An admixture can be defined as a chemical compound that is added in very small quantities to concrete, mortar or grout at the time of batching or mixing, to produce some desired characteristic or modification to the mix
and/or to the mature concrete, mortar, or grout.
While the use of admixtures in the UK has increased significantly in recent years, this country still lags far behind Continental Europe, the USA and other developed countries. There is an increasing demand for higher quality/performance concrete and properly selected admixtures are more or less essential to achieve this.
The main types of admixtures in general use are:
It should be noted that BS 8110—the Structural Use of Concrete, refers to pigments as an admixture; but PD.6534:1993, Guide to the use in the UK of ENV206:1992—Concrete, clause 4.5 includes pigments under the heading of ‘Additions’. In this book they are included under ‘Admixtures’.
The general use of admixtures is covered by various Codes, and by BS 5328, Parts 1 and 3, and by ENV 206 (draft European Standard). The ENV puts an upper limit on the use of admixtures in a mix at 5% by mass of the cement, and a lower limit of 0.2%. The ENV also requires that admixtures in liquid form, when the dosage exceeds 3 litres/m3 of concrete, shall be taken into account when calculating the water/cement ratio of the mix.
The British Standards are performance specifications.
These are covered by BS 5075: Part 1 and ENV 934–2 (draft European Standard). These only cover accelerators for concrete; there is no BS for accelerators for mortar and grout.
BS 5075: Part 1, defines an accelerating admixture as:
A material that increases the initial rate of reaction between cement and water & thereby accelerates the setting and early strength development of the concrete.
Some accelerators contain chloride as an active ingredient and when this is the case, the BS requires that the chloride content shall be stated by the manufacturer. This is important because Standards now strictly limit the chloride iron content of concrete which contains ferrous metals (see BS 5328: Parts 2 and 3).
The performance requirements include stiffening times and minimum compressive strength as percentage of the control mix. The 24-hour strength is set as 125% and the 28-day strength as 95%.
A retarding admixture is defined in BS 5075: Part 1 as:
A material that decreases the initial rate of reaction between cement and water and thereby retards the setting of concrete.
For mortars, a similar definition is applicable. The relevant UK Standard is BS 4887: Part 2. The draft European Standard is ENV 934–2. It should be noted that BS 4887: Part 2, states that the Standard covers building mortars and rendering, but not mortar for floor screeding.
The retarders are in the form of liquids and are organic compounds and the exact composition varies from manufacturer to manufacturer.
For concrete, these admixtures are covered by BS 5075: Part 1 which defines a ‘normal water-reducing admixture’ as:
A material that increases the fluidity of the cement paste without significantly affecting the air content and thereby increases the workability of the concrete at constant water/cement ratio, or permits concrete to be made with a decreased amount of water while maintaining equal workability, with a consequent increase in strength.
The Standard also covers ‘accelerating water-reducing admixtures’ and ‘retarding water-reducing admixtures’. These admixtures combine the two described functions.
These admixtures are covered by BS 5075: Part 3, and by the draft European Standard ENV 934–2. The UK Standard defines such admixtures as:
An admixture, that when added to a hydraulic binder concrete, imparts very high workability or allows a large decrease in water content for a given workability.
The increase in workability is dramatic, as the concrete flows, and this is measured as described in BS 1881: Part 105. This super workability only lasts for a limited period, generally about two to four hours.
The very high workability obtained (150–200mm slump) ensures that the concrete is virtually self-compacting. Concrete containing superplasticizers is used for a number of purposes which include:
The basic principles underlying the improvement in workability in normal plasticizing admixtures apply to superplasticizers but on a much greater scale. Simply expressed, the admixture particles are negatively charged and are adsorbed onto the surface of the hydrating cement particles, which also become negatively charged. As negatively charged particles repel each other, the cement particles are dispersed and workability thereby is greatly increased.
The two main basic types of superplasticizers are sulphonated naphthalene-formaldehyde condensates, and sulphonated melamineformaldehyde condensates.
This type of admixture is defined in BS 5075: Part 2 as:
An admixture that causes a controlled and stable quantity of air to be incorporated during the mixing of concrete, without significantly effecting the setting of the concrete.
The draft European Standard is ENV 934–2.
The action of the entrained air is to either reduce the water requirement of the mix with constant workability or to increase the workability with a constant water/cement ratio. The presence of the entrained air also increases the resistance of the concrete to freezing and thawing (frost attack). However, there is a reduction in compressive strength compared with a control mix of the same mix proportions and water/cement ratio.
The actual reduction in compressive strength depends on a number of factors, but a figure of 4% for each 1% of air entrained is often used as a guide. It should also be noted that if the cement content of the mix exceeds about 350 kg/m3 difficulties are likely to arise in entraining the air. The size of the bubbles of entrained air is about 50 microns or 0.05mm.
Air entraining admixtures for use in mortars are covered by BS 4887: Part 1, in which they are referred to as ‘air-entraining (plasticizing) admixtures’. These mortars are used for bedding masonry units and for rendering. In addition to improving workability, the air entrainment also increases the frost resistance of the mortar.
The relevant British Standard is BS 1014: Pigments for Portland cement and Portland cement products.
The Standard sets out requirements for sampling, use as a powder or as a dispersion, composition, and the effect of the pigment on the setting time and strength of Portland cement products.Table 1 of the Standard lists seven pigments, of which four are oxides of iron, one is carbon black, one is chromic oxide and one is titanium dioxide.
The principal pigments in use are oxides of iron and carbon black, and are in the form of very fine powders. The average values for the particle sizes suitable for use in concrete and mortar are:
These can be compared with Portland cement—1.0 to 50 microns, and sand 150 to 5000 microns (1 micron=0.001mm). While pigments are not used to any great extent in the repair of concrete, their use, when necessary, can give rise to complaints and disappointment. This is usually due to lack of understanding of the limitations of pigmented concrete/mortar compared to pigmented coatings. For example, complete uniformity of colour cannot be obtained due to a number of factors, of which the principal are: