695

EXPERIMENTAL STUDIES OF FLEXURAL STRENGTH AND MECHANICS PERFORMANCE FOR RECYCLED CONCRETE BEAM

Shouchang DENG (1) (2), Fang YU (3)

(1) School of Civil Engineering and Architecture and Mechanics, Central South University of Forestry and Technology,Hunan Province , Changsha 410004,China

(2) Department of Architecture and Civil Energineering, Huizhou Zhou University, Guandong Province, Huizhou 516007, China

(3) School of City Management, Hunan City University 413000, China Abstract: The intent of this paper is to investigate the flexural behavior of 4 recycled concrete simply-supported beams with same percentage of reinforcement and axial compressive strength but different replacement ratio of recycled aggregates. Besides the flexural capacity of normal section, the characteristics of both deflection and cracking are analyzed. Based on the experimental results, the following conclusions are drawn. In bending, the recycled concrete beam also has elastic, cracking, yield and ultimate point. The average strain measured on cross-section obliges to the plane section assumption. Under the same conditions, the cracking and ultimate moment of recycled concrete beam are similar to conventional concrete beam. After verifying the formula suggested by present code for Design of Concrete Structures(GB50010-2002) for recycled concrete beams, a conclusion can be drawn that the ultimate moment of recycled concrete beam can be estimated by the formula suggested by present code, whereas the cracking moment, the deflection and the width of crack can not. Keywords: recycled concrete beam, compressive strength, bending strength, loading, tress, strain, deflection, crack, elastic module, mechanical performance

1. INTRODUCTION XiaoJianzhuang and LanYang[1] completed three roots of the tests about flexural property

of recycled concrete beams, the replacement rates of the recycled coarse aggregate respectively were 0,50 and 100% .

Huang Qing[2] discusses the basic experimental research method of an ordinary concrete flexural frame member, the parameter is ratio of reinforcement, design for five different ratio of reinforcements, in all 15 roots of recycled concrete beams, 3 roots are under-reinforced beams, the others are balanced-reinforced beams.

K.I shill[3] and other researchers who studied flexural characteristics of recycled concrete beams, found that the difference about bearing capacity between recycled concrete beams and

696

ordinary concrete is not large, but the crack of recycled concrete beam is wider than ordinary concrete beam.

Ajdukiewicz [4] completed the bending test on 12 roots recycled concrete beams, including three common contrastive concrete beams, the concrete compressive strength was divided into low strength (30 ~40MPa), medium strength (50 ~60MPa), and high strength (80 ~ 90MPa). Experimental results show that the difference between bearing capacity of recycled concrete beams and the same intensity of common concrete beam is not large, but the deflection of recycled concrete beams is obvious larger than natural aggregate concrete beams, the deflection is larger by 10%-25% under the normal condition, and larger by 30%-50% under the limit condition.

Maruyama[5] and other researchers studied bending resistance through 12 roots recycled concrete beams. The main concern was on water cement ratio of concrete, aggregate composition, and mixing expanding agent. In the experiment, the cracking bending moment, the maximum crack interval, the maximum crack width, the deflection under the ultimate load, and plastic deformation are measured. Research shows that the bending resistance of recycled concrete beam is not larger than ordinary concrete beam, and the crack’s interval of recycled concrete beam is smaller than ordinary concrete beam；The use of expanding agent reduced crack’s width, crack’s width measured value decreased by 20-30% compared to the theoretical value. The deflection of recycled concrete beam is larger than ordinary concrete beam.

Yagishita[6] and other researchers made four roots concrete beams, one of them is common concrete beam’s contrastive beam. The compressive strength of concrete was 30-32 MPa, tensile strength was 2.4-2.8 MPa. Test results show that the medium grade’s initial crack bending moment of recycled concrete beam is the largest, the other two recycled concrete beams are different degree of below normal concrete beams; The medium grade’s ultimate flexural capacity is the largest, every beams’ load- deformation curve are consistent.

In addition, Dolara[7] and other reseachers studied force analysis of recycled concrete prestressed beams. Results show that the deformation of recycled concrete prestressed beams is significantly increased compared to normal concrete beam, the more content of the recycled aggregates, the larger the deformation.

In order to master the physical and mechanical properties of recycled concrete beams, the authors have completed a group of experiments with respect to flexural bearing loading capacity so that they can deeply study the performances of recycled concrete beams that are made by recycled coarse aggregates or coarse and fine aggregates.

2. DESIGN AND FABRICATION OF SPECIMEN

2.1 Specimen design Recycled concrete bending experiment is composed of 4 beams, Number RCL1 beam is

in contrast to ordinary natural coarse aggregate, Number RCL2 and Number RCL3 both are natural yellow sand of recycled concrete beam, that recycled coarse aggregate replacement rates are 50％ and 100％respectively. Number RCL4 is called complete regenerated beam, whereby the recycled coarse aggregates replacement rate is 100％, and fine aggregates are also recycled fine aggregates.

