THE MIX-DESIGN METHOD

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mix-design shall be performed in a manner that makes it possible to evaluate how the available materials shall be combined to receive appropri­ate properties. From chapter 2 it can be concluded that it is possible to succeed with a self-compacting concrete mix in a wide spectra of proportions between the constitu­ents. It has been reported that a well working SCC can be produced with a powder content ranging from 445 to 605 kg/m3, which is a relatively large spread.

In addition, it has been shown that no ultimate amount of the constituents exists to succeed with SCC, which is probably an effect of the importance of taking both the properties of the available materials and the application area into consideration when composing SCC. A mix-design method that makes it possible to choose appropriate combinations of the available material with respect to demanded workability and performance can thus increase the possibility to produce more cost-effective concrete. If the properties of each constituent are utilized as much as possible, the chance to succeed with a SCC mix within reasonable economical frames will probably increase.

3.2 Basic definitions

As mentioned in chapter 1 and 2, fresh concrete can be regarded as solid particles sus­pended in water. However, depending on particle size, type and its influence on the concrete properties, for some reasons it can be simpler to regard concrete in different phases during the mix-design process, which all are closely related. The following defi­nitions are used here:

Concrete: water, cement, mineral additives, superplastiziser, fine aggregate and coarse aggregate.

Paste: water, cement, mineral additives and superplastiziser

Micro mortar: cement, mineral additives, superplastiziser and fine aggregate particles from 0 to 0.25mm

Mortar: cement, mineral additives, superplastiziser and fine aggregate up to the size limit for fine aggregate.

Aggregate: all aggregate particles ranging from 0 to the maximum grain size where fine aggregate is defined as 0-4mm and coarse aggregate is defined as particles >4mm.

It can be concluded that the phases are closely related, e.g. parts of the aggregate and the paste is both a part of the micro mortar. The definition is made on a technical basis,

which facilitates the possibility to trace and evaluate the effect from different types of material on the total concrete behaviour.

3.3 Method and basic assumptions

A micro mortar optimization method and an aggregate optimization method is pro­posed and fully described in the appended papers A and B. The most essential parts from these two papers are briefly described here, where focus is on how to use received information in concrete mix-design. The proposed method is developed based on a pragmatic perspective. It is an experimentally based concrete mix-design method with the main focus held on how material related properties, important for the fresh con­crete, can be evaluated, and how received information can be used for concrete pro­duction. The effect on concrete properties from the available materials and how a mix shall be chosen regarding its field of application has thus been in focus. Material charac­terization methods are chosen regarding their simplicity to both procure and use be­cause the suggested method shall be able to adopt without expensive laboratory equip­ments.

The mix-design method proposed in is based on the following assumptions:

· Concrete can be regarded in different phases, defined in section 3.2, which are closely related. Properties and amounts from one phase are in direct relation to an­other phase, which means that the connection between different phases to a concrete mix is an essential part.

· The main effect of superplastiziser is to disperse agglomerated particles in the cement paste (Aitcin et al., 1994). The agglomeration is a result of several types of interac­tions; Van der Waals interaction between particles, electrostatic interactions and in­teractions involving water molecules (Legrand and Wirquin, 1992). The effect of su­perplastiziser on the finest particles in concrete, i.e. the micro mortar, are here ex­perimentally evaluated and optimized based on workability tests, here performed with a Marsh cone test together with a mini slump test.

· The finest part of the aggregate will consume both water and superplastiziser and will thus influence the fresh mortar properties.

· The excess paste theory is applied in two steps and experimentally evaluated; 1) The micro mortar shall contain an excess of cement paste to ensure sufficient workability and 2) the mortar or micro mortar shall overfill the coarse aggregate to ensure suffi­cient concrete workability.

· Powers theory that there is a most appropriate fine aggregate content for a given water to powder ratio is here applied by packing tests in combination with a defined connecting parameter ë25.

3.4 Regarding concrete in different phases

The micro mortar phase and the aggregate phase are here suggested to be optimized separately. It facilitates the possibility to ensure workability step by step from the paste phase via micro mortar, which includes the combined effect of paste and fine aggregate,

to a concrete matrix. The proposed method makes it possible to evaluate the effect of cement paste on micro mortar, and then combining the micro mortar to the aggregate skeleton by a material based relation.

When regarding SCC as a two phase material, a common approach is to use paste tests to find the optimal superplastiziser dosage for a given combination of cement and min­eral additives. The received combination is then added to an aggregate skeleton chosen due to some demands, often based on its maximum degree of packing. The optimized paste is then added to the total aggregate skeleton and overfilled to receive sufficient workability. Domone (2006b) is proposing how mortar fluidity, tested for a given amount of fine aggregate, can be interpreted with the Marsh cone test together with the mini slump test.

However, paste tests performed that excludes the fine aggregate will be difficult to translate and use in a concrete mix, since the finest part of the aggregate, according to Powers (1932), also influence the water and superplastiziser demand needed for a cer­tain workability. However, even if the fine aggregate are included in the paste tests, and then denoted mortar or micro mortar, its relevance will only be valid if the pro­portions of the constituents chosen from the micro mortar tests can be further trans­lated and used in a concrete mix. To further increase the relevance and thus increase the potential in performing micro mortar tests, it is favourable if the fluidity can be interpreted and chosen with respect to the actual field of application.

The most essential part when working with phases separately is thus identified as:

1) the phase comprises the finest particles in concrete shall include all particles in the chosen size area, i.e. cement, mineral additives, water, superplastiziser and the finest part of the aggregate.

2) it shall be able to translate the proportions of the constituents and its corresponding properties chosen in one phase to a concrete mix.

3) the highest potential can be reached if the available materials can be combined to fulfill a demanded workability, based on the structural element to be cast.

The micro mortar optimization method and aggregate optimization method proposed in this thesis are both designed in a manner that makes it possible to directly connect the micro mortar phase with the aggregate phase with retained proportions of the con­stituents. A connecting parameter, called ë25, has been introduced, which describes the relation between the finest part of the fine aggregate and the total content of cement and mineral additives. This parameter is shown to be one of the key parameters both for the micro mortar properties and when combining the constituents to a concrete mix.

3.5 Traditional mix-design of concrete

Concrete mix-design means a method to choose the constituents in different amounts in order to achieve a concrete mix with sufficient properties for the current application. The mix shall fulfill demands in fresh, young and hardened phase. The most common

mix-design method used in Sweden is based on Alexanderson och Buö (1970). The basis is the demands on compressive strength where the water-to-cement ratio law stated by Abrams (1918) is used to receive a particular strength requirement and in addition, air content is chosen regarding stated demands. Concrete is regarded as two different phases; cement paste volume and aggregate volume. All constituent volumes are here expressed as m3 (constituent)/m3(concrete) or simply a dimensionless ratio. The sum of all constituents is the concrete mix, which gives

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The law of water-to-cement ratio, Abrams law, is an accepted tool to choose appropri­ate amount of cement and water to receive a demanded compressive strength. Aggre­gate are often divided into different fractions, fine and coarse aggregate. The mutual proportion between these two is based on the total grading curve, demands on worka­bility and properties of the chosen aggregate.

As a basic mix-design approach Alexandersons method can be used also in case of SCC with adjustments in the proportions between the constituents in comparison to normal vibrated concrete. The challenge with SCC mix-design is to know how to choose appropriate combinations and proportions between aggregate, cement, water and min­eral additives to fulfill stated demands on fluidity and performance.

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