Variation in mechanical properties of natural and recycled aggregate concrete as related to the strength of their binding mortar

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1 Available online at Materials and Structures 38 (August-September 2005) Variation in mechanical properties of natural and recycled aggregate concrete as related to the strength of their binding mortar G. F. Kheder 1 and S. A. Al-Windawi 2 (1) University of Al-Mustansiriya, Department of Civil Engineering, Iraq (2) Baghdad, Iraq Received: 22 April 2004; accepted: 21 September 2004 ABSTT Experimental work was performed to study the effect of binding mortar strength on the mechanical properties of recycled natural aggregate concrete mixes as well as reference corresponding natural aggregate concrete mixes. The moduli of elasticity of both and were found to be higher than that of corresponding mortar by about 40% and 10% respectively, for all compressive strengths investigated. It was possible to reach compressive strength for of 53.5 MPa. The ratios of compressive strength of or to that of mortar varied between ( ) and ( ) respectively, these ratios decreased with the increase in compressive strength. Also from the results of compressive strength, it was found that the ratios cylinder/cube compressive strengths of and mortar were smaller than those of. The ranges of values obtained were ( ) and ( ) for and mortar respectively, while for this ratio ranged between ( ), these values were obtained for compressive strengths ranging between 15 to 55 MPa. It was found that it is better to relate the cylinder/cube strength ratio to the modulus of elasticity of the concrete or mortar rather than to its compressive strength. The flexural strength showed an opposite trend, the ratios of and to that of mortar ranged between ( )% and ( )% respectively. These ratios increased with the decrease in compressive strength of mortars. On the other hand, the splitting tensile strength of was higher than that of and mortar for all strength levels investigated. The ratio of to mortar splitting tensile strength ranged between ( ), while this ratio for ranged between ( ). Finally, several regressions were developed that can relate the mechanical properties of the three materials investigated RILEM. All rights reserved. RÉSUMÉ Un travail expérimental a été réalisé pour étudier l effet de la résistance à la compression de mortiers avec liant sur les propriétés mécaniques de mélanges de bétons recyclés et de mélanges de bétons de référence élaborés à partir de granulats naturels. Les modules d'élasticité des bétons de granulats naturels et recyclés se sont avérés plus élevés que celui d un mortier de référence, respectivement de 40 et 10%, pour toutes les résistances à la compression étudiées. La résistance à la compression des bétons de granulats recyclés a pu atteindre 53,5 MPa. Les rapports entre les résistances à la compression des mélanges de bétons naturels ou recyclés et celle du mortier ont varié de (1,05-1,56) à (1,02-1,26), respectivement ; ces rapports baissaient en fonction de la hausse de la résistance à la compression. On a également trouvé à partir des résultats d essais de résistance à la compression, que les rapports de résistance à la compression de cylindres ou cubes de mortiers et bétons de granulats recyclés étaient inférieurs à ceux des bétons de granulats naturels. Les valeurs obtenues allaient de (0,71-0,84) à (0,69-0,75), respectivement pour les bétons de granulats recyclés et les mortiers, contre (0,81-0,92) pour les bétons de granulats naturels ; ces valeurs étaient obtenues pour des résistances à la compression comprises entre 15 et 55 MPa. On a établi qu il était préférable de mettre en relation le rapport de résistance des cylindres ou cubes avec le module d élasticité du béton ou mortier plutôt qu avec sa résistance à la compression. La résistance à la traction a mené à une tendance contraire : les rapports des bétons de granulats naturels et recyclés étaient compris respectivement entre (0,72-0,95)% et (0,61-0,80)%. Ces rapports ont augmenté avec la baisse de résistance à la compression des mortiers. D autre part, la résistance à la traction par fendage des bétons de granulats naturels était plus élevée que celle des bétons de granulats recyclés et des mortiers pour toutes les résistances étudiées. Les rapports de résistance à la traction par fendage pour les bétons de granulats naturels allaient de 1,13 à 1,69, contre (0,87-1,36) pour les bétons de granulats recyclés. Enfin, plusieurs régressions mettant en relation les propriétés mécaniques des trois matériaux étudiés ont été développées. 1. INTRODUCTION Except for structures, which have to be preserved as monuments, greater number of them has to be demolished eventually. Millions of tons of concrete debris are also generated by natural disasters. For example, the annual production of demolished waste is estimated to be about 50 million tons in the European Economic Communities [1], about 60 million tons in the United States [2], and about 12 million tons in Japan [3]. Concrete accounts for nearly 75% of RILEM. All rights reserved. doi: /14216

