Experience with new admixture technologies in high performance concrete

Transcription

1 Experience with new admixture technologies in high performance concrete D King*, Degussa Construction Chemicals - MBT Singapore 27th Conference on OUR WORLD IN CONCRETE & STRUCTURES: August 2002, Singapore Article Online Id: The online version of this article can be found at: This article is brought to you with the support of Singapore Concrete Institute All Rights reserved for CI Premier PTE LTD You are not Allowed to re distribute or re sale the article in any format without written approval of CI Premier PTE LTD Visit Our Website for more information

2 2J'!i Conference on OUR WORLD IN CONCRETE &STRUCTURES: August 2002, Singapore Experience with new admixture technologies in high performance concrete D King, * Degussa Construction Chemicals - MBT Singapore Summary Superplasctisers based on new chemistries originally developed in Japan have established a strong track record of successful use in Asia. This new range of superplasticisers, sometimes called "hyperplasticisers" is described generically as "polycarboxylates" (PC) or "comb polymers". This news technology was introduced into the region in Since then it has developed track record to solve a variety of concrete and concreting problems in major construction projects. It is also specified by generic or performance on many new infrastructure projects. Benefits arise in economic very high strength, slump retention, environmental and self-compacting capability with associated cost savings and better properties of concrete. Introduction The developments of admixtures that significantly enhance the properties of concrete have evolved over many decades. Until recently almost all admixtures for concrete have been developed from the by products of other industries such as paper and soap. Superplasticisers are derivatives of organic chemistry in wider industries using, melamine, naphthalene, formaldehyde and other organic chemicals. These chemicals have and still do play an important role in the concrete technology. They act on different components of cement during the hydration process (table 1). During the 1980's chemistry was developed in Japan to meet the requirements of the construction industry for high performance concrete. Concrete that would allow structures to be designed with very high strength, low permeability, waterproof properties and the ability to retain workability in hot conditions during the transportation and placing periods. As a result significant cost savings have been achieved in the construction process with fewer problems for the engineer and a market opportunity for material suppliers including the concrete industry. These cost savings are based on, truncated construction programmes, fewer repairs and reduction in labour and equipment. In areas of high land costs, such as Hong Kong, the use of high strength concrete enables the design of taller buildings in concrete linked with more useable floor space. 2. Development of Admixtures for Concrete in Hot Climates The capability to reduce the water content of placeable concretes, to control set and to minimize slump loss are key requirements for concretes in hot climates. High ambient temperatures are common in all ASEAN countries and in Hong Kong the only cooler times are between November and March. Local aggregates are crushed rock coarse and fine. Most concrete is produced at ready mixed concrete plants with comparatively long delivery times, especially due to traffic congestion. A typical mix design for Grade 30/20 concrete with 75mm slump could contain 40-45% crushed rock fines (CRF) kg/m3 Portland Cement and water content of kg/m3. Consequently, almost all concretes contain a water-reducing and set controlling admixture. Plasticisers, or water reducing admixtures (WRA), based on hydroxylated carboxylic acids give typical water reductions of -5%; lignosulphonate based plasticisers give up to 12%. 293

