September 2006

By: Subash Paudel- Structural Engineer at Commonwealth

DEVELOPMENT OF VERY HIGH STRENGTH CONCRETE

“THE ART OF AN ENGINEER”

ABSTRACT

The use of high strength concrete has become a common practice in many applications throughout the world for many decades, especially for high-rise buildings, long span bridges and repair and rehabilitation works. Moreover, during the last decade, developments in mineral and chemical admixtures have made it possible to produce concrete with relatively much higher strength than was thought possible. High strength concrete is not a revolutionary material; rather, it is a development of normal strength concrete. Hence, special attention and detail considerations are required to produce high strength concrete. This article deals with the techniques used in making of high strength concrete. The article is purely based on laboratory works on Asian Institute of technology, Thailand .

INTRODUCTION

High strength concrete seems to have become the key word in today's concrete technology. In the early 1940s, 30 N/mm 2 (at 28 days) was considered to be the representative of high strength concrete. This level jumped to 50 N/mm 2 in the late 1950s and early 1960s. Concrete strengths of 100-130 N/mm 2 is now being viewed as the criteria for high strength. Just how far we can go to reach an ultimate in strength in the future is nobody's guess. High strength concrete is among the most significant ideal materials available in the market to rehabilitate and enhance the performance of the nation's crumbling infrastructure such as assisting the widespread problems of deteriorated bridge structures and tall

Table 1: Classification of high strength concrete

THEORETICAL CONSIDERATIONS

High strength concrete may be modeled as three phase composite materials, the three phases being (i) the hardened cement paste; (ii) the aggregate; and (iii) the interfacial zone between the hardened cement paste and the aggregate. For high strength concrete these three phases must all be optimized, which means that each must be considered explicitly in the design process and all the components of concrete mixture must be pushed to their critical limits. Under the compressive load, there are three possible failures of concrete components, which lead to the failure of the concrete specimen. These are matrix failure, coarse aggregate failure and bond of matrix and coarse aggregate failure as shown in figure 1. The key concept for making high strength concrete is to prevent these failures as much as possible.

The matrix consists of hardened cement gel (cement + water) and fine aggregate. According to Abrams' law, low water cement ratio (W/C) has to be used in order to increase the strength of matrix as shown in figure 2.

(a)	(b)	(c)
Figure 1: Modes of failure of concrete specimen under compressive loads. (a) Matrix Failure (b) Aggregate Failure (c) Bond Failure.				Figure 2: Abrams' Law, the Relationship between Compressive strength and Water/Cement Ratio.

However, low amount of water causes loss of workability of fresh concrete, which cannot be compacted properly and leads to dropping of strength. The balance between W/C ratio and workability is another ‘art' to be applied by an engineer. Therefore, high-range water reducing agents or superplasticizers are added to maintain the workability of fresh concrete while using low W/C .The function of superplasticizer is to disperse the cement particle which allows even small amount of water to flow and react with cement particles easily.

Impurities, acid and base in water can harm concrete properties especially strength. Hence the water used in high strength concrete is the same as normal concrete, i.e. potable water. The quality control of water in high strength concrete must be more stringent than water in normal strength concrete in order to guarantee the quality requirement for the high level of strength.

When cement and water interact, a number of products are formed, but only two of these are important, calcium hydroxide [Ca(OH) 2 ] and calcium silicate hydrate [CSH]. The calcium hydroxide is the weaker part and the calcium silicate hydrate is the stronger part and is the ¢ glue ¢ that holds the paste, mortar and concrete together. To diminish or remove the weaker part, pozzolanic material mainly containing Silicon Dioxide (SiO 2 ) is used to react with Ca(OH) 2 . The product of this reaction will be stronger part, CSH. Therefore pozzolanic materials can improve concrete strength by converting the weaker part to the stronger part. The examples of pozzolanic materials are Condensed Silica Fume (CSF), fly ash, rice husk ash etc. Among these materials, CSF has been proved as the best pozzolanic material, as far as strength is concerned. Since the particle size of CSF is one hundred times smaller than cement particle, the particle of CSF will fill the microscopic void between cement particles leading to dense concrete and ultimately to high strength. Thus, CSF will enhance strength of concrete by two mechanisms, pozzolanic reaction and micro-filler effect.

