Ensuring strength and durability through slump retention

Manufacturers have been adopting various technologies to ensure slump retention in concrete. One of the effective methods is the use of admixtures. The admixture technology continues to grow as new challenges arise. Charles S Jones elaborates on one such aspect on managing longer workability in concrete without it affecting properties such as strength and durability.

Due to rising traffic and changing weather conditions, concrete producers not only find it very difficult to maintain the consistency of concrete but also to deliver concrete that can satisfy the performance requirement under stipulated costs. The average lead time for a commercial ready-mixed concrete to be delivered to any site is anywhere between 150 – 210 minutes. One of the solutions to counter the effect of delay is to use concrete admixtures.

The growth on the use of admixtures in concrete can be largely attributed to the understanding of factors that affect the durability of the concrete. Over the years, the change of admixture technology from lignosulphonates to napthalene sulphonates to poly carboxylates stands testimony to this transformation. Also, the insight gained on the contribution of supplementary cementing materials such as flyash, slag, silica fume, etc, have increased the use of admixtures in concrete today. Modernisation and mechanisation have also added new dimensions to the growth of this industry and to the launch of various new technologies thereto. The shortage of skilled labour has made self-compacting and smart dynamic concrete the preferred choice for most builders.

The movement of rural population to urban areas has taken its toll not only on the infrastructure but also in its management and maintenance. The traffic condition in metros in general, despite the number of flyovers being constructed, has become increasingly difficult to maneuver through. This has given rise to a unique concern for the concrete fraternity when it comes to maintaining the consistency of the concrete. The environment mandate exercised by the government bodies also adds to this problem.

A decade ago, typical lead times (slump retention time) to ship the concrete from one point to the other was about 2 hours for a city like Mumbai. This has changed dramatically and today it is impossible to have a lead time of less than 3 hours. It is a big challenge for producers to provide concrete that is not only pumpable at 3.5 hours but is also able to set within 24 hours, so that de-stripping of form-work is possible. The split dosing of admixtures have been heavily discouraged by consultants and clients, owing to the fear concrete not setting properly. This has pushed producers to over-design the concrete with extra cement content so that the mix can accommodate extra water in case of a slump drop during transit. The addition of water into the mix is a very sensitive issue and requires a good degree of understanding of the consistency of concrete. In the absence of a qualified engineer, it can lead to a severe strength drop and durability concerns. Neither the purchaser nor the concrete producer can guarantee the quality of the concrete with this system. This has led to consultants resorting to specify the grade of concrete marking it higher than required, which in turn, has led to an unnecessary increase in costs without any guarantee of the quality.

This unfortunate situation arises c only because concrete is specified for its performance requirement at the site rather than the performance and quality right from batching to pouring point. If care is taken to specify the quality and performance of the concrete in terms of initial slump and final slump, rheology, etc, then such concerns can be easily addressed.

Super retention technology -Suretec

Traditional super-plasticizers (napthalene-based) are the cheapest admixtures available but are not capable of addressing the issues in M20 – M40 grades, where long workability is required without affecting the strength. The Poly Carboxylic Ethers (PCE) based admixtures called hyper-plasticisers have paved the way for newer concrete technology and the demand for such admixtures has been ever increasing. PCEs in general, impart better control over the rheology of the concrete and that’s one of the reason such admixtures are always used for producing self-compacting, smart dynamic concrete. With the advent of the Super-Retention Technology by SureTEC, second generation PCE based admixtures by BASF, this issue of long workability and early strength gain have now been addressed.

Table 1: Details of cement

SureTEC provides improved short and long- term performance of concrete by controlling the two distinct features essential for high quality concrete, extended workability and high early strength. The SureTEC concept ensures that the concrete is of high quality as originally specified; the consistency of the concrete remains the same from the batching plant, to the delivery and placing, followed by its hardening process. Generally, in situations where longer retention is required, the concrete is designed at a higher slump and reaches the minimum slump requirement after the stipulated retention period. With SureTEC, it is now possible to make a concrete with fairly lower slump, where the slump fall is gradual so that the concrete placed is relatively better in its rheology. The demand for extreme performances of admixture has led to excellence in engineered polymers, which provide new understanding of chemical-cement interaction. It is an extension of the competencies from organic to inorganic chemistry of cement and concrete. This facilitates stronger water reduction for improved workability, faster hydration for rapid strength, longer workability retention time and better rheology control for easy form filling.

