WATER WITHIN LIGHTWEIGHT AGGREGATE CONCRETE AND ITS RELATION TO AUTOGENOUS SHRINKAGE

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1 WATER WITHIN LIGHTWEIGHT AGGREGATE CONCRETE AND ITS RELATION TO AUTOGENOUS SHRINKAGE S.A. Kristiawan Department of Civil Eng.Sebelas Maret University, Senot Sangadji Department of Civil Eng.Sebelas Maret University, Abstrat Autogenous shrinkage of lightweight aggregate onrete (LAC) has been investigated with the aims of studying if water within LAC is effetive in preventing autogenous shrinkage as suggested by Bentz s model. By alulating ratios of water supplied by lightweight aggregate (LA) at various degrees of saturation to water required for maximum hydration and plotting these against ultimate values of autogenous shrinkage, it seems that only when the ratio is high (above 3.5) then the water within LAC supplied from LA is immediately ready to fill the empty pores and in turn, reduing autogenous shrinkage. The ase is also onfirmed when ratios of total void (porosity) of onrete to total volume of water within onrete are plotted against autogenous shrinkage. Keywords: autogenous shrinkage, Bentz s model, lightweight aggregate, porosity. INTRODUCTION In the progress of hydration onrete onsumes moisture in the apillary pores when external uring is absent. This phenomenon, referred to self desiation, bring about internal relative humidity to drop and in turn gives rise to formation of water-air menisi. The presene of menisi auses tensile stresses and as a result onrete shrinks. The shrinking of onrete instigated by self desiation is termed as autogenous shrinkage. Autogenous shrinkage observed in high-strength onrete ould be in the magnitude of 700 mirostrain (Tazawa and Miyazawa, 1994). The high magnitude of this type of shrinkage will be a soure of problem in the utilisation of high-strength onrete in the struture. As the movement of strutural onrete by autogenous shrinkage is restrained, it will indue tensile stress leading to raking of onrete. A suggestion was proposed by previous investigators for examples Kohno et al (1999) and reently Jensen et al (2001) and Bentur et al (2001) to use lightweight aggregate with the intention of reduing autogenous shrinkage. Kohno showed that the redution depends on type, moisture ontent and quantity of lightweight aggregate. The explanation is that the spae originally oupied by ement partiles and water is gradually replaed by the spae filled by hydration produts. The spae not taken up by solid omponents suh as the unhydrated ement partiles or hydration produts onsists of apillary pores. At the early stage of hardening, most of the apillary pores and most of lightweight aggregate are fully saturated with water. As the hydration reation progresses, the apillary water are onsumed to form new and fine apillary pores. However, the internal relative humidity in the apillary pores is not lowered due to ontinuous supply of moisture from lightweight aggregate. As a result, the ement paste does not shrink. The relationship of moisture provided by lightweight aggregate and the amount of autogenous shrinkage redution is of interest. Bentz et al (1999) derived an equation for determining the replaement level of normal-weight aggregate by lightweight aggregate to ensure adequate water for omplete uring of the onrete. In this ase, omplete uring means that the ement reahes the maximum degree of hydration that is possible, given the spae limitations for forming hydration produts in low w/ ratio systems. The volume of water per ubi meter of onrete needed to be supplied by the lightweight fine aggregate (LWFA) depends on the mixture proportions of the onrete in the following manner. Let CS denote the hemial shrinkage ourring during the hydration of the ement; typially, this value is on the order of 0.06 kg H 2 O per kg ement hydrated. The amount of water neessary will depend on this quantity, as well as the ement ontent, C f in kg ement/m 3 onrete, and the w/ ratio for the mixture proportions. For w/ ratios below 0.40 (typial of an HPC), omplete hydration annot be ahieved and the maximum degree of hydration, MEDIA TEKNIK SIPIL/Juli 2006/83