2 702 G.F. Kheder, S.A. Al-Windawi / Materials and Structures 38 (2005) all construction materials and it follows that it should also account for the major percentage of demolished wastes. Depletion of natural aggregate sources, stricter environmental laws and waste disposal problems are factors that make the use of reclaimed concrete as recycled aggregate an attractive proposition. The use of recycled aggregate concrete () started mainly about 60 years ago, when large quantities of concrete debris became available from World War II damaged structures, and suddenly a great need for aggregate rose up when these structures were to be re-constructed or repaired. Previous research works [2-4] reported that had a compressive strength of 5% to 25% lower than that of natural aggregate concrete (). This difference is smaller at low W/C ratios (higher strength levels). Ravindrariah and Tam [5], and Tavakoli and Soroushian [6] reported that the compressive strength of is governed by the strength of the new mortar, while the flexural and splitting tensile strengths of showed no clear trends when compared with those of the original concrete of similar composition. They concluded that the flexural and splitting strengths of could be higher than those of. In addition, Tavakoli and Soroushian concluded that conventional relationships between flexural, splitting tensile and compressive strengths must be modified for. 2. RESEARCH SIGNIFICANCE In this research work, it was aimed to study the effect of mechanical properties of the binding mortar with a compressive strength in the range of MPa on those of corresponding recycled aggregate concrete mixes, as well as reference natural aggregate concrete mixes, and also to investigate the possibility to obtain with relatively high compressive strength. In addition, it was attempted to obtain mathematical relationships that can be used to predict these properties for the three types of mixes: mortar,, and. 3. EXPERIMENTAL WORK 3.1 Materials The materials used in this experimental work were: 1. Ordinary Portland cement. 2. Siliceous sand with fineness modulus of Natural gravel with maximum size of 20 mm. 4. Superplasticizer of the form melamine formaldehyde condensate. The recycled coarse aggregates were produced by casting concrete with compressive strength in the range of 20 to 25 MPa, using the materials mentioned above. This concrete was cured, then crushed and sieved to different size fractions, and then these fractions were mixed to yield recycled coarse aggregate with grading similar to that of the natural gravel. All materials used in this work conformed to the requirements of the ASTM Specifications [7, 8]. 3.2 Concrete and mortar mixes Two groups of concrete mixes, and, were produced using natural sand. These mixes were designed according to ACI 211 mix proportioning method. In the design of the and the mixes, the slump of all concrete mixes is limited to 80 to 100 mm, and variable W/C ratios were investigated to obtain different strength levels. A third group of mortar mixes, with mix proportions similar to those of group mixes, except that they were cast without coarse aggregate (i.e., the mortar mixes had the same water:cement:sand ratios as the mixes). Each of these three groups was divided into two parts; the first did not include any admixture, while the mixes of the second part contained 3% superplasticizer by weight of cement. The superplasticizer was added to reduce the W/C ratio of the mix only, to increase the strength of the mixes. The slumps of all concrete mixes were kept constant at 80 to 100 mm. Table 1 gives full details of the mixes investigated. It is important to mention here that all the aggregate used were in saturated surface dry condition, so that aggregate absorption will not affect the mix proportions. 3.2 Testing of concrete Four main mechanical properties of the concrete and mortar mixes were studied. These properties were the modulus of Table 1 - Notations and mix proportions of, and mortar mixes Mix Type Mix Notation Mix Proportions water:cement:fa:ca* Superplasticizer (%) :1:1.33: :1:1.92: :1:2.53: :1:3.16: :1:3.84: S :1:1.42: S :1:2.03: S :1:2.66: S :1:3.31: Natural aggregate concrete Recycled aggregate concrete S :1:4.00: :1:1.11: :1:1.64: :1:2.18: :1:2.74: :1:3.33: S :1:1.20: S :1:1.75: S :1:2.31: S :1:2.88: S :1:3.50: M :1: M :1: M :1: M :1: M :1: MS :1: MS :1: MS :1: MS :1: MS :1: * FA = Fine aggregate CA = Coarse aggregate

