High performance concrete (HPC) has been used more widely in recent years due to the increasing demand for durable concrete. Structures that are exposed to aggressive environments reveal that high strength concrete alone cannot guarantee long-term performance. Concrete is required to exhibit performance in the given environment. However, there has been no established method whereby the mixture proportions of concrete can be optimized according to the required performance. The methods adopted for design of conventional concrete mixes are not directly applicable to HPCs. Several methods have been proposed over the years for the proportioning of mineral admixture – based HPC mixes. The methods namely ACI, modified ACI, DOE etc are found to be suitable for designing HPC mixes especially in cold countries where temperature hardly goes beyond 25οC.

India, being a tropical country has different environment in its different parts. Tropical countries usually receive significant rainfall during only some part of the year leading to substantial variation in the level of humidity in many parts of the tropics. So the variation in temperature and humidity has profound effect on the properties of HPC such as strength and durability since the mix proportions are usually decided at laboratory conditions. Therefore, HPC mix design in tropical climate need to be given special attention to incorporate the variation in its properties.

In the present study, an attempt has been made to study the effect of humidity and temperature on workability and strength properties of M50 grade HPC by exposing it to varying humidity and temperature conditions in a chamber where controlled conditions for different humidity and temperature are mainted are monitored. The results indicate, there is a significant effect on strength and workability of HPC mixes due to variation and temperature and humidity.

Kumbhar, P.D. Asst. Professor, Dept of Civil Engg,, K.E.Society's, Rajarambapu Institute of Technology, Rajaramnagar,

Murnal, P. Professor and Head, Dept of Applied Mechanics, Govt College of Engg, Satara

Introduction

These days, more concrete is used in infrastructure projects. High cost of such projects coupled with non replacement possibilities put higher emphasis on durability. Normal concrete, though versatile is not very suitable against severe aggressive conditions, chemical conditions and thermal stresses. High strength concretes were introduced few years back to take care of strength requirement for such highly durable structures. However, structures exposed to aggressive environments have revealed that high strength of concrete alone cannot guarantee long-term performance. This fact has led to the development of high performance concrete (HPC)1. In the present scenario, HPC is emerging as a construction material which will serve the basic dual purpose of strength and durability. However, the basic method of mix design of HPC has not yet been established as it includes other admixtures to serve the requirements of fresh and hardened concrete. These admixtures include silica fume, fly ash and plasticizer or superplasticizer2.

India has very aggressive and corrosive climatic conditions. In some areas, it rains heavily for more than four months a year. Concrete is constantly subjected to various external destructive forces like humidity, heat, cold, industrial pollution, rains etc. So, concrete structures are subjected to gradual deterioration, leading to leaching, carbonation, cracks, separation plaster and corrosion of steel reinforcement, causing substantial reduction in the life of the building and with high costs on repair and maintenance. The long stretch of coastal belt causes saline winds and the summer months are very hot. The problem is aggravated by pollution from increasing industrialization, auto emissions and chemical attacks. Over a period of time, concrete loses its ability to protect itself; cracks and leaks develop and steel reinforcement corrodes, becoming vulnerable to structural failures3.

India, being a tropical country has different environments with varying humidity and temperature conditions in its different parts. The weather and environmental conditions at the time of casting of the concrete may require variation in the proportions. Since the water / binder ratio significantly affects the properties of concrete in both fresh and hardened state, the variation in the temperature and humidity of the surroundings could significantly affect the properties if the water/binder ratio as per the mix-design is used. The water / cement ratio and minimum cement content may also have to be varied for durability considerations. Under such conditions high performance concrete has become quite popular in recent years. However, various required performance attributes of HPC, including strength, workability, dimensional stability and durability, often impose contradictory requirements on the mix parameters to be adopted, thereby rendering the concrete mix design a very difficult task. There are many methods of mix design but the British or American methods will not be applicable for our country as the specific relationships constituting figures and tables are based on their materials4. The conventional mix design methods are no longer capable of meeting the stringent multiple requirements of HPC. The mix design of HPC cannot be based on general tables and graphs. The mix has to be developed for the specific application and for the given set of ingredients5.

