Engineering of Self Compacting Concrete

Subrato Chowdhury, & Sandeep Kadam, UltraTech Cement Limited, Andheri (East), Mumbai

Cementitious material is the lifeline of modern infrastructure. Increasing demand for concrete in newer applications leads to engineer the properties of concrete at fresh and hardened state.

One of the most important performance criteria for concrete is the fluidity at fresh state. Appropriate fresh state properties are achieved by engineering suitably the theology of concrete. Such engineering is achieved by incorporating chemical & mineral admixtures into cementitious system. The development of self-compacting concrete is primarily achieved by designing the appropriate theology using different cementitious system, admixtures, etc.

Self-compacting (or consolidating) concrete (SCC) is a particular concrete mix which has a special performance requirement of self–consolidation or compaction at the time placement. However, at the hardened state, there is not much difference in terms of mechanical properties and durability between SCC and other type of concrete mixes viz. high performance concrete (HPC), normal strength concrete (NSC), etc.

The important aspects of achieving the functional requirements (filling ability, passing ability and resistance to segregation) of SCC are related with:

• Appropriate characterization of ingredients
• Mix proportion
• Mixing method
• Placement

This paper would discuss the effect of characteristics of individual ingredients, different approaches for mix proportioning and the mixing method on the overall performance of the SCC mix in fresh state, especially on its theology. The effect of method of placement, especially in terms of the pressure exerted on the formwork will also be discussed.

Introduction

Concrete is a suspension of aggregates in cement paste (1). A suspension is self-flowing if it flows under its own weight. Additionally, it is to ensure–uniform suspension of solid particles during casting and thereafter until setting (2). The above perspective induces the definition of self-compacting (or, consolidating) concrete (SCC), as a concrete mix, which in fresh state, has the ability to fill the formwork and encapsulate reinforcing bars only through the action of gravity i.e. self-weight at the time of placement without any external energy inputs from vibrators, tampering or similar actions and with maintained homogeneity at the time of placement (3). SCC can be used in most application where traditional vibrated concrete, such as conventional normal strength concrete (NSC), high performance concrete (HPC) is used. Two principal advantages of SCC are improved homogeneity of fresh concrete that leads to more durable concrete at hardened state as well as higher productivity in terms of pouring of concrete, and improvement in working condition and less noise pollution (4, 5).

The difference between the SCC and vibrated concrete exists in the performance requirements during fresh state; not much in terms of properties at harden state such as strength, durability. SCC is engineered to fill all the space within the formwork passing through the reinforcements or other obstruction without segregation. This attributes to three important functional requirements related to workability of the concrete mix: filling ability, resistance to segregation, and passing ability (3). Filling ability is the high fluidity and deformability to ensure adequate flow under selfweight. Resistance to segregation is the ability of the particle suspension (in fresh state) to maintain homogeneity throughout the mixing, transportation, and placement process. Passing ability is the ability to pass obstacles, narrow opening and closely spaced reinforcement bar without getting blocked by interlocking of aggregate particles (3). Filling ability and passing ability of a fresh concrete mix depend on its fluidity and resistance of segregation on the homogeneity. Additionally, the paste or mortar has to deform well too. The yield stress and plastic viscosity generally characterizes such theological behavior of fresh concrete mix. Fluidity is inversely proportional to the yield stress, while plastic viscosity has direct proportionality on homogeneity. Contact and collision between aggregates as well as the interparticle friction increase with the decreases in relative distance between aggregates particles in the concrete mix, resulting in the blockage of aggregate particles (6). Limiting coarse aggregate volume increases inter-particle separation and reduces the inter-particle friction and collisions resulting in minimization of the blockage leading to improvement in passing ability.

The increase of paste volume with emphasis to low water powder ratio (w/p) in presence of compatible chemical admixtures further strengthens the fluidity and helps in attaining homogeneity. Adequate homogeneity improves viscosity of the mix, which in turn enhances the segregation resistance. An optimum balance between fluidity and viscosity is the key to achieve efficient selfcompacting characteristics of the concrete mix at fresh state. In SCC, the powder contains binder component consisting of ordinary Portland cement (OPC), mineral admixtures like flyash along with/ without filler material like limestone powder, dolomite etc. To achieve moderate plastic viscosity and low yield value, multiple chemical admixtures are required. Special chemical admixture like viscosity modifier admixture (VMA) is used for controlling the viscosity of the mix and superplasticizer for lowering the yield stress. In addition, the characteristics of fine and coarse aggregates play very important role on the yield stress of the mix.

