STUDY ON OPTIMIZATION OF HIGH PERFORMANCE CONCRETE ADMIXTURES

Liu Junlong,Ma Haiyan,Li Qiang,Chen Shudong,Zhang Yunqing

(1.Collegeof Aerospace Engineering,NUAA,29 Yudao Street,Nanjing,210016,P.R.China;2.Department of Architecture and Civil Engineering,Zhejiang College of Construction,Hangzhou,311231,P.R.China)

INTRODUCTION

With the advancement of science and technology and the sustainable development,demands on high strength and excellent durability of concrete or concrete structure are increasing.High-strength concrete and high performance concrete(HPC)attract world-wide attention and serve as significant construction materials for many great buildings such as highway bridges,seafloor tunnels and dams[1-2].Water reducing agent,especially high range water reducing agent is an important component of high-strength concrete and HPC[3-6].However,in the selection of HPC admixtures,which one should be chosen,naphthalene-based water reducer or polycarboxylate-based water reducer? In prefabrication of concrete works,people always want to improve the concrete strength by using accelerating agent.But what is the effect?

The influences of naphthalene series high range water reducing agent and polycarboxlic series of high performance water reducer on the strength and the vapor-cured characteristics of concrete added with sodium sulfate hardening accelerating agent are discussed and specified.

1 EXPERIMENT

1.1 Raw materials

(1)Cement: Portland cement P.II 52.5R produced by Dongguan Huarun Cement Manufactory in Guangdong,whose physical and mechanical properties are tested and shown in Table 1.The chemical composition of cement clinker is listed in Table 2.

(2)Coarse aggregates: crushed granite,whose maximum particle size is 16 mm and packing density and apparent density are 1 410 kg/m3and 2 565 kg/m3.

Table 1 Properties of cement

Table 2 Chemical composition of cement clinker %(in weight)

(3)Fine aggregates: river sand, whose packing density and apparent density are 1 480 kg/m3and 2 605 kg/m3.Its mud content is less than 1%and fineness modulus is 2.55,Gradation II,belonging to medium sand.

(4)Mineral admix tures: grade I fly ash produced by Shenzhen Shajiao Thermal Power Plant,S95 grade of ground blast furnace slag produced by Lianda Gaoxin Building Materials Factory in Pingxiang,and micro silicon powder(Aiken)produced in Shanghai,in which the content of SiO2 is greater than 90%. The performance indexes of fly ash and fine-slag are shown in Table 3.

(5)Water reducing agents:Maidi MNF-3 accelerating water reducing agent(powder)produced in Shenzhen,JM-B Naphthalene series high range water reducing agent(powder)produced by Jiangsu Bote New materials Co.Ltd.and JM-PCA(I)polycarboxlic series of high range water reducing agent(aqueous solution agent)produced by Jiangsu Bote New materials Co.Ltd.The water reducing rate of the first one possibly achieves above 12%,and the content of sodium sulfate in it is more than 14.5%.This admixture is formed by Naphthalene series high range water reducing agent and anhydrous sodium sulfate which contains a large quantity of unevenly-mixed white aggregates of sodium sulfate. The water reducing rate of the second one is above 20%,and the content of sodium sulfate and the chloride ion in it are less than 3%and 0.05%.The last one contains low alkalinity and no chloride ion,and its water reducing rate is more than 35%.

(6) Hardening accelerating agents:anhydrous sodium sulfate,white powder.

(7)Water:Tap water.

1.2 M ix proportions

Table 4 shows the mix proportions of HPC,for different samples as No.1,2,3,4,5,which are added with JM-B Naphthalene series high range water reducing agent, MNF-3 hardening accelerating and water reducing agent,JM-PCA(I)polycarboxlic series of high range water reducing agent and hardening accelerating agent.The test aims to find out the effects of naphthalene-based water reducer and polycarboxylate-based water reducer,analyze the harm of sodium sulfate hardening accelerating agent and determine the principles for selection of HPCadmixtures.