Temperature is one of the important factors which affect the durability of concrete. Concrete structures deteriorate more rapidly when exposed to hot environment. High temperature associated with other factors like high humidity has significant effect on the durability of concrete6. Hence, it has become necessary to study the effect of such environmental conditions on workability and strength properties of high performance concrete. The present experimental work deals with study of effect of varying humidity and temperature conditions on workability and strength properties of M50 grade high performance concrete by exposing it to varying humidity and temperature conditions in a room controlled for specified humidity and temperature ranges.

Experimental Investigations

Materials

The materials used in making HPC mixes along with their various properties have been given in Table 1.

Mix Design of HPC

The mix design of HPC was done by using the guidelines of IS Code method (IS10262-1982)8. The design stipulations and the data considered for mix design HPC has been presented below.

Characteristic Strength, fck (MPa)    : 50

Max Size of Coarse Aggregate    : 20mm (Crushed)

[Fraction I-60%, 20mm-12.5mm]

[Fraction II-60%, 12.5mm-10mm]

Degree of Quality Control    : Good

Type of Exposure    : Severe

Degree of Workability    : 0.95 (Compaction Factor)

Target Mean Strength (f'ck), MPa    : fck + t x S = 50+1.65x5 = 58.25 Where,

fck = characteristic compressive strength at 28 days,

S = standard deviation, and

t = a statistic, depending upon the accepted proportion of low results and the number of tests; for large number of tests, the value of 't' is given in Table 2 of IS 10262-1982 code.

Mix Proportions

Mix proportion of M50 grade HPC mix was obtained by making certain modifications in the mix proportion arrived at using the guidelines of IS Code method. The target mean strength is determined by considering the standard deviation value as recommended by IS 456-20009 Code. The mix proportion was obtained without considering any addition or replacement of mineral admixture (i.e. micro silica).

Effect of Humidity and Temperature on Properties of High Performance Concrete
Figure 1: Room Controlled for Humidity and Temperature conditions

After several trials, a cement content of 450 kg/m3 and w/b ratio of 0.33 were finalized based on 28 days compressive strength gain of HPC mix and desired workability properties (slump/flow). Thus, for making HPC mixes a cement content of 450 kg/m3 and a w/b ratio of 0.33 were used along with optimum content of micro silica as mineral admixture. After carrying out several preliminary mix trials, the optimum contents of micro silica at 10% and superplasticizer at 1.55%, both by weight of cement, were found to give desired workability and strength properties. The w/b ratio was calculated by dividing the weight of mixing water by combined weight of cement and micro silica. The final mix proportion was arrived at by altering the ratio of fine aggregate to coarse aggregate and is expressed as parts of water: cement: fine aggregate: coarse aggregate as given by 0.33: 1:1.75:2.50.

Preparation of HPC Mix

The required quantities of all the ingredients were taken by weigh batching, with appropriate coarse aggregate fractions and mineral admixtures. Mixing of the ingredients was done in a pan mixer as per the standard procedure. A reference mix was prepared under the prevailing humidity and temperature condition in a room controlled for specified humidity and temperature conditions (Fig.1), using a water-binder ratio of 0.33 and suitable superplasticizer content (by weight of cement) in order to get desired workability. The workability of the concrete was studied by conducting slump and flow tests as per the standard procedure10 (Fig.3 and Fig.4). Standard cube specimens of 150mm x 150mm x 150mm size were cast using the procedure described in IS 516 code11 and were immediately covered with plastic sheet and kept there for 24 hours and then released in water tank for 28 days curing.

Effect of Humidity and Temperature on Properties of High Performance Concrete Effect of Humidity and Temperature on Properties of High Performance Concrete
Figure 2: Slump Test Figure 3: Flow Test
All the HPC mixes were prepared using the same mix proportion, w/b ratio and superplasticizer dose under different combined humidity and temperature conditions. A humidity range of 20–90% and a temperature range of 30οC, 35οC and 40οC were considered for study of workability and strength properties of HPC mixes.

The humidity was defined by considering a permissible variation of ± 5% whereas the temperature was defined by considering a permissible variation of ±0.5οC. Thus, a humidity of 50% means the humidity variation which has a range of 15%, 20%, 25% and a temperature of 30οC means the temperature variation which has a range of 29.5οC-30οC- 30.5οC. The details of workability (slump and flow) properties of the mixes prepared under various humidity and temperature conditions along with their quality are given in the Table.2.

