Concrete admixtures at work
Sep 19, 2019
There is no doubt that the interaction of the various input materials used in the formulation of concrete has become more complex over the years. Their correct application needs the knowledge of experienced concrete technologists – professionals capable of intermatching the requirements of the construction process, the laying conditions and the specified concrete properties. This article offers a number of examples of how this is done in practice.
Admixtures have become an integral part of modern concrete technology. Without them, developments in this field would have followed a completely different direction. By assisting in the advent of novel types of concrete, such as self-compacting or ultra-high-strength grades, they have opened up new areas of application for concrete construction and also made this more economical in many fields. Looking back, we find the first DAfStb (German Committee for Reinforced Concrete) code of practice for free-flowing concrete regulating the use of plasticisers on the construction site was issued in 1986. With the development of PCE-based plasticisers, this code is now out of date. Laying times of three to four hours can be easily achieved these days. At that time, there was talk of so-called “superplasticisers”, which improve the flowability of fresh concrete without significantly impairing its cohesiveness. Flowing concrete was understood to be fresh concrete with good flowability and sufficient cohesiveness.
Concrete plasticizers, superplasticisers, air-entraining agents, stabilisers, retarders, solidification and hardening accelerators, sealants and pressing agents are the main functional promoters used in practice today. At around 80 per cent, liquefying admixtures (plasticisers and superplasticisers) account for the largest share in terms of volume. This is illustrated by the chart on page 9 which shows the sales development of liquefying admixtures in Germany and the development in the share of PCE-based superplasticisers. The development of concrete admixtures is closely linked to the steady and ongoing further development of concrete technology. The requirements for kilometre-high towers in hot desert states are different from those for concrete offshore platforms in the polar seas. However, the introduction and implementation of Germany’s primary concrete standard (EN 206-1 in conjunction with DIN 1045-2) with the extension of the consistency classes up to a slump rating of F6 have also led to the further development of admixtures. The trend observed worldwide is towards easily workable, softer concretes. The following are just a few examples of the
advantages of using admixtures:
- Concretes produced with superplasticisers are easy to work, reduce the degree of compaction required and thus also diminish the noise nuisance on inner city construction sites.
- Stabilisers enable the laying of underwater concrete without segregation. Thus, sealed excavation pit bottoms can be produced in an environmentally friendly manner without pumping out groundwater.
- Air-entraining agents enable concretes to withstand extreme frost and the kind of attack caused by de-icing salts.
New superplasticiser systems are available today that effectively reconcile opposing requirements (low w/c value, good consistency over 90 min and very good early strength development enabling stripping after just 24 h). This makes the addition of superplasticiser on site unnecessary, reduces the occurrence of error and saves valuable time. Yet concrete admixtures are occasionally blamed for problems in concrete construction – prematurely, as it often proves. There is no doubt that the interaction of the various input materials used in the formulation of concrete has become more complex over the years. The erstwhile three-material system consisting of cement, aggregates and water has meanwhile developed into a multi-phase complex requiring effective coordination and tuning of the cement, additive and admixture components. As a rule, fresh concrete today is assessed only on the basis of its consistency (slump rating or compactability) and the information on the delivery note. This means its rheological behaviour cannot be adequately described and evaluated. Other relevant performance characteristics such as “stickiness”, pumpability and, in particular, mix stability cannot be determined with this initial examination. Their correct application therefore requires the knowledge of experienced concrete technologists – professionals capable of intermatching the requirements of the construction process, the laying conditions and the specified concrete properties.