3 Over the past four decades, there has been a progressive improvement in admixture technology. Superplasticisers, or high range water reducing admixtures (HRWA), were introduced into the region in the early 1970's. The conventional Superplasticisers still commonly used today are broadly categorized into four groups; sulphonated melamine-formaldehyde condensates (SMC); sulphonated naphthalene-formaldehyde condensates (SNC); modified lignosulphonates (MLS); and others, including blends of materials (Ramanchandran V.S. & Malhotra V.M ). In simple terms, they are believed to act in two ways; by affecting the interparticle forces between cement grains and the hydration mechanisms of the mineral components of cement. Generally, SMC and SNC based Superplasticisers have a practical upper limit on water reduction of about 30% (Rixom M. R. and Mailvaganam N.P. 1986). Superplasticisers based on combinations of raw materials with improved performance, including reduction in slump loss, were introduced in the 1980's ( Payne J. and Dansfield J.M. 1985). However, they have limitations. They can produce high strength and flowing concretes, but workability can drop off with time during transportation so that site re-dosing may be required. Such Superplasticisers have been successfully used in HPC in Hong Kong in many major projects, such as Check Lap Kok airport and the Tsing Ma Bridge (Payne J. and Kay W., 1995) 3. New Admixture Technologies New technologies have been developed in Japan, where the first patents were registered in the early 1980's ( Tanaka Y. et al 1996), to produce high performance polycarboxylate polymers for use in admixtures. These polymers are based on new chemistries, are manufactured synthetically, and are designed specifically for use in concrete. Advanced performance characteristics are achieved by designing the molecular architecture of the polymers (Sugiyama T et al 1995). Advanced performance Superplasticisers are based on a patented set of polymers, known Water reduction v. dosage 1.4 ~ ~ CD.. 1. SMC ClC fti CD 0.8!II e.snc 0 CD 0.6 't:i Co) Ope CD 0.4 > i 0.2 & % water reduction generically as modified polycarboxylic ethers. These new materials have exceptional properties. They act as powerful cement dispersants, giving extremely efficient plasticising and water reducing effects, so that water reductions of 40% are achievable. This is due to additional dispersion mechanisms not provided by conventional Superplasticisers (Ohta A. et al 1996). 294

4 Minimal retardation "10 Water reduction Polycarboxylates give minimal retardation, even at high dosages. With appropriate mix design, loss of workability can be negligible during transportation of concrete. This results from the unique chemistries built into the molecular structure of the comb polymers ( Ohta A. 1998). They are equally effective when dosed normally or when added into the concrete mix water when batching. They contain low alkali levels, thus minimising the risk of alkali - silica reactivity (ASR). They are neutral so are not caustic to the skin. 4 High Performance Concrete (HPC) Polycarboxylate based Superplasticisers offer a range of technical and economical solutions not previously available for the production of high performance concretes (HPC). Examples include higher early strength concretes, higher ultimate strength concretes, and more durable concretes for extreme environments (Khurana R 1998). In Hong Kong successful trials have been established with Grade 80 and Grade 100 concrete. In addition Polycarboxylate admixtures have been successfully tria lied for use in major bridges where there is the risk of chloride ingress and where long term surface appearance is important. For the building owner, architect and specifying engineer, more durable concrete can reduce building maintenance costs, reduce the life cycle cost of the structure, and allow planning for structures with a service life of greater than 50 years. Improved engineering properties of HPC concretes can offer additional cost savings such as reduced column diameters, increased beam spans, and increased structural loading capability. The ability to greatly reduce water contents has particular advantages for the precast industry, where very high early strengths can be achieved, and heat-curing costs can be reduced or eliminated (Torresan I. and Khurana R. 1998). Self-compacting concrete (SCC) using polycarboxylate based Superplasticisers has over a decade of experience in Japan, and has become widely used in Europe and has now gained experience in the region, especially Singapore and Hong Kong (Bartos et al 1999). Considerable experience of SCC mix designs using poycarboxylates has also been gained in UK. The principal advantage of the new technology is that it can greatly reduce slump loss (Khurana R. and Scabini S. 1998). For contractors, direct cost savings from using SCC include reduced manpower at site, faster placing, good compaction even with congested reinforcement. Faster finishing, and excellent surface finishes. For the ready-mixed concrete producers, this technology offers reduced risk from re-tempering at site, lower plant maintenance costs, faster turn-round of trucks, easier pumping, increased haulage radials, and virtually no segregation or bleeding. Self- Compacting Zero Energy Concrete The fact concrete can be designed to be compacted with zero energy (no vibration at all) is beneficial to all parties, especially the contractor and the developer. 295