3 G.F. Kheder, S.A. Al-Windawi / Materials and Structures 38 (2005) Table 2 - Mechanical properties investigated and methods of testing Test Specification Type of Specimen Number of Specimens Compressive Strength ASTM C39 Cylinder mm 3 BS 1881: Part 116 Cube mm 3 Flexural Strength ASTM C78 Prism mm 3 Splitting Tensile Strength ASTM C496 Cylinder mm 3 Modulus of Elasticity ASTM C469 Cylinder mm 2 elasticity, compressive strength, flexural strength and splitting tensile strength. Table 2 shows details of the tests conducted, specifications, types and number of specimens tested for each different mix. Two types of concrete specimens for determining the compressive strength were tested, namely cylinders and cubes. This was done to investigate whether the cylinder to cube compressive strength ratio of and mortar is the same as that of the or not, as there was evidence at the beginning of this work that this ratio is different and need to be considered in evaluating concrete test results. 4. EXPERIMENTAL RESULTS AND DISCUSSION 4.1 Modulus of elasticity Fig. 1 shows the relationship between binding mortar modulus of elasticity and the modulus of elasticity of corresponding and mixes. The results show that the static modulus of elasticity of both and are found to be higher than that of mortar. For all strength levels investigated the modulus of elasticity of and mixes exceeded that of the binding mortar by about 40% (between 38 to 48%) and 10% (between 7 to 14%) respectively. The modulus of elasticity of exceeded that of by about 20 to 25% for all strength levels investigated. Based on experimental study for prediction of a relationship or modulus of elasticity (MPa) y = x y = x modulus of elasticity (MPa) Fig. 1 - Relationship between modulus of elasticity of or and corresponding mortar modulus of elasticity. between the modulus of elasticity of concrete and that of its constituents, Zhou et al. [9] and Zhang and Gjørv [10], reported that concrete may be treated as a simplified two component composite: cement mortar and coarse aggregate. The modulus of elasticity of concrete is thus related to the modulus of cement mortar and coarse aggregate, to the volume fraction of the coarse aggregate and to the paste-aggregate bond strength. For normal weight aggregate, The aggregate, generally, has higher modulus of elasticity than the cement paste or mortar, hence, the higher the modulus of elasticity of coarse aggregate or the higher the coarse aggregate content is, the higher the modulus of elasticity of concrete will be. Also crushed coarse aggregate with rough texture will produce better bond and will result in higher modulus of elasticity of concrete. Taking these facts into consideration, there will be no wonder why the recycled aggregate in a new concrete usually leads to a lower modulus of elasticity as compared with that of natural aggregate concrete. This is in fact attributed to a number of reasons, first, the amount of weak bond areas in the is significantly more than in. Apart from having bond areas between the crushed gravel joints. Second, the recycled aggregate contains higher amounts of faults, including many micro cracks in the old mortar and the gravel particles, created during the crushing of the recycled concrete. Third, the decrease in the mean size of the recycled aggregate (i.e. the decrease in the maximum size and the proportion of coarse aggregate particles) owing to breaking up of the recycled aggregate particles by losing large amounts of adhered mortar during handling and mixing processes. Indeed this effect of the aggregate was also confirmed by Oluokun et al. [11] who stated that the modulus of elasticity of concrete, in general increase with the increase in the coarse aggregate size and proportion. The following relationships were found to relate the modulus of elasticity of the two types of concrete and to that of the corresponding mortar mix cn E cm E = (r = 0.977) (1).1348 cr E 1 cm E = (r = 0.998) (2) In order to test the significance of Equations (1) and (2), Student s T test was used. The minimum value required for the coefficient of variation is (for 1% level of significance, 10 data points and a regression with one independent variable), which is well below the coefficients of variations values obtained (0.977 and 0.998). The variation in the modulus of elasticity and cylinder compressive strength of, and mortar mixes are shown in Fig. 2. It is seen that as the compressive strength increases, the modulus of elasticity also increases but with a decreased rate, particularly for mortar, this is because proportioning concrete to favor compressive strength is usually done by increasing its cement content and

