Concrete Containing More Than Two Admixtures

D. K. Kulkarni, Selection Grade Lecturer, Civil Engineering Department, Rajarambapu Institute of Technology, Rajaramnagar, Islampur. Dr. K.B. Prakash,

Portland pozzolana cement and locally available sand and aggregates were used

Professor Civil Engineering Department K. L. E Society's College of Engineering & Technology, Belgaum.

Concrete is a material often used in the construction of highrise buildings. In case of unexpected fire, the concrete elements such as columns, beams, etc. will be subjected to extreme temperatures and needs assessment of their performance after fire. Hence, it is important to understand the changes in the concrete properties due to extreme temperature exposures.

In this paper, an attempt is made to find out the effect of sustained elevated temperature on the properties of concrete containing more than two admixtures. The following combinations of admixtures are used in this experimentation work.

• Superplasticiser + Air Entraining Agent + Accelerator
• Superplasticiser + Air Entraining Agent + Retarder
• Superplasticiser + Air Entraining Agent + Waterproofing Compound
• Superplasticiser + Air Entraining Agent + Shrinkage Reducing Admixture
• Superplasticiser + Air Entraining Agent + Viscosity Modifying Admixture

The tests are conducted to evaluate the strength characteristics of concrete like compressive strength, tensile strength, flexural strength, and impact strength of concrete when it is subjected to a temperature of 600°C for 6 hours.

Introduction

One of the greatest advantages of concrete as a building material is its remarkable resistance to fire. The distress in concrete due to fire manifests in the form of cracking and spalling of the concrete surface¹. Concrete though not a refractory material is incombustible and has good fire resistant properties². The property of concrete to resist the fire reduces damage in a concrete structure whenever there is an accidental fire. In most of the cases the concrete remains intact with minor damages only. The reason being low thermal conductivity of concrete at high temperature and hence limiting the depth of penetration of fire damage. But when the concrete is subjected to high temperature for long duration, the deterioration of concrete takes place³. Concrete has been widely used as construction materials in buildings and other industrial structures for a long time. The recent technological advances have extended its use to special applications like aircraft engine test cells, tube jet runways, nuclear reactor vessels and missile launching pads, which have to endure higher tempratures⁴.

Chemical admixtures play a key role in the production of concrete with enhanced performance also known as High Performance Concrete or HPC. In conjunction with mineral additives, such as silica fume, chemical admixtures have enabled major improvements in many of the properties of concrete, particularly, compressive strength and durability.

Now-a-days the concrete is called upon for the use in various tricky situations and the concrete has to show a resistive nature for all the special situations for which it is used. In such circumstances, it becomes necessary to use two or more than two admixtures simultaneously in concrete.

Experimental Programme

The main aim of this experimentation work is to find the effect of sustained elevated temperature on the properties of concrete containing more than two admixtures. The following combinations of admixtures have been selected for the studies on concrete:

• Superplasticiser + Air Entraining Agent + Accelerator (S+AEA+A)
• Superplasticiser + Air Entraining Agent + Retarder (S+AEA+R)
• Superplasticiser +Air Entraining Agent + Waterproofing Compound (S+AEA+W)
• Superplasticiser +Air Entraining Agent + Shrinkage Reducing Admixture (S+AEA+SRA)
• Superplasticiser +Air Entraining Agent + Viscosity Modifying Admixture (S+AEA+VMA)

Portland pozzolana cement and locally available sand and aggregates were used in the experimentation. The specific gravity of fine and coarse aggregate was 2.66 and 2.85 respectively. The experiments were conducted on a mix proportion of 1: 1.26:2.51 with w/c = 0.41 which corresponds to M20 grade of concrete. The admixtures and their chemical content and dosages used in the experimentation are shown in Table 1.

