Effect of Sulphate Attack on Self compacting Concrete

P. Kathirvel, Faculty & Research Scholar, Dept. of Civil Engg., KLN College of Engg., Madurai; M. Kapilarasan, Senior Design Engineer, E&G op, L&T, ECC Div., Chennai; M. Shahul Hameed,Faculty & Research Scholar, Dept. of Civil Engg., Sethu Institute of Technology, Kariapatti; Dr. A. S. S. Sekar, Asst. Professor, Dept. of Civil Engg., Alagappa Chettiyar College of Engg. & Tech., Karaikudi.

In Limestone and Granite Quarries, considerable amounts of limestone and quarry dust powder is being produced as by-products of stone crushers. Large Quantity of both type of powder are being collected and utilization of this by-product is a big problem from the aspects of disposal, due to environmental pollution and health hazards. These fines can efficiently be utilized as viscosity enhancers, particularly in Self–compacting Concrete (SCC) applications. Thus the Successful utilization of these powders in SCC could turn this material in to a valuable resource.

The present experimental investigation aims to study the Durability of SCC with Partial replacement of cement by Quarry and limestone (dust) Powder by (10%, 20%, and 30%) and comparing the properties like Density Variation, Compressive Strength, Water Sorptivity for 28, 60, 90 and 120 Days age with respect to control SCC.

Introduction

To achieve SCC properties, the concrete mix should contain lower volume of coarse aggregate(1). SCC requires higher powder content, lesser quantity of coarse aggregate, high range Superplasticizer and VMA to provide Stability and fluidity to the concrete mixes. The flow characteristic increases with increasing VMA. Compressive strength, flexural strength and Split tensile strength decreases with increased addition of VMA

Substitution of 10% of cement with Quarry limestone powder (QLP) improved the compressive strength of cement pastes(2), which can be accepted as a positive factor in utilization of QLP in Self–compacting paste applications. QLP can successfully be used in production stage of proper SCC mixtures. Incorporation of QLP at the same cement content generally reduced the superplasticizer requirement and improved the 28 days compressive strength of SCCs. Normal strength SCC (~30 Mpa) mix that contain approximately 300-310 kg of cement per metre cube can be successfully prepared by employing high amount of QLP. However, substitution of high amount of cement with QLP reduced the strength values.

The SCC has become widely used standard concrete rather than a special concrete(11). The new structural design and construction systems are making full use of SCC in durable and reliable concrete structures. Different testing methods to test high-flowability, resistance against segregation, and passability(6). It is difficult to develop SCC with CA content higher than 45% or lower than 15% of the total aggregate(5). SCC properties can be achieved even for a very high percentage of sand content (up to 85%). Strength and workability properties of SCC containing different combinations of admixtures improved as a dosage of superplasticizer in them increased(4). No adverse effects observed in the combination of admixtures, mainly because of using the admixtures from the same manufacturer.

Optimum water/cement ratio for producing SCC is in the range of 0.84–1.07 by volume(8). The ratios above and below this range may cause blocking or segregation of the mixture, respectively. Self-compactability test method stipulations are not universally accepted rules. Degree of toleration depends on the engineering judgment, material type and variety. Proper concrete mixtures can be produced by trial and error method. Limestone was the most common addition in most of the cases. Approximately half the cases used a viscosity-modifying agent (VMA) in addition to superplasticizer and could therefore be considered as a combined type of SCC, which are generally more robust than mixes without a VMA(7). It was found that the quarry dust could be used successfully in the production of SCC(12). However, due to its shape and particle size distribution, mixes with quarry dust required a higher dosage of superplasticizer to achieve the flow properties.

The optimum dosage of superplasticizer for cement with fillers was about 4.38% of weight of cement(13). Loss in Mass and Compressive strength of concrete cubes were found to be negligible when under marine and Sulphate attack(3).

Material Properties

Cement

Ordinary Portland cement of 53 Grade was used in this investigation. The specific gravity of cement was 3.13.

Lime Stone Powder and Quarry Dust Powder

In general, powder is referred to as materials with particle sizes less than 0.125mm. Here locally available limestone Powder with particle less than 0.125mm having a specific gravity of 2.53 was used.

Aggregate crushers produce significant amounts of crusher dust during the process of producing 20-10mm nominal size aggregates. Utilization of stone crusher dust, a waste material, has not been made in concrete. The reason for this is the greater specific surface area, excess of material smaller than 150 mm and the associated increase in water demand of concrete. However, with the availability of high range water reducers, the increased water demand of the crusher dust can be accommodated without increasing the water-cement ratio. The crusher dust collection for this entire experimental programme was done from locally available crushers. Quarry dust not only reduces the cost of construction but also the impact on environment by consuming the material generally considered as a waste product with few applications. The specific gravity of crusher dust is 2.30.

