Development of high-strength self-compacting concrete with fly ash: a four-step experimental methodology

R Gettu*, Universitat Politecnica de Catalunya Spain

J Izquierdo, Universitat Politecnica de Catalunya Spain P C C Gomes, Universitat Politecnica de Catalunya Spain

A Josa, Universitat Politecnica de Catalunya Spain  

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27th Conference on OUR WORLD IN CONCRETE & STRUCTURES: 29 - 30 August 2002, Singapore

Development of high-strength self-compacting concrete with fly ash: a four-step experimental methodology

R Gettu*, Universitat Politecnica de Catalunya Spain J Izquierdo, Universitat Politecnica de Catalunya Spain

pee Gomes, Universitat Politecnica de Catalunya Spain A Josa, Universitat Politecnica de Catalunya Spain

Abstract

The development of high-strength self-compacting fly-ash concrete is a positive contribution to sustainable c;oncrete technology . The present work details a mix design methodology for such· concretes based on four steps where simple test procedures are used . Self-compacting concrete with a 90-day compressive strength of about 100 MPa has been obtained.

keywords: self-compacting concrete, high strength concrete , mix design, fly ash

1. Introduction

According to Mehta (1], the three fundamental elements for supporting an environmentally-friendly concrete technology for sustainable development are the conservation of primary materials , the enhancement of the durability of concrete structures, and a holistic approach to the technology.

Regarding the conservation of primary materials, reductions in the consumption of cement, aggregates and water, along with the use of waste materials and industrial by-products , are the principal actions to be taken in order to reduce the utilization of non-renewable resources and the negative impact on the environment. Along these lines, Malhotra (2] has demonstrated that high­volume fly ash concrete is one of the best value-added uses of a waste material. The prolongation of the useful life of a structure obviously delays the need for the construction of a replacement. This can be achieved by developing concrete that is more durable and of higher performance. In this respect, the use of high strength concretes with low water-cement ratios (w/c) generally results in better structural performance. The holistic approach to concrete technology emphasizes the need to consider all the critical aspects of the fabrication, utilization and life of the material in order to reduce wasteful practices and environmental impact, while minimizing the cost-benefit ratio . The success of the holistic approach depends on the ease with which environmentally-friendly materials can be introduced , accepted and utilized in structures. This has to be promoted through education , research, standards development, modification of design codes and prototype demonstration projects .

The development of high-strength self-compacting concrete with fly ash is a positive contribution to the sustainability of concrete technology. The use of fly ash reduces the demand for cement, fine fillers and sand [3) , which are required in high quantities in self-compacting concrete (SCC). Moreover, the incorporation of fly ash also eliminates the need for viscosity-enhancing chemical admixtures. The low wlc of the concrete leads to higher durability, in addition to better mechanical integrity, of the structure. More importantly, defects due to improper or inadequate vibration for compacting the concrete , which often reduce the durability of the structure, are completely avoided [4] . For applying the holistic approach to SCC technology, several actions have to be initiated, which include the development of new testing and quality control procedures, liberalization of design codes with restrictions on the use of high quantities of fly ash and superplasticizers, and user education. On the

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other hand, there are obvious benefits in the use of SCC, including the reduction in noise pollution and health-related problems for construction workers, which are attractive in the holistic sense .

In the present work, a mix design procedure based on four steps, each of which consists of simple tests , is discussed and applied . Locally-available cement, aggregates and superplasticizer are employed, along with fly ash. The self-compactability is judged through tests, on the fresh concrete, for flowability (i.e. , slump flow and funnel flow time), ability to pass through gaps (i.e ., L-box flow and blocking) and absence of segregation (i .e., coarse aggregate density variation) . The compressive strength of the hardened concrete was measured at 7, 28 and 91 days to evaluate the maximum strengths that could be attained.

2. Mix Design Methodology

The methodology used for the mix design of the high-strength SCC (HSSCC) is an extension of an approach proposed by Toralles-Carbonari et at. [5] for high strength silica fume concrete and later modified by Gomes et al. [6] for SCC. Within this approach, concrete is considered as a two-phase material consisting of a paste that provides fluidity and cohesion, and an aggregate skeleton that provides the mechanical integrity. The procedure is outlined in Figure 1, where sp/c and fa/c denote, respectively, the solid superplasticizer and the fly ash dosages, with respect to the weight of the cement.

