- Email: [email protected]
Influence of fly ash and silica fume on the consistency retention and compressive strength of concrete subjected to prolonged agitating
Construction and Building Materials 25 (2011) 1277–1281
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Influence of fly ash and silica fume on the consistency retention and compressive strength of concrete subjected to prolonged agitating Sß akir Erdog˘du ⇑, Caner Arslantürk, Sß irin Kurbetci Department of Civil Engineering, Karadeniz Technical University, 61080 Trabzon, Turkey
a r t i c l e
i n f o
Article history: Received 11 June 2009 Received in revised form 20 August 2010 Accepted 2 September 2010 Available online 29 September 2010 Keywords: Consistency Workability Slump loss Superplasticizer Fly ash Silica fume
a b s t r a c t Good workability at construction site is essential for high quality concrete since concretes of bad workability are prone to yield low strength and poor durability properties since placement and consolidation procedures cannot be performed properly. Nevertheless, this is what usually encountered in practice because slump loss is indispensable particularly when long delivery times are the case. This would be more pronounced when mineral additives are incorporated into the concretes. In this study, concretes of C25/30 class with fly ash and silica fume were produced and slumps were measured with time elapsed. At the end of each agitating period, the slumps of the mixtures were restored to the initial slumps using a superplasticizer and specimens were thereafter prepared for strength measurement. The effectiveness of using fly ash and silica fume in concrete in relation with slump loss was sought by determining the amount of superplasticizer used. Regardless of the mixture recipe, it can be concluded that as the total amount of cementitious materials increases in the concrete mix of the same initial slump, the slump loss with the elapsed time decreases. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction For ready-mixed concrete deliveries, prolonged agitating cannot be avoided in some cases such as breakdown of the truck mixer or flat tire, traffic jamming, long distance delivery or delays in placing, consolidation, and finishing operations [1]. Due to prolonged agitating, particularly in dry and hot weather conditions, part of mixing water in fresh concrete evaporates which in turn leads to increased slump loss. Slump loss appears to be one of the main reasons hinders concrete to possess enough strength and being durable as such concretes cannot be placed and consolidated properly. The depletion of mixing water is one of the main causes for slump loss in fresh concrete and it is theoretically involves chemical and physical processes. The loss of consistency during the dormant stage is mainly attributed to the physical coagulation of cement particles rather than to chemical processes. In the period in which the slump loss is occurring, the tricalcium aluminate reacts with gypsum. The product develops into crystalline structure and is distributed in the mass [2]. As a result, a certain amount of free water in fresh concrete mix is used up due to the hydration process of the cement and evaporation, leading to stiffening of the mixture [3,4]. There are of course other effects for slump loss like the depletion of superplasticizer in the pore solution. ⇑ Corresponding author. Tel.: +90 462 377 2051; fax: +90 462 377 2606. E-mail address: [email protected] (S ß. Erdog˘du). 0950-0618/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2010.09.024
When placement and consolidation cannot be performed properly, stiff concretes usually contain large pores and this in turn would result in a substantial decrease in strength [5]. Due to the friction between the ingredients of the concrete mixture arises during delivery and hydration of the cement may result in a temperature rise in the truck mixer which this in turn involves a reduction in free water of fresh concrete [6]. Concrete stiffens as free water reduces, which this in turn leads to a decrease in workability [7]. The practice of retempering is frequently performed to restore the initial slump and keep concrete workable at construction site in order to cope with the need for expediting the casting operations and reducing the consolidation effort. Retempering may be performed with water only or with a plasticizer or with a combination of water and plasticizer [8,9]. Retempering with water alone would result in a substantial strength loss since extra water increases the water to cement ratio of concrete mix [10]. However, retempering with a combination of water and a plasticizer would be beneficial in terms of the strength loss experienced [11]. Consistency improvement with plasticizers is obviously advantageous considering the strength gain of concrete; however, it is stated elsewhere [12–15] that the rheological properties of fresh concrete may vary depending on the type and amount of plasticizer used. This could be more complicated for concretes containing fly ash and silica fume particularly when long delivery times is the issue [16,17]. Ravina [16] in his work indicated that prolonged agitating of concrete has two technological aspects that should be kept in
Sß. Erdog˘du et al. / Construction and Building Materials 25 (2011) 1277–1281
1278
mind. One is slump loss; the other is the increase in compressive strength up to a certain mixing time. In this study, it is aimed to clarify the influence of fly ash and silica fume on the slump loss and/or consistency improvement and the strength of concrete subjected to prolonged agitating. Different combinations of concrete mixes were tested as compared to the usual practice of mixes to monitor the variation in relation with slump loss depending on agitation duration. To do this, mineral additives at different ratio were added to the mix instead of replacement as opposed to the common practice. Concrete mixes of C25/30 class having approximately 200 ± 10 mm initial slump with and without mineral additives of fly ash and silica fume were produced. At the end of each agitating period, 150 mm cubes from each mix were prepared for compressive strength measurements. Concretes were also produced to measure the standard 28-day compressive strength. The results obtained were compared with those of without fly ash and silica fume.
reducer at a ratio of 1% by weight of cement was used. The mix proportions of all concrete mixes, along with the water to binder ratios (w/b) are given in Table 2. All mixes were prepared in a laboratory at 22 ± 1 °C and with a R.H. of 70%. All materials used for the production of concrete mixes were weighed and stored in these conditions for 24 h prior to mixing. Concrete temperatures measured at the end of each agitating period were ranged between 25 ± 2 °C. The concrete mixes were initially mixed for 5 min in a mixer to ensure uniformity. To provide continuous agitation for concrete, a standard rotating drum mixer at a low speed was used; with a rotating speed of 4 rpm. Samples were taken after 5, 30, 60 and 90 min of agitation. Prior to sampling, at the end of each agitating period, a superplasticizer was used to restore to the initial slump measured at the end of 5 min of mixing. The drum was approximately run for 2 min more to agitate the fresh concrete to ensure uniformity before measuring slump. In case this was not accomplished in the first trial, a second trial was tried. The amounts of superplasticizer required for this process for each agitating period are given in Table 3. Specimens of 150-mm cubes were prepared for compressive strength testing. The cubes were compacted by light vibration on a vibrating table, covered with polyethylene sheeting and kept for 24 h in the same environment. The following day, the specimens were stored in a water tank at 21 ± 1 °C until tested for compressive strength at the age of 28 days. Strength was determined as the average of three specimens. For each mix recipe as given in Table 2, one set of concrete mix for slump test, and another set for addition superplasticizer requirement test were produced.