Table 3 Perf ormance indexes of fly ash and ground blast furnace slag

Table 4 Mix proportions of HPC

1.3 Test method

The raw materials are mixed by a compulsory mixer,then the fresh state slump and the 1 h slump are measured. Afterwards, concrete prisms with size of 100 mm× 100 mm× 400 mm are prepared and sealed with plastic sheets.They are then separated into two groups for different kinds of curing conditions. The steam cured group is firstly put into a tunnel kiln for steam curing,during which fast curing system is adopted,namely the specimens are put directly in steam curing at a constant temperature of 85—90°C for 4 h,without the periods of increasing and decreasing temperature.Then the specimens are taken out of the kiln and demoulded,afterwards they are standardly cured for 28 d.The standard cured group is demoulded after 24 h in room temperature and then standardly cured for 28 d. The standard curing condition is saturated Ca(OH)2 solution at the temperature of 20°C. The compressive strength is then determined by a 2 000 kN press machine,and the values are modified by a mutilation of 0.95 to be translated to the standard cubic compressive strength.

2 RESULTSAND DISCUSSIONS

2.1 Effects of admixtures on workability of HPC

Table 5 shows the initial and 1 h slump of concretes with different admixtures.The results demonstrate that for No.1,2,4,5 concretes,although the water-to-binder ratios are similar,the slump and slump loss are different.When naphthalene-based water reducer with high content of sodium sulfate or sodium sulfate hardening accelerating agent is added,the slumps of HPCare smaller and the speed of loss is faster.Although the water-to-binder ratio is relatively low,the workability of HPC with polycarboxlic series of high performance water reducer is superior to the one with naphthalene series high range water reducing agent and its initial slumps and slump loss are smaller. Therefore,when preparing for HPC,the priority should be given to polycarboxylate-based water reducer. If naphthalene series high range water reducing agent is added,the content of sodium sulfate must be limited.

Table 5 Initial and 1 h slumps of HPC mm

2.2 Effects of admixtures on compressive strength of standard cured HPC

Fig.1 shows the compressive strength of standard cured HPC which added with different admixtures. The results demonstrate that for No.1,2,4,5 concretes,which have the similar water-to-binder ratio,the strengths of HPC are quite different.When naphthalene-based water reducer with high sodium sulfate or sodium sulfate hardening accelerating agent is added,the compressive strength of HPC decreases.It is the reason why international bid was invited for concrete admixtures in the year when China introduced the complete sets of technology of Daya Bay Nuclear Power Station in Shenzhen from abroad.One of the key technical indexes for naphthalene series high range water reducing agent was that the content of sodium sulfate must be limited strictly.At that time,no admixtures in China could meet the bid demand,so investors had to import a large quantity of high rang ewater reducing agents from foreign countries.And it has been an important event in the development of admixtures in China.

Fig.1 Compressive strength of standard cured HPC

HPC with polycarboxlic series of high range water reducing agent can achieve a much lower water-to-binder ratio while its strength is higher.Therefore,for HPC polycarboxlic series of high performance water reducer should be given priority.

2.3 Effects of admixtures on compressive strength of steam cured HPC

Fig.2 shows the compressive strength of steam cured HPC samples of No.1,3,4 and 5.Results show that compared with strength of the standard cured specimens,as shown in Fig.1,steam curing reduces the compressive strength of HPC.When concrete specimens are added with naphthalene series high range water reducing agent,the compressive strength decreases by 14%.When concrete specimens are added with polycarboxylate-based water reducer, the compressive strength reduces by 19%.When 5%sodium sulfate hardening accelerating agent is added,the compressive strength of steam cured HPC shows no enhancement compared with the corresponding standard cured specimens. It demonstrates that when preparing HPC,the use of hardening accelerating agent should be prudent and not used simultaneously with naphthalene-based water reducer. It is naphthalene-based water reducer with high concentration and low sodium sulfate content that is recommended.Therefore,for steam cured HPC naphthalene series high range water reducing agent with low sodium sulfate should be given priority,and polycarboxylate-based water reducer should not be added.

Fig.2 Compressive strength of steam cured HPC

3 CONCLUSIONS

(1)In order to ensure the workability and high strength of HPC,polycarboxlic series of high performance water reducer should be given priority.If naphthalene series high range water reducing agent is added,the content of sodium sulfate in it should be strictly limited.

(2)For steam cured HPC,naphthalene series high range water reducing agent with low sodium sulfate should be given priority and polycarboxylate-based water reducer should not be added.

(3)Sodium sulfate hardening accelerating agent cannot increase the strength of HPC,but reduce it.So the use of this kind of early strength should be prudent.

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