RECYCLED AGGREGATE CONCRETE



To obtain good quality concrete using recycled aggregate it is necessary to follow the minimum requirements defined by the respective Building Standards. Acceptable properties of aggregates are an elemental base for concrete quality, however adequate mix proportions and concrete production methods are highly important in concrete quality too. Recycled aggregates composed of original aggregates and adhered mortar. The physical properties of recycled aggregates depend on both adhered mortar quality and the amount of adhered mortar. The adhered mortar is a porous material; its porosity depends upon the w/c ratio of the recycled concrete employed.

Crushing concrete to produce coarse aggregate for the production of new concrete is one common means for achieving a more environment-friendly concrete. This reduces the consumption of the natural resources as well as the consumption of the landfills required for waste concrete. The crushing procedure and the dimension of the recycled aggregate have an influence on the amount of adhered mortar. The density and absorption capacity of recycled aggregates are affected by adhered mortar and they must be known prior to the utilization of recycled aggregates in concrete production in order to control properties of fresh and hardened concrete.

The absorption capacity is one of the most significant properties that distinguish recycled aggregate from raw aggregates, and it can have an influence both on fresh and hardened concrete properties.

The utilization of recycled sand was avoided, due to its absorption capacity, which would no doubt produce a shrinkage effect M.Etxeberria et al. If recycled aggregates are employed in dry conditions the concrete’s workability is greatly reduced due to their absorption capacity. Some researchers argue that the recycled aggregates should be saturated before use A. Nealen, S. Schenk.

In general the workability of recycled aggregate concretes is affected by the absorption capacity of the recycled aggregates. The shape and texture of the aggregates can also affect the workability of the mentioned concretes. This depends on which type of crusher is used Shokry R, Siman A.

The aggregate particles of recycled concrete compare well to conventional mineral aggregates in that they possess good particle shape, high absorption, and low specific gravity. Recycled aggregate concrete has also been shown to have no significant effect on the volume response of specimens to temperature and moisture effects. However, the presence of gypsum in the concrete rubble, which is used as aggregate for new concrete, can produce an expansive reaction with the cement matrix due to the concentration of sulphate ions.

The density of concrete made with recycled aggregate shows opposite properties with the normal concrete. Its density is less than normal concrete. It has been found that the workability of recycled concrete is low. With the increase of recycled aggregate in concrete mixture, the values of toughness, plastic energy capacity and elastic energy capacity decreases. Frost resistance of recycled aggregate concrete has been proven not to differ from that of the conventional concrete.

STRENGTH OF REINFORCED CONCRETE WITH RECYCLED COARSE AGGREGATE

Strength of reinforced concrete is defined as the maximum load or stress it can carry. Concrete is strong in compression but weak in tension. Because of this, concrete structures with exception of pavement are designed on the assumption that concrete carries little or no tension but compression while reinforcements are designed to withstand tension.

With the rapid advancement of concrete technology, high strength concrete is being increasingly widely used in the construction of high-rise buildings and other reinforced concrete structures. However, the tensile and shear strength of high strength concrete do not increase in proportion with the compressive strength. And of the two strength properties, the shear strength is of particular importance because the tensile strength is not normally relied on for carrying load but shear is unavoidable in beam column framework. Although there has been a rapid growth of interest in high strength concrete, current specifications for the shear strength of reinforced concrete beams in the American concrete institute [ACI] Building code and British standard are based on results of beam tests done using concrete with relatively low compressive strength.

Compressive Strength Of Concrete With Recycled Coarse Aggregate

The compressive strength of concrete is evaluated by the concrete’s 28 days cube strength. BS: 1881: part 3 requires that the specimen load per unit area sustained by a concrete specimen before it fails in compression.

BS 1881 stipulates that the usual test is the crushing of a 150mm cube in a compression machine loaded at the rate of 15Nmm-2 without reinforcement. However, when the maximum size of aggregate does not exceed 19mm. 100mm cubes can be used for laboratory work. Most structural concrete are proportioned to have strength of 20-30Nmm-2 at 28 days.

