Efficiency of lightweight aggregates for internal curing ... - Springer Link

Materials and Structures/Mat6riaux et Constructions, Vol. 35, March 2002, pp 97-101

Efficiency of lightweight aggregates for internal curing of high strength concrete to eliminate autogenous shrinkage S. Zhutovsky, K. Kovler and A. Bentur Technion - IsraelInstitute of Technology, Israel

Paper received:March 12, 2001; Paperaccepted:June 17, 2001

A B S T R A C T

R I~ S U M f:

High Strength Concretes (HSC) with extremely low binder (w/b) ratios are characterized by high cracking sensitivity which is a consequence of increased autogenous shrinkage. The major reason for autogenous shrinkage - self-desiccation - cannot be eliminated by traditional curing methods. The application of the concept of internal curing by means of saturated lightweight aggregate was applied and shown to be effective in eliminating autogenous shrinkage. The present paper describes an approach to optimize the size arid porosity of the lightweight aggregate to obtain effective internal curing with a minimum content of such aggregate.

Les b~tons de haute r&istance qui ont un rapport eau/liant extr~mement bas, sont caract&is& par une haute sensibilit~ a la fissuration, qui est l'une des consdquences d'un retrait endog~ne plus important. La raison principale du retrait endog~ne - l'auto-dessiccation - ne peut ~tre ~lirain& par les m~thodes conventionnelles de cure. L'application du concept de la cure interne au moyen de granulats l~gers satur& a dt~ pratiqu& et s'est av&& effective pour ~liminer le retrait endog~ne. Cet article d&rit une approche visant a am~liorer la taille et la porosit~ du granulat l~ger afin d'obtenir une cure interne effective avec un volume minimum d' un tel granulat.

1. I N T R O D U C T I O N

means to control the effectiveness ofautogenous curing. It is well known that self-desiccation is induced by the emptying of pores due to chemical shrinkage of the hydrated water. Hence, the amount of water required in the internal reservoirs of the lightweight aggregates to completely eliminate self-desiccation can be calculated from chemical shrinkage as follows [8]:

water to

Self-desiccation is a phenomenon inherent to high strength concretes, HSC, with low water/binder, w/b, matrix. As a consequence, HSC exhibits substantial autogenous shrinkage, which can lead to early-age cracking in restrained components. In practice, components are restrained, to one degree or another. An effective strategy to overcome this problem is the use of presoaked lightweight aggregates as internal water reservoirs. This concept was explored by several investigators [i-7] and was assessed quantitatively with respect to strength development and elimination of autogenous shrinkage and the resulting stresses under restrained conditions. In these studies the primary emphasis was placed upon an investigation of the effects of the replacement level of normal weight coarse aggregates by saturated lightweight ones, and the degree of water saturation of the lightweight aggregate. These parameters provide the

Wcur =C'O~max "CS

where: Wcur - water content (kg/m3); C - cement content (kg/m3); ~max -maximum degree of hydration; CS - chemical shrinkage (kg water/kg cement hydrated). Studies reported in recent years suggested that the amount of water within the aggregates which was required for the elimination of self-desiccation and autogenous shrinkage was considerably higher than that calculated from Equation (1). Calculations of the content of internal water curing based on Equation (1) provide val-

~AS: 'E~ (TAC): ~2.TDF

1359-5997/02 9 1LILEM

(1)

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Materials and Structures/Mat~riaux et Constructions,Vol. 35, March 2002

ues in the range of 18 to 23kg/m 3 for the systems reported in references [3-7]. However, in none of them was this content sufficient, and the levels required were at least 30 to 40 kg/m 3 or more. The explanation for this apparent discrepancy is that not all the water in the aggregates can become effective to counteract self-desiccation. Several factors can be considered, such as: (i) Aggregate pore size: if it is very fine water may not migrate readily into the surrounding paste, and (ii) The spacing between the aggregate particles: if it is too large the paste surrounding the aggregates may not be accessible to the water in the aggregate within a reasonable time (this limitation can be quantified in terms of the diffusivity of the water into the surrounding matrix which becomes denser with time). These influences may be expressed in a simplified engineering approach in terms of an efficiency term, q , which is a factor in the range of 0 to 1, describing the portion of water in the aggregate which can become available for internal curing. In the view of the above, this factor is the result of complex function and is not only dependent on the properties of the aggregate. Accordingly, the content of lightweight aggregate, LWA, in units of kg per m 3 of concrete required to eliminate self-desiccation by the internal water in the aggregates can be calculated:

LWA=

W~ur ep.s. n

2. E X P E R I M E N T A L 2.1 Lightweight aggregate for internal curing

The lightweight aggregate used was pumice sand, which was sieved and divided into three fractions: 0.15 m m < d < 1.18 m m ("Pumice0"), 1.18 m m < d < 2.36 ("Pumice1") and 2.36 m m < d < 4.75 m m ("Pumice2"). The pore content of the pumice was evaluated by soaking in boiling water, and measuring the absorption as a function of time. Saturation was reached in about 48 hours, and the absorption values after 120 hours of absorption are provided in Table 1. It is of interest to note the proportionality between the size and pore content: the larger size particle possesses larger porosity. This is quite characteristic to foamed aggregates, where larger and coarser pore structure can be built into the bigger aggregate. Table 1 - Absorption valued of the lightweight aggregates after 120 hours of soaking Absorption Aggregate %weight %volume 17.3 Pumice0 13.0

(2)

where: - aggregate water absorption by weight (kg water per kg of dry aggregate); S - degree of saturation of aggregate; 11 - efficiency factor (i.e. the fraction of water absorbed in a saturated aggregate that can become effective to counteract self desiccation). Ideally, one would like to develop aggregates for internal curing where the efficiency factor, q, is 1, and the water absorption is as high as possible, to minimize the aggregate content required to obtain effective internal curing. To achieve this, one would resort to aggregates of a small size and a pore structure which is large and coarse. Small size of particles would minimize the distance between the reservoirs, making the paste volume to be more accessible to the water in the internal reservoirs. Coarse pore structure of the aggregate is beneficial to make the water more readily dischargeable into the surrounding paste. A large pore volume in the aggregate may have two opposing influences: it may enable to minimize the lightweight aggregate content, but at the same time it will increase the spacing between the aggregate. Thus, from the point of view of the effectiveness of the aggregate, there may be an optimal pore volume for a given aggregate size. The object of the present study was to explore the possibility to obtain lightweight aggregate having maxim u m internal curing efficiency (i.e. q =1), that could effectively eliminate autogenous shrinkage, but with an aggregate content which is as small as possible, without detrimental effects on strength. In view of the considerations outlined above the size range explored was that of fine aggregates, smaller than 5 mm.

PumiceI

19.0

24.9

Pumice2

26.7

32.3

2.2 Variables studied

The theoretical amount of water required to compensate for self-desiccation can be calculated from Equation (1). For the mixes described below, which were studied for 7 days of sealed curing, the water needed for compensating for self desiccation at this time can be calculated from Equation (1), using a degree of hydration of 65%, which was determined to be the approximate value for the systems studied here. The level of chemical shrinkage reported in literature [9] can be taken as 0.06 kg water per kg cement hydrated. Thus, based on eq.1, the following amount of water for efficient internal curing was calculated for the mixes studied here, which had a cement content of 506 kg/m3: W~,,e, = 506(kgcement).0.65.0.06 kg water ,~20kg water kg cementhydrated Two types of variables were evaluated in this study: (a) Mixes with fully saturated pumice aggregates of different sizes, added at a content which provided internal water content in the concrete of 20 kg/m 3. (19) Mfixes with the larger size pumice aggregate, pumice2, which was added in a fully saturated state. The content was such that it provided internal water content of 10, 20 and 30 kg/m3 (i.e. 50%, 100% and 150% of the water required for full compensation of the self-desiccation).