Locally available Crushed granite aggregate with a maximum nominal size of 12mm with fineness modulus of 7.48 and specific gravity as 2.78 and bulk density 1486 Kg/m3 have been used. Fine aggregate used for the study should be properly graded to give the minimum voids ratio and shall be free from deleterious materials like clay, silt content and chloride contamination. River sand is normally preferred over crushed sand since in the former particle shape is fully water worn by attrition which helps in reduction of water content of mix and also lesser resistance to pumping. Locally available sand passing through 4.75mm sieve with fineness modulus of 2.85, Bulk Density as 1720 Kg/m3 and specific gravity 2.64 which falls under grading zone II were used for the entire investigation.

Chemical Admixtures

High performance concrete superplasticizer (Conplast SP430), based on aqueous solution of lignosulphonates, organic polymer to reduce water cement ratio for a required workability.

Conplast SP 430 is a ready to use admixture that is added to the concrete at the time of batching. The maximum effect is achieved when it is added after the addition of 50 to 70% of the water and it must not be added to the dry materials. Conplast SP430 is differentiated from conventional superplasticizers in that it is based on aqueous solution of lignosulphonates, organic polymer with long lateral chains. This greatly improves cement dispersion. At the start of the mixing process the electrostatic dispersion occurs but the presence of the lateral chains, linked to the polymer backbone, generate a steric hindrance which stabilizes the cement particles capacity to separate and disperse. This mechanism provides flowable concrete with greatly reduced water demand.

Glenium Stream 2 is a viscosity modifying admixture which is used in combination with the Super Plasticizers in order to guarantee maximum efficiency. Glenium Stream 2 consists of a mixture of water-soluble polymers which is adsorbed onto the surface of the cement granules, thereby decreasing the viscosity of the water and influencing the rheological properties of the mix thereby reducing segregation.

Mix Design

A simple mix design method was proposed by Nan Su et al (2001) for the development of self– compacting concrete & same was used in this study for M30 concrete. Assuming Packing factor of 1.18, the mix proportion arrived is given in Table 1.

Test Methods

The investigation was carried out by varying the quarry dust and limestone content (replacement for cement). Table 1 Give the details of the workability test results for different mixes taken for the investigations.

The observations are given below:

1. While replacing cement by limestone powder by 10% the flow properties was high when compared to control SCC and also the passing ability and filling ability also increased. For 20% and 30% it was within the permissible limit recommended by EFNARC. But it was slightly decreased when compared to control SCC and 10% lime replacement.
2. While replacing cement by 10% quarry dust the flow properties was high when compared to control SCC. For 20 and 30% the flow properties decreased when compared to 10% quarry replacement but satisfies the acceptable limits.

Workability test results were found out as significant as per the recommendations given by EFNARC. The tests were repeated for every replacement by keeping the superplasticizer, VMA, water-powder ratio constant through out the investigation. Immediately after conducting the fresh concrete tests on SCC (Fig. 1 – 4), the concrete cubes were casted for durability study (Fig 5). The tests conducted were carried out at hardened states of SCC were Compressive strength test, Density variation test and Water Sorptivity test.

Sorptivity is defined as the rate of movement of a wetting front through a porous material. The water sorptivity test involves the uni-directional absorption of water into one face of a pre-conditioned concrete sample (Fig 6). At predetermined time intervals, the sample is weighed to determine the mass of water absorbed, and the sorptivity is determined from the mass of water absorbed of time. The lower the water sorptivity index, the better is the potential durability of the concrete.

Results and Discussions

Compressive Strength Vs Duration of Immersion

Figures 7 to 12 show compressive strength of the specimens kept in water and 5% of Sodium sulphate solution. Fig 1 shows the comparison of compressive strength of cubes (QD) placed in water and 5% of Sodium sulphate solution. The figure shows that the compressive strength on specimen decreases by average of 0.5% at 28 days. The decrease in compressive strength continuous as the duration of immersion increases.


At 90 days the compressive strength of control specimens, 1Q, 2Q, 3Q decreased by 1.7%, 1%, 1.7% and 2.3% respectively. For 120 days test results the decrease in compressive strength of control specimens, 1Q, 2Q, 3Q is by 2.2%, 2%, 2.3% and 2.5% respectively. Here the decrease in compressive strength of SCS and 3Q is high when compared to other specimens. The specimen 1Q shows good resistant to sulphate attack when compared to other specimens.