Testing of Specimens

After 28 days curing period, the specimens were taken outside the curing tank and were tested under a compression testing machine of 2000KN capacity. The crushing loads were noted and the average compressive strength of three specimens is determined. The compressive strength values of specimens subjected to different combined humidity and temperature conditions has been presented in Table 3.

Result and Discussion

Effect of Humidity and Temperature on Properties of High Performance Concrete
Figure 5: Strength Variation at different Relative Humidity (Temp 30OC)

To examine the effect of humidity and temperature on properties of HPC, the specimens with reference mix proportion have been exposed to different humidity and temperature conditions during mixing and casting. From Table 2, it is observed that slump and flow significantly increase in humidity for a given temperature. This means that the increase in surrounding humidity is responsible for maintaining the water content in the mix thereby increasing the workability. From Figures 5 to 6, it is observed that the compressive strength of a given mix is significantly affected by variation in temperature and humidity. There is a general tendency of reduction in compressive strength with increasing humidity (Fig.5 to Fig.7). All the specimens have gained the target mean strength of 58.25MPa. The effect of temperature on compressive strength with varying humidity conditions is indicated in Fig.8. The humidity conditions have been broadly classified into low (30%), medium (50%) and high (80%) humidity levels. From Figure 8, it is observed that there is a general tendency of increase in compressive strength with increase in temperature.

Effect of Humidity and Temperature on Properties of High Performance Concrete
Figure 6: Strength Variation at different Relative Humidity (Temp 35oC)

Effect of Humidity and Temperature on Properties of High Performance Concrete
Figure 7: Strength Variation at different Relative Humidity (Temp 40oC)

Effect of Humidity and Temperature on Properties of High Performance Concrete
Figure 8: Effect of Temperature on Compressive Strength of HPC

The concrete specimens subjected to different humidity conditions at 35OC have shown some deviations in compressive strength results, however a trend of reduction of compressive strength has been observed.

Conclusion

  • The HPC mixes can be designed using existing IS Code method of mix design with some modifications to achieve a specified target strength using locally available materials and appropriate dosages of superplasticizers and by incorporating micro silica.
  • The behavior of the design concrete mix is significantly affected by variation in humidity and temperature both in fresh and hardened state.
  • It is necessary to develop some correction schemes for the designed mix, which would be applied to the mix at site depending on the site environmental conditions.

References

  • Shridhar, R. (2002), "Use of Chemical Admixtures in HPC for Durable Structures," The Indian Concrete Journal, September, pp.579-580.
  • Krishnan,B., Singh, A., and Singhal, D.(2006), "Mix Design of High Performance Concrete and Effects of Different Types of Cement on High Performance Concrete," Proceedings of National Conference on High Rise Buildings: Materials and Practices, New Delhi, October, pp.11-18.
  • Yargal, S.C., Ravikumar, C.M. and Babu Narayan K.S.(2006), "Design of Various Grades of Concrete in Coastal Environment: A Case Study," CE &CR, December, pp74-83.
  • Maiti S.C., Agarwal, R.K. and Kumar R. (2006), "Concrete Mix Proportioning," The Indian Concrete Journal, December, pp.23-26.
  • Rama Rao, G.V. and Seshagiri Rao, M.V.(2005), "High Performance Concret Mix Proportioning with Rice Husk Ash as Mineral Admixture," NBM&CW, January, Vol.10 (7), pp.100-108.
  • Patnaikuni, I. and Roy, S.K. (1997), "High Strength-High Performance Concrete for Tropical Climates," International Conference on Mainte- nance and Durability of Concrete Structures, March, JNT University, Hyderabad (AP), pp.148-150.
  • BIS Code IS: 383-1970. "Specification for Coarse and Fine Aggregates from Natural Sources for Concrete. (Second Revision), September 1993.
  • BIS Code IS: 10262-1982. "Code of Practice for Recommended Guidelines of Concrete Mix Design." March 1998.
  • BIS Code IS: 456-2000. 'Code of Practice for Plain and Reinforced Concrete (fourth revision).' July 2000.
  • BIS Code IS: 1199–1959. "Code of Practice for Methods of Sampling and Analysis of Concrete." November 1991.
  • BIS Code IS: 516–1959. "Code of Practice for Methods of Tests for Strength of Concrete." (Reaffirmed 1999).
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