4 704 G.F. Kheder, S.A. Al-Windawi / Materials and Structures 38 (2005) Modulus elasticity (MPa) decreasing the coarse aggregate content, therefore the increase in the modulus of elasticity will be less than the increase in the compressive strength. Finally, the relationships between, and mortar modulus of elasticity and their cylinder compressive strengths are given below: cn 5.323f cn E = (r = 0.982) (3) cr 4.993f cr E = (r = 0.998) (4) m 6.631f cm E = (r = 0.995) (5) 4.2 Compressive strength y = x y = x y = x Cylinder compressive strength (MPa) Fig. 2 - Relationship between modulus of elasticity of, or mortar and their cylinder compressive strength. From the preliminary results of compressive strength tests, there was evidence that the shape of specimen, cylinder or cube differed for or mortar as compared with. So it was decided to investigate this point to determine which specimen is better to be used, or to find a new relationship between cylinders and cubes compressive strengths. Fig. 3 shows the relationship between compressive strength of cylinder and the ratio of cylinder/cube strengths for all the three groups investigated:, and mortar. From this figure, it can be seen that the exhibited the highest ratio, with the range of ( ) for cylinders compressive strengths between MPa (25-60 MPa cube strength). On the other hand, for the and mortar mixes, this ratio was smaller, with the ranges of ( ) and ( ) respectively. These ratios corresponded to cylinder compressive strength ranges of MPa (23-64 MPa cube strength) for mixes and MPa (18-56 MPa cube strength) for the mortar mixes. This difference in the cylinder/cube strength ratio for and mortar as compared to that of is simply due to the lower modulus of elasticity and higher Poisson s ratio of these two materials [12]. In this is due to the lower coarse aggregate content and more micro-cracks present within the concrete (between the old mortar and the coarse aggregates due to crushing of concrete), while in the mortar Cylinder to cube compressive strength ratio y = x y = x y = x Cylinder compresive strength (MPa) Fig. 3 - Relationship between cylinder to cube compressive strength ratio and cylinder compressive strength. mixes, this is due to the absence of coarse aggregates. The lower modulus of elasticity and higher Poisson s ratio will result in higher lateral tensile strains and higher restraining effect of the testing machine platens during the crushing of concrete cube specimens. This will result in higher measured compressive strength when cube specimens are used, resulting from the compressive tri-axial state of stress generated in the concrete specimens, while the cylinder specimens will be subjected to pure uni-axial state of stress as the cylinder height to diameter ratio equals to 2 (exceeds 1.732). Thus using concrete cubes for testing or mortar will result in overestimated strength. As a result, the ratios of cylinder/cube strength recommended by different standards or codes of practice should not be used when or mortar mixes are tested. Equations (6) to (8) simply relate the cylinder/cube strength ratio to the cylinder compressive strength for the three types of mixes;, and mortar, respectively: R = f cn (r = 0.954) (6) R = f cr (r = 0.975) (7) R = f cm (r = 0.892) (8) Another look has been taken to the relation between the compressive strength of concrete cylinders and cubes and its relation to the modulus of elasticity of the mix, Fig. 4 shows the relationship between this ratio and the modulus of elasticity of all the investigated mixes. From this figure, it can be clearly seen that the cylinder/cube compressive strength ratio depended directly on the modulus of elasticity of the concrete or mortar, and this ratio can be related to the modulus of elasticity by the following unique equation: R = E c (r = 0.922) (9) From Fig. 4 as compared with Fig. 3, it is very clear that cylinder/cube compressive strength is better be related to the modulus of elasticity than to compressive strength as usually done by different codes of practice or researchers [13]. So in this work the cylinder compressive strength will be used rather than the cube strength to describe other properties.