Component Selection (locally available and/or

environmentally-friendly materials) ~r ,Ir

I cement CEM I 52.5 R, w/c S; 0.4 I

aggregates: sand, gravel

~ Determination of superplasticizer type and

dosage (sp/c), and fly ash dosage (f/c)

•Paste Composition

I

~ Determination of the sand/aggregate ratio

•Aggregate Skeleton

I .. ...

Determination of the Mix Composition Through Tests with Varying Paste

I Volumes

Figure 1. Outline of the Mix Design Methodology

First of all, the w/c is maintained at a low value (i.e., w/c s; 0.4) to ensure high mechanical performance and durability; the w/c is progressively decreased from 0.4 if the desired strength level is not achieved. Within the first step, the superplasticizer type and dosage are fixed, and in the second step, the fly ash dosage is optimized . For these two steps, the optimization is based on achieving a paste with high fluidity and good cohesion (for avoiding segregation). In the third step, the proportions of the aggregate skeleton are obtained for a minimum void content in the dry uncompacted state. Generally, clean sand and gravel with a maximum aggregate size of 10-20 mm are used . For such a binary combination , only the optimum sand/gravel ratio has to be determined . In the final step, tests are performer:i on concrete mixes of varying paste volumes, all with the optimized cement paste and sand/gravel ratio, for determining the concrete composition that exhibits self-compactability and high compressive strength.

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2.1 Step 1: Superplasticizer dosage

In the first step, the Marsh cone test is used to determine the optimum superplasticizer dosage (sp/c). The test consists of determining the time needed for a certain volume of paste to flow through the Marsh cone for varying sp/c. Here, a metal cone with an aperture of 8 mm is used, as in Figure 2a, where 1 liter of paste is introduced and the time taken for 500 ml to flow out of it is measured. The flow times (T) are plotted in logarithmic scale with respect to splc and the optimum splc is defined as the saturation point beyond which the flow time does not decrease significantly (see Figure 2b). For an objective definition, the saturation point is taken as the dosage where the internal angle in the CUNe is 1400±100. This procedure defines the practical limit of the superplasticizer dosage in terms of paste fluidity [5-7]. It can also be used to compare the effectiveness of different superplasticizers when various choices are available.

Since the fly ash dosage is unknown in the first step, the tests are repeated for different fly ash dosages; for example, tests can be made on a reasonable range of falc in increments of 0.1.

(a) (b)

Saturation Point

.j o '" a. = 140 ± 10"

;,

... o

sp/c(%)

Figure 2. Determination of the optimum superplasticizer dosage

2.2 Setp 2: Fly ash dosage

The mini-slump test [8] is used in the second step to choose the optimum fly ash dosage and, consequently, the paste composition with the prescribed w/c. Here, the mold in the shape of a truncated cone, shown in Figure 3, is filled with paste. The mold is then lifted and the paste is allowed to flow over the base plate. The diameter of the final spread is measured, along with the (spread) time to reach a diameter of 115 mm (T115). The test is performed for pastes with different falc, all with the corresponding saturation superplasticizer dosages determined in the Marsh cone tests of Step 1. The paste with the maximum fly ash dosage that exhibits a spread diameter of 180±10 mm and a T 115­value of 2-3.5 seconds is chosen as the optimum. These limits for the mini-slump properties have been proposed [6] to ensure a paste with good fluidity and moderate cohesion.

(a) (b) 19 mm

~IH 1 ..6.4 mm

22.2 mm II \\ 15.9 mm1 ~ ~

/ \ !I'E E .....

.... It)

\ <Xi ! M

!

I- "I38.1 mm

Figure 3. Mini-slump test: (a) dimensions of mold and (b) final spread of paste

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---------

2.3 Step 3: Aggregate proportions

The minimum void content of the aggregate skeleton is determined by filling a large container (here, a 5-liter cylinder) with dry mixes of varying sand/gravel ratios, without compaction . The unit weight of the aggregate mix is determined in each case and the void content is calculated using the densities of sand and gravel. The combination with the minimum void content is taken as the optimum sand/gravel ratio . This procedure is expected to account for the shape, texture and granulometries of the aggregates [5] .

2.4 Step 4: Tests on concrete to determine the optimum composition

In the final step, concretes with different paste volumes (larger than the void content of the aggregate skeleton) and the optimum sand/gravel ratio are fabricated and tested . First of all , the self­compactability is evaluated on the fresh concrete through tests such as these used here: • The flowability is measured with the slump flow [9) (see Figure 4a) and the V-funnel tests [10) (see

Figure 4b) . The respective requisites are a final spread of 60-75 cm and a flow time of 10±3 s. Additionally, the time taken for the slump flow spread to reach 50 cm (T50) should be 5±2 s.