2. Experimental program and procedure
3. Test results and discussion
2.1. Materials used A Turkish Portland cement (similar to CEM I), Type F fly ash, silica fume and a crushed calcareous aggregate of 25 mm maximum size with a specific gravity of 2.65 were used in producing concrete. The chemical analysis and physical properties of silica fume, fly ash and cement, as provided by the manufacturer, are given in Table 1. For restoring slump at the end of each agitating period, an ASTM C 494 Type F melamine-based superplasticizer with a specific gravity of 1.21 was used. 2.2. Program layout and procedure In this study, 20% and 30% fly ash added, 25 kg of fly ash substituted (approximately 7%), and 10% silica fume added concrete mixes of C25/30 class with an initial slump of approximately 200 ± 10 mm were produced. The initial slump of all mixes was set at 200 ± 10 mm by adjusting the amount of mixing water. A normal water
Table 1 Chemical and physical properties of cement, silica fume and fly ash used. Composition (%)
Cement
Silica fume
Fly ash
CaO SiO2 Al2O3 Fe2O3 MgO SO3 Free CaO LOI Insoluble residue
63.41 20.22 5.67 2.91 0.96 2.92 1.20 3.32 0.93
1.09 86.66 0.25 0.65 7.98 1.61 – 4.75 –
3.08 55.18 19.55 10.58 5.86 0.70 – 1.04 –
Mineralogical compound (%) Tricalcium silicate Dicalcium silicate Tricalcium aluminate Tetracalcium aluminoferrite
51.2 16.7 10.1 8.9
– – – –
Physical properties Specific gravity Specific surface (Blaine) (cm2/g)
3.07 3564
2.40 –
3.1. Slump loss of concrete mixes with time elapsed The slump loss of each concrete mix measured at the end of each mixing period is shown in Fig. 1. As seen from the figure, slump loss for all mixes is increased as the mixing time elapsed. Depending on the mixing duration, concrete mix replaced with 25 kg of fly ash has the least residual slump, which is smaller than 25% of initial slump (50 mm) at the end of 90 min of agitation. This is followed by concrete mixes of 10% silica fume added and concrete mixes without mineral additive, respectively; with an average residual slump of only 40% of initial slump (80 mm) at the end of 90 min of agitation. Concrete mixes of 20% fly ash added have a residual slump of about 50% of initial slump (100 mm) at the end of 90 min of agitation. The residual slump is the highest for concrete mixes of 30% fly ash added, with a value approximately 65% of initial slump (130 mm) at the end of 90 min of agitation. From the illustration, it can clearly be seen that fly ash addition has a slump retention capacity and this may be attributed to the spherical shape of fly ash particles. Due to their spherical shape, fly ash particles reduce the internal friction between the ingredi-
Table 3 Admixture required for restoring the initial slump of mixes (kg/m3). Mix recipe
2.09 2550
Reference mix 20% FA added 30% FA added 7% FA replaced 10% SF added
Agitating duration (min) 30
60
90
3.1 1.5 0.7 3.5 3.3
4.6 2.3 1.2 4.7 4.2
5.4 3.1 2.6 5.6 4.8
Table 2 Ingredients, mix proportions and water/binder ratios of all concrete mixes. Mix recipe
Reference mix 20% FA added 30% FA added 7% FA replaced 10% SF added a
The quantities of materials used in concrete mixes (kg/m3) Water
Cement
Fly ash
Silica fume
Admixture
Aggregate
w/ba
224 214 227 212 229
350 350 350 325 350
– 70 105 25 –
– – – – 35
3.50 4.20 4.55 3.50 3.85
1705 1641 1561 1727 1652
0.65 0.52 0.51 0.62 0.60
The water content of the admixture is included in water/binder ratio (w/b).
Sß. Erdog˘du et al. / Construction and Building Materials 25 (2011) 1277–1281
with respect to cement and hence a great amount of water is needed to feed up hydration reactions. High water demand of silica fumed concrete can be counteracted by using a superplasticizer instead of using relatively large amount of additional water; greatly reduces the strength of concrete [19].
30 No mineral added 20% Fly ash added
25
30% Fly ash added 7% Fly ash substituted
Slump (cm)
3.2. Effect of slump restoring on compressive strength
10% Silica fume added
20
15
10
5
0
0
20
40
60
1279
80
100
Mixing Time (min) Fig. 1. Slump loss measured for at the end of different agitation period.