In making high strength concrete, sand is used as fine aggregate. Better strength of concrete is obtained using smaller particle size. However, using only very fine sand will trap the water during mixing process and this effect causes loss of workability and unacceptable compaction of fresh concrete. Therefore, sand should be well graded in the range of 150-600 m m size. The gradation of sand also makes concrete uniformly dense. Type of sand has an insignificant effect on the strength of concrete. Nevertheless, the shape of sand effects the workability. Round shape particles increase workability while the effect is reverse for angular shape particles. Therefore, river sand is used in making high strength concrete.

Type, size and shape are the main factors that affect the strength of coarse aggregate. In high strength concrete, crushed rocks are used as coarse aggregate. Size of crushed aggregates should be between 5-10 mm. Bigger stones will contain more micro cracks induced during crushing process while the smaller ones mostly have thin or slender shape. Both factors will decrease the strength of aggregate. The shape of crushed aggregates should be neither thin nor round. Round shape causes slip between matrix and stones under loading. Therefore, angular shape stones are preferred. Figure 3 shows different shapes of crushed aggregates rock.

(a)	(b)	(c)
Figure 3: Three Different Shapes of Coarse Aggregate (Crushed Rock) (a) Slender, (b) Thin and (c) Angular.

The bond of matrix and coarse aggregate is of immense importance and can be increased using angular shape of coarse aggregate as mentioned above. The rough surface of coarse aggregate will enhance friction between matrix and coarse aggregate. The micro-filler effect of CSF also reduces the amount of trapped water on the surface of coarse aggregate which helps in increasing friction between matrix and coarse aggregate .

MATERIALS PREPARATION

After deciding the mix proportion, the preparation of materials was started with coarse aggregate and sand. To obtain 4.75 mm coarse aggregate size, mechanical sieving machine was used. Then coarse aggregates were selected piece by piece to ensure that there is no bad shape (thin or smooth-rounded), bad color (yellow or brown) and bad properties (cracked and porous). The coarse aggregates were also selected at various sizes ranging from 4.75 mm to 9.5 mm. Sand was also sieved by mechanical sieving machine to obtain particle size between sieve No. 8 (2.36 mm) and No. 100 (150 m m). Then the sand was sieved by hand for separate particle sizes that retained on sieve No. 16 (1.18 mm), No. 30 (600 m m), No. 50 (300 m m), and No. 100 (150 m m). The selected coarse aggregate and sand were washed with water to ensure that there is no trace of dust, impurities, oils, etc. on their surface and soaked in clean water for 24 hours. After that coarse and fine aggregate were allowed to dry in open air to get saturated surface dry (SSD) condition .

If the aggregates are not to be used immediately, they must be packed in the plastic bag. The quality of water used also effects the strength of concrete.Clean, dust free water must be used in this process.. Cement and silica fume must be completely dry.

PROCEDURE FOR MIXING, CASTING

At first, the mixer drum, moulds, and concrete casting hammer and equipment were cleaned and checked for working accurately to avoid the disturbance in the casting process. All the required materials, admixtures, and water were re-weighed and stored in nearby containers. The mixing process was as follows :

(a) The coarse aggregate, cement, silica fume and sand were poured into the drum for dr y mixing (shown in figure 4). To ensur e well and faster distribution of particles the coarse aggregate was placed as the first layer, cement and silica fume as the second layer and the third layer was sand. The dry mixing was carried out for 5 minutes to ensure both the good distribution of particles and the well - coated aggregates with dry cement .

Figure 4: Dry Mixing of Concrete

(b) The water was divided into two parts. One half was mixed with superplasticizer and then slowly poured into the drum while the mixer was running. It took about 10 minutes.

(c) The remaining half of water was slowly poured into the drum, until cement, CSF, and the sand completely became mortar gel covering all coarse aggregates . At this stage, the dry cement and sand particles tend to disperse to the bottom edge of the mixer drum due to inertia forces. Therefore, after every 2 to 3 minutes, the mixing had to be stopped for some 30 seconds so as to dig these dispersed particles back to the center of the drum. This process took 10 minutes. It is advised that total time of stage a, b, and c should not exceed 30 minutes.

