Essence of admixtures in concrete

Transcription

1 Essence of admixtures in concrete East African 1 st Cement, Concrete, and Energy Summit March 17-19, 2016 Addis Ababa, Ethiopia Yoseph Birru Gebregiorgis (PhD), Executive director, Ethiopian Construction Project Management Institute

2 Contents Introduction Background Cement replacement materials Benefits of admixtures Challenges in the use of admixtures Commonly exhibited Undesirable characteristics Recommendations to tackle the Challenges Current state of the art on admixtures Advances in concrete technology Essence of admixtures in sustainable development Summary

3 Introduction It is now an established fact that durable concrete means concrete with fewer microcracks ( μm) Microcracks allow ingress of external agents leading to the deterioration, distress, and destruction of the structure. They can be reduced by using pozzolanic or cementitious materials, collectively called mineral admixtures, to replace cement in concrete.

4 Introduction Admixtures are those ingredients added to concrete mixture immediately before or during mixing in order to alter some performance behaviors of concrete both in the fresh and hardened state.

5 Introduction The term includes all siliceous and aluminous materials, which, in finely divided form and in the presence of water, react chemically with the calcium hydroxide generated during cement hydration to form additional compounds possessing cementitious properties.

6 Background Raw materials are changing Demands on modern concrete are increasing Budgets are shrinking The best value from the cementitious materials in a mixture

7 Background Global goals of sustainability fixed by the Intergovernmental Panel on Climate Change (IPCC) aims at reducing CO 2 emissions in developed countries in 2050 by a factor of 4 first reduced by a factor of 2 by 2020 Against that goal, the global scenario on sustainability of the cement industry predicted by the World Business Council for Sustainable Development (WBCSD) envisages reduction in CO 2 emission only by a factor of 2 by Replacement of Portland cement by mineral admixtures leads to sustainability as the mineral admixtures are mostly industrial and agricultural wastes The technological shift will require not only changes in concrete raw materials and mix design, but also new building techniques, using less materials to obtain the desired structural properties

8 Estimated Cement Demand, CO 2 Generation, and Potential for CO 2 Reduction Using Mineral Admixtures, in Major Regions in 2020.

9 Cement replacement materials Several types of materials are in common use, Several types of materials are in common use, some of which are by-products from other industrial processes, and hence their use may have economic advantages. However, the main reason for their use is that they can give a variety of useful enhancements of or modifications to the concrete properties. All the materials have two common features: 1. Their particle size range is similar to or smaller than that of portland cement; 2. They become involved in the hydration reactions

10 Cement replacement materials They can be supplied either as individual materials and added to the concrete at mixing, or as pre-blended mixtures with the Portland cement. The former case allows choice of the rate of addition, but means that an extra material must be handled at the batching plant; a pre-blended mixture overcomes the handling problem but the addition rate is fixed.

11 1. Mineral admixtures They may be naturally occurring materials, They may be naturally occurring materials, industrial and agricultural wastes or byproducts, or materials that require less energy to manufacture. The mineral admixtures so vast which includes Pulverized Fuel Ash (PFA), Blast Furnace Slag (BFS), Silica Fume (SF), Rice Husk Ash (RHA), MetaKaolin (MK). Besides the chemical (pozzolanic or cementitious) reaction, the mineral admixtures also act physically.

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13 2. Chemical admixtures Chemical admixtures are used in concrete to affect either the fresh or hardened concrete properties. While a wide variety of admixtures are available to the industry, air entraining admixtures, with set-controlling admixtures (accelerators and retarders) and water reducers or plasticizers used under specific circumstances.

14 Chemical reaction of admixtures The action mixtures can be explained in a simplified manner as follows: Cement + water = hydrated paste + Ca(OH) 2 Mineral admixture + water = slurry Mineral admixture + Ca(OH) 2 + H 2 O = Calcium-silicate-hydrate or C-S-H through secondary hydration. Calcium hydroxide which is considered as a weak link in the concrete structure gets consumed.

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16 Relationship between the w/c ratio and strength W/C Ratio Strength Workability Low High Low High Low High

17 Benefits of admixtures Capable of imparting considerable physical and Capable of imparting considerable physical and economic benefits (initial & life-cycle cost). Concomitant savings, for example, in the cost of labour required to effect compaction, in the cement content which would otherwise be necessary, or in improving durability without the use of additional measures. Enables the use of concrete under circumstances where previously there existed considerable, or even insuperable, difficulties. Possibility of using a wider range of ingredients in the mix.

18 Benefits of admixtures Due to the development of powerful high-range water-reducing and viscosity modifying admixtures, it is now possible to very efficiently and economically build high-rise concrete structures. Concrete can be pumped from the first to the highest floor no longer necessary to use cranes to transport and place concrete.

19 Challenges in the use of Admixtures 1. Mentality of admixtures as silver bullets. 1. Mentality of admixtures as silver bullets. 2. Availability of large varieties of admixtures for same function and difficulties in dosage. 3. Admixtures whose performance is known from experience at normal ambient temperatures may behave differently at very high or very low temperatures. 4. Admixtures, whose performance when used separately is known, may not be compatible when used together.