ents of concrete and this in turn results in a considerable increase in fluidity of the concrete mix [18]. On the other hand, the use of fly ash obviously increases the volume of paste in the mixture; this in turn improves workability greatly. Hence, slump loss may be taken under control somehow by adding fly ash to the concrete mix. The reason why the mix substituted with 25 kg of fly ash had the least residual slump would rather be due to lean and different paste content. In relation with slump retention, silica fume do not indicate the same trend compared to fly ash since a residual slump of 50% of initial slump (100 mm) is experienced even for 30 min of agitation. This may be attributed to the silica fume particles of being very small compared to cement particles. This means that the specific surface area of silica fume is rather large
The compressive strengths measured at the end of 28-day are illustrated in Fig. 2 as an average of three 150-mm cubes. The figure illustrates the compressive strengths of concrete cubes obtained at the end of each agitating period right after restoring the initial slump. As seen from the figure, concrete of replaced with 25 kg of fly ash gave the lowest initial compressive strength while silica fume added concretes gave the highest strengths. The 20% and 30% fly ash added concretes gave initial compressive strengths greater than that obtained from concretes without mineral additive. Additional fly ash increases the volume of paste phase in the concrete mix and hence this in turn obviously increases the workability of fresh concrete since the magnitude of frictional forces between the ingredients of concrete decrease. Due to better workability, concrete of high volume of paste having a high compressive strength is quite straightforward. Another reason for concrete of substituted with 25 kg of fly ash having lower strength compared with those of 20% and 30% fly ash added concretes may be attributed to quite different water to binder ratios that they have. While the water to binder ratios are 0.52 and 0.51 for concretes of 20% and 30% fly ash added, respectively; it is 0.62 for concrete replaced with 25 kg of fly ash. The 10% silica fume added concretes gave highest compressive strength compared to other mixes. This is because silica fume is a relatively fine mineral additive compared to cement and fly ash and hence it functions as filler in the concrete mixture [20]. On the other hand, the pozzolanic activity of silica fume has a relatively high contribution to the strength as well. Clearly, both of these properties make silica fume a must for producing high strength concrete [21,22]. The most important outcome of this illustration is that, compared to the initial compressive strengths, there is a slight increase in the compressive strength for all concrete mixes as the mixing time is elapsed. This might be attributed to the depletion of the
50
8.0
45
7.0
Amount of superplasticizer (kg/m³)
Compressive Strength (MPa)
No mineral added
40 35 30 No mineral added
25
20% Fly ash added 30% Fly ash added
20
7% Fly ash substituted 10% Silica fume added
15 10
0
20
40
60
20% Fly ash added 30% Fly ash added
6.0
7% Fly ash substituted 10% Silica fume added
5.0 4.0 3.0 2.0 1.0
80
Mixing Time (min) Fig. 2. The 28-day compressive strength of concrete mixes.
100
0.0
0
20
40
60
80
100
Mixing Time (min) Fig. 3. The amount of superplasticizer required for restoring the initial slump.
Sß. Erdog˘du et al. / Construction and Building Materials 25 (2011) 1277–1281
1280
6.0
Maximum mixing period: 90 min
Amount of superplasticizer (kg/m³)
5.0
Ambient temp.: 22°C Slump: 20±1cm
4.0
3.0
2.0
Regression equation: y = 0.38 x - 0.35 Corr. coefficient: r = 0.91
1.0
0.0
0
2
4
6
8
10
12
14
16
Slump loss (cm) Fig. 4. Statistical relationship between superplasticizer used and slump loss.
mixing water in fresh concrete due to hydration and evaporation prior to molding the specimens. The slump loss due to hydration and evaporation was compensated using a superplasticizer at the end of each agitating period without compromising the workability. This is because a reduction in mixing water or water to binder ratio would normally lead to an increase in strength as long as placing and consolidation procedures are properly performed. This is a good indication of how important it is to retemper concrete to its initial slump using a superplasticizer in order to have a compressive strength equal or greater than its initial strength. Another reason for increased compressive strength as time elapsed is that the air content in concrete mixes decreases for longer periods of mixing; this normally results in higher strengths, again as long as proper placing and consolidation procedures are put into operation [23]. 3.3. Amount of superplasticizer required for consistency improvement The amount of superplasticizer required to restore the initial slump of each concrete mix at the end of each agitating period is illustrated in Fig. 3. As seen from Fig. 1, the slump loss of each concrete mix increases as the mixing time is elapsed. Therefore, it is quite straightforward that, even with different quantities, an increased amount of super plasticizer is required for restoring the initial slump of each concrete mix. Based on this illustration, the quantities of super plasticizer required for restoring the initial slumps of concrete mixes with no mineral additive, 25 kg of fly ash replaced and 10% silica fume added is relatively greater than those needed for concrete mixes of 20% and 30% fly ash added. For instance, the amount of superplasticizer required for consistency improvement of concrete mix of 25 kg of fly ash replaced at the end of 60 min of agitation is about three times the amount required for concrete mix of 30% fly ash added. In other words, adding higher amount of fly ash to the concrete mix necessitates lesser amount of superplasticizer to restore the initial slump of the mix compared to mixes without mineral additive, with 7% fly ash replaced and 10% silica fume added. The amount of superplasticizer used for restoring the initial slump of concrete mix of 10% silica fume added up to an agitating period of 60 min is almost the same amount as used for concrete mix with no mineral additive
and for concrete of 25 kg of fly ash replaced. This might be attributed to the quick depletion of the mixing water in fresh concrete due to hydration as greater amount of silica fume participated in the reaction in unit time since the specific surface area of the silica fume is relatively large [20]. At the end of 90 min of agitating, however, the amount of superplasticizer needed for restoring the initial slump of concrete mix of 10% silica fume added is lesser than the amount of superplasticizer needed for concretes with no mineral additive and with 25 kg fly ash replaced (7% fly ash replacement). This clearly indicates that incorporating silica fume in concrete mix is beneficial in relation with slump retention particularly when long delivery times are into consideration. It can also be seen from the same graph that as the amount of fly ash addition to the concrete increases the slump retention increases as well. Fig. 4 provides an general overview on the relationship between the amounts of superplasticizer required to restore the initial slump of each concrete mix of different composition and slump loss for a maximum agitating period of 90 min. The mixes given here are different in composition; however, they are quite similar in workability. Therefore, it is possible to perform such a statistical evaluation to yield some overall conclusions regarding the work carried out. As seen from the graph, the relationship increases linearly with a statistically significant correlation coefficient of 0.91. In other words, the amount of superplasticizer used to restore the initial slump of concrete mixes with different composition is consistent with the slump loss, at least for an agitating period of 90 min. Based on the relationship, for example, for restoring a slump loss of 80 mm, which is the most usual case in practice, an amount of approximately 2.69 kg/m3 superplasticizer is needed for restoring the initial slump of about 200 mm. It should be reminded that this is an experimental finding of a laboratory condition therefore it may not surely represent the actual working conditions, let say, a ready-mixed concrete plant or a jobsite since there are several factors that may affect the extent of slump loss. For example, the air content of the concrete mix decreases when a laboratory mixer is used, and hence, additional amount of superplasticizer is needed to compensate for the slump loss due to the decreased air content. Therefore, the findings obtained are valid only within the experimental conditions of this study and have to be justified with the results obtained in the actual concreting conditions. The illustration is quite interesting in respect with the vast difference between the amounts of superplasticizer used to restore the initial slump of concrete mixes of different composition and it can also be seen that it is quite beneficial to incorporate as much fly ash as possible in concrete mix in order to reduce superplasticizer required for restoring the initial slump at the jobsite as long as strength requirement is met.