2 α max, an be estimated as (w/)/0.40. Then, the volume of water, V wat that is onsumed during hydration due to hemial shrinkage is given by Eq. [1]: 3 3 C f CSα max V ( m water / m onrete ) =... [1] wat Where ρ is the density of water (1000 kg/m 3 ). Denoting the porosity of the LWFA by Φ LWFA and its saturation (0 1) by S, the total volume fration of LWFA neessary, V LWFA, is given by Eq. [2]: V LWFA V = wat S. Φ LWFA ρ [2] The ratio of this quantity to the volume fration of aggregate in the original mixture proportions is the required frational replaement by LWFA. This equation assumes that all of the water in the LWFA will be readily aessible to the surrounding ement paste. Furthermore, this equation implies that as long as internal moisture/water within onrete is higher than that onsumed during hydration autogenous shrinkage will be prevented. This researh observed autogenous shrinkage of lightweight aggregate onrete (LAC) at various lightweight ggregate (LA) degree of saturation. The results are then used to larify Bentz s model by assessing the amount of water available within LAC and that onsumed during hydration in relation to the magnitude of autogenous shrinkage. The results indiate that the higher available water within LAC ompared to that needed for hydration will not guarantee in preventing autogenous shrinkage. This is in ontrast to that assumed by Bentz s model. METHODS Conrete Ingredients and their proportion LAC investigated in this researh was made from mixing of Ordinary Portland Cement, sand, LA, gravel and water at proportion listed in Table 1. Absorption harateristi of LA used thorough the researh is about 13.73%. Casting and Preparation of Speimen Cylinder moulds made from PVC were prepared to ast onrete speimens. They were 75 mm in diameter and 275 mm in height. Inside the ylinder, a plasti sheet was attahed on the ylinder wall. At least, two speimens were ast for eah mix proportion. The speimens whih have been taken out from the ylinder moulds were then marked at the points where the Deme points would be plaed. For a single speimen eight Deme points were needed; four Deme points at equidistane of 90 o were loated on the upper part of ylinder (at a distane of 37.5 mm from the top) and another four were loated parallel to those of the upper at a gauge length of 200 mm. A referene bar was used to aurately set the Deme points at 200 mm gauge length. The Deme points were pasted on speimens using plasti steel paste. Measurement of Autogenous Shrinkage Autogenous shrinkage was measured using a Deme Gauge with resolution of 1 miron. Fig.1 shows the Deme Gauge used to test speimen. The proedure is as follows: Deme Gauge was positioned on a pair of Deme points at a gauge length of 200 mm. The reading of Deme Gauge was noted down. Five readings were taken and their average reading for that partiular pair of Deme points was alulated. This average reading was onsidered as the representation of the true reading for that pair of Deme points. The proedure was repeated on another three pairs of Deme points on the same speimen. The average reading from four pairs of Deme points on eah speimen was then alulated. The reading was taken at the age of 1, 3, 5, 7, 10, 14, 17, 21, 24 and 28 days. Table 1. Mix proportion for produing 1 m 3 LAC used in this researh Sample Identifiation Cement (kg) Sand (kg) LA (kg) Gravel (kg) water/ement ratio LAC %-0% LAC %-25% LAC %-50% LAC %-75% LAC %-100% LA degree of saturation % 0% 25% 50% 75% 100% 84/ MEDIA TEKNIK SIPIL/Juli 2006

The hange in length of the speimen was omputed from the differene between the first and subsequent readings.

Measurement of autogenous shrinkage using Deme Gauge RESULTS AND DISCUSSION Autogenous Shrinkage The observed autogenous shrinkage up to 28

The autogenous shrinkage patterns observed on all mixes suggest that autogenous shrinkage inreases at dereasing rate and will finally reah ultimate value.