Tavakoli M., Soroushian P. studied that concrete made with 100% of recycled coarse aggregate with lower w/c ratio than the conventional concrete can have larger compression strength. When the w/c ratio is the same the compression strength of concrete made with 100% of recycled aggregate is lower than that on conventional concrete. For the recycled aggregate concrete it will be necessary to add more cement in concrete made with 100% of recycled aggregate in order to achieve the same workability and compression strength as conventional concrete. Any variation in concrete production or in the properties of the constituents used produces a variation of strength in the resultant concrete.

TENSILE STRENGTH OF CONCRETE WITH RECYCLED COARSE AGGREGATE

This is of great importance in the design of concrete roads, railways etc. Concrete members are also required to withstand tensile stresses resulting from restraint to contraction due to drying or temperature variation.

Unlike metals it is difficult to measure concrete strength in direct tension and indirect methods have been developed for assessing this property. The concrete strength in direct tension is evaluated by split cylinder test. This method entails diametrically loading a concrete cylinder in compression along its entire length (BS 1881: part 4). The load induces tensile stress over the loaded diametrical plane and the cylinder splits along the loaded diameter. The magnitude of the induced tensile stress at failure in expressed as:

fct = 2F/(pi*Ld) ———————————– (2.1)

Where; F = Applied load

L = Length of cylinder

d = Diameter of cylinder

FLEXURAL STRENGTH OF CONCRETE WITH RECYCLED COARSE AGGREGATE

The flexural strength of concrete is known as the modulus of rupture that is used to evaluate tensile strength as determined from tests on beams. The standard size of beams for flexural tests according to (BS 1881) is 150mm x 150mm x 700mm. However, the American society for testing materials (A.S.T.M) stipulates that the length of the beam should be at least 50mm longer than three times its depth and its width should be not more than one and half times its depth. The minimum depth of width should be at least three times the maximum size of aggregate and not less than 50mm. This is determined from simply supported beam loaded at the third points. The resulting bending moment induces compressive and tensile stresses in the top and bottom of the beam respectively. The beam flexural strength is given as:

fct = FL/(b*d*d) ————————————- (2.2)

where; F = Applied load

L = Effective span

b,d, = Breadth and depth of beam respectively.

The strength in bending is the extreme stress on the tensile side of a point at the point of failure. The ultimate strength of under reinforced beams in flexure is insensitive to the model used to represent the stress-strain relationship. It is only where failure occurs by crushing of concrete in compression that the different stress-strain relationship may result in different calculated ultimate strength.

SHEAR CAPACITY OF REINFORCED CONCRETE WITH RECYCLED COARSE AGGREGATE

The strength of a material is normally given in terms of unit stress or internal force on unit area. The point at which yielding starts is generally expressed as unit stress. A safety factor is applied to either of these stresses to determine a unit stress that should not be exceeded when the member carries design load. That unit stress is known as allowable stress or working stress. In working stress design to determine whether a structural member has adequate load carrying capacity, the maximum unit stress produced by design loads in the members for each type of internal force, tensile, compressive or shearing has to be computed and compared with the corresponding allowable unit stress.

Shear is generally used as a measure of the beams ability to resist principal tensile stresses. It is of paramount importance to note that the strength of a beam in shear depends on the compressive strength, tensile strength, shearing strength of the concrete as well as the amount and distribution of the shear reinforcement and even to some extent on the longitudinal reinforcement and also on the bond between the concrete and reinforcement. In reinforced concrete, the term shear stress refers to the stress in the concrete while bond stress refers to the shearing stress between steel and concrete at the surface of the steel bars. The intensity of each can be computed using the flexure formulae. Shear transfer in reinforced concrete beams relies heavily on the tensile and compressive strength of concrete. The shear in a reinforced concrete beam without shear reinforcement is carried by a combination of three main components.

(i). Concrete in the compression zone.