4. Concluding remarks Test results suggest that as the amount of fly ash increases in the concrete mix with a certain initial slump, then, the slump retention capacity of the fresh concrete mix increases as the agitating period increases. The rate of slump loss of concrete mix with 7% fly ash substituted is relatively high compared to the other concrete mixes such as 20% and 30% fly ash added. This would rather be due to lean and different paste content. A considerable increase in the compressive strength of concrete mixes is possible to obtain by using a superplasticizer to restore the initial slumps of concrete mixes as long as placing and consolidation procedures are properly performed. The strength gain obtained at the end of 90 min of agitation period ranges from 15% to 20% for all mixes. The amount of superplasticizer required for consistency improvement or for restoring the initial slump is directly related to the composition
Sß. Erdog˘du et al. / Construction and Building Materials 25 (2011) 1277–1281
of the concrete mix. For example, the amount of superplasticizer required for restoring the initial slump of 30% fly ash added concrete mix is relatively small with respect to those of with no mineral additive and with only 7% fly ash substituted concrete mixes. The relation between the amounts of superplasticizer used for restoring the initial slump of concrete different in composition is quite straightforward with a statistically significant correlation coefficient of 0.91. Based on the relationship, an amount of 2.69 kg/m3 of superplasticizer is required for restoring a slump loss of 80 mm. However, it should be pointed out that this is valid only for the laboratory conditions of this study rather than for being representative of construction site conditions. Therefore, the relationship has to be justified by the data obtained at the jobsite since the experimental conditions in the laboratory may differ quite a lot from those at the jobsite. References [1] Ravina D, Soroka I. Slump loss, compressive strength of concrete made with WRR and HRWR admixtures and subjected to prolonged agitating. Cem Concr Res 1994;24(8):1455–62. [2] Previte RW. Concrete slump loss. ACI J Proc 1977;74:361–7. [3] Soroka I, Ravina D. Hot weather concreting with admixtures. Cem Concr Comp 1998;20(2–3):129–36. [4] Chandra S, Björnström J. Influence of superplasticizer type and dosage on the slump of Portland cement mortars-Part II. Cem Concr Res 2002;32(10):1613–9. [5] Al-Gahtani HJ, Abbas AGF, Al-Amoudi OSB. Concrete mixture design for hot weather: experimental and statistical analyses. Mag Conc Res 1998;50:95–105. [6] Ravina D. Retempering of prolonged-mixed concrete with admixtures in hot weather. ACI J Proc 1975;72:291–5. [7] Meyer LM, Pernechio F. Theory of concrete slump loss as related to the use of chemical admixtures. ACI Concr Int 1979;1:36–43.
1281
[8] Ravina D. Slump retention of fly ash concrete with and without chemical admixtures. ACI Concr Int 1975;17:25–9. [9] Baskoca A, Ozkul MH, Artırma S. Effect of chemical admixtures on workability and strength properties of prolonged agitated concrete. Cem Concr Res 1998;28(5):737–47. [10] Alhozaimy AM. Effect of retempering on the compressive strength of readymixed concrete in hot-dry environments. Cem Concr Comp 2007;29(2):124–7. [11] Kırca Ö, Turanlı L, Erdog˘an TY. Effect of retempering on consistency and compressive strength of concrete subjected to prolonged agitating. Cem Concr Res 2002;32(3):441–5. [12] Erdog˘du Sß. Effect of retempering with superplasticizer admixtures on slump loss and compressive strength of concrete subjected to prolonged agitating. Cem Concr Res 2005;35(5):907–12. [13] Bonen D, Sarkar SL. The superplasticizer adsorption capacity of cement pastes, pore solution, and parameters affecting flow loss. Cem Concr Res 1995;25(7):1423–34. [14] Hanehara S, Yamada K. Interaction between cement and chemical admixture from the point of cement hydration, absorption behaviour of admixture, and paste rheology. Cem Concr Res 1999;29(8):1159–65. [15] Ozkul MH, Baskoca A, Artirma S. Influence of prolonged agitation on water movement related properties of water reducer and retarder admixture concretes. Cem Concr Res 1997;27(5):721–32. [16] Ravina D. Effect of prolonged agitating on compressive strength of concrete with and without fly ash and/or chemical admixtures. ACI Mater J 1996;93:451–6. [17] Duval R, Kadri EH. Influence of silica fume on the workability and the compressive strength of high-performance concretes. Cem Concr Res 1998;28(4):533–47. [18] ACI Committee 226.3R-87, Use of fly ash in concrete. ACI, Detroit; 1990. [19] Ganesh Babu K, Surya Prakash PV. Efficiency of silica fume in concrete. Cem Concr Res 1995;25(6):1273–83. [20] ACI Committee 226, Silica fume in concrete. ACI Mater J 1987;158–66. [21] Sarkar SL, Aitcin PC. Dissolution rate of silica fume in very high strength concrete. Cem Concr Res 1987;17(4):591–601. [22] Shannag MJ. High strength concrete containing natural pozzolan and silica fume. Cem Concr Comp 2000;30(3):399–406. [23] Neville AM. Properties of concrete. 3rd ed. England: Longman Scientific & Technical; 1986. p. 480.