PCE admixtures have been widely used in the recent past, especially in self-compacting concretes. They are characterised by short principal chairs (the backbone) with long side chairs. Considering the many properties of such a polymer, such as the molecular weight of the whole polymer, backbone or side chairs, ionic strength, chemical composition, production parameter, an extra ordinary variety of special performing polymers is feasible. Figuratively, admixtures could be perceived to be consisting of building blocks, (Fig. 1) of differing configuration of molecular structure. ‘A’ block controls slump retention, ‘B’ early strength, ‘C’ final strength and ‘D’ water reduction. Either the building blocks are combined or formulated. Functionally ‘A’ controls the absorption speed of the PCE to the cement surface while it is not retarding. Block ‘B’ enhances the natural hydration process without influencing the hydration products. Contrarily, ‘C’ enhances strength by influencing the hydration and lastly, ‘D’ is a conventional water reducing component of PCE. Using the modular approach, a seemingly vast number of combinations are possible. An understanding of the individual polymers help in formulating a product that can literally have no drop in slump throughout the transportation and once the setting starts, hydrates much faster to achieve early age strengths.

Experiment using Suretec

M-25 grade of concrete mix was designed for typical Mumbai aggregates and details of cement used for trials conducted are given in Table 1 and 2. The control used BNS type admixture while the PCE admixture (SureTEC) PCE-1 was used for comparison. Various dosages of PCE-1 were tried out with a target of 150mm slump at four hours, and the same was evaluated for strength values.

Table 2: Mix designed with aggregates for construction Mumbai

Observations

While the initial slump was 230 mm (collapse) for the BNS admixture, it was only 175 mm for the PCE-1 which had the SureTEC component in it. The control mix with BNS had bleeding upto 30 minutes but yielded a slump of 200 mm slump; the SureTEC admixture continued to have almost the original slump. The SureTEC PCE continued to remain at almost the same slump even up to 120 minutes, with a slight drop of about 10 mm. The traditional BNS mixture, however, had reduced to 150 mm by the end of two hours which is a fall in slump by 80 mm. Moreover, the slight increase in slump seen at 30 minutes is due to the phenomenon that one of the components of the formulation was acting in a delayed manner. The BNS admixture-based mix was able to withstand a pumpable slump only upto 180 minutes and went on to about 20 mm with the mix becoming very stiff. On the other hand, the SureTEC PCE had good pumpable consistency even after four hours. The setting characteristics of both the mixes were studied to understand the effect these admixtures had on the setting. While the BNS admixture had ten and 18 hours as the initial and final setting time, the SureTEC admixture showed improved performance of nine and 15 hours respectively. The retarding effect of BNS formulations were very evident from these results, whereas the SureTEC formulations were unaffected by the admixture chemistry. It is also evident from the strength results that the early strength (24 hours) with SureTEC formulations are much higher compared to that of the BNS admixtures, whereas in the later age strengths, there was not much of a difference. This means that the SureTEC admixtures allows the cement to hydrate at a faster rate compared to the traditional BNS based admixtures.

Conclusion

The benefits that can be drawn out of the use of SureTEC admixtures could be immense and will help not only the concrete producer but also the client and the consultants in ensuring the ultimate quality of the concrete that goes into the formwork. For a consultant specifying the mix, the quality of concrete and strength achievement is assured. For the owner and client, the durability of the concrete is ensured as there is no re-tempering of concrete either with water or admixture. For a concrete producer, there is tremendous saving not only terms of QC time, cost of concrete, etc, but also in terms of mix optimisation. This can very well provide a concrete that sets early and develops strength faster. Therefore, specifying the performance of the concrete through the transit period needs to be emphasised at all the sites, since it brings about durability and better strength.

Charles S. Jones, Head of Business Segment Management, BASF India Limited