• The passing ability is determined using the L-box test [11] (see Figure 4c), where the specified requisites are the time needed to flow up to 20 cm (TL2o), which should be HO.5 s, the time needed to flow up to 40 cm (TL40) , which should be 2.5±0.5 s and a ratio between the heights of the concrete at each end or the blocking ratio (RB), which should be 2 O.B. The gap between the rebars is set here at 42 .5 mm.

• The resistance against segregation is determined using a U-shaped pipe of 16 cm diameter (see Figure 4d) through which the concrete is made to flow, without compaction. After the concrete sets, the pipe is opened and samples of about 10 cm thickness are extracted from different locations, and washed over a 5 mm sieve to remove the mortar. The remaining gravel is dried and the coarse aggregate content is determined for each sample. The segregation ratio (RS) is defined as the ratio between the lowest coarse aggregate content obtained and the theoretical coarse aggregate content. It is expected that RS 2 0.90 .

(a) (b)

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E (J

Q

'" .E

£ Cl

., ~

~d" 6.5cm 1'>

(c) (d)

, ' , 100 '

, ~ 570 t;

.:: . .:..:......................~..-.....-...~ ...................................:

\.,. i 'k/ ':: _______ .r h / i .:- ", ••' . ··'~·.•o······:,./..··\·i.····......·.. ·..···.. ···',·..:o:·"....·_...,<' . ' ..: ::a)' :1 ~: :

460 B(()

., Figure 4. Apparatus used in the self-compactability tests: (a) slump flow, (b) V-funnel for flow time

measurement, (c) L-box for checking blockage and (d) U-pipe for segregation evaluation

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600

570

In addition to the tests of self-compactability, tests can be performed to evaluate the compliance of the other requisites. Here, a minimum compressive strength of 50 MPa at the age of 7 days has been prescribed, and in addition the strengths at 28 and 90 days have also been determined. Standard 150x300 mm moist-cured cylinders, cast without any compaction, were used in these tests.

3. Materials Used

The materials used in the tests are a cement of type CEM I 52.5R (European Standard ENV 197­1 :92) , a vinyl copolymer superplasticizer (with a density of 1.14 glml and solid content 25 %), fly ash with the characteristics shown in Table 1, and crushed limestone sand and gravel (with a maximum aggregate size of 12 mm). The superplasticizer is always considered in terms of its solid or active component and its water content is accounted for in the w/c . Distilled water was employed in the tests of pastes, to avoid any influence of impurities.

Table 1. Characteristics of the fly ash

% Retained in 200 ~m sieve 0.24 % Passing through 63 ~m sieve 87.0 Blaine Surface area (cmLIQ) 2935 Density (g/cm~) 2.58 Water demand at normal consistency in Vicat penetration test

0.19

4. Results

In the application of the mix design method described earlier, Marsh cone and mini-slump tests are performed on pastes in Steps 1 and 2. The wlc ratio was set at 0.4, and Marsh cone flow times were determined for falc-values up to 0.6 and different sp/c. The results obtained for pastes with good fluidity (see Figure 5a) are shown in Figure 5b. In general, the increase in the fly ash dosage, falc, increases the flow time (i .e., an upward shift of the flow time curves), implying a decrease in the fluidity, as expected. The saturation splc, beyond which the flow time does not increase significantly, is indicated for each falc by the corresponding internal angle. The increase in falc tends to increase the saturation splc or the superplasticizer demand for maximum fluidity.

2.2

2 200

1.8 so'~ r~ ~ Qt 80 :§; ~ 1.6 60 '" e

OJ ~ 'I:E w/c =0.40 - Fly ash 40 ~'I: ].4 ¢

falc =0.0~ ~ c •E3 falc =0.3~ 1.2 20 ~ falc =0.4

-8 falc =0.5~ falc = 0.6 8+

0.8 (a) (b) 0 0.2 0.4 0.6 0.8 1.2

sp/c (%) - Vinyl mpolymer

Figure 5. Use of the Marsh cone test: (a) discontinuous flow in pastes with good fluidity and (b) flow time curves for varying fly ash dosages

The mini-slump tests were performed on pastes with each of the fa/c-values considered above, at the respective saturation superplasticizer dosages. The results are summarized in Table 2, where the

221

unit weight of each paste is also reported. Considering the range of 2-3.5 s prescribed for the spread time (T11S), the pastes with falc from 0.4 to 0.6 satisfy the requisite. Note that all the pastes had final spreads within the prescribed range of 170-190 mm. Since one of the objectives of this work is to maximize the use of fly ash, the paste with falc = 0.6 is considered as the optimum.