5 G.F. Kheder, S.A. Al-Windawi / Materials and Structures 38 (2005) Cylinder to cube compressive strength ratio y = x Modulus of elasticity (MPa) or compressive strength (MPa) y = x y = x compresive strength (MPa) Fig. 4 - Relationship between cylinder to cube compressive strength ratio of, or mortar and their modulus of elasticity. On the other hand, Fig. 5 shows the relationship between the cylinder compressive strength of the binding mortar and that of or. From this figure, it can be seen that at lower strength of mortar (13.2 MPa), the ratio of strength to that of mortar is 1.56 (the is about 7.3 MPa higher than corresponding mortar mix), while the corresponding ratio for mix is 1.26 (the was about 3.4 MPa higher than corresponding mortar mix). On the hand, for the higher mortar strength level (52.4 MPa), the difference between the and compressive strengths compared to that of mortar decreased significantly, thus the ratio of and compressive strengths to that of mortar are 1.05 and 1.02 only (the and the mixes are only about 3.2 MPa and 1.1 MPa higher than the corresponding mortar mix respectively). As a result of the above observations, it can be said that for low strength binding mortar, the compressive strength of the and exceeded that of the corresponding binding mortar, while for high strength binding mortar, the compressive strength of all three materials (the mortar, and mixes) approached each other. The reasons for this behavior can be summarized in the following points: 1- The has higher modulus of elasticity than and mortar, thus the cylinder specimen with lower modulus of elasticity under uni-axial compression load will undergo larger tensile strain deformation in directions lateral to the direction of loading, thus will fail under lower levels of stress. Thus the will have higher strength than and both will be stronger than mortar. 2- The natural aggregate has a smooth surface texture, whereas the recycled aggregate has some crushed gravel particles. At high strength levels, the binding mortar is strong, so the mixes will suffer from matrixaggregate bond weakness compared to and mortar. Therefore the strength of all the mortar, and the will approach each other. 3- The natural aggregate is much stiffer than the recycled aggregate, hence in high strength mortar matrix, the strong mortar will exert higher stresses on the coarse aggregate particles, before the collapse of the binding matrix, therefore the strong natural aggregate will act as a stress raising discontinuities in an otherwise Fig. 5 - Relationship between compressive strength of or and corresponding compressive strength of mortar. homogeneous matrix, which creates non uniform internal state of strain and stress distribution, and by varying their degrees of restraint imposed on the surrounding matrix, produce stress levels much higher than the imposed load as compared with recycled aggregate or mortar mixes [14]. 4- The elastic modulus of the recycled aggregate is generally closer to that of the matrix, in the high strength mortar mixes, but lower than that of the natural aggregate. Hence when the is subjected to a uniaxial compressive load, the aggregate particles are subjected to a lateral confinement of the surrounding matrix, and therefore the recycled aggregate will benefit from the multi-axial state of stress more than the natural aggregate. 5- The recycled aggregate as compared with the natural aggregate has a smaller aggregate mean size, due to the crushing of the recycled aggregate particles with the releasing of the attached old mortar during the mixing process (especially in mixes with low W/C ratios). This will increase the strength of concrete, due to the higher surface to volume ratio of the aggregate particles, which will affect the concrete strength positively. The two relationships below relate the cylinder compressive strength of and to the corresponding compressive strength of the binding mortar cn = 3.535f cm f (r = 0.990) (10) cr = 1.813f cm f (r = 0.987) (11) 4.3 Flexural strength A comparison of flexural strength results of, and mortar is shown in Fig. 6, from which it can be seen that the flexural strength of both and was always lower than that of the corresponding mortar mix. The ratios of concrete/mortar flexural strengths ranged between ( ) and ( ) for and, respectively. The ratio increased with the decrease of the strength of the mix. This behavior can be attributed to the sensitivity of the flexural strength of concrete mixes due to the presence of coarse aggregate. Incorporation of coarse aggregate in concrete