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Influence of fly ash and silica fume on the consistency retention and compressive strength of concrete subjected to prolonged agitating Sß akir Erdog˘du ⇑, Caner Arslantürk, Sß irin Kurbetci Department of Civil Engineering, Karadeniz Technical University, 61080 Trabzon, Turkey
a r t i c l e
i n f o
Article history: Received 11 June 2009 Received in revised form 20 August 2010 Accepted 2 September 2010 Available online 29 September 2010 Keywords: Consistency Workability Slump loss Superplasticizer Fly ash Silica fume
a b s t r a c t Good workability at construction site is essential for high quality concrete since concretes of bad workability are prone to yield low strength and poor durability properties since placement and consolidation procedures cannot be performed properly. Nevertheless, this is what usually encountered in practice because slump loss is indispensable particularly when long delivery times are the case. This would be more pronounced when mineral additives are incorporated into the concretes. In this study, concretes of C25/30 class with fly ash and silica fume were produced and slumps were measured with time elapsed. At the end of each agitating period, the slumps of the mixtures were restored to the initial slumps using a superplasticizer and specimens were thereafter prepared for strength measurement. The effectiveness of using fly ash and silica fume in concrete in relation with slump loss was sought by determining the amount of superplasticizer used. Regardless of the mixture recipe, it can be concluded that as the total amount of cementitious materials increases in the concrete mix of the same initial slump, the slump loss with the elapsed time decreases. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction For ready-mixed concrete deliveries, prolonged agitating cannot be avoided in some cases such as breakdown of the truck mixer or flat tire, traffic jamming, long distance delivery or delays in placing, consolidation, and finishing operations [1]. Due to prolonged agitating, particularly in dry and hot weather conditions, part of mixing water in fresh concrete evaporates which in turn leads to increased slump loss. Slump loss appears to be one of the main reasons hinders concrete to possess enough strength and being durable as such concretes cannot be placed and consolidated properly. The depletion of mixing water is one of the main causes for slump loss in fresh concrete and it is theoretically involves chemical and physical processes. The loss of consistency during the dormant stage is mainly attributed to the physical coagulation of cement particles rather than to chemical processes. In the period in which the slump loss is occurring, the tricalcium aluminate reacts with gypsum. The product develops into crystalline structure and is distributed in the mass [2]. As a result, a certain amount of free water in fresh concrete mix is used up due to the hydration process of the cement and evaporation, leading to stiffening of the mixture [3,4]. There are of course other effects for slump loss like the depletion of superplasticizer in the pore solution. ⇑ Corresponding author. Tel.: +90 462 377 2051; fax: +90 462 377 2606. E-mail address: [email protected] (S ß. Erdog˘du). 0950-0618/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2010.09.024
When placement and consolidation cannot be performed properly, stiff concretes usually contain large pores and this in turn would result in a substantial decrease in strength [5]. Due to the friction between the ingredients of the concrete mixture arises during delivery and hydration of the cement may result in a temperature rise in the truck mixer which this in turn involves a reduction in free water of fresh concrete [6]. Concrete stiffens as free water reduces, which this in turn leads to a decrease in workability [7]. The practice of retempering is frequently performed to restore the initial slump and keep concrete workable at construction site in order to cope with the need for expediting the casting operations and reducing the consolidation effort. Retempering may be performed with water only or with a plasticizer or with a combination of water and plasticizer [8,9]. Retempering with water alone would result in a substantial strength loss since extra water increases the water to cement ratio of concrete mix [10]. However, retempering with a combination of water and a plasticizer would be beneficial in terms of the strength loss experienced [11]. Consistency improvement with plasticizers is obviously advantageous considering the strength gain of concrete; however, it is stated elsewhere [12–15] that the rheological properties of fresh concrete may vary depending on the type and amount of plasticizer used. This could be more complicated for concretes containing fly ash and silica fume particularly when long delivery times is the issue [16,17]. Ravina [16] in his work indicated that prolonged agitating of concrete has two technological aspects that should be kept in
Sß. Erdog˘du et al. / Construction and Building Materials 25 (2011) 1277–1281
1278
mind. One is slump loss; the other is the increase in compressive strength up to a certain mixing time. In this study, it is aimed to clarify the influence of fly ash and silica fume on the slump loss and/or consistency improvement and the strength of concrete subjected to prolonged agitating. Different combinations of concrete mixes were tested as compared to the usual practice of mixes to monitor the variation in relation with slump loss depending on agitation duration. To do this, mineral additives at different ratio were added to the mix instead of replacement as opposed to the common practice. Concrete mixes of C25/30 class having approximately 200 ± 10 mm initial slump with and without mineral additives of fly ash and silica fume were produced. At the end of each agitating period, 150 mm cubes from each mix were prepared for compressive strength measurements. Concretes were also produced to measure the standard 28-day compressive strength. The results obtained were compared with those of without fly ash and silica fume.
reducer at a ratio of 1% by weight of cement was used. The mix proportions of all concrete mixes, along with the water to binder ratios (w/b) are given in Table 2. All mixes were prepared in a laboratory at 22 ± 1 °C and with a R.H. of 70%. All materials used for the production of concrete mixes were weighed and stored in these conditions for 24 h prior to mixing. Concrete temperatures measured at the end of each agitating period were ranged between 25 ± 2 °C. The concrete mixes were initially mixed for 5 min in a mixer to ensure uniformity. To provide continuous agitation for concrete, a standard rotating drum mixer at a low speed was used; with a rotating speed of 4 rpm. Samples were taken after 5, 30, 60 and 90 min of agitation. Prior to sampling, at the end of each agitating period, a superplasticizer was used to restore to the initial slump measured at the end of 5 min of mixing. The drum was approximately run for 2 min more to agitate the fresh concrete to ensure uniformity before measuring slump. In case this was not accomplished in the first trial, a second trial was tried. The amounts of superplasticizer required for this process for each agitating period are given in Table 3. Specimens of 150-mm cubes were prepared for compressive strength testing. The cubes were compacted by light vibration on a vibrating table, covered with polyethylene sheeting and kept for 24 h in the same environment. The following day, the specimens were stored in a water tank at 21 ± 1 °C until tested for compressive strength at the age of 28 days. Strength was determined as the average of three specimens. For each mix recipe as given in Table 2, one set of concrete mix for slump test, and another set for addition superplasticizer requirement test were produced.