3 The hange in length of the speimen was omputed from the differene between the first and subsequent readings. Autogenous shrinkage was then alulated from the hange in length divided by the original length (200 mm). Fig.1. Measurement of autogenous shrinkage using Deme Gauge RESULTS AND DISCUSSION Autogenous Shrinkage The observed autogenous shrinkage up to 28 days is plotted against time and presented in Fig.2. The autogenous shrinkage patterns observed on all mixes suggest that autogenous shrinkage inreases at dereasing rate and will finally reah ultimate value. And it is this ultimate value, whih is the interest of designer. The ultimate value might be obtained after ontinues measurement of autogenous shrinkage whih may take up to 1 year. 300 Autogenous Shrinkage (mirostrain) LAC %-0% LAC %-25% LAC %-50% LAC %-75% LAC %-100% Age (days) Fig. 2. Autogenous shrinkage of LAC up to 28 days measurement. 650 Ultimate Autogenous Shrinkage (mirostrain) LA Degree of Saturation LA Degree of Saturation (%) Fig.3. Ultimate autogenous shrinkage predited by adopting ACI.209. R-92 method MEDIA TEKNIK SIPIL/Juli 2006/85

4 To redue the time for determining ultimate autogenous shrinkage by diret measurement, it is desirable to have shrinkage-time expression, whih an be used to predit the ultimate value from shortterm measurement. By adopting ACI 209.R-92 method as desribed in Kristiawan et al (2006) the ultimate values of autogenous shrinkage for all mixes are estimated and the results are given in Fig.3. Ratio of Water Available from Saturated LA to Water Required for Maximum Hydration and Its Relation to Autogenous Shrinkage Using Bentz model as desribed earlier, it is possible to alulate the amount of water required (V wat ) for maximum hydration of eah mix. The water required will be assumed enough for internal uring during self-desiation. The results of alulation are tabulated, in Table 2. The table also shows the available water from saturated water in LA (V WLA ). V WLA is alulated by onsidering the LA degree of saturation. Theoretially, maximum hydration an be ahieved as long as the available water is higher than that required for hydration. Giving this into onsideration, most of the available water from the LA is higher than that required for hydration. apillaries tension are generated and autogenous shrinkage is expeted to be low. However, this is In turn, it is expeted that when hydration proeeds water pores onsumed for this reation is immediately replaed by that in the LA; there is no drop of relative humidity in the pores and as a result no not always the ase as seen Fig. 4. When the ratio of V WLA to V wat is higher than 1, it means that the water available from LA is greater than that needed for hydration. However, the graphs suggest that from 25% to 50% degree of saturation, even though the ratio inreases but the ultimate autogenous shrinkage is also inreased. Only after the ratio is very high (above 3.5) the redution of ultimate autogenous shrinkage is observed. It seems that water in LA will readily available to fill the empty pores due to self-desiation when the ratio of V WLA to V wat is high. Table. 2. Calulated water for hydration (V wat ) and available water from LA (V WLA ) Sample Identifiation V wat V WLA V WLA/V wat LAC %-0% LAC %-25% LAC %-50% LAC %-75% LAC %-100% Ratio of Total Volume of Water to Total Volume of Porosity and Its relation to Autogenous Shrinkage In this setion, the water available for hydration is alulated not only due to that in the LA but also due to that from the original water for mixing. This water mixing is obviously dependent on the w/ ratio. The autogenous shrinkage ould be prevented if the emptying pores as a result of ontinues hydration is refilled with water. The refilling of pores with water is possible only if the amount of water available in the mix both from mixing water and that in the LA is greater than the total porosity of onrete Ultimate Shrinkage (10-6 ) Series V WLA /V wat Fig.4. Relationship between ultimate autogenous shrinkage and V WLA /V wat 86/ MEDIA TEKNIK SIPIL/Juli 2006