20 Challenges in the use of Admixtures 5. Sensitivity to changes in concrete mixing procedure on performance of admixtures. 6. Even if two admixtures are compatible when introduced into the mix, they may interact adversely if they come into contact with one another prior to being introduced into the mixer.

21 Commonly exhibited undesirable characteristics Some concretes exhibit undesirable characteristics because of incompatibility among different concrete materials. Undesirable characteristics include: 1. Early loss of workability (early stiffening) 2. Delayed set (retardation) 3. Early-age cracking due to excessive autogenous and drying shrinkage of concrete 4. Lack of proper air-void system (Kohn and Tayabji, 2003: 62).

22 Recommendations to tackle the Challenges 1. Selection of reputed suppliers with time tested good performance, staff with professional experience who can guide on effective application/use of admixture in right way. 2. One should ensure supplied with test certificate, manufacturing date and its chemical composition that comply to specifications given by the authorities.

23 Recommendations to tackle the Challenges 3. Generally, admixtures from the same 3. Generally, admixtures from the same manufacturer will have been tested for compatibility with each other and are therefore less likely to present problems. To minimize incompatibility problems, Kohn and Tayabji (2003: 63) recommend using admixtures only from a single manufacturer and keeping dosages under the manufacturers recommended maximums, and using only cementitious materials that meet project specifications and/or standards. 4. It is essential to use trial mixes for any combination of admixtures.

24 Current state-of-the-art on admixtures The current state of the art of the The current state of the art of the science and technology of concrete admixtures made possible to explain not only the fundamental mechanisms of the actions of the most important admixtures, but also to design specific new admixtures to improve the particular properties of both fresh and hardened concrete. The time is long past when different industrial byproducts were selected by trial and error as concrete admixture.

25 Advances in concrete technology During the last 4 decades, concrete technology has made considerable progress - due to utilization of efficient admixture than some improvement in cement In the 1970's even advanced countries such as USA and Canada, concrete with 30 MPa and a slump of 100 mm were typically used as high strength Today MPa concrete having 200 mm of slump is in commonplace 200 MPa ultra-high strength concretes have been achieved Concrete can be pumped from the very first floor to top of skyscrapers 40 MPa or more selfcompacting concrete has been used Very light box girder bridges and very thin section concrete paving for roads and airfields has been made possible

26 Essence of admixtures in sustainable development Sustainable development of cement and construction industry in relation to environment impact is one of the biggest challenge. It is known that the production of a ton of Portland cement expels an almost equal mass of CO 2 into the atmosphere. The cement industry contributes about 5% of the total anthropogenic CO 2 emissions globally.

27 The principal sources of green house gas generation in the cement industry are the manufacturing process, fossil fuels, transport, and power.

28 Unfortunately for Earth's climate, however, the most widely used form of that material today 'portland' cement is made by roasting limestone and clay in giant kilns in a process that sends nearly a tonne of carbon dioxide skywards for every tonne of final product.

29 Environment concern we are now more and more concerned by the environmental impact of civil engineering structures, which favors the use of low W/C concretes that require the use of admixtures. Judicious use of concrete admixtures can result in a significant reduction of carbon footprint of concrete structures.

30 Illustrations Let us suppose that to support a given load L we decide to build two unreinforced concrete columns - one with a 25 MPa and the other with a 75 MPa concrete. The cross-sectional area of 25 MPa, is 3 times larger than that of 75 MPa. Therefore, to support the same load, it will be necessary to place 3 times more concrete, which means more CO 2.

31 Case studies It is generally believed that concrete made It is generally believed that concrete made from fly-ash cement is stronger than the conventional variety, so designers can use less of it. Studies on CeraTech (non-portland cement & concrete US based company) demonstrated that in a typical three-storey, 4,600-squaremetre building, the use of fly-ash cement would reduce the total volume of concrete by 183 cubic metres and the total mass of steel reinforcing bars by about 34 tonnes; it would also divert 374 tonnes of fly ash from landfills and reduce CO 2 emissions by 320 tonnes.

32 Summary & Recommendations The micro-filling (physical) and pozzolanic (chemical) effects of supplementary cementing materials (SCM) contribute to the densification of microstructure, particularly in the Interfacial Transition Zone (ITZ) of HSHPC. Improvement of concrete durability is one of the major benefits offered by the SCM.

33 Summary & Recommendations The understanding of the materials aspects of the mineral admixtures and their impact on the hydration, strength, and durability of concrete will make a positive contribution, encouraging greater and more fruitful utilization of these and even other wastes in cement and concrete, and lead to the sustainable growth of both the cement and construction industry, on the one hand, and the wasteproducing industries, on the other.

34 Summary & Recommendations The big carbon reductions will come only when next-generation cements are embraced by the construction industry's thousands of independent producers, engineers, architects, city planners and building inspectors. And that means lowering the perceived risk of choosing greener cements over their time-tested, conventional counterparts.

35 Summary & Recommendations Attitude might change if more countries adopt Attitude might change if more countries adopt tax levies or cap-and-trade schemes that make the cost of emitting carbon much higher than it is now. More tangible, near-term way to overcome the reluctance is to build demonstration structures, such as bridges, roads and buildings, to prove the real-world viability of the new cement and concrete materials (gov t may take initiative on public projects). Implementing pilot projects on studies carried out for graduate thesis in HE of Ethiopia.

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