limestone is highly efficient in this regard….”
However, debates about this phenomenon are on-going with further research required.
2.2.4. Super-Plasticiser (SP)
All SCC mixes include a SP by necessity to ensure the fluidity of mixture. SP are water-soluble organic polymers which have to be synthesised, using a complex process, to produce long molecules of high molecular mass. These long molecules wrap themselves around cement particles giving them a negative charge and causing the cement particles repel each other. This results in de-flocculation and dispersion of the cement particles resulting in improved workability of the concrete mix. SP do not fundamentally alter the structure of the hydrated cement paste, the main affect being a better distribution of cement particles and better hydration.
[Neville. 2011]
Figure X. Dispersion of cement particles due to electrostatic repulsion.
[Amziane et al. (2013), Chapter 1, Figure 1.4]
At a given water/cement ratio, increasing the water content to increase the flow-ability would reduce the strength of the concrete. The dispersing action of the SP increases the flow-ability of the mix without negatively affecting the strength.
Small dosages of SP, 1 to 3 litres per 1m3 of concrete, are commonly used but this varies depending on the required flow-ability and the water/cement ratio of the mix.
2.2.5. Viscosity Modifying Admixture (VMA)
VMAs are high molecular weight, water-soluble organic polymers incorporated within SCC mixes to reduce the risk of separation of the heterogeneous constituents. It is used to
enhance the viscosity of the cement paste without the need to reduce the water content.
[Khayat and Ghezel. 2003]
VMAs do not alter any properties of the mix with the exception of the viscosity. This increased viscosity and thickening of the mix prevents segregation. Its inclusion makes the concrete more tolerant to water content variations, maintaining the plastic viscosity and segregation resistance, making the concrete more robust. However, an overuse makes the concrete too cohesive and thick, reducing the fluidity of the mix.
VMAs are more commonly used with SP, either as individual admixtures or one complete solution. In this combination, the dispersing action of the SP enhances the flow whilst the VMA provides stability to prevent segregation.
Most importantly, VMAs are not a substitute for poor quality constituents or a poorly designed mix.
2.3. Workability of SCC
Rheology (Bingham 1929), is the science of deformation and flow. The dynamic yield point τ_0 and plastic viscosity η, the resistance of the concrete to flow under external stress, are two parameters used in Bingham’s linear behaviour model and the equation relates the shear rate γ ̇ to the shear stress τ,
τ=τ_0+ηγ ̇
The static yield point corresponds to the shear stress necessary to induce the material to flow. For SCC to flow it has to overcome the yield stress which is sufficient to break bonds. After flowing at a given time, equilibrium is reached and the flow stops. At this point equilibrium exists in the mix between the applied shear stress and the bonds.
Khayat and Ghezel. (2003) state that SCC should exhibit a low yield value to ensure high
flow-ability and moderate viscosity to secure high resistance to segregation.
Amziane et al. pp, 38. (2013) cited Svermove et al. (2003) and D’Aloia et al. (2006), have shown that SP and VMA have a direct effect on the static yield point and viscosity. The yield point, which reflects a greater or lesser resistance to flow, is directly affected by the introduction of a SP and VMA.
A study conducted by Barrack et al. (2008) on the influence of the paste constituents on rheological properties of cement paste, determined the hierarchical influence of constituents, shown in Figure X.
SP and VMA are most influential factor in yield stress variation as shown in Figure X. The addition of a SP lowers the yield point of the mix, prevents flocculation, causing flow to occur at a lower applied shear stress. For the concrete to spread, the SP is the dominate constituent, with the size of the spread related to the water/binder ratio. The higher the ratio the greater the spread diameter. [Barrack et al. 2008 cited Kordts and Breit 2003.]
Barrack et al. (2008) goes on to state that the VMA does not seem to play an important role in the spread variation. It acts on the stability at rest rather than during flow.
Figure X. Hierarchical influence of constituents [Barrack et al. (2008), Table 9, pp. 20.]