2. Experimental program and procedure
3. Test results and discussion
2.1. Materials used A Turkish Portland cement (similar to CEM I), Type F fly ash, silica fume and a crushed calcareous aggregate of 25 mm maximum size with a specific gravity of 2.65 were used in producing concrete. The chemical analysis and physical properties of silica fume, fly ash and cement, as provided by the manufacturer, are given in Table 1. For restoring slump at the end of each agitating period, an ASTM C 494 Type F melamine-based superplasticizer with a specific gravity of 1.21 was used. 2.2. Program layout and procedure In this study, 20% and 30% fly ash added, 25 kg of fly ash substituted (approximately 7%), and 10% silica fume added concrete mixes of C25/30 class with an initial slump of approximately 200 ± 10 mm were produced. The initial slump of all mixes was set at 200 ± 10 mm by adjusting the amount of mixing water. A normal water
Table 1 Chemical and physical properties of cement, silica fume and fly ash used. Composition (%)
Cement
Silica fume
Fly ash
CaO SiO2 Al2O3 Fe2O3 MgO SO3 Free CaO LOI Insoluble residue
63.41 20.22 5.67 2.91 0.96 2.92 1.20 3.32 0.93
1.09 86.66 0.25 0.65 7.98 1.61 – 4.75 –
3.08 55.18 19.55 10.58 5.86 0.70 – 1.04 –
Mineralogical compound (%) Tricalcium silicate Dicalcium silicate Tricalcium aluminate Tetracalcium aluminoferrite
51.2 16.7 10.1 8.9
– – – –
Physical properties Specific gravity Specific surface (Blaine) (cm2/g)
3.07 3564
2.40 –
3.1. Slump loss of concrete mixes with time elapsed The slump loss of each concrete mix measured at the end of each mixing period is shown in Fig. 1. As seen from the figure, slump loss for all mixes is increased as the mixing time elapsed. Depending on the mixing duration, concrete mix replaced with 25 kg of fly ash has the least residual slump, which is smaller than 25% of initial slump (50 mm) at the end of 90 min of agitation. This is followed by concrete mixes of 10% silica fume added and concrete mixes without mineral additive, respectively; with an average residual slump of only 40% of initial slump (80 mm) at the end of 90 min of agitation. Concrete mixes of 20% fly ash added have a residual slump of about 50% of initial slump (100 mm) at the end of 90 min of agitation. The residual slump is the highest for concrete mixes of 30% fly ash added, with a value approximately 65% of initial slump (130 mm) at the end of 90 min of agitation. From the illustration, it can clearly be seen that fly ash addition has a slump retention capacity and this may be attributed to the spherical shape of fly ash particles. Due to their spherical shape, fly ash particles reduce the internal friction between the ingredi-
Table 3 Admixture required for restoring the initial slump of mixes (kg/m3). Mix recipe
2.09 2550
Reference mix 20% FA added 30% FA added 7% FA replaced 10% SF added
Agitating duration (min) 30
60
90
3.1 1.5 0.7 3.5 3.3
4.6 2.3 1.2 4.7 4.2
5.4 3.1 2.6 5.6 4.8
Table 2 Ingredients, mix proportions and water/binder ratios of all concrete mixes. Mix recipe
Reference mix 20% FA added 30% FA added 7% FA replaced 10% SF added a
The quantities of materials used in concrete mixes (kg/m3) Water
Cement
Fly ash
Silica fume
Admixture
Aggregate
w/ba
224 214 227 212 229
350 350 350 325 350
– 70 105 25 –
– – – – 35
3.50 4.20 4.55 3.50 3.85
1705 1641 1561 1727 1652
0.65 0.52 0.51 0.62 0.60
The water content of the admixture is included in water/binder ratio (w/b).
Sß. Erdog˘du et al. / Construction and Building Materials 25 (2011) 1277–1281
with respect to cement and hence a great amount of water is needed to feed up hydration reactions. High water demand of silica fumed concrete can be counteracted by using a superplasticizer instead of using relatively large amount of additional water; greatly reduces the strength of concrete [19].
30 No mineral added 20% Fly ash added
25
30% Fly ash added 7% Fly ash substituted
Slump (cm)
3.2. Effect of slump restoring on compressive strength
10% Silica fume added
20
15
10
5
0
0
20
40
60
1279
80
100
Mixing Time (min) Fig. 1. Slump loss measured for at the end of different agitation period.