Figure 5: Fresh Concrete ready for Casting

(d) Fresh concrete (shown in figure 5) was poured into cylinder moulds (10 x 20 cm) in 8 layers , each layer being compac ted by a 1 inch steel rod ( 25 times ) . Each mould of concrete was put on vibrating table for 2- 3 minutes to increase the gradation and allow water to appear. C ompaction methods are very important in case of high strength concrete, because the W/C ratio of the mix is very low and the workability is also low though superplasticizer was added, the ordinary compaction can not be sufficient. The concrete should be left 0.5 cm higher than the mould for next leveling step. The frequecy of vibration which effects the degree of compaction is the another factor to be careful. High frequency causes bleeding and seggregation that decreases strength and low vibration causes less compaction that makes to remain more air voids decreasing strength again.

(e) The surface of each concrete specimen was carefully leveled by a metal plate with compressive force to make plain, stiff and smooth contact surface. This stage is also vital because uneven sample will result in high stress concentration during the loading test, and this specimen may fail below the desired strength. The four concrete specimens were numbered, and carefully kept in open air for 24 hours.

PROCEDURE FOR CURING

After curing in open air for 24 hours, the specimens were taken out of the moulds. In order to check the development of strength in different curing condition, two specimens were put into a water container and placed in the oven at temperature range of 80 0 C - 90 0 C (hot curing). The other two specimens were put into water bucket and place in normal condition (water cooling) for three days, after that they were also placed into hot-curing.

Table 2: Mix proportion

Before the test date 24 hours earlier, the water bucket containing specimens were taken out of hot curing oven to put into normal condition for 12 hours. After that time, the specimens were cleaned and placed in open air 12 hours . This is important to reduce the effect of Moisture State on concrete strength.

MIX PROPORTIONS AND TEST RESULTS

One specimen was taken to test after 3 day curing to check the strength development, and the 3 day-compressive strength was 130.30 N/mm 2 Other three specimens were taken for testing after 11 day curing. Before the test, each specimen was washed and dried, dimensions checked and numbered. For the testing, the rubber cap was supplied on top of the specimen for uniform distribution of stress over the contact surface. The test results are shown in table 3.

The second trial mix came to be with more interesting results. In this case Basalt is used as coarse aggregate in place of limestone.

For the two specimen in heat curing (in oven) for 7 days and then 3 days hot water curing and then normal curing in remaining days, 28 days compressive strength is tested to be as shown in table 5.

Figure 6: Specimen After Testing

Table 4: Mix Proportion for Second Trial.

Table 5: Compressive Testing Results of Second Trial

CONCLUSIONS

This article presents the background and techniques in making of high strength concrete by using local fine and coarse aggregates.These results show a good development of compressive strength over time, and after 28 days curing, the strength can be expectedto increase even more than 150.00 N/mm 2 , which is in the range of Ultra High Strength Concrete. These results

are obtained from laboratory and not from industrial production or on site-work and focused only on the compressive strength aspect without satisfying the workability and the economic requirements. To achieve the strength as required and calculated from theory, the production of high strength concrete needs very high quality control. It can be concluded that good quality control in general concrete mix increases the strengths 10-20% and to develop high strength requires micro care and attentions in each steps which is the art of an engineer.

REFERENCES

(1) Abrams, D. A., ¢ Design of Concrete Mixtures ¢ , Bulletin 1, Structural Material Research Laboratory, Lewis Institute, Chicago, USA, Dec. 1918.

(2) Neville, A.M., ¢ Properties of Concrete ¢ , Fourth and Final Edition, Longman Group Limited, England , 1995.

(3) Mehta, P.K.and Monteiro Paolo, J.M., ¢ Concrete: Microstructure, Properties and Materials ¢ , Second Edition, The McGraw-Hill Companies Inc., USA , 1993.

(4) Nimityongskul, P., Plengkhom, K. and Panichnava, S., ¢ Making of High Strength concrete in Thailand ¢ , Proceedings, The 1 st National Concrete Conference, 14-16 May 2003, Thailand, P. 7-16 (in Thai).

(5) Report on The 3 rd High Strength Concrete contest,(Elephant Power Concrete), organized by Faculty of Civil Engineering, King Mongkut's Institute of Technology, Thonbhuri, and Siam Cement Industry Limited, August,23-September 20, 2002, Thailand (in Thai).