Table 2. Results of mini-slump tests

falc 0.3 0.4 0.5 0.6 saturation splc (%) 0.3 0.3 0.4 0.5 final spread (mm) 188 170 195 183 spread time T115 (s) 1.7 2.0 2.5 2.7 unit weiQht of paste (kQ/liter) 1.95 2.00 2.02 2.05

For optimizing the aggregate skeleton, the unit weights of dry mixes of sand and gravel with varying proportions were determined . Using the density of each component, the void content in each mix was computed. It was found that the minimum void content of 35% corresponded to a mix with the sand:gravel ratio of 52.5:47.5, which was considered as the optimum.

In the final step of the mix design procedure, concrete mixes with the optimized paste composition and the optimum sand:gravel ratio were fabricated with varying paste contents; the paste content was varied from 40 to 45%. The mix proportions and the corresponding properties are given in Table 3. Note that the water added includes that needed for saturating the aggregates for saturated surface-dry conditions, and excludes the water content of the superplasticizer and the humidity of the aggregates. It can be seen that the fresh density is in the nonnal range for concrete even though the paste volumes are relatively high. From the first two tests (Le., slump flow and V-funnel), to obtain a concrete with adequate flowability, the paste volume has to be higher than 43%, when the slump flow reaches the range of 60-75 cm, and the Tso and V-funnel flow times are within the prescribed ranges. In terms of the passing ability, the concretes with 43 and 45% paste volumes satisfy the requisites of flow times and blocking ratio in the L-box test. In the U-pipe test, only the concretes with higher paste volumes could flow through the pipe, and both exhibited good resistance against segregation (i.e., RS > 0.90).

Table 3. Trial concrete mixes and their properties

Mix proportions (l<g/111.1_ Paste voltlme (%) 40 42 43 45 Cement 407 428 438 458 Fly ash (fa/c =0.60) 244 257 263 275 Water added 181 188 191 190 Superplasticizer 7.5 7.9 8.1 8.4 Sand (0-5 mm) 815 788 774 748 Aggregate (5-12 mm) 761 736 723 698

Results of tests on fresh concrete Density (kQ/m1 2410 2404 2400 2398 Slump flow (cm 44 48 58 66 Slump flow time Tso (s) - - 4.1 3.6 V-funnel flow time (s) 31 13 10 12 L-box with a gap of 42.5 mm

TL20 (s) 2.8 1.8 1.3 1.3 TL40 (s) 6.3 4.3 2.9 3.0 RB (h2/h1) 0.0 0.2 0.8 0 .9

Segregation in U-pipe

RS - - 0.96 1.02

Compressive strength (I\/IPa) at 7 days 60.4 (± 1.4%) 63.1 (± 1.3%) 60 .7 (± 1.6%) 60 .8 (± 1.2%) at 28 days 70.4 (± 1.3%) 74 .5 (± 0.7%) 69 .5 (± 1.1 %) 68.2 (± 1.0%) at 91 days 96.2J:I- 2.7%) 100.1 (:I- 2 .8%l 100.5 (:+ 3.4%) 98 .6 J.+ 2.3%)

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The results of the compressive strengths obtained at the ages of 7, 28 and 91 days are also given in Table 3 as mean values and coefficients of variation (in %) obtained from 3 specimens. In general , the variability is low and the levels of strength achieved are satisfactory in all the mixes. This implies that the wlc does not have to be reduced further and can be maintained at 0.4.

Since the requisites in terms of self-compactability are more critical than the strength limit, it can be concluded that the concrete with a paste content of 45% can be considered as the optimum mix, as far as the present study is concerned. Further improvements can undoubtedly be made through trials based on this mix to maximize the fly ash dosage, minimize the superplasticizer and cement dosages, etc.