ents of concrete and this in turn results in a considerable increase in fluidity of the concrete mix [18]. On the other hand, the use of fly ash obviously increases the volume of paste in the mixture; this in turn improves workability greatly. Hence, slump loss may be taken under control somehow by adding fly ash to the concrete mix. The reason why the mix substituted with 25 kg of fly ash had the least residual slump would rather be due to lean and different paste content. In relation with slump retention, silica fume do not indicate the same trend compared to fly ash since a residual slump of 50% of initial slump (100 mm) is experienced even for 30 min of agitation. This may be attributed to the silica fume particles of being very small compared to cement particles. This means that the specific surface area of silica fume is rather large
The compressive strengths measured at the end of 28-day are illustrated in Fig. 2 as an average of three 150-mm cubes. The figure illustrates the compressive strengths of concrete cubes obtained at the end of each agitating period right after restoring the initial slump. As seen from the figure, concrete of replaced with 25 kg of fly ash gave the lowest initial compressive strength while silica fume added concretes gave the highest strengths. The 20% and 30% fly ash added concretes gave initial compressive strengths greater than that obtained from concretes without mineral additive. Additional fly ash increases the volume of paste phase in the concrete mix and hence this in turn obviously increases the workability of fresh concrete since the magnitude of frictional forces between the ingredients of concrete decrease. Due to better workability, concrete of high volume of paste having a high compressive strength is quite straightforward. Another reason for concrete of substituted with 25 kg of fly ash having lower strength compared with those of 20% and 30% fly ash added concretes may be attributed to quite different water to binder ratios that they have. While the water to binder ratios are 0.52 and 0.51 for concretes of 20% and 30% fly ash added, respectively; it is 0.62 for concrete replaced with 25 kg of fly ash. The 10% silica fume added concretes gave highest compressive strength compared to other mixes. This is because silica fume is a relatively fine mineral additive compared to cement and fly ash and hence it functions as filler in the concrete mixture [20]. On the other hand, the pozzolanic activity of silica fume has a relatively high contribution to the strength as well. Clearly, both of these properties make silica fume a must for producing high strength concrete [21,22]. The most important outcome of this illustration is that, compared to the initial compressive strengths, there is a slight increase in the compressive strength for all concrete mixes as the mixing time is elapsed. This might be attributed to the depletion of the
50
8.0
45
7.0
Amount of superplasticizer (kg/m³)
Compressive Strength (MPa)
No mineral added
40 35 30 No mineral added
25
20% Fly ash added 30% Fly ash added
20
7% Fly ash substituted 10% Silica fume added
15 10
0
20
40
60
20% Fly ash added 30% Fly ash added
6.0
7% Fly ash substituted 10% Silica fume added
5.0 4.0 3.0 2.0 1.0
80
Mixing Time (min) Fig. 2. The 28-day compressive strength of concrete mixes.
100
0.0
0
20
40
60
80
100
Mixing Time (min) Fig. 3. The amount of superplasticizer required for restoring the initial slump.
Sß. Erdog˘du et al. / Construction and Building Materials 25 (2011) 1277–1281
1280
6.0
Maximum mixing period: 90 min
Amount of superplasticizer (kg/m³)
5.0
Ambient temp.: 22°C Slump: 20±1cm
4.0
3.0
2.0
Regression equation: y = 0.38 x - 0.35 Corr. coefficient: r = 0.91
1.0
0.0
0
2
4
6
8
10
12
14
16
Slump loss (cm) Fig. 4. Statistical relationship between superplasticizer used and slump loss.
mixing water in fresh concrete due to hydration and evaporation prior to molding the specimens. The slump loss due to hydration and evaporation was compensated using a superplasticizer at the end of each agitating period without compromising the workability. This is because a reduction in mixing water or water to binder ratio would normally lead to an increase in strength as long as placing and consolidation procedures are properly performed. This is a good indication of how important it is to retemper concrete to its initial slump using a superplasticizer in order to have a compressive strength equal or greater than its initial strength. Another reason for increased compressive strength as time elapsed is that the air content in concrete mixes decreases for longer periods of mixing; this normally results in higher strengths, again as long as proper placing and consolidation procedures are put into operation [23]. 3.3. Amount of superplasticizer required for consistency improvement The amount of superplasticizer required to restore the initial slump of each concrete mix at the end of each agitating period is illustrated in Fig. 3. As seen from Fig. 1, the slump loss of each concrete mix increases as the mixing time is elapsed. Therefore, it is quite straightforward that, even with different quantities, an increased amount of super plasticizer is required for restoring the initial slump of each concrete mix. Based on this illustration, the quantities of super plasticizer required for restoring the initial slumps of concrete mixes with no mineral additive, 25 kg of fly ash replaced and 10% silica fume added is relatively greater than those needed for concrete mixes of 20% and 30% fly ash added. For instance, the amount of superplasticizer required for consistency improvement of concrete mix of 25 kg of fly ash replaced at the end of 60 min of agitation is about three times the amount required for concrete mix of 30% fly ash added. In other words, adding higher amount of fly ash to the concrete mix necessitates lesser amount of superplasticizer to restore the initial slump of the mix compared to mixes without mineral additive, with 7% fly ash replaced and 10% silica fume added. The amount of superplasticizer used for restoring the initial slump of concrete mix of 10% silica fume added up to an agitating period of 60 min is almost the same amount as used for concrete mix with no mineral additive
and for concrete of 25 kg of fly ash replaced. This might be attributed to the quick depletion of the mixing water in fresh concrete due to hydration as greater amount of silica fume participated in the reaction in unit time since the specific surface area of the silica fume is relatively large [20]. At the end of 90 min of agitating, however, the amount of superplasticizer needed for restoring the initial slump of concrete mix of 10% silica fume added is lesser than the amount of superplasticizer needed for concretes with no mineral additive and with 25 kg fly ash replaced (7% fly ash replacement). This clearly indicates that incorporating silica fume in concrete mix is beneficial in relation with slump retention particularly when long delivery times are into consideration. It can also be seen from the same graph that as the amount of fly ash addition to the concrete increases the slump retention increases as well. Fig. 4 provides an general overview on the relationship between the amounts of superplasticizer required to restore the initial slump of each concrete mix of different composition and slump loss for a maximum agitating period of 90 min. The mixes given here are different in composition; however, they are quite similar in workability. Therefore, it is possible to perform such a statistical evaluation to yield some overall conclusions regarding the work carried out. As seen from the graph, the relationship increases linearly with a statistically significant correlation coefficient of 0.91. In other words, the amount of superplasticizer used to restore the initial slump of concrete mixes with different composition is consistent with the slump loss, at least for an agitating period of 90 min. Based on the relationship, for example, for restoring a slump loss of 80 mm, which is the most usual case in practice, an amount of approximately 2.69 kg/m3 superplasticizer is needed for restoring the initial slump of about 200 mm. It should be reminded that this is an experimental finding of a laboratory condition therefore it may not surely represent the actual working conditions, let say, a ready-mixed concrete plant or a jobsite since there are several factors that may affect the extent of slump loss. For example, the air content of the concrete mix decreases when a laboratory mixer is used, and hence, additional amount of superplasticizer is needed to compensate for the slump loss due to the decreased air content. Therefore, the findings obtained are valid only within the experimental conditions of this study and have to be justified with the results obtained in the actual concreting conditions. The illustration is quite interesting in respect with the vast difference between the amounts of superplasticizer used to restore the initial slump of concrete mixes of different composition and it can also be seen that it is quite beneficial to incorporate as much fly ash as possible in concrete mix in order to reduce superplasticizer required for restoring the initial slump at the jobsite as long as strength requirement is met.