6 706 G.F. Kheder, S.A. Al-Windawi / Materials and Structures 38 (2005) or flexural strength (MPa) y = 1.379x y = x flexural strength (MPa) Fig. 6 - Relationship between flexural strength of or and corresponding flexural strength of mortar. does not change the essential microstructure of the cement gel, but generally tends to convert what might otherwise be considered on a moderately large scale as a homogeneous matrix into a heterogeneous system. Similar observations were reported by Jones and Kaplan [15], who stated that the flexural strength of concrete was generally lower than that of the corresponding mortar. would thus seem to set the upper limit to the flexural strength of concrete, and the presence of coarse aggregate generally reduces this strength. On the other hand, the properties of coarse aggregate such as particle shape and surface texture are also found to play an important role in controlling the flexural strength of concrete, particularly, at lower W/C ratios [16]. Taking these findings into consideration, it is possible to provide some explanations for the variation in flexural strength for concrete of various qualities, which show that the difference in strength between and increase only a little with the decrease in W/C ratio, on the contrary to what was anticipated. This seems to be primarily attributed to the behavior of the uncrushed gravel in concrete under flexural loads, since the modulus of rupture of concrete is a direct function of the aggregate characteristics. This effect was confirmed by Giaccio et al. [17] whom reported that the matrix-aggregate bond strength became especially important under tensile (flexural) loads rather than compressive loads. Whereas: the modulus of rupture of concrete is generally lower than that of its matrix, and higher than its bond strength. This is because the tensile bond strength of the aggregate-matrix interface is much lower than the tensile strength of the matrix itself. The difference between the two tends to be smaller for higher W/C ratios. However, for lower W/C values, the pattern of variation of the relationship differs, the bond strength being particularly affected by small changes with the decrease in W/C ratio, while the tensile strength in flexure is affected by higher degree [18]. As mentioned earlier, the flexural strength of both and is generally lower than that of mortar at all strength levels. Furthermore, the difference in the flexural strength between and with respect to mortar increases as the W/C ratio decreases (strength level increase). This is expected, since the strength of mortar in flexure depends on several factors, and the most dominant one is the mortar class. When stressed by short term static loads, the mortar will develop elastic strains up to a certain loading stage, then plastic strains will develop caused mainly by the micro-cracking process which takes place inside the hardened cement paste mass. The loading stage at which the micro-cracks are initiated is a function of the mortar class. Low strength mortar has marked plastic properties within the tensile zone and the process of the cement paste micro cracking begins at low loading stages, whereas, high strength mortar behaves almost elastically till fracture [18]. Thus, it can be seen that the presence of coarse aggregate in the concrete frame, will prevent the concrete from reaching the ultimate tensile strength of the mortar matrix, particularly at lower W/C ratios. This is due to the initiation of micro-cracks at the interface between the aggregate and the surrounding mortar at a relatively lower tensile stress. From the results obtained, Fig. 6 shows the relationship between the flexural strength of or and that of corresponding mortar were obtained: rn 1.379f rm f = (r = 0.947) (12) rr 1.163f rm f = (r = 0.937) (13) On the other hand, the relationship between the flexural strength and compressive strength of, and mortar were obtained and shown in Fig. 7. From this figure, it can be seen that the ratio of flexural/compressive strength of the three types of mixes,, and mortar ranged between (11-17)%, (10-15)% and (16-25)%, respectively. This ratio was found to increase with the decrease in compressive strength. The following relationships between the flexural and compressive strengths for the three types of mixes were obtained and given in Equations (14-16) rn 0.610f cn (r = 0.930) (14) rr 0.762f cr (r = 0.943) (15) rm 0.846f cm (r = 0.981) (16) 4.3 Splitting tensile strength The relationship between the splitting tensile strength of or on one hand, and that of the corresponding mortar mixes on the other are given in Fig. 8. From this Flexural strength (MPa) 9 8 y = x y = x y = 0.762x Compressive strength (MPa) Fig. 7 - Relationship between flexural strength of, or mortar and cylinder compressive strength.