In regards to the viscosity of mix, the cement is the most influential factor as it is the principle component. For powder type SCC, to maintain the stability of the concrete and prevent segregation, a viscous flow, the fine particles have to be increased. Alternatively, for the VMA type the same effect is achieved with the inclusion of a VMA.
When using the additives simultaneously, an optimum combination of the two must be found. The proportion combination has to be determined so that the yield point remains low, to induce flow, whilst maintaining a viscous form, to reduce gain sedimentation. The VMA stabilises the system of particles generated by the SP. [Barrack et al. 2008]
An experimental study Yammine (2007) showed that the volume of paste within a SCC mix is commonly set between 35-40%. At this large volume of paste, the yield point is low, aggregates are lubricated and friction contact is minimised providing the desired workability for SCC. At paste volumes lower than this, concrete fluidity is low, the static yield point is high and aggregate interactions are dominated by direct friction contact [Amziane et al. pp, 15. 2013].
2.4. Durability of SCC
Often the issue of durability is overlooked as engineers make the assumption that strong concrete is also durable. Concrete structures are vulnerable to both physical and chemical deterioration and therefore have to not only achieve the desired mechanical requirements, such as compressive strength, but also the durability requirements. SCC is subjected to the same durability requirements as conventional concrete. For a given exposure class in BS EN 206-1,
SCC is subjected to the same performance requirements.
The flow-ability required for a SCC mix, obtained with the inclusion of more fines and a SP, leads to reductions in trapped air content with improves the compaction and hence, the hardened strength of the concrete. Roziere and Khelidj pp. 202. (2013) concluded “The increased paste volume slightly worsens the mechanical properties, but does not seem to have a significant impact on durability”.
Experimental evidence by Kanellopoulos et al. (2012) has shown that SCC with cement replacement material showed noticeable improvements in its durability, especially with
chloride-ion permeability. This is attributed to the surplus calcium silica hydrate formed during the pozzalanic reaction, which densified the microstructure resulting in lower open porosities.
The densification of the microstructure, and because of the low content of calcium hydroxide (GGBS reacts with calcium hydroxide) the resistance to sulphate attack is improved. Further evidence shows that concrete binder with high slag content have a good resistance to sulphate rich waters, due to the low content of calcium hydroxide. A study conducted by Higgins showed that for a 60% cement replacement with slag, the resistance of concrete is greatly improved that that of cement alone. [Roziere and Khelidj, pp. 200. 2003. cited Higgins, D. 2003]
Mohammed et al. (2013) states “Carbonation of the concrete is considered one of the major concrete durability problems…” Carbonation occurs when, Carbon dioxide CO2 penetrates the concrete and reacts with the hydrated cement, reacting most readily with Portlandite Ca(OH)2 and hydrated calcium silicates, forming Calcium Carbonate CaCO3 and reducing the pH from 13 down to as 8.3 [Neville, pp. 499].
In high pH conditions the cement paste forms a thin passivity layer of oxide, protecting it from rust and corrosion. This is known as passivation. However, carbonation reduces the pH to a value below the passive threshold of steel. When this occurs the protective oxide film is removed and corrosion takes place.
Roziere and Khelidj, pp. 164 (2003) cited Baron, J (1996) advises against high proportions of binder replacement with slag, no more than 50% due to the negative influence on carbonation resistance. Neville (2013) cited Osbourne (1996), Thomas and Mathews (1992) states that high slag contents lead to a greater depth of carbonation. However, for replacement levels less than 50%, the marginal increase in carbonation is negligible. [Neville. 2003. cited Bier. 1987]. The effect can strongly depend on good curing.
2.5. Advantages of SCC
SCC offers many environmental, economic and safety advantages in comparison to traditional concrete. These include faster construction, improved quality and the elimination of noise due to vibration.
Mechanical vibration is an unhealthy task and prolonged exposure can cause what is known as ‘white finger’ due to poor blood circulation. SCC offers a solution to this as it consolidates under its own weight without the need for external vibration. Therefore, noise levels are also significantly reduced on construction sites and in pre-cast factories. Walraven (2003), cited Dekkers (2001) states that the noise has decreased from an average of 93dB to far below the critical level of 80dB since the introduction of SCC in pre-cast factories.