(ii). Dowelling action of tensile reinforcement.

(iii). Aggregate interlock across flexural crack.

In flexural members in particular the shear resisting mechanism interacts intimately with the bond between concrete and the embedded reinforcement and the anchorage of the reinforcement. With regard to the way in which the shear strength of concrete increases with the compressive strength, the ACI Building Code assumes that the nominal shear capacity (shear capacity of beam without shear reinforcement) is essentially a function of the square root of the compressive strength while the British Standard takes it as being proportional to the cube root of the compressive strength. It is thus apparent that even for normal-strength concrete there is no universally accepted relationship between the shear strength and compressive strength.

In most of the aforementioned codes, the shear capacity of a reinforced concrete member without shear reinforcement is calculated using empirical formulae which are based principally on experimental results obtained using simply supported beams.

In recycled aggregate concrete (RAC), the recycled aggregate from field-demolished concrete for example can be relatively weaker than a typical natural aggregate, and hence can yield reduced shear strength. Han et al. studied the shear behavior of beams made with RAC. They concluded that using the current ACI code equations, the shear strength of RAC can be overestimated. However, 10 out of the 12 beams they reported were tested under a relatively low span to depth ratio (£ 2.0), and hence can be considered deep beams, an unlikely target for RAC use.

Khaldoun Rahal observed that the shear strength of concrete depends significantly on the ability of the coarse aggregate to resist the shearing stresses. He compares the average strength of recycled aggregate concrete (RAC) with normal aggregate concrete (NAC). The cube and cylinder compressive strengths and the indirect shear strength are included in the comparison. On the average, the RAC cube strength was 88.4% of that of the normal concrete. Similarly, the cylinder compressive strength and the indirect shear strength of RAC were 92.2% and 87.7% of those of NAC, respectively. In general, there is a 10% decrease in strength when recycled coarse aggregates are used. This observation is similar to that by Ravindraraj which measured a 9% decrease in compressive strength and departs from that of Yamato et al. who observed that the decrease in strength was 20%, 30% and 45% for recycled coarse aggregate replacement of 30%, 50% and 100%, respectively. The significant difference in strength reduction shows the effect of the various factors such as the source of the recycled aggregate and shows the need to test local materials for the actual behavior.

Masaru Sogo et al. Experimental results indicate that reinforced recycled concrete (RRC) beams without stirrups is decreased in shear strength by 20% and empirical design code overestimates the shear strength of recycled aggregate concrete. On the contrary, shear strength of RRC beams with stirrup is almost the same as those of conventional concrete beams in experimental results, while modified truss theory underestimates the ultimate shear strength of beams by 30% on the condition that shear failure is defined by yielding of stirrup.

DURABILITY OF RECYCLED AGGREGATE CONCRETE

The mechanical properties and the durability characteristics of recycled aggregate concrete (RAC) must be observed to ensure proper use of the recycled material. There have been numerous studies concerned with the mechanical and durability properties of RAC. Tests have shown that the mechanical properties depend on the properties of the recycled concrete used to produce the aggregate and on the percentage replacement of coarse aggregates in the new concrete.

Ravindraraj suggested equations for the modulus of elasticity that give an average decrease of 15% for NAC and RAC of similar cylinder compressive strengths. Due to the wide variation in the properties of the available resources, properties using local materials need to be observed in order to gain the required confidence in the performance of the new material.

RAC has been shown to be weaker than a similar concrete made of natural aggregate and could hence be softer in that the strain at peak stress could be larger than the typically assumed value of 0.002 Khaldoun Rahal. Hence the modulus of elasticity and the strain at peak stress of locally produced RAC need to be studied to obtain the necessary confidence required for structural use.

FACTORS AFFECTING THE STRENGTH OF CONCRETE WITH RECYCLED COARSE AGGREGATE

The strength of concrete is affected by a number of factors such as:

CONSTITUENT MATERIALS

(A) Water-Cement Ratio: Most concrete properties are influenced by water contained in the cement paste. This determines the workability and the strength of concrete. The required quantity of water depends primarily on the maximum size, shape and surface characteristics of the aggregates. The amount of cement required for a particular concrete strength varies considerably for different aggregates. This depends of the strength and modulus of deformation of the aggregate particular and also on the free water content required for adequate workability.