4. Concluding remarks Test results suggest that as the amount of fly ash increases in the concrete mix with a certain initial slump, then, the slump retention capacity of the fresh concrete mix increases as the agitating period increases. The rate of slump loss of concrete mix with 7% fly ash substituted is relatively high compared to the other concrete mixes such as 20% and 30% fly ash added. This would rather be due to lean and different paste content. A considerable increase in the compressive strength of concrete mixes is possible to obtain by using a superplasticizer to restore the initial slumps of concrete mixes as long as placing and consolidation procedures are properly performed. The strength gain obtained at the end of 90 min of agitation period ranges from 15% to 20% for all mixes. The amount of superplasticizer required for consistency improvement or for restoring the initial slump is directly related to the composition
Sß. Erdog˘du et al. / Construction and Building Materials 25 (2011) 1277–1281
of the concrete mix. For example, the amount of superplasticizer required for restoring the initial slump of 30% fly ash added concrete mix is relatively small with respect to those of with no mineral additive and with only 7% fly ash substituted concrete mixes. The relation between the amounts of superplasticizer used for restoring the initial slump of concrete different in composition is quite straightforward with a statistically significant correlation coefficient of 0.91. Based on the relationship, an amount of 2.69 kg/m3 of superplasticizer is required for restoring a slump loss of 80 mm. However, it should be pointed out that this is valid only for the laboratory conditions of this study rather than for being representative of construction site conditions. Therefore, the relationship has to be justified by the data obtained at the jobsite since the experimental conditions in the laboratory may differ quite a lot from those at the jobsite. References [1] Ravina D, Soroka I. Slump loss, compressive strength of concrete made with WRR and HRWR admixtures and subjected to prolonged agitating. Cem Concr Res 1994;24(8):1455–62. [2] Previte RW. Concrete slump loss. ACI J Proc 1977;74:361–7. [3] Soroka I, Ravina D. Hot weather concreting with admixtures. Cem Concr Comp 1998;20(2–3):129–36. [4] Chandra S, Björnström J. Influence of superplasticizer type and dosage on the slump of Portland cement mortars-Part II. Cem Concr Res 2002;32(10):1613–9. [5] Al-Gahtani HJ, Abbas AGF, Al-Amoudi OSB. Concrete mixture design for hot weather: experimental and statistical analyses. Mag Conc Res 1998;50:95–105. [6] Ravina D. Retempering of prolonged-mixed concrete with admixtures in hot weather. ACI J Proc 1975;72:291–5. [7] Meyer LM, Pernechio F. Theory of concrete slump loss as related to the use of chemical admixtures. ACI Concr Int 1979;1:36–43.
1281
[8] Ravina D. Slump retention of fly ash concrete with and without chemical admixtures. ACI Concr Int 1975;17:25–9. [9] Baskoca A, Ozkul MH, Artırma S. Effect of chemical admixtures on workability and strength properties of prolonged agitated concrete. Cem Concr Res 1998;28(5):737–47. [10] Alhozaimy AM. Effect of retempering on the compressive strength of readymixed concrete in hot-dry environments. Cem Concr Comp 2007;29(2):124–7. [11] Kırca Ö, Turanlı L, Erdog˘an TY. Effect of retempering on consistency and compressive strength of concrete subjected to prolonged agitating. Cem Concr Res 2002;32(3):441–5. [12] Erdog˘du Sß. Effect of retempering with superplasticizer admixtures on slump loss and compressive strength of concrete subjected to prolonged agitating. Cem Concr Res 2005;35(5):907–12. [13] Bonen D, Sarkar SL. The superplasticizer adsorption capacity of cement pastes, pore solution, and parameters affecting flow loss. Cem Concr Res 1995;25(7):1423–34. [14] Hanehara S, Yamada K. Interaction between cement and chemical admixture from the point of cement hydration, absorption behaviour of admixture, and paste rheology. Cem Concr Res 1999;29(8):1159–65. [15] Ozkul MH, Baskoca A, Artirma S. Influence of prolonged agitation on water movement related properties of water reducer and retarder admixture concretes. Cem Concr Res 1997;27(5):721–32. [16] Ravina D. Effect of prolonged agitating on compressive strength of concrete with and without fly ash and/or chemical admixtures. ACI Mater J 1996;93:451–6. [17] Duval R, Kadri EH. Influence of silica fume on the workability and the compressive strength of high-performance concretes. Cem Concr Res 1998;28(4):533–47. [18] ACI Committee 226.3R-87, Use of fly ash in concrete. ACI, Detroit; 1990. [19] Ganesh Babu K, Surya Prakash PV. Efficiency of silica fume in concrete. Cem Concr Res 1995;25(6):1273–83. [20] ACI Committee 226, Silica fume in concrete. ACI Mater J 1987;158–66. [21] Sarkar SL, Aitcin PC. Dissolution rate of silica fume in very high strength concrete. Cem Concr Res 1987;17(4):591–601. [22] Shannag MJ. High strength concrete containing natural pozzolan and silica fume. Cem Concr Comp 2000;30(3):399–406. [23] Neville AM. Properties of concrete. 3rd ed. England: Longman Scientific & Technical; 1986. p. 480.