Overview of SCC

The work on SCC had started in 1988 in Tokyo University, Japan. The Japanese concept spread through Asia and to Europe around 1993 (7). This concept is well accepted in USA now. A few points are important with regard to engineering of structures using SCC mix to satisfy the intended specification.

These are:

1. characterization of the ingredients
2. mix proportion technique to achieve desired characteristics
3. mixing method
4. effect of method of placement, especially the pressure exerted on formwork.

The above points are deliberated in the following sections of the paper.

Characterization

Ingredient characterization exhibits different aspects depending upon the background of the users. These concepts range from that of the scientist, who thinks of it in atomic terms, to that of the concrete technologists, who thinks of it in terms of properties of concrete in fresh and harden state, procedure of construction and quality assurance, etc. Characterization of an ingredient deal with those features of the material like composition, structure, etc that are significant for a particular preparation, study of properties or use etc. The three basic functional requirements of SCC mix at fresh state, i.e. filling ability, passing ability and resistance to segregation could be assessed in terms of the theological characteristic like yield stress and plastic viscosity.

An appropriate ingredient characterization helps to achieve the performance behavior of SCC at both fresh and hardened states. General-purpose Ordinary Portland Cement (OPC) is suitable to be the main cementitious constituent for SCC. It is also well–established that compatibility between superplasticizer and OPC plays important role on the rheological characteristic of mortar. Certain chemical compounds of OPC clinker such as alkali (Na2O, K2O); sulphate (SO3) has significant influence on such compatibility (8). The presence of mineral admixtures has a definite role on the performance of paste, especially format ion of micro mortar. The micro-mortar formations is involved with all particles below the size of 125¼, chemical admixture and water (6). Flyash is commonly used mineral admixture in SCC. Particle size distribution of flyash, chemistry of flyash and presence of un-burnt coal particles has enough impact on fluidity and deformability of mortar for SCC (10). The bulk solid volume of the fly ash also has significant impact on the rheology. Low lime content flyash improves the fluidity of the paste (9). The flow value increased as the bulk solid volume of flyash is increased (9, 10). High belite content OPCwas used at the initial years of SCC without any application of VMA (3). However, high alite content high strength OPC may be desirable for achieving high strength SCC along with appropriate replacement level of OPC by mineral admixture.

The compatibility of multiplechemical admixtures present along with mineral admixtures needs a serious attention towards satisfactory performance of rheological properties as well as hydration kinetics that has bearing on hardened properties.

The fine aggregate is one of the major components of paste formations. Well-graded fine aggregate is desirable. The size of coarse aggregate in SCC is 5 to 20 mm. However, the size of the aggregate is decided based on the size of the opening such a spacing of reinforcement bar (1). Larger the aggregate size more the driving force for flow would be required. Blocking will occur if the maximum size of the aggregate is large as well as the content of the larger size aggregates is high. Crushed stone aggregates require more paste volume for nonblockage criteria compared with the natural gravels. Higher packing density of aggregates reduces demand of superplasticizer (1, 2). Extensive works on characterization of ingredients like OPC, fly ash and fine aggregates for SCC were carried out by authors and are published elsewhere. (8,10,11,12).

Ingredients for self-compacting concrete shall satisfy the respective codal specifications. Findings of the works, on characterization of ingredients, carried out by authors are summerised here.

Ordinary Portland Cement

Clinkers may have different levels of alkali and sulphate concentrations, but the corresponding OPC shows fairly the same levels of sulphate owing to addition of gypsum during grinding process. Alkali and sulphate content of the clinker not that of cement binder, has influence on the rheology of mortar for SCC.

Initial flowability and viscosity of mortar mixes are not influenced by the alkali and sulphate content of the clinkers irrespective of the dosages of flyash. The initial flowability decreases and viscosity increase with elapsed time for all the cement replacement levels and types of OPC.