"Concrete admixtures are also often held responsible for problems in concrete construction: mostly premature.”Eugen Kleen, Head of R&D mineral building materials and concrete admixtures at MC
Mix stability values
Segregations occurred in canal lock chamber walls (during the construction of the Wusterwitz lock), causing the Federal Institute for Hydraulic Engineering (BAW) to implement a range of ad hoc measures, including recommendations regarding the use and handling of PCE-based superplasticisers. These ad hoc measures were introduced in May 2015 as regulations supplementary to the “Additional Technical Contract Conditions for Hydraulic Engineering Structures of Concrete and Reinforced Concrete (ZTV-W LB 215)”. In BAW Bulletin 01/2015 “Problems with the mix stability of concrete” and the A1 supplement from 2018, it is stated that the vibration energy introduced has a very significant influence on mix stability. The resistance of the concrete is affected by many factors such as consistency, choice of an appropriate aggregate grading curve or sufficient glue content. Nevertheless, it was assumed at the time that “the use of superplasticisers based on PCE can have a particular influence on the stability of the mixture”. In general, stabilising admixtures can exert a positive effect on sedimentation stability. This presupposes ensuring optimum metering of the stabiliser, as fluctuations in the amount added (too high or too low doses) can lead to a reduction in stability. Too short mixing times and/or too low mixing intensity can also lead to an inhomogeneous concrete composition and impair mix stability. This may occur if admixtures (e.g. superplasticisers) have not been sufficiently solubilised during production in the ready-mix concrete plant, causing excessive dosing to occur. But what is actually meant when we refer to the stability and robustness of a fresh concrete mix? A stable, fresh concrete is one that is appropriately resistant to segregation; i.e. when exposed to expected vibration, pumping and working effects, the mix remains sufficiently homogeneous so as to ensure that the essential properties of the hardened concrete are reliably achieved.
Sedimentation stability (as part of mix stability) refers to the sinking of the coarse aggregate. The fresh concrete must remain stable to the extent that there is no or only limited unwanted separation between the glue or mortar phase and the coarse aggregate. The tendency of individual components of fresh concrete to separate from each other is referred to as segregation propensity. This phenomenon essentially depends on the effects acting on the concrete and its ability to withstand those effects. Water separation (bleeding), which eventually causes porous solution to accumulate on the surface of the fresh concrete, is invariably regarded as an undesirable form of segregation. Robustness, on the other hand, refers to the ability of a concrete to react “benignly” (predictably) to unplanned influences, such as fluctuations in water content, batch-related variations in the source materials, changing environmental conditions, delays in the construction process or accidentally prolonged vibration. In summary, a stable fresh concrete can be described as follows:
- Minimal water separation, bleeding
- Minimal segregation of cement glue/mortar
- Minimal sedimentation of coarse aggregates
- In the case of air-entrained concrete, minimal foaming
First of all, a concreting schedule has to be worked out which defines the stresses arising from layer thickness, placement openings, free fall height, length of horizontal movement (drifting), vibration energy (type and duration), immersion points of the vibrating equipment, etc.
A robust concrete aligned to this can be produced, e.g. by selecting a suitable aggregate grading curve and a sufficient binder or glue content on the basis of extended suitability tests. Elements incorporated into a recommended extended suitability test may include:
- Determination of the air entrained over a simulated transport period
- Test after initial mixing and then again after 10 min extended mixing time
- Test after extended compaction time
- Testing with elevated fresh concrete temperature
The previously described properties of mix stability or segregation propensity cannot yet be conclusively defined using generally recognised test methods. Work is currently being carried out at the Institute for Building Materials in Hannover (ifB) with the aim of establishing practical tests of these relevant fresh concrete properties. One such approach is aligned to introducing a sedimentation test to determine the mix stability of concrete under the influence of vibration. The focus here is on the development of a mobile “sedimentation device” suitable for construction sites, which enables variable control of the compaction parameters. Initial prototypes of the so-called “sedimentation pot” are already available.
Concurrent with the development of the test procedure and the trialling of the test apparatus, a number of further fundamental questions concerning the requirements and the testing of fresh concrete are currently being discussed, for example in a “Fresh Concrete” working group set up by the DAfStb. Present discussions are revolving around questions such as what constitutes complete compaction.
- The complete compaction of the fresh concrete is one of the basic prerequisites for reliable achievement of the desired hardened concrete properties and thus represents one of the most important process criteria in concrete-laying.
- As the consistency class increases, the compaction effort required for complete de-aeration decreases. The compaction method chosen must also be aligned to the consistency of the fresh concrete.
These and other open questions regarding the compaction parameters, such as duration and intensity (frequency/amplitude) with a given compactor, still need to be clarified before theory can be transferred into practice.