5 The properties of concrete are greatly enhanced and the characteristics of the finished structure improved because the concrete is fully compacted. The incidents of honeycombing and "blow holes" are minimized, thus reducing or eliminating the need for repairs. Surface finish is improved to an extent where an architect may reconsider surface coatings strategy. Labour costs are significantly reduced and because of the elimination of the noise of vibrators concrete can be poured in unsocial hours. In the pre-cast factory the cost savings are more profound. Additional shifts can be planned and steam curing can be eliminated to achieve high early strengths. A whole new world of opportunities in concrete placement and design philosophy is open. see mix design criteria Cementitious contents: Horizontal > 400kglm3 Vertical > 450kglm3 Fines <315 micron: Horizontal> 650kglm3 Vertical> 750kg/m3 Use of cement replacements High fine aggregate (50% w/w FA Continuous overall aggregate grading see placing criteria Workability Slump flow test appropriate (not slump) Slump flow must be high (> mm) Head> 2 metres above pour Single point placement preferable Tremie used where practical A further benefit is the ability to produce watertight concrete. Because lower water cement ratios «0.35) can be achieved with self-compacting concrete, the permeable void is Significantly reduced and testing for water permeability has demonstrated improved performance over traditional water proofing admixture types together significant reduced costs. 5. Experience with Polycarboxylate Admixtures in Asia The market share for polycarboxylate polymers in Japan in 1997 was estimated to be 45% of all Superplasticisers sold. A large number of major projects have been completed using the new technology. Examples include the Akashi Bridge, completed in 1998, which is the longest suspension bridge in the world with a central span of 2000 metres. 1.4 million cubic metres of SCC was used for the foundation work. The Trans Tokyo Bay project, completed in 1998, is a combination of tunnel and bridge, linked by a man-made island 0.25 million cubic meters of SCC was used in its construction. Other notable contracts using these new admixtures include the Fukuoka dome Stadium Building (0.20 million cubic metres), the Nanko Tunnel (0.12 million cubic metres) and the JK Tokai Central Towers (0.22 million cubic metres). In China for the 384 metre high 77 storey Di-Wang Building, built in Shenzen by Kumagai-Gumi in 1996, a polycarboxylate based superplasticiser was specified for 20,000 cubic metres of high strength concrete. Architects were American Design Associates and K.Y. Cheung DeSign Associates, and consultants were Maunsell Consultants Asia, For the World Plaza Building in Pudong, Shanghai, the main contractor Nippon Steel placed 5000 cubic metres of high strength concrete using a polycarboxylate based superplasticiser for pumping where good slump retention was required. Cement content was 450kg/m3, at a water/cement ratio of Compressive strength was 70 MPa at 28 days. In Korea at Young Jong Island near Incheon, a large concrete tank to hold LNG is under construction. Work started in March 1998 and will finish in December The constructors, Hyun Dai and Kajima, require a high workable concrete which can be pumped, and retain its workability for two hours. The only solution found was by using a polycarboxylate based superpasticiser. The concrete is required to be self compacting, with a flow of 68+/-5cm, pumpability by V-funnel 20+/ seconds, with a flow retention of 3+/-cm for 2 hours. The cement was a belite type from Ssang Yong, and limestone powder and a cohesion promoting admixture were included in the mix design. 52,000 cubic metres of concrete were placed. 296

6 In the Philippines a second bridge has been constructed between Mactan and Cibu. High workability grade 40 MPa concrete with extended slump retention was required for the bridge piles. The main contractor, Kajima and the consultants specified a polycarboxylate based superplasticiser, with exceptional slump retention characteristics and controlled retardation. The project was completed in August Over 20,000 cubic metres of concrete have been placed. The mix design used 415kg/m3 cement with a water cement ratio of 0.43 with an admixture dosage of 1.3% by weight of cement. 6. Experience with Polycarboxylate Based 5uperplasticisers in the Local Region The first use of this technology in Hong Kong was for HPC concrete placed into columns on the new Kowloon Station in The requirements were for high strength concrete Grade 60, with a high workability, suitable for pumping, and with greater than two hours workability retention. The solution found was to use a polycarboxylate superplasticiser at a dosage of 7 litres 1m3. Concrete was supplied by Hong Kong Concrete, the main contractor was China State Contractors, the consultant was Ove Arup and the client was the Mass Transit Railway Corporation (MTRC). Concrete results more than met the specified requirements. The ready-mixed company reported excellent slump retention and pumping pressures at site were about 40% below normal. Typical results achieved Grade 60 WIC ratio 0.34 Initial slump 220mm 2 hours >200mm Compressive strength at 7 days >70 MPa Compressive strength at 28 days >90 MPa Typical results achieved Hong Kong MTRC Grade A Grade 40 WIC ratio 0.38 Initial slump 220mm 2 hours 210mm Compressive strength at 7 days >50 MPa Compressive strength at 28 days >70 MPa BSS Water absorption < 1.5% Chloride permeability Very low Permeability E 90 ~ 80 ~ 70 i 60 i 50 ~ 40 CII 'lii 30! 20 ~ 10 a CII 0 C Polycarboxylate SNC No admixture A series of test programmes were carried out with MTRC (Mass Transit Railway Corp) to determine the suitability of a polycarboxylate superplasticiser for concrete to meet the specification 297

