Study Of The Admixture-Cement Compatibility For Self Compacting Concrete
Supradip Das, Consultant
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 resulted in highly durable concrete could be made possible with the addition of superplastizers based on Napthalene formaldehyde & Melamine formaldehyde. Nowadays, admixture is an essential component in any modern concrete formula, and plays a significant role in the sustainable development of concrete technology in the future. With the advent of poly carboxylate based superplasticizer, it has been possible to make Self-compacting concrete (SCC). Also known as self-consolidating concrete, is a highly flowable, nonsegregating concrete that 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 superplasticiser from two sources & compares their compatibility as a case study. The paper also discusses various methods adopted for measuring flow characteristics using V-flow & U-flow tests. This study was carried out to compare the compatibility & 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 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 behaviour was also very important as the concrete to be in high workable condition to travel around 40 kilometres and requires very high slump retention to ease placing characteristics. The performance of both PCE-based superplasticizers were studied, analyzed and compared. The addition of superplasticizers with mineral admixtures like fly ash, silica fume, etc. to concrete imparts a high performance and workability to it, even at very small water cement ratios. But to get the maximum benefit from this amalgamation of concrete and admixtures, the incompatibility issues between these two need to be studied. In the present work, the aim is to find the optimum dosage for two different PCE based superplasticizers, one of which was tested with VMA with a particular grade of cement using flow tests as per EFNARC. Over the course of time, the concrete industry has to cope with the day to day challenges and the use of mineral and chemical admixtures has become a magic wand for the same. To achieve the desired properties in a high quality concrete, these admixtures, particularly PCE based superplasticizers, are added to the cement. These superplasticizers are high range water reducers used for the proper dispersion of the cement particles in a concrete suspension [Ramachandran (1995)]. This has made possible to achieve Self Compacting Concrete (SCC). SCC was developed in Japan to reduce labour in the placement of concrete & 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 increases the 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 the 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 the economic loss. Meeting the above requirement of SCC in a mega project has been a challenge to commercial ready mix concrete producers. Difficulties in maintaining uniform production quality, longer slump retention, self-compactability without segregation, excellent rheology is inevitable. So selection of right admixture, the right type of concrete making material and its trial run is most important in achieving the desired result. Trials Site trials were carried out using two brands of PCE base admixture for M25 & M30 grade concrete. In this study only M25 has been reported. In both the cases, coarse aggregate of 10mm down size has been used. Mix Design For M25 grade self compacting concrete, the stipulated design is given below: 1. Characteristics Compressive Strength 25 at 28 days, N/mm2 2. Max size of course aggregates, mm 10 3. Target strength at 28 days, N/mm2 31.6 Same mix design was used with both the admixture and is given in Table 2.
Two different brands of PCE-based admixtures were used to compare workability in terms of flow behaviour as well as other parameters. One is based on caboxylated co-polymer and the other one 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 ingredients; 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. 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. The addition of microsilica 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 articles7, 38. Brief descriptions of some of the less common methods, particularly the three segregation potential tests, are described below.
M25 grade concrete using both the admixtures was tested in a controlled laboratory for workability in terms of rheology using various test methods stipulated in EFNARC & recommended in Table 4. The details of the test along with the results are discussed below: 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 parts through the narrow-opening can be obtained using the V-funnel test. However, it is reported5 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 time5. 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 or 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 carried was carried out a preliminary test. The results are given in Table No. 5.
U Box Tests
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 litres of concrete, and then the gate is lifted, allowing the concrete to flow upwards into the other sections. The height of the concrete in both sections is 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
The apparatus is shown in the figure 3. The apparatus consists of a 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 is 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 stops, 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 of 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 of the filling ability. The section of bar can be of different diameters 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 the Result
If the concrete flows as freely as water, at rest it will be horizontal, so H2/H1=1. Therefore, if 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 below in Table5
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 a 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.
Self compacting concrete (SCC) with both the admixture was tested as per the guidelines of EFNARC codes as we do not have in Indian Standard specification on SCC. The flow properties at a low water cement ratio using half of the dosage has been very high as compared to SNF based superplasticizer which was used in initial stages. The P.C. based admixtures coupled with VMA will definitely facilitate the production of cohesive concrete free from bleeding & segregation.
In achieving very cohesive & 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 have 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.
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