SCC also contributes to shorter construction periods as it can be placed at a faster rate, even in highly reinforced zones, and therefore potential cost savings. Labour resources are also used more efficiently as the operatives previously tasked with placing and vibrating the concrete can be put to work on other tasks.
Walraven (2003) further states that SCC is therefore denoted as silent, healthy and a tolerant concrete.
SCC offers an improved and superior finished product [Goodier 2003].
Nan Su et al. (2003) states that the high fluidity, self-compacting ability and segregation resistance of SCC all greatly contribute to the reduced risk of honeycombing. The finished quality is independent of mechanical vibration and therefore the influence of bad workmanship and the likelihood of honeycombing is significantly reduced [Walraven 2003]. Goodier (2003) continues by stating that less skilled labour is required for SCC to be placed, finished and made good after casting.
The inclusion of GGBS and limestone powder, both of which are waste by-products reduces the amount of cement required providing both economic and environmental benefits.
Amziane et al. pp,14. (2013) states “Limestone filler… it is not very costly and is available in large quantities”.
Another benefit of SCC is the durability potential is greater than that of conventional VC.
This is due to the increased fines within the mix that densifies the microstructure.
[Kanellopulous et al. 2003]
Walraven (2003) concludes that the cost of production is decreased by the introduction of SCC whereas the labour conditions and quality of the finished concrete improve.
2.6. Experimental Formulation of SCC
For a conventional VC mix, achieving a SCC of similar strength, for both powder and VMA type, the first step is to replace a percentage of cement with GGBS. The amount will depend on the hydration requirements of the concrete, with a higher replacement reducing the initial strength gain and decreasing the peak hydration temperature.
For powder type SCC, the next step is to increase the paste volume by replacing a percentage of the CA with LP. About 35-40% paste volume is an educated starting point with previous experimental evidence [Yammine. 2007] showing that at paste percentages lower than this, concrete fluidity is too low. At this point it is expected that the mix would have some fluidity to it but still be very thick and dense, still very similar to that of a conventional VC.
The third step is to introduce a SP into the mix. How much is dependent of the volume of the mix and the required flow-ability.
For VMA type SCC, after the inclusion of GGBS in the mix, a VMA – SP solution is added to the mix. The amount added, similar to that of SP, is dependent of the volume of the mix and the required flow-ability.
To determine if the requirements of SCC are satisfied and whether the mix achieves its required consistency class in its fresh state, the mix is tested as per the British Standards tests. Based on the results the aggregates, LP and SP are adjusted accordingly. With the VMA type, the only changing variable is the VMA – SP.
Importantly, when developing any type of concrete, a key parameter is the water/binder ratio. Karihaloo (2015) states that to achieve a 50MPa concrete the critical water/binder ratio is 0.55.
Powder Type
Replace a percentage of cement with GGBS
Reduced the CA and replace with LP
Introduce a SP, starting with a small dose
VMA Type
Replace a percentage of cement with GGBS
Introduce a VMA – SP solution, starting with a small dose
3. Experimental Procedure
3.1. Laboratory Preparation
A full Health and Safety brief was given and a Risk Assessment completed before commencing with work in the laboratory. Hand, eye and respiratory protection were used throughout the sieving and trial mixing process. Hearing protection was used as and when deemed necessary to do so.
The course aggregate, fine aggregate and limestone powder were all sieved to the required size range with the use of a vibratory sieve. The materials were dry prior to sieving.
Coarse Aggregate (gravel): 2mm to 20mm, with an equal distribution between 2 to 10mm and 10 to 20mm.
Fine Aggregate (sand): < 2mm.
Limestone Powder: < 2mm
When trial mixing, the cement drum was dampened immediately prior to use but kept free from excess moisture as to not affect the water content of the mix. After each trial mix the drum was cleaned to avoid contamination between the mixes.