Admixture-Cement Compatibility For Self-Compacting Concrete

An admixture is now an essential component in any modern concrete formula and plays a significant role in sustainable development of concrete technology.

Dr. Supradip Das, Consultant – Admixture, Waterproofing, Repair & Retrofitting, New Delhi

The application of concrete admixtures and the advancement of admixture technology have promoted the development of numerous new concrete technologies in the past few decades. Some of the properties such as high strength concrete, retention of slump resulting in high durable concrete can be made possible with the addition of superplastizers based on Napthalene formaldehyde and Melamine formaldehyde.

With the advent of poly carboxylate based superplasticiser, it has now been possible to make self-compacting concrete (SCC). Also known as self-consolidating concrete, this highly flowable, non-segregating concrete can spread into place, fill formwork, and encapsulate even the most congested reinforcement, all without any mechanical vibration. As a high-performance concrete, SCC delivers these attractive benefits while maintaining all of concrete's customary mechanical properties and durability characteristics.

This paper studies 3rd generation poly-carboxylate (PCE) as a superplasticiser from two sources and compares their compatibility as a case study. It also discusses various methods adopted for measuring flow characteristics using V-flow and U-flow tests. This study was carried out to compare the compatibility and self-compacting high performance concrete for an infrastructure building in NCR.

PCE based admixture has a powerful dispersion capability and flexibility in molecular design is far superior in achieving very high workable concrete. It has the capacity of producing concrete at a very low water-cement ratio that too with high workability. PC based admixtures are specifically being used to cater to different challenging requirements, such as high strength, high durability, high workability and long workability retention, etc. The study of flow behavior was also very important as the concrete to be in high workable condition to travel around 40 kilometer & requires very high slump retention to ease placing characteristics.

In the present work, the aim is to find the optimum dosage for two different PCE based superplasticizers, one of which was tested with Viscosity Modifying Agent (VMA) with a particular grade of cement using flow tests as per EFNARC.

Introduction

To achieve the desired properties in a high-quality concrete, mineral and chemical admixtures, particularly PCE based superplasticizers, are added to the cement. These superplasticizers are high range water reducers used for proper dispersion of cement particles in a concrete suspension [Ramachandran (1995)]. This has made possible the achievement of Self Compacting Concrete (SCC).

SCC was developed in Japan to reduce labour in placement of concrete and also to achieve vibration / compaction free concrete. In the study, trials were carried out using a very low dosage of PCE based admixture (0.38%) coupled with low water-cement ratio in the concrete (0.40 - 0.44) without affecting its strength and workability. The low water cement ratio also increased durability of the concrete.

These days, due to the availability of different types of admixtures and cement in the market, there is flexibility in choosing the right composition of the concrete according to the desired parameters, keeping in mind the overall economy and environmental safety. Admixtures, especially new superplasticizers are being developed regularly, which dramatically change the properties of concrete. But if there is incompatibility between the cement and the admixture, it may cause rapid loss of workability, excessive quickening/retardation of setting, and low rates of strength gain, in addition to economic loss.

Meeting the above requirement of SCC in mega projects has been a challenge to commercial ready mix concrete producers. Difficulties in maintaining uniform production quality, longer slump retention, self-compactability without segregation, and excellent rheology are inevitable. So, selection of the right admixture, right type of concrete making material, and its trial run, are most important in achieving the desired result.

Trials

Site trials were carried out using two brands of PCE based admixtures for M25 & M30 grade concrete. In this study only M25 has been reported. In both the cases, a coarse aggregate of 10mm down size has been used.

Mix Design

For M 25 grade self-compacting concrete, the stipulated design is given Table 1.

M 25 grade self-compacting concrete

Same mix design was used with both the admixtures and is given in Table 2.

M 25 grade self-compacting concrete

Admixture

Two different brands of PCE based admixture were used to compare the workability in terms of flow behaviour and other parameters. One is based on carboxylated copolymer and the other is with modified cellulose based VMA.

The conventional method of improving the stability of flowing SCC is to increase the fines content by using a large amount of filler, reactive or inert. Of late, however, attempts are being made to reduce the fines content (and paste content) to the levels of normal concrete (in doing so, reducing the potential for creep and shrinkage) and use viscosity modifying agents (VMAs) to improve the stability. Current research shows that SCC produced with low powder content and VMA had similar fresh concrete properties as SCC with high powder contents produced without VMA21.

VMAs have been in use for a long time22. They were mainly used for underwater concreting in the past, but are now also used in self-compacting concrete. Most VMAs have polysaccharides as active ingredient; however, some starches could also be appropriate for control of viscosity in SCC23,24.

The sequence of addition of VMA and superplasticizer into the concrete mixture is important. If VMA is added before the superplasticizer, it swells in water and it becomes difficult to produce flowing concrete. To avoid this problem, VMA should be added after the superplasticizer has come into contact with the cement particles. Another method of addition is to disperse the superplasticizer in mixing water, and then add VMA to this mixture. In this case Brand B was premixed with VMA.