Fig 8 shows the comparison of compressive strength of cubes (Lime) placed in water and 5% of Sodium sulphate solution. As for the specimens immersed in Sodium sulphate solution, there is gradual decrease in compressive strength up to 120 days. The compressive strength of the specimens, SCS, 1L, 2L and 3L decreases by an average of 0.4%, 0.7% for 28 and 60 days respectively. The decrease in compressive strength of the specimens SCS, 1L, 2L and 3L at 90 days decreases by 1.7%, 0.6%, 1.3% and 2.7% respectively. By the 120 days results the SCS and 3L specimen decreases by 2.2% and 3.5% respectively. The decrease in compressive strength is high when compared to 1L and 2L specimens. The compressive strength of the specimens 1L shows slight decrease in strength by 1.5% showing good resistance to sulphate attack when compared to other specimens.

Density Variation Vs Duration of Immersion

Figs 13 to 18 show the density variation and duration of immersion in water and 5% of Sodium sulphate solution.


The figures show that the density of the specimens increased by an average of 0.5% at 60 days. This might be due to the water absorption of the specimens. After 60 days the density of the specimens, immersed in 5% of Sodium sulphate solution start decreasing. This shows the beginning of the deterioration of the specimens.


Fig 13 shows the density variation of the control specimens and specimens replaced with lime (10%, 20% and 30%) kept in water and 5 % Sodium sulphate solution. At 90 days of testing the density of the specimens CS, 1L, 2L and 3L decreased by 0.4%, 0.10%, 0.20% and 1% respectively. At 120 days of testing the density of the specimens CS, 1L, 2L and 3L decreased by 2%, 0.5%, 1.2% and 2.4% respectively. The specimen 1L and 2L shows slight decrease in density when compared to the other specimens.


Fig 14 shows the density variation of the control specimens and specimens replaced with Quarry dust (10%, 20% and 30%) kept in water and 5 % Sodium sulphate solution. At 90 days of testing the density of the specimens SCS, 1Q, 2Q and 3Q decreased by 0.5%, 0.2%, 0.3% and 0.5% respectively. At 120 days of testing the density of the specimens SCS, 1Q, 2Q and 3Q decreased by 2%, 0.8%, 1.1% and 1.6% respectively. The specimen 1Q and 2Q shows slight decrease in density when compared to the other specimens.

Water Sorptivity Vs Duration of Immersion

Figs 19 to 24 show the water sorptivity of the specimens kept in water and 5% of Sodium sulphate solution.

The figures are drawn for 28, 60, 90 and 120 days sorptivity results. Based on the results obtained it can be seen that there is negligible or no change in water sorptivity from 28 day to 60 days. After that, there is reduction in water sorptivity, which denotes the reduction in permeation of water as time goes on.


From the Fig 19, the 120 days test results of the water sorptivity specimen SCS, S1L, S2L, S3L are decreased by 1.6%, 0.5%, 1.2%, and 2% respectively. At 120 days of testing the Sorptivity specimens SCS, S1Q, S2Q and S3Q decreased by 1.6%, 0.8%, 1.1% and 1.2% respectively. Though there is saturation of water, due to the deterioration cement matrix, there is reduction in water sorptivity.

Conclusion

In this investigation, the self compacting properties were found to be good for cement replacement by limestone powder by 10%. As the percentage of quarry dust increases, the workability properties of SCC decreased with reduction in strength. The flow properties of all the replacements were satisfying the recommended values given by EFNARC.

Compressive strength of replacement of 10% Lime was 5 percent higher than the control specimens. Addition of limestone powder increases the sulphate resistance up to 10% which is 0.5 percent higher than that of concrete without replacement of cement by limestone. Reduction in density was 1.5 percent lesser for replacement of cement by 10% lime when compared to of concrete without replacement of cement by limestone.

The reduction in density was 1 percent lesser for replacement of cement by 10% quarry dust when compared to of concrete without replacement of cement by quarry dust. In sorptivity the reduction in density was 1 percent lesser for both replacement of cement by 10% lime and quarry dust powder. The result of the study indicated that the replacement of cement with 10% lime improved the durability of Self–compacting concrete. The loss in mass, compressive strength and sorptivity of cubes were found to be negligible under Sulphate attack. It was observed that limestone and quarry dust powder resists Sulphate attack within tolerable limits.

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