7 requirements for Category A concrete. This programme included casting one cubic metre test pieces, which were cored for testing. Full compliance was achieved. Typical results for water permeability tested to ISO 7031 showing a significant improvement compared to a conventional SNC type superplasticiser. The water absorption (BS 1881: part 122:1983) requirement of <1.5% was also easily achieved. A polycarboxylate based superplasticiser was used in approximately 50,000 m3 of Grade 40, slump 200mm concrete supplied to MTRC Contract 612 for walls for the cut and cover tunnel. Contractors were Gammon and China State JV and the concrete supplier was Gammon Ready mixed Concrete. Other projects which have used polycarboxylate based Superplasticisers in Hong Kong include a residential high rise building located at Good Shepherd Road, Ho Man Tin. For the columns approximately 1600 cubic metres of Grade 60 concrete was used, placed by pumping at 200mm slump. Consultant was B.L. Wong & Partners. Contractor was China State Engineering Corporation, who also own the building. Concrete supplier was Hong Kong Concrete Co. Ltd. In Singapore polycarboxylate admixtures have been successfully used in high performance concrete for the construction of the MHA Complex at Irradwaddy Road, with Takenaka Corporation as main contractor. Also the East West Line MRT Contract 1217 for the L TA, and the undersea cable trench project at Jurong Island. Specifiers in the region are now recognizing that the use of this new admixture technology in concrete has many technical, environmental and overall cost benefits in civil engineering, building and underground construction requiring the most impermeable and watertight concrete with the lowest shrinkage. 7. References Ramachandran V.S. and Malhotra V.M., (1995). "Concrete Admixtures handbook Superplasticisers", p.410. Rixom M.R. and Mailvaganam N.P., (1986) "Chemical Admixtures for Concrete"2 nd edition; University Press. Payne J. and Dransfield J.M., (1985) "Development and Use of Superplasticisers"Civil Engineering, August. Payne J. and Kay W., (1995) "Specialist Products in High Performance Concrete", Structural Engineering, March. Tanaka Y., Matsuo S., Ohta A and Ueda M., (1996) " A new admixture for high performance concrete". Symposium, Tokyo and Dundee. Sugiyama T., Ohta A., and Tanaka Y.,(1995), "Synthesis of polycarboxylate cement dispersant and their dispersing mechanisms" Symposium Polymer Science, Osaka. Ohta A., Sugiyama T., Tanaka Y., (1996), "Dispersing mechanism of polycarboxylate-based dispersant for cement" Pacifichem, Hawai. Rossetti and Torresan, (1996), "Action mechanism of superplasticisers of the new generation"fip Naples. Torresan I. and Khurana R., (1998), "New high performance superplasticiser for elimination of steam curing in precast concrete", FIP98, Amsterdam. Ohta A. (1998), "Slump retention of new polycarboxylate-based Superplasticisers", Cement Technology Conference, Tokyo, May. Khurana R. (1998), "Admixtures for ready mixed high strength and durable concrete", ERMCO 1ih European Congress, June. Bartos P.J. and Grauers M. (1999), "Self-compacting Concrete", Concrete Journal. Khurana R. and Scabini S. (1998), "New generation of superplasticisers for long slump retention in ready-mixed concrete", ERMCO 1 ih European Congress. Payne J. Experience with new admixture technology in Hong Kong, USCT,