The fine aggregate, cement and coarse aggregates were dry mixed in a mixer for 60 seconds. The required quantity of fibers and hybrid fibers were added into the dry mix and again the entire mass is mixed homogeneously for another 60 seconds. At this stage approximately 80% of calculated quantity of water (w/c = 0.41) was added into the dry mix and agitated for 3 minutes. Now the superplasticiser was added in the remaining 20% water and this liquid was added to the concrete. The concrete was mixed again in the mixer, after which the remaining two more admixtures were added and homogeneously mixed. This homogeneous concrete mass was poured into the moulds which were kept on the vibrating table. The concrete was consolidated in three layers by using just the required vibration time needed for a good compaction. After consolidation the top surface was finished smooth and covered with wet gunny bags. After 12 hours, the specimens were demoulded and transferred to the curing tank wherein they were allowed to cure for 28 days.

For compressive strength test, the cubes of dimensions 150 X 150 X 150 mm were cast and were tested under compression testing machine as per I S 516-1959⁵. For tensile strength test, the cylinders of diameter 100 mm and length 200 mm were cast and were tested under compressive testing machine as per I S 5816- 1999⁶. For flexural strength test the beams of dimensions 100 X 100 X 500 mm were cast and were tested on an effective span of 400 mm with two point loading as per I S 516-1959⁵. For impact test four different test methods are referred in the literature7. Drop weight method being the simple method, was adopted to find the impact energy. Impact strength specimens were of dimensions 250 X 250 X 30 mm. A steel ball weighing 13.03 N was dropped from a height of 1 m on the centre point, which was kept on the floor. Number of blows required to cause first crack and final failure were noted down. From these number of blows, the impact energy was calculated as under. Impact energy = w h N (N-m)

Where w = Weight of steel ball = 13.03 N

h = Height of drop = 1 m

N = Number of blows required for first crack or final failure as the case may be.

After 28 days of curing, the specimens were transferred to the electric furnace wherein they were maintained at 6000 C for 6 hours. After 6 hours they were cooled to room temperature and then tested for their respective strengths.

Test Results

Table 2 gives the compressive strength test results of concrete with different combinations of admixtures. It also gives percentage increase or decrease of compressive strength w.r.t. reference mix. The variation of compressive strength is depicted in the form of graph as shown in Figure 1.

Table 3 gives the tensile strength test results of concrete with different combinations of admixtures. It also gives percentage increase or decrease of tensile strength w.r.t. reference mix. The variation of tensile strength is depicted in the form of graph as shown in Figure 2.

Table 4 gives the flexural strength test results of concrete with different combinations of admixtures. It also gives percentage increase or decrease of flexural strength w.r.t. reference mix. The variation of flexural strength is depicted in the form of graph as shown in Figure 3.

Table 5 gives the impact strength test results of concrete with different combinations of admixtures. It also gives percentage increase or decrease of impact strength w.r.t. reference mix. The variation of impact strength is depicted in the form of graph as shown in Figure 4.

Discussion on Test Results

It has been observed that the concrete produced from the combination of admixtures (S+AEA+R) show maximum compressive strength when subjected to 6000C for 6 hours. This is followed by the combination of admixtures (S+AEA+A), ( S + A E A + W ) , (S+AEA+SRA), and (S+AEA+VMA). The reference mix without any combination of admixtures shows the least compressive strength. The percentage increase in the compressive strength of the above said combinations w.r.t. reference mix are respectively 45.07%, 32.65%, 25.07%, 15.76%, and 7.48%.

It has been observed that the concrete produced from the combination of admixtures (S+AEA+R) show maximum tensile strength when subjected to 600°C for 6 hours. This is followed by the combination of admixtures (S+AEA+A), (S+AEA+W), (S+AEA+SRA), and (S+AEA+VMA). The reference mix without any combination of admixtures shows the least tensile strength. The percentage increase in the tensile strength of the above said combinations w.r.t. reference mix are respectively 55.35%, 53.02%, 51.63%, 47.91%, and 13.48%.