5. Conclusions

A mix design procedure for developing high-strength self-compacting concretes with fly ash has been presented . The procedure consists of four steps, all of which are based on Simple tests. Each step serves to fix one of the mix parameters and progressively optimize the composition of the concrete , considering it to be a paste-aggregate composite. The paste is optimized in the first two steps by using the Marsh cone and mini-slump tests, where the superplasticizer and fly ash dosages are chosen for a given water-cement ratio (i.e ., 0.4) . In the third step, the aggregate skeleton is optimized for a minimum void content in the dry uncompacted state. In the fourth and final step, tests are performed on concrete with varying paste volumes to evaluate the self-compactability and other requisites . In this step, the paste volume is chosen , defining the final concrete composition . The self­compactability of the fresh concrete is evaluated through the slump flow and V-funnel tests for flowability, the L-box test for passing ability and a U-pipe test for segregation resistance. In terms of the mechanical properties, the compressive strengths of the concretes obtained here surpass 50 MPa at 7 days and 90 MPa at the age of 91 days.

A positive contribution to the sustainable growth of concrete technology is made by the use of fly ash and low wlc to obtain high-strength concrete that can be placed with less noise-pollution and reduced health risks for the workers. Moreover, the properties of such concretes would lead to more durable structures. For a more holistic treatment of the applicability of such concretes , test standards for their characterization and quality control should be developed by considering methods such as those used here.

Acknowledgements

Dr. V.M. Malhotra suggested the use of fly ash in SCC to the first author in July 2001 , which motivated the present work and ongoing research at the UPC along these lines. Partial financial support from the CICYT projects P898-0298 and TRA99-0788, and the CICYT and European Commission FEDER grant 2FD97-1973-C02-02 to the UPC are gratefully appreciated . The materials used were donated by Cementos Molins, SIKA and Uniland Cementera.

References :

[1] Mehta, PK, Concrete Technology for Sustainable Development - An Overview of Essential Principles, Concrete Technology for Sustainable Development in the Twenty-First Century, Ed . P.K. Mehta, Cement Manufacturers' Association , New Delhi , India, 1999, 1-22.

[2] Malhotra, V.M., and Bilodeau, A. , High-Volume Fly Ash System: The Concrete Solution for Sustainable Development, Concrete Technology for Sustainable Development in the Twenty-First Century, Ed . P.K. Mehta, Cement Manufacturers' Association , New Delhi , India, 1999, 43-64.

[3] Khurana, R. , and Saccone, R., Fly Ash in Self-Compacting Concrete, Fly Ash, Silica Fume, Slag and Natural Pozzolans in Concrete, ACI SP-199, Ed. V.M. Malhotra, American Concrete Institute, Farmington Hills, Michigan, USA, 2001,259-274.

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[4] Okamura, H., Ozawa, K., and Ouchi, M., Self-Compacting Concrete, Structural Concrete, Vol. 1, No.1, 2000,3-17.

[5] Toralles-Carbonari, B., Gettu, R, Agull6, L., Aguado, A., and Aceiia, V., A Synthetic Approach for the Experimental Optimization of High Strength Concrete, Proc. 4th Intnl. Symp. on utilization of High Strength/High Performance Concrete, Eds. F. de Larrard and R. Lacroix, Presses ENPC, Paris, 1996,161-167.

[6] Gomes, P.C.C., Gettu, R, Agull6, L., and Bernad, C., Experimental Optimization of High-Strength Self-Compacting Concrete, Proc. Second Intnl. Symp. on Self-Compacting Concrete, Eds. K. Ozawa and M. Ouchi, COMS Engineering Corporation, Fukui, Kochi, Japan, 377-386.

[7] Agull6, L., Toralles-Carbonari, B., Gettu, R, and Aguado, A., Fluidity of Cement Pastes with Mineral Admixtures and Superplasticizer - A Study Based on the Marsh Cone Test, Mater. Struct., Vol. 32, 1999, 479-485.

[8] Kantro, D.L., Influence of Water Reducing Admixtures on Properties of Cement Pastes - A Miniature Slump Test, Cem. Concr. Aggregates, Vol. 2, 1980, 95-102.

[9] JSCE, Method of Test for the Slump Flow of Concrete, Standards of the Japan Soc. of Civil Engineers, F03, 1990.

[1 0] Ozawa, K., Sakata, N., and Okamura, H., Evaluation of Self Compactability of Fresh Concrete ­Using the Funnel Test, J. Japan Soc. of Civil Engineers, Vol. 23, No. 490, 1994, 71-80.

[11] Petersson, 0., Billberg, P., and Van, B.K., A Model for Self-Compacting Concrete, Production Methods and Workability of Concrete, Eds. P.J.M. Bartos, D.L. Marrs and D.J. Cleand, E&FN Spon, London, 1996,483-492.

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