(B) Aggregate: The particle grading of the aggregate mainly affect the quantity of mixing water required for adequate workability. Increase in the proportion of fines increases the water requirement and this leads to a lower concrete strength unless the cement content is increased. It should be noted that the utilization of recycled sand was avoided, due to its absorption capacity, which would no doubt produce a shrinkage effect.

(C) Expansive Additive: Addition of expansive additive increases the shear strength by 10% in any cases of aggregate type. This can be explained by that the axial force, which is produced by expansion of concrete, makes wider compressive zone and narrower cracking width. The wider compressive strength zone improves the shear resistance and the narrower cracking width improves the interlocking performance.

Masaru Sogo et al. shows the effect of water-cement ratio and aggregate type on shear strength of recycled concrete [RC] beams without shear reinforcement which is obtained by V/(bd), where V denotes shear force at diagonal cracking, compared with shear strength of virgin concrete [VC], the shear strength of (RC) indicated 20% decrease at most. This decrease can be explained mainly by the weakness of aggregate without the roundabout crack on the surface of the aggregate, which should reduce fracture energy and interlocking effect, while the effect of the difference of compressive strength on the shear stress is included. Reducing water-cement ratio increases the shear strength and it can be concluded that reducing water-cement ratio improve the shear property of RC beams with recycled aggregate.

ABSORPTION CAPACITY

The absorption capacity is one of the most significant properties that distinguish recycled aggregate from raw aggregates, and it can have an influence both on fresh and hardened concrete properties. The absorption capacity of recycled aggregates are affected by adhered mortar and they must be known prior to the utilization of recycled aggregates in concrete production in order to control properties of fresh and hardened concrete. Recycled sand will be avoided, due to its absorption capacity, which would no doubt produce a shrinkage effect. Workability of recycled aggregate concretes is affected by the absorption capacity of the recycled aggregates.

INFLUENCE OF METHOD OF CRUSHING

The crushing method (mechanical or manual) of concrete to produce coarse aggregate for the production of new concrete is one of the factors that affect the strength of the concrete. The crushing procedure and the dimension of the recycled aggregate have an influence on the amount of adhered mortar.

INFLUENCE OF METHOD OF PREPARATION

During the period of mixing concrete ingredients, care must be taken to ensure that a consistent and homogenous mass is obtained to avoid low quality concrete and adequate care taken during placing and compaction to minimize the probability of occurrence of bleeding, segregation and honey comb.

INFLUENCE OF CURING

The quality of concrete is determined by the manner in which curing is accomplished. Evaporation of water entails cessation of hardening of a fraction of binder grains that are as yet failed to hydrate causing air to take up their spaces forming supplementary voids in the texture of the hardening concrete.

FAILURE MODES OF CONCRETE BEAM WITH RECYCLED AGGREGATE

It is a well-known fact that flexural members can fail not only from bending moment but also due to its low shear resistance and absence of shear reinforcement. Lawrence observed that the primary cause of failure in beams may be due to splitting along the longitudinal reinforcement in tension zone caused primarily by transverse shear in the reinforcement or due to crushing in the zone resulting from combined state of shear and compression in the concrete or from a critical state of combined stress without significant relative rotation of the segment. it was observed that the mode of failure behaviour of recycled concrete beams were similar to normal concrete beams.

Failure of reinforced concrete beams is classified under two main groups:

(a) Flexural failure.

(b) Diagonal failure.