Two different brands of PCE based admixture

Effective addition of VMA in concrete is an application-related issue, because of the relatively low proportions of VMA needed to stabilize the superplasticised concrete. Unless the VMA is uniformly dispersed across the entire volume of concrete, it cannot perform the intended function. At present, VMA is packaged in water-soluble bags that can be added directly at the concrete mixer. The other alternative is to prepare a suspension of VMA in water (saturated with superplasticizer) before adding into the concrete mixture. Addition of micro silica also improves the stability of suspensions of these polysaccharides. Properties of both the chemicals are given in Table 3.

Rheological Properties of Self Compacting Concrete

Since the concrete produced is flowing in nature, Filling ability, passing ability and stability of mixtures can be considered as the distinguishing properties of fresh SCC. These requirements are not common to conventional concrete and, therefore, are handled through special tests. These tests should be done carefully to ensure that the ability of SCC to be placed remains acceptable. The flow properties of SCC have been characterized. Based on their experience with SCC, researchers have suggested limits on test values. Table 4 lists the common testing methods and recommended values, as drawn from some research articles. Brief descriptions of some of the less common methods, particularly the three segregation potential tests, are described below.

Self Compactibility Tests

M 25 grade concrete using both the admixtures were tested in a controlled laboratory for workability in terms of rheology using various tests methods stipulated in EFNARC & recommended in Table 4.

various tests methods stipulated in EFNARC & recommended

As per Table No. 4, flowability is measured mostly using ‘slump flow’ test, which is simple and reliable. An estimate of the viscosity and the ability to part through the narrow-opening can be obtained using the V-funnel test. However, it is reported that a number of factors, in addition to the viscosity, (namely, the deformation capacity (slump flow), size distribution and amount of coarse aggregate, and the shape of coarse aggregate) affect the V-funnel flow time. These effects have not been quantified, particularly the effect of aggregate shape. As stated earlier, the study of aggregate shape and its influence on various SCC properties could be helpful in improving the scope for SCC with marginally unsuitable aggregates.

Adams Cone Test / Spread Tests

Since the concrete produced is flowing in nature, the flowability of SCC is measured at site by "spread” using a modified slump test (ASTM C 143). The spread (slump flow) of SCC typically ranges from 455 to 810 mm depending on the requirements for the project ( Fig 1 ). In this case, the spread test was carried out a preliminary test. The results are given in Table No. 4

the flowability of SCC is measured at siteFigure 1: Measurement of slump flow or spread at site

As per Table 4, passing ability is measured by U Box & L Box method. This test assesses the flow of the concrete and also the extent to which it is subjected to blocking by reinforcement or resistance in blocking the concrete.

U Box Tests

U Box TestsFigure 2: U Box Test Apparatus
The test was developed in Japan to measure the filling ability of SCC. Some time the apparatus is called “box shaped” test. The apparatus consists of a vessel that is divided by a middle wall into two compartments; an opening with a sliding gate is fitted between the two sections. Reinforcing bar with nominal diameter of 134 mm are installed at the gate with centre to centre spacing of 50 mm. This creates a clear spacing of 35 mm between bars. The left hand section is filled with about 20 liter of concrete then the gate is lifted and the concrete flows upwards into the other section. The height of the concrete in both sections are measured.

Interpretation of the result:

If the concrete flows as freely as water, at rest it will be horizontal, so H1-H2=0. Therefore the nearest this test value, the ‘filling height’, is to zero, the better the flow and passing ability of the concrete.

L Box Test on Self Compacting Concrete

L Box TestsFigure 3: L Box Test
The apparatus is shown in the figure 3. The apparatus consist of rectangular section box in the shape of an ‘L’, with a vertical and horizontal section, separated by a movable gate, in front of which vertical length of reinforcement bar are fitted. The vertical section is filled with concrete, and then the gate lifted to let the concrete flow into the horizontal section. When the flow has stopped, the height of the concrete at the end of the horizontal section is expressed as a proportion of that remaining in the vertical section. It indicates the slope of the concrete when at rest. This is an indication passing ability, or the degree to which the passage of concrete through the bars is restricted. The horizontal section of the box can be marked at 200mm and 400mm from the gate and the times taken to reach these points measured. These are known as the T20 and T40 times and are an indication for the filling ability. The section of bar can be of different diameters and are spaced at different intervals, in accordance with normal reinforcement considerations, 3x the maximum aggregate size might be appropriate. The bar can principally be set at any spacing to impose a more or less severe test of the passing ability of the concrete.

Interpretation of Result:

If the concrete flows as freely as water, at rest it will be horizontal, so H2/H1=1. Therefore the nearest this test value, the ‘blocking ratio’, is unity, the better the flow of concrete. The EFNARC suggested a minimum acceptable value of 0.8. T20 and T40 time can give some indication of ease of flow, but no suitable values have been generally agreed. Obvious blocking of coarse aggregate behind the reinforcement bars can be detected visually. compiled results are given in Table 5.

the concrete flows as freely as water

Analysis of Test Results

All the relevant workability aspects of SCC, viz., flowability, passing ability, and segregation resistance were evaluated using various recommended methods in EFNARC. In spite of the large number of test methods stipulated in the spec, the study tried combination of methods in both the cases. It was very difficult make a substantial comparison between the two products. It is pertinent to mention here that the Brand B significantly reduced the segregation in the concrete.