98

Zhutovsky, Kovler, Bentur

2.3 Concrete composition

Table 2 - Mix proportion of concretes (kg/m3) * Cement Mix Water

Sand

Gravel

Pumice

HSC mixtures with w/b ratio of 0.33 were Reference (REF) 506 167 572 1145 0 tested. The cement was ASTM type 1. The coarse aggregate was crushed dolomite of 2.36 Reference (WSAREF) 506 167 574 1162 0 (572) (1145) mm < d < 9.5 m m with water absorption capacity 1.5% by weight. The fine aggregate was natural PumiceO+2OkgWater 506 167 268 1162 174 "eumiceO (20)" (267) (1145) (154) sea sand with grain size below 0.6 m m and water absorption capacity 0.4% by weight. The superPumicel+2Okg Water 506 167 362 1162 125 plastisizer used was of the naphthalene formalde"Pumice1 (20)" (361) (1145) (105) hyde sulfonate type at a content of 1.5% by Pumice2+lOkg Water 506 167 492 1162 48 weight of cement. In the mixes with lightweight "Pumice2 (10)" (490) (1145) (38) aggregates, sand (in saturated surface dry, SSD, Pumice2+2OkgWater 506 167 410 1162 95 conditions) was replaced with equal volume of "Pumice2 (20)" (408) (1145) (75) lightweight aggregate (in SSD conditions). Pumice2+3OkgWater 506 167 328 1162 143 Two reference mixes were produced and "Pumice2 (30)" I (327) (1145) (113) tested, without any lightweight aggregate. In Values in parentheses are dry weights the first mix, the aggregates were air dry (REF) and in the second one they were presoaked to SSD conditions (WSAREF). The amount of absorbed water in the normal aggregate in the WSAREF 3. RESULTS mix is 19 kg/m 3. Mix proportions are given in Table 2. 3.1 Free shrinkage of reference mixes For the evaluation of size effect, Pumice0, Pumice1 and Pumice2 aggregates were used. Their content was The two reference mixes with dry and SSD aggreadjusted so that they contained the same water content of gates contained the same mixing water content. The 20 kg per cubic meter. This adjusted content corresponds shrinkage-time curves are shown in Fig. 1. to 53.0%, 36.0% and 28.6% sand replacement by volume, Both specimens show early expansion, followed by respectively. Mix proportions are given in Table 2. autogenous shrinkage. Early expansion is frequently The effect of the total water content in the lightreported even in isothermal tests. It could be the result weight aggregates was studied for one lightweight aggreof deviation from strict isothermal conditions at early age gate size, Pumice2. Calculated water quantities of 10, 20 where heat liberation is particularly intense, and it could and 30 kg per cubic meter of concrete were introduced be the consequence of re-absorption of bleed water with Pumice2, which corresponds to replacement perimmediately after setting. centages of 14.3%, 28.6% and 42.9% sand replacement The higher expansion and lower autogenous shrinkby volume respectively. age of WSAREF can be accounted for by the absorbed water c o n t e n t in the normal weight aggregate. Theoretically, the content of 19 kg/m 3 of this absorbed 2.4 Testing procedures water is sufficient to eliminate self-desiccation and autogenous shrinkage. However, the fact that this does not Free shrinkage tests were conducted using the testing happen, and only part of the self-desiccation is compenapparatus described in [10], which consists of two paralsated for, suggests that not all of the absorbed water is lel molds of identical size connected to a computerized measuring system. This sys~0o tem allows deformation measurement immediately after casting. 50 Concrete was mixed in a pan mixer and directly cast into the molds of the testing apparatus. In order to minimize friction 0 between the mold and the specimen, a polyethylene film was placed between -S0 them. The specimens were cured in sealed conditions in a room at a constant temper-I00 ature of 30 _+1~ Free shrinkage in sealed conditions was determined in bar specimens with size of -150 24 48 72 96 120 144 168 40 x 40 x 1000 mm. Cube specimens of Time, hr 50 m m size were used to determine compressive strength. Detailed strength results Fig. 1 - Free shrinkage of sealed reference mixes with dry vs. water-saturated aggregate. will be reported in a future publication. 99

Materials and Structures/Mat6riaux et Constructions,Vol. 35, March 2002

4~

g

g ---

s

Time, hr

Fig. 2 - Free shrinkage of sealed pumice mixes with various grain size.

100

one may conclude that the efficiency factor q was almost I in the coarser aggregate, and smaller in the other ones, with 11 decreasing with decrease in aggregate size. From the point of view of the spacing between the aggregates, one would have expected a better performance from the smallest aggregate, since the calculated average spacing is 0.43, 1.54 and 3.68 m m for the Pumice0, Pumice1 and Pumice2, respectively. A smaller space between the aggregates would make it more likely that a higher volume of the cement paste will be accessible to water from this aggregate reservoir. The fact that the opposite was observed suggests that another factor is adversely affecting the efficiency here: perhaps a smaller pore size distribution in the aggregate, making it more difficult for water to diffuse outward. A study of the pore size distribution to validate such hypothesis is currently under way.