7 G.F. Kheder, S.A. Al-Windawi / Materials and Structures 38 (2005) or splitting strength (MPa) y = x y = x splitting strength (MPa) Fig. 8 - Relationship between splitting strengths of or and corresponding splitting strength of mortar. figure, it can be seen that the ratio of concrete/mortar splitting tensile strength varied widely with mortar strength. For, this ratio varied between ( ) of that mortar with compressive strengths between 52.4 and 13.2 MPa respectively. The corresponding ratios for were between ( ). From this comparison, it can be seen that the splitting tensile strength of is always greater than that of mortar, while that of is higher than that of mortar in the low-strength mixes, and becomes lower than mortar in the high-strength mixes. It is evident from these results that unlike the compressive strength, the mortar s tensile strength cannot contribute directly to making the splitting strength or the flexural strength of matches that of, since the measured splitting tensile strength and flexural strength of remains lower than that of the corresponding by about 25% and 20% respectively, even at high concrete compressive strengths. Thus there is no doubt that the presence of coarse aggregate particles in the mixture can form a major source for most of the variations taking place within the specimen under splitting tensile loads. This is analogous to the conclusion reached by Hughes and Chapman [19], who stated that the aggregate inclusion have a fundamental role in influencing the splitting tensile behavior of concrete materials through maintaining the dimensional stability, strength, and unity of the system. The presence of the coarse aggregate particles, the inherent discontinuities, and the bond cracks, generally, enables concrete materials to sustain more deformations beyond maximum tensile strain. Based on these findings, there will be no wonder that all specimen and those of with W/C ratios higher than 0.50 exhibited apparently higher splitting tensile strengths than those of the corresponding mortars. In such cases, the natural aggregate or the recycled aggregate is generally stronger than the matrix. When the or are subjected to tensile stresses. Cracks typically propagate in the matrix or along the matrix-aggregate interface. In addition, propagation of cracks will be arrested by aggregate particles, resulting in the meandering and branching of cracks, a process that can absorb a considerable amount of energy [10]. This effect intensifies as the aggregate size increases, especially greater than 5 mm [17]. On the other hand, For mixes with W/C ratio less than 0.5, the binding mortar becomes strong enough compared to the recycled aggregate, therefore the recycled aggregate becomes the weak point in the composite, thus the will exhibit lower splitting tensile strength than mortar. On the other hand, the showed a lower splitting tensile strength than the, the difference ranged between 12.9 to 23.5%. This is due to the fact that the recycled coarse aggregate is much more deformable than the high quality natural aggregate. The reason behind that is attributed mainly to the inherent inferior characteristics of the recycled aggregate particles, which normally consist of considerable amounts of porous old mortars of different qualities, hence form zones of weakness in the concrete composite. In addition, the presence of micro-cracks in some of the aggregate particles resulted from crushing the old concrete from which the recycled aggregate is generated. Taking this behavior into account, it seems that shows almost the same tendency as for lightweight-aggregate concrete [5, 20]. Short and Kinniburgh [21] reported that the appearance of cracks in light weight concrete is quite different from that in gravel concrete, and indicates, by its more regular linear form that fracture is caused by tensile stresses in aggregate particles themselves, as well as the fracture of the matrix, while in gravel concrete the strength and stiffness of the aggregate itself are usually high when compared with those of the cement matrix. Therefore failure in tension will very rarely occur as a result of fracture of the aggregate, but almost invariably because of the breakdown of the bond between the matrix and the surface of aggregate or fracture of the matrix itself. Comparing the fracture surfaces of both the and obtained, showed that most of the failure in natural aggregate concrete occurred along the interface between the mortar and the aggregate particles, while in the failure plane goes through or around the aggregate. This type of failure causes somewhat a more abrupt collapse of concrete, which may explain why is more brittle than. Fig. 9 shows the influence of the splitting tensile strength of mortar upon those of and. The following relationships were found to relate these properties: tn 1.979f tm f = (r = 0.993) (17) tr 1.741f tm f = (r = 0.983) (18) The relationships between splitting strength and compressive strengths of the, and mortar mixes are shown in Fig. (9). It is clearly seen that the splitting strength of and mortar mixes is more affected by the increase in compressive strength when compared with. The tensile splitting strength of exceeded that of by about 20 to 25% for all strength levels investigated. The following relationships were obtained relating these two properties: tn 0.328f cn tr 0.568f cr (r = 0.978) (19) (r = 0.992) (20) tm 0.218f cm (r = 0.988) (21)