The concrete was also tested for compressive strength & both concrete could cross the requirement easily. Strength result has not been reported as the study confined to evaluate the rheological behaviour of concrete in terms of workability.

Conclusion

Self compacting concrete (SCC) with both the admixtures was tested as per the guidelines of EFNARC codes as we do not have Indian Standard specifications on SCC

The flow properties at low water cement ratio using half of the dosage has been very high as compared to SNF based superplasticiser which was used in the initial stages.

The P.C. based admixtures coupled with VMA definitely facilitate production of cohesive concrete free from bleeding and segregation.

In achieving very cohesive and uniform concrete, viscosity modifier such as fly ash was used in combination.

The advancement in admixture technology has played a significant role in the development of concrete technology. The advanced PC co-polymer-based admixtures has demonstrated various performance benefits and technical advantages over conventional superplasticizers in meeting the diversified challenging technical requirements of various high performance concrete technologies for infrastructure construction.

Two PCE based admixtures were taken in this study keeping the brand and grade of cement constant throughout. Both the admixtures behaved alike in the rheological parameters except the second one which gave slightly better cohesive concrete. V funnel test at 5min to check segregation shows better with the admixture with VMA. This concludes the superiority of PCE based superplasticizers on SNF based superplasticizers. However, a cost analysis is only beneficial when project site requirements are not very specific. For specific and critical requirements of any project, it is recommended to use specially recommended admixtures at recommended dosages for given mix design.

References

  1. Ramachandran, V.S. (1995) Concrete Admixtures Handbook–Properties, Science, and Technology, 2nd Edition, William Andrew Publishing, ISBN 0- 8155-1373-9 p. 121
  2. The Concrete Portal, Self Compacting Concrete (2013)
  3. Pamnani N., Patel P., Pitroda J., Verma (2013), Comparison and Optimization of Dosage of Different Super-Plasticizers for Self Compacted Concrete Using Marsh Cone, IJEIT 2 .
  4. The European Guidelines for Self-Compacting Concrete - Specification, Production and Use (EFNARC' May 2005)
  5. Chiara F Ferraris, Lynn Brower, Workability of Self Compacting Concrete
  6. P K Mehta, Advancements in Concrete Technology, “Concrete International”, Vol 21, No 6,
  7. Malhotra, V. M, Superplasticizer: their effect on fresh and hardened concrete, “Concrete International”, Vol 3, No 5, pp. 61-81.
  8. Jeknavorian, J. J. et al, Use of polycarboxylate based high range water reducers in commercial concrete, “Proceedings of the European Ready-Mix Conference”, Lisban, 1998
  9. Ramachandran, V.S, Concrete Admixture Handbook, Properties, Science &Technology, Second Edition, Noyes Publications, N.J., .
  10. Jeknavorian, J. J. et al, Condensed Polycyclic Acid-Aminated Polyester Polymers as Superplasticizer for concrete, 5th CANMET/ACI International Conference on Superplasticizers and Other Chemical Admixtures in Concrete, ACI SP-173, Ed. Malhotra.
  11. Verma Ajay & Shankar Kumar saha, Brief summary on self compacting concrete (A new generation concrete) - study report made at Maneri Bhali Hydro electric project 2005
  12. Johann Plank & Christian Hirsch, Impact of zeta potential on early cement hydration phase on superplasticizer adsorption, Cement & Concrete Research 37(2007), pp537-542.
  13. Mario Collepardi, Enco, Engineering Concrete, Ponzano Veneto (Italy), Admixture Enhancing Concrete Performance, 6th International Congress, Global Construction, Ultimate Concrete Opportunities, Dundee, U.K.
  14. Das Supradip, Superplasticizer – The new generation Chemical Admixture Construction Journal of India, Feb2003, pp 24 - 25
  15. Das Supradip – Construction Chemicals – The recent trends, Construction journal of India,June 2002,
  16. Dhir R K & Mccarthy M J, Admixture for High Performance Concrete, pp. 291-300
  17. Tanaka Y., Ohta A., Sugiyama T.: Polycarboxylate−Based Advanced Superplasticizers for High Performance Concrete. Proceedings of the International RILEM Conference "The Roleof Admixtures in High Performance Concrete" Monterrey, Mexico, March 1999,
  18. IS : 9103-2004 – Specification of admixture for concrete
  19. Athira Ajay, P Ramaswamy & M Nazeer, Department of Civil Engineering, TKM College of Engineering, Kollam, Kerala, India - "A study on compatibility of superplasticizers with high strength blended cement paste" : 5th International Conference IOP.
Acknowledgment: This article has been reproduced from the ICI journal with the author's kind permission.
ICCT, May - June 2024
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