4o[

3.3 Effect of internal curing water content

2001

The effect of internal curing water content introduced with lightweight fine aggregate of a single size fraction, on the 40 early-age deformations, was studied for the 0 24 48 72 ~ 120 1~ 168 coarser aggregate Pumice2. This size Time, hr aggregate was chosen because its efficiency Fig. 3 - Free shrinkage of sealed Pumice2 mixes with different internal curing is about 1. The results in Fig. 3 can be water content. explained on the basis of this efficiency: increasing the content above 20 kg/m 3, which is the one available for internal curing. If self-desiccation is taken required theoretically for eliminating self-desiccation, as the value of shrinkage after the peak at early age does not change the shape of the curve, as the extra expansion, one can concluded that the 19 kg/m 3 of water is not needed. On the other hand, lower water absorbed water could compensate for only 43.4% of the content results in greater autogenous shrinkage, as there autogenous shrinkage. is not sufficient water to eliminate self-desiccation, -20.

'

~

3.2 Effect of the size of lightweight aggregate

4. CONCLUSIONS

The effect of the lightweight aggregate size for mixes in which the internal water content level is equal to that calculated for elimination of self-desiccation (i.e. 20 kg/m3) is shown in Fig. 2. If one estimates the level of autogenous shrinkage as the reduction in strain from the peak at the early swelling, than a correlation can be established between the aggregate size and its influence on reduction in the autogenous shrinkage: the best effect was observed with the coarser aggregate (Pumice2(20)) which almost eliminated the autogenous shrinkage. The smallest aggregate (Pumice0(20)) was the least effective. Since the content of internal water in all the systems with the lightweight aggregate were the same and equal to the calculated value needed to eliminate self-desiccation,

This experimental study clearly shows the benefits of using fine lightweight aggregate for autogenous curing. The efficiency of fine pumice aggregate, i.e. the fraction of absorbed water available for internal curing, can approach 1, i.e. 100% efficiency. This efficiency was achieved with pumice aggregate of size in the range of 2.36 to 4.75 m m with 26.7% absorption. Similar pumice aggregates with finer size and smaller absorption values were less effective. The differences in the effectiveness of the different size aggregates were discussed in terms of the spacing between the aggregates and their pore structure.

100

Zhutovsky, Kovler, Bentur [6] van Breugel, K., Outwerk, H. and de Vries, J., 'Effect of mixture composition and size effect on shrinkage of high strength concrete', Proceedings of International RILEM Workshop on Shrinkage of Concrete, Paris (2000). [7] Lura, P., van Breugel, K. and de Vries, H., 'Moisture exchange as a basic phenomenon to understand volume changes of lightweight aggregate concrete at early age', Proceedings of International RILEM Workshop on Shrinkage of Concrete, Paris (2000). [8] Bentz, D. P. and Snyder, K. A., 'Protected paste volume in concrete. Extension to internal curing using saturated lightweight fine aggregate', Cementand ConcreteResearch29 (1999) 1863-1867. [9] Geiker, M., 'Studies of Portland cement hydration: measurements of chemical shrinkage and a systematic evaluation of hydration curves by means of dispersion model', Ph.D. Thesis, Technical University of Denmark (1983). [10] Kovler, K., 'Testing system for determining the mechanical behavior of early age concrete under restrained and free uniaxial shrinkage', Mater. Strnct. 27 (1994) 324-330.

REFERENCES [1] Vaysburd, A. M., 'Durability of lightweight concrete bridges in severe environments', ConcreteInternational 18 (7) (1996) 33-38. [2] Weber, S. and Reinhardt, H.-W., 'A new generation of high performance concrete: Concrete with autogenous curing', Advanced Cement BasedMaterials 6 (1997) 59-68. [3] Takada, K., van Breugel, K., Koenders, E. A. B. and Kaptijn, N., 'Experimental evaluation ofautogenous shrinkage of lightweight aggregate concrete', Int. Workshop on Autogenous Shrinkage of Concrete,June 13-14, 1998, Hiroshima, Japan. [4] Bentur, A., Igarashi, S. and Kovler, K., 'Prevention ofautogenous shrinkage in high strength concrete by internal curing using wet lightweight aggregates', Accepted for publication, Cement and ConcreteResearchl

[5] Schwesinger, P. and Sickert, G., 'Reducing shrinkage in HPC by internal curing by using pre-soaked LWA', Proceedings of International Workshop on Control of Cracking in Early-Age Concrete, Tohoku University, Japan (2000).

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