5 Table 3 shows total volume of water (TVW) and total volume of porosity of onrete (TVP). TVW is alulated from the amount of mixing water per m 3 onrete (V w- ) and V WLA. For example, for mix LAC %-100%; the ement ontent is kg/m 3 onrete and w/ = 0.6. The mixing water per m 3 onrete is 0.6 x = kg. This equals to a volume of m 3. Meanwhile, the TVP onsists of porosity of LA (P LA ) and porosity of onrete (P ). P LA is obtained from absorption harateristi of LA and P an be alulated using Expression 3 as follows (Neville and Brooks, 1987): P = w 1 ρ f 0.17h+ Af + 1 ρ a A + w + a..[3] where w,,a, A f and A represent the amount of water, ement, entrapped air, fine aggregate and oarse aggregate, respetively, in onrete. Meanwhile, ρ f and ρ are the speifi gravity of fine and oarse aggregate, respetively, and h represents degree of hydration. Most of TVW is greater than TVP. This fat ensures that there is water available for refilling of the pores when it is drying due to hydration onsumption. Consequently, autogenous shrinkage is expeted lower with higher ratio of TVW to TVP. However, Fig. 5 does not onfirm this. The explanation seems to be similar to that suggested in previous setion. Other fat that may affet this phenomenon is that some of the water is lost during mixing beause of evaporation and bleeding. Table 3. Calulated available water (TVW) and total porosity of onrete (TVP) for mixes series 3 Sample Identifiation TVW TVP TVW/TVP LAC %-0% LAC %-25% LAC %-50% LAC %-75% LAC %-100% CONCLUSION Water supplied by lightweight aggregate has been assumed to be immediately available for refilling the empty pores due to self-desiation. Based on this assumption, Bentz s developed model for quantifying the amount of lightweight aggregate for the purpose of preventing autogenous shrinkage of onrete. However, this researh suggests that only when the ratio of water supplied from lightweight aggregate to water required for hydration is high (above 3.5) then this assumption is valid as seen from the redution of autogenous shrinkage. The ase is also true for the relation between the ratio of total porosity to total volume of water against autogenous shrinkage. ACKNOWLEDGMENT The researh is part of the study on the autogenous shrinkage of lightweight aggregate onrete funded by Indonesia Toray Siene Foundation (ITSF) through its 11 th Siene and Tehnology Researh Grant (STRG) The authors also would like to thanks to student of Civil Engineering Department, University of Sebelas Maret who took part in the experimental works: Heti Ultimate Shrinkage (10-6 ) Series TVW/TVP Fig.5. Relationship between ultimate autogenous shrinkage and ratio of TVW/TVP MEDIA TEKNIK SIPIL/Juli 2006/87

6 REFERENCES Bentur, A, Igarashi, S and Kovler, K, 2001, Prevention of autogenous shrinkage in highstrength onrete by internal uring using wet lightweight aggregates, Cement and Conrete Researh, vol. 31, pp Bentz, DP and Snyder, KA,1999, Proteted paste volume in onrete; Extension to internal uring using saturated lightweight fine aggregate, Cement and Conrete Researh, vol. 29, pp Jensen, OM and Hansen, PF,2001, Autogenous deformation and RH-hange in perspetive, Cement and Conrete Researh, vol. 31, pp Kristiawan, SA and Sangadji, S, 2006, Autogenous shrinkage of lightweight aggregate onrete at various w/ ratios, lightweight aggregate ontents and degrees of saturation, Indonesia Toray Siene Foundation (ITSF) Seminar on Siene and Tehnology, Shangri-La Hotel, Jakarta, 1 February Kohno, K, Okamoto, T, Isikawa, Y, Sibata,T, and Mori, H,1999, Effet of artifiial lightweight aggregate on autogenous shrinkage of onrete, Cement and Conrete Researh, vol. 29, pp Neville, AM and Brooks, JJ,1987, Conrete Tehnology, Longman Sientifi And Tehnial. Tazawa, E and Miyazawa, S,,1994,"Autogenous shrinkage of onrete and its important in onrete tehnology",proeeding of the Fifth International RILEM Symposium on Creep and Shrinkage, E & FN SPON, pp / MEDIA TEKNIK SIPIL/Juli 2006

Prediction of Temperature and Aging Effects on the Properties of Concrete

Prediction of Temperature and Aging Effects on the Properties of Concrete Predition of Temperature and Aging Effets on the Properties of Conrete Jin-Keun Kim 1* and Inyeop Chu 1 1 KAIST, Republi of Korea * E-mail: kimjinkeun@kaist.a.kr ABSTRACT For the sustainable onrete strutures,