FLEXURAL TENSION FAILURE OF CONCRETE BEAM WITH RECYCLED AGGREGATE

The first flexural cracks appear at the points of maximum moment. As the applied load continues to increase, the flexural cracks gradually increase in number as well as length and usually propagate vertically towards the centroidal axis of the beam. The first type of crack is often referred to as a web-shear crack, the second type being identified as a flexural-shear crack. The flexural crack causing the inclined crack is the initiating flexural crack. Secondary cracks results from splitting forces developed by the deformed bars when slip between concrete and steel reinforcement occurs, or from dowel action forces in the longitudinal bars transferring shear across the crack.

DIAGONAL TENSION AND COMPRESSION FAILURE OF CONCRETE BEAM WITH RECYCLED AGGREGATE

In beams where shear effect is significant, diagonal cracks are formed due to diagonal tension resulting from a combination of shearing and flexural tension. It is evident that the value of diagonal tension is generally indeterminate. For this reason, it is the practice now for beams with shear reinforcement to calculate the value of vertical shearing unit stresses developed in the beams noting of course that the actual diagonal tension is considerably greater than the vertical shearing stress. In fact, most failures that are termed shear failure are diagonal tension failure, shear bending failure, shear bond failure, and occasionally compression failure. The diagonal crack starts from the least flexural crack and more inclined under the shear loading.

A diagonal tension failure is defined as an inclined crack in the shear span extending from tensile reinforcement towards the nearly concentrated load and intersecting the level of tensile reinforcement at an angle of approximately 450. This occurs when the shear force is relatively large and bending moment rather low. It is usually when the load is close to the support. Large diagonal cracks propagate from the support to the load as shown in the figure 2.1(a)

image

The manner in which inclined cracks develop and grow and the type of failure that subsequently develops is strongly affected by the relative magnitude of the shear stress and flexural stress.

Angelakos et al. Experimental data has shown that in normal strength concrete, the shear crack propagates in the hardened cement matrix, and around the relatively stronger coarse aggregate. In higher strength concrete where the matrix is relatively stronger, the shear crack passes through the matrix as well as the aggregate, forming a smoother crack surface.

Etxeberria M. et al. studied the failure mode of recycled concrete beam and observed that the failure of the concrete derives from its weakest point. The weakest point being in these medium strength concretes, the recycled aggregates themselves. In medium strength conventional concretes, the interface is the weakest point, however this is not the case when the concrete is made with recycled aggregates, as what happened in high strength concretes where the failure is through the aggregates. His research also shows splitting tensile failure of concrete made with a high amount of recycled aggregates. The failure happened through the recycled aggregates (the recycled aggregates being the weakest point) producing two similar symmetric faces, the failure never happened in the new interfacial transition zone.

Belen Gonzalez-F. et al. studied the behaviour of concrete beam made with conventional and recycled coarse aggregate, each specimen without shear reinforcement exhibited an initial flexural crack at the centre of the specimen and subsequent flexural cracks away from that section. As the applied load was increased, one of the flexural cracks extended into a diagonal crack near one of the supports, or a diagonal crack formed abruptly at the mid-height of the beam within the shear span. After the formation of the diagonal crack, brittle failure occurred. The specimens with shear reinforcement showed the same crack pattern as the specimens without shear reinforcement until the formation of diagonal cracking, but showed higher load-carrying capacity following it. Considerable splitting cracks along the tension reinforcement were observed, especially in recycled concrete beams. The inclusion of an appropriate amount of minimum shear reinforcement (maintaining an appropriate stirrups spacing) controlled these horizontal-splitting cracks and resulted in improved shear response. Recycled concrete beams present almost the same shear force at failure and deflections as the conventional concrete beams with an equal amount of transverse reinforcement, as the amount of transverse reinforcement increases and the stirrup spacing decreases, the shear force at stirrup yield increases. However, a rise in the amount of transverse reinforcement obtained by increasing the stirrup diameter and maintaining the stirrup spacing causes lower shear forces at stirrup yield. It was also noted that only the recycled concrete beams achieved the shear force at stirrup yield in the spans. The ratio of shear force at failure to shear force at cracking indicates that the recycled concrete beams reached shear force at cracking in earlier load stages than conventional concrete beams.