It has been observed that the concrete produced from the combination of admixtures (S+AEA+R) show maximum flexural strength when subjected to 6000C for 6 hours. This is followed by the combination of admixtures (S+AEA+A), (S+AEA+W), (S+AEA+SRA), and (S+AEA+VMA). The reference mix without any combination of admixtures shows the least flexural strength. The percentage increase in the flexural strength of the above said combinations w.r.t. reference mix are respectively 111.03%, 77.93%, 35.17%, 30.34%, and 9.65%.

It has been observed that the concrete produced from the combination of admixtures (S+AEA+R) show maximum impact strength when subjected to 6000C for 6 hours. This is followed by the combination of admixtures (S+AEA+A), (S+AEA+W), (S+AEA+SRA), and (S+AEA+VMA). The reference mix without any combination of admixtures shows the least impact strength. The percentage increase in the impact strength of the above said combinations w.r.t. reference mix are respectively 77.77%, 55.56%, 44.43%, 33.33%, and 11.10%.

This may be due to the fact that the addition of combination of admixtures induce more workability thus making the compaction a perfect one. This makes the concrete more dense which is ultimately responsible for increase in the strengths. The addition of AEA creates small air bubbles in the concrete. These induced air bubbles can resist the expansion of concrete due to temperature.

Conclusions

It can be concluded that the combinations of admixtures used in the experimentation such as (S+AEA+R), (S+AEA+A), (S+AEA+W), (S+AEA+SRA), and (S+AEA+VMA), do not have any compatibility problems either with respect to the properties of fresh concrete or hardened concrete. It can also be concluded that the maximum strength of concrete can be obtained with the combination of admixtures (S+AEA+R) when subjected to 6000C for 6 hours. This is followed by the combinations of admixtures (S+AEA+A), (S+AEA+W), (S+AEA+SRA), and (S+AEA+VMA). Hence it can be recommended to use any combinations of admixtures on the site to suite the situations.

Acknowledgment

The authors would like to thank Dr.(Mrs) S. S. Kulkarni, Principal, RIT, Sakharale and Dr.S.C.Pilli, Principal, KLE Society's College of Engg. & Technology, Belgaum for giving all the encouragement needed which kept our enthusiasm alive. Thanks are also due to the management authorities and others who constantly boosted our morale by giving us all the help required. Thanks are also due to authorities of MBT Pvt.Ltd(Degussa) Mumbai India for supplying the required admixtures.

References

• Lakshmipathy M and Balachandar M, "Studies on the effects of elevated temperature on the properties of high strength concrete containing supplementary cementatious materials," Proceedings of the International Conference on recent advances in concrete and construction technology, Dec 7-9, 2005, SRMIST, Chennai, India. pp. 539-554
• Balamurugan P and Perumal P, "Effect of thermoshock on bond strength of HPC, "Proceedings of the International Conference on recent advances in concrete and construction technology, Dec 7-9, 2005, SRMIST, Chennai, India. pp. 555-556
• Sashidhar C, Sudarsana Rao H, Ramana N.V and Vaishali Gorpade, "Studies on SIFCON subjected to elevated temperature," Proceedings of the International Conference on recent advances in concrete and construction technology, Dec 7-9, 2005, SRMIST, Chennai, India. pp.567-576
• Anbuvelan K, Dinesh M, Kumaravel K, Thiyagarajan A and Sureshkumar N, "Sustained elevated temperature effects on post peak flexural strength of high strength concrete containing polypropylene fibers," Proceedings of the International Conference on recent advances in concrete and construction technology, Dec 7-9,2005, SRMIST, Chennai, India. pp. 577-590
• I S : 516-1959 "Methods of tests for strength of concrete," Bureau of Indian Standards, New-Delhi.
• I S : 5816-1999 "Splitting tensile strength of concrete method of test," Bureau of Indian Standards, New-Delhi
• Balsubramanain, K. et al, "Impact resistance of steel fiber reinforced concrete," The Indian concrete Journal, May 1996, (pp 257-262).