Low sulphate content of clinker increases the flow ability and reduces viscosity irrespective of alkali content. Alkali content of clinkers has similar trend of effect on flowability and viscosity but this influence is not as significant as that of sulphate. OPC from low sulphate bearing clinkers and cement replacement level of 50% and above by flyash is vulnerable to the risk of segregation. Low sulphate content increases the filling ability of concrete mixes.

Flyash

The flow of the mortar is affected adversely with flyashes having higher percentage of particle size above 90ì, and the mortar becomes unfit for the purpose. The flow is enhanced with fly ashes having higher percentage of particle size below 45ì.

Flyash with high lime and sulphate content is not suitable for producing SCC as it decreases the flow and increases the viscosity; non-cohesiveness of the mortar is also increased significantly. Flyash with higher LOI, i.e. the higher carbon content, is not a suitable mineral admixture for SCC mortar. It affects the rheology adversely making the mix highly viscose aswell as non-cohesive.

Higher quantity of fly ash could result in adjustment of chemical admixture to lower dosages for achieving appropriate flow and viscosity of mix. Flyash of appropriate characteristics acts as flow enhancing and viscosity reducing agent in SCC mortar. Increase in flyash quantity neutralizes the negative impact of high sulphate and high alkali content of OPC clinker as well as the size fraction of fine aggregates on the rheology of mixes. Though quantity of flyash does not significantly influence the initial spread and viscosity of mixes, its increase in value helps in retention of higher spread diameter and lower viscosity.

Fine Aggregate

Initial viscosity of mortar mixes is influenced by the size fraction of fine aggregate. The finer fraction of sand reduces flowability and increases viscosity of mortar mix. Lower quantity of fines in fine aggregate accentuates the possibility of segregation.Ingredients characterized and found suitable by mortar rheology experiments are suitable for selfcompacting concrete.

Mix Proportioning Method

A number of methods for proportioning SCC mix have been developed over the years with primary attention to produce satisfactory self–compacting properties but with less attention to the properties at hardened state. Most of the methods those are presently available may have some inherent limitations, either in terms of ingredients for which they have been shown to be suitable or in terms of the range of concretes that can be produced. These methods are of varying complexity and may require wide range of information on the effect of each ingredients on the mechanics of SCC mixes. In general, the SCC mix proportioning methods consider volume as the key parameter because of the importance of the need to fill over the voids in between the aggregate particles by the paste.

Different mix-proportioning methods can be grouped in having two categories of approaches. The basic steps of first category are determination of quantity of coarse aggregate, and then deriving appropriate quality of mortar compatible for SCC mix. While in the second approach, the suitable mortar mix is first proportioned and then quantity of coarse aggregates is determined. The mixes proportioned by both these categories can further be subdivided in to three types; powder type, VMA type and mixed type. In first type cement content is very high, mineral admixture content is very low to none and no VMA is used. The second type method results in almost equal quantity of cement and mineral admixture, and high quantity of VMA is required for maintaining homogeneity of the mix though superplasticizer requirement comes down significantly compared to the first type. Mineral admixture content in the third type mix is about one–third of the powder content and a lower quantity of VMA is used (13).

Okumara and Ozawa of University of Tokyo developed most probably the first method of SCC mix proportion in 1995 [3, 14]. Their method is also known as general method. This is a step-by-step method in which VMA is not used. First the quantity of coarse aggregate, per unit volume of concrete mix, is set at 50% of the dry rodded weight. The required mortar volume is determined taking into consideration the air content in the mix. The fine aggregate content is worked out about 50% of the resulting mortar volume. The water/powder ratio and superplasticizer dosages of the mortar are adjusted until the minimum relative flow area of 5 and relative flow rate between 0.9 – 1.1 are achieved using mortar spread and V–funnel test respectively [3]. The mix proportion thus arrived at is tested for selfcompactability by concrete funnel test and slump flow test. The mix is considered satisfactory from selfcompatibility consideration if it exhibits slump flow of 650mm and relative flow rate between 0.5 and 1.0. This method is applicable to a limited range of Japanese materials; 5-20 mm sized coarse aggregates, fine aggregates of size less than 5mm, and high belite Portland cement. The air-entraining agent was used. Criterion related to concrete strength is not included in this mix proportioning method. This method falls under first category and produces only powder type mix.