8 708 G.F. Kheder, S.A. Al-Windawi / Materials and Structures 38 (2005) Splitting strength (MPa) CONCLUSIONS y = x y = x y = x Compressive strength (MPa) Fig. 9 - Relationship between splitting strengths of, or mortar and cylinder compressive strength. 1. The modulus of elasticity of and exceeded that of corresponding mortar by about 40% and 10%, respectively, for all compressive strengths investigated, in addition, the modulus of elasticity of is about 20-25% lower than. This fact must be taken into consideration in design in addition to strength requirements when using. 2. The compressive strength of depends largely on the W/C ratio of the mix. It was possible to reach a compressive strength of 53.5 MPa by the use of binding mortar with strength of 52.4 MPa. The corresponding strength was 55.2 MPa. 3. The cylinder/cube compressive strength ratios in and mortar were significantly lower than that of. It is recommended to use cylinder strength rather than cube strength in design or predicting properties of or mortar as cube strength is more affected by the concrete strength level as compared with the cylinder strength. This is due to the lower modulus of elasticity of or mortar compared with. This will produce a multi-axial state of stress in compression in the cube specimen, thus the cube strength will be largely affected by the lateral confinement provided by the machine platens, which will result in higher measured concrete compressive strength. Also, it was found that the relationship of the cylinder/cube compressive strength ratio for all three types of mixes investigated can be related directly by a unique relationship to the modulus of elasticity of the concrete or mortar regardless to the type of aggregates used or mix proportions. In previous literature this ratio was related to concrete compressive strength. In this work it was obvious that it was not possible to relate the cylinder/cube strength ratio the compressive strength by a unique relationship for all types of the three materials. The ratio of compressive strength of and to that of mortar ranged between ( ) and ( ) respectively, the ratio decreased with the increase of the compressive strength level. 4. The flexural strengths of both and were lower than that of mortar by about (5 28)% and (20 39)%, respectively. The difference decreased with the increase in compressive strength of the mix. 5. The splitting tensile strength of was higher than that of mortar by (13 69%), this difference increased by the decrease of strength, while splitting strength was lower than that of mortar for mixes of high strength, and with the decrease in compressive strength, it became higher than that of mortar, the difference was between (-13 36)%. 6. Twenty-one relationships were obtained relating the mechanical properties of, and mortar. These relationships had correlation coefficients ranging between ( ), which are significantly higher than the required value of for level of significance of 1% (confidence 99%). NOTATION E cm, E cn, E cr : Modulus of elasticity of mortar,,, respectively f ' cm, f ' cn, f ' cr : Cylinder compressive strength of mortar,,, respectively f rm, f rn, f rr : Modulus of rupture of mortar,,, respectively f tm, f tn, f tr : Splitting tensile strength of mortar,,, respectively R: Ratio of cylinder to cube compressive strength r: Coefficient of correlation REFERENCES [1] Environmental Resources Limited, Demolition waste an examination of the arising, end uses and disposal of demolition wastes in Europe and the potential for further recovery of materials from these wastes, a report prepared for EEC, DG 12. Environmental resources limited (Construction Press, London, 1980). [2] Wilson, D.G., Foley, P., Wiesman, R. and Frondistou- Yannas, S., Demolition debris quantities, composition and possibilities for recycling, Proceedings of the Fifth Mineral Waste Utilization Symposium, Chicago, April [3] Yoshikane, T., The instances of concrete recycled for base coarse material in Japan, Proceedings of the second International RILEM Symposium on Demolition and Reuse of Concrete and Masonry, Tokyo, 1988 (Chapman and Hall Ltd., London-New York, 1988) [4] Hansen, T.C., Recycled aggregate and recycled aggregate concrete, Second state-of-the-art report, RILEM TC-DRC. Mater. Struct. 19 (111) (May-June 1986) [5] Ravindrarajah, R.S. and Tam, T.C., Methods of improving the quality of recycled aggregate concrete, Proc. of the 2 nd International RILEM Symposium on Demolition and Reuse of Concrete and Masonry, Tokyo, Japan, 1988, [6] Tavakoli, M. and Soroushian, P., Strength of recycled concrete made from crushed concrete coarse aggregate, Concrete International 5 (1) (Jan. 1983) [7] ASTM Standard, Cement, lime and gypsum, Part [8] ASTM Standard, Concrete and Aggregate, Part

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