Bui, et al [15] introduced a new approach for the proportioning of SCC that essentially falls under the second category and can produce combined and VMA type mixes. The approach is based on the paste rheology model, which is built on the combination of the criteria of minimum apparent viscosity, minimum flow and optimum flow viscosity ratio. The effect of aggregate properties and content has been considered to develop a new paste model for SCC. The model developed by testing wide range of concrete composition also provides a basis for quality control and further development of mineral and chemical admixtures. Polycarboxylate based superpla–sticizer was used and a viscosity modifying agent was used in some mixes. Relationship between viscosity and flowability of paste, with aggregate spacing were developed using average aggregate diameter 5.675 mm.

For different paste volume, water binder ratio, cement content, flyash content, admixtures, the flow of each paste were plotted against viscosity. The limits for segregation and low deformability zone were also plotted. Bui et al defined three zones for mix proportion with the help of these plots. One extreme zone is segregation zone in which flow is very high and viscosity is low. Other extreme zone is low deformation zone where viscosity is high and flow is low. The satisfactory zone falls in between these two extreme zones. The paste rheology, which is falling within the satisfactory zone, was considered appropriate for the purpose of selfcompatibility. Subsequently, the unit volume was achieved by addition of aggregates into the paste without any additional adjustment.

Mixing Method

The ingredients of vibrated HPC mix and SCC mix are similar except for VMA. The HPC mix is manufactured adopting multistage mixing method. It has been observed that mixing method has significant influence on the properties of the concrete mix both in hardened and fresh state (16, 17). Hardly any information is available in this respect for SCC mix.

Form Pressure

SCC results in higher form pressure because of its extreme fluidity showing nearly Newtonian behavior (18). The method as well as rate of casting dominates the form pressure (19). The traditional vibrated concrete results in lower form pressure than SCC having same casting rate. The correlation between form pressure and casting rate is relatively linear. When concrete is placed using pump and if the pumping is done from bottom it creates more anchor pressure than that when pumping from top (18). The anchor force due to pump filling from bottom doubles than that when filling from top, the reason is that the pressure from pump adds to the pressure of concrete. The relation between concrete pressure and optimal rate of pouring calls for further study to establish their inter-relation (20).

Leemann and C. Hoffmann investigated the pressure exerted by SCC on formwork both at laboratory scale and at field (20). They studied the formwork pressure caused by SCC with varying workability and conventional concrete filling the formwork from top in the laboratory and the pressure of SCC pumped into the formwork at its base was determined in a field study. The studies conclude that the maximum pressure of filled into a formwork from top is dependent on the casting speed and rate of the continuous pressure decrease of the SCC already cast. SCC pumped into the formwork a tits base can locally surpass hydrostatic pressure.

Concluding Remarks

SCC mix engineering starts with balancing between high fluidity and high segregation resistance to achieve appropriate self-compacting properties, hardened state properties as well as optimized behavior of the suspension within the formwork. Meticulous selection and characterization of locally available ingredients are the key to engineer the rheology of SCC.

• The constituents of the material have significant impact on the concrete rheology and hydration kinetics of SCC mix. The approach for characterization of SCC mix leading to defined acceptance criteria needs further work.
• The characterization in terms of physical and chemical properties of ingredients of powder, aggregates and their influence on the behavior of SCC is essential.
• Selection of appropriate chemical admixtures, its dosages, its chemical compatibility with powder are issues to be addressed further.
• A detailed investigation on the effect of curing regime on the properties of SCC at hardened state needs further investigation.
• The form pressure in SCC is few folds more and different compared with vibrated concrete. More work is needed to under stand the relation between pump and concrete pressures.
• Few of the areas like adjustment for mix proportioning procedure, use of local aggregates, mixing methodology, online controlling of rheology, prediction of strength and durability, need to be looked into.

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Acknowledgment

The article has been reproduced from the SEWC’07 proceeding with the kind permission from the SEWC organisers.