A test experiment was carried out in order to study the recycled fine aggregate influence on recycled concrete beam normal section stressed and deformation performance. The

697

dimensions of the 4 bending beams are 150mm×300mm. All of the structural element beam have the same reinforcement. Configuration of 2φ 12 handling reinforcement in compression zone, configuration of 2φ 16 longitudinal tensioned reinforcement in tension zone, concrete cover depth is 25mm, reinforcement stirrup is φ 6@100. Tensile reinforcement ratio is ρ＝0.976%, in shearing force and bending moment zone , reinforcement ratio is svρ ＝1.1%. Specimen parameter is showed in Tab. 1. Each specimen of longitudinal reinforcement design, 1-1 section reinforcement and 2-2 section reinforcement are as showed in Fig. 1.

Table 1 Specimen parameters table

Coarse Aggregates Replacement Ratio (％)

Number Section Size（mm）

Longitudinal Reinforcement

Reinforcement Ratio ρ（%）

Span L0 (m)

0 RCL1 150×300×3000 2φ 16 0.976 2.8 50 RCL2 150×300×3000 2φ 16 0.976 2.8

100 RCL3 150×300×3000 2φ 16 0.976 2.8 100 (complete regeneration)

RCL4 150×300×3000 2φ 16 0.976 2.8

2φ16

φ6@1002φ122φ12φ6@100

2800

100 100

φ 6@ 10 0

2φ 16

2φ 12

2φ 16

b) Section 1-1 c) Section 2-2

Fig. 1: Sections reinforcement drawing of the beam 2.2 Specimen fabrication

The batch 4 root bending beams are casted in “Hunan City University Structural Laboratory” according to the concrete design mixing proportion[8-11] and construction drawings on October 15, 2008. The specimens were made indoor, and cured for 28 days,

698

being watered daily, each specimen had 3 standard cube of concrete blocks（150mm×150mm×150mm）and 9 prismoidal blocks （150mm×150mm×450mm）, standard test blocks and specimens were cured at 28 days under the same conditions. Concrete design strength was C30. The design of concrete is showed in Tab. 2and Tab. 3. Using blender mixing concrete, wood form molding, and inserting the vibrator to vibrate the concrete.

3. TEST PLAN 3.1 Test loading equipment

In order to eliminate the effect of shear stress to normal section stressed capacity, in this experiment, beam form the pure bending of the length of 1.2m. The experimental equipment is shown in Fig. 2. The beams’ bearing end is fixed hinge- bearing, the other end is scroll hinge- bearing, which can make proper movement in a horizontal direction. With 30 tons of hand screw two-point loading, strength is read by 200 tons of sensors. Gradual loading process, before the application of the load, an 8kN preliminary load(every level 2KN is added) is added. This is to test and make sure all measuring instruments are functioning properly, the relationship between deformation and load remained stable. Initially, when loading, a stress of 2KN is added at each level until more vertical cracks appear on the structural member by macroscopic observation. Then change and increase the stress added on every level to 5KN. The test operation should be strictly according to the 《Concrete Structure Standard Test Method》(GB 50152-82-92). Fig. 2 and Fig. 3 shows the loading device diagram.

a) Facade of loading equipment b) Prifile of loading equipment and instruments

Fig. 2: Loading device real diagram

699

分配梁

传感器

100100

2800

8001200800

a) Structural diagram of loading equipment

100100

2800

8001200800

b) Loading schematic diagram

Fig. 3: Schematic diagram of loading equipment

3.2 Arrangement of deformation measuring point In order to determine deflection change of the beam in the process of loading, each test

beam layout has five dial indicators as Fig. 4, Number1 and Number5 dial indicators are at the end of the structural member support. Number 2 and Number 4 dial indicators are at the beam bottom where there is concentrated force, and Number 3 dial indicator is at beam bottom mid-span. The measuring range of Number1 and Number5 dial indicator is 10mm, the measuring range of Number3, Number4 and Number5 dial indicators is 50mm. The position of deflectometer installation should be burnish smoothly, and clean it with acetone, and then stick to glass sheet (30mm×30mm) with the fast drying glue 502 to ensure the smoothness of the measuring point.

100100 800800 1200

Fig. 4: Layout of dial indicator

Sensor

Steel beam

700

3.3 The arrangement and glue of strain gauges

Reinforcement strain measuring point layout on longitudinal tensile reinforcement of girder span section, NumberS1, S2 as Fig. 5, layout one piece of strain gauge above every steel; concrete strain measuring point mainly layout at two side of section concrete in mid-span, at the top of beam mid-span section and at the bottom of beam mid-span section respectively layout one piece of Number C1 ～C9. The layout of strain gauges is showed as Fig. 6.

2800

8001200800

、

Fig. 5: Measuring points layout of strain gauge on steel

2800

a) Elevation layout of strain gauge on concrete（front）

2800

(

b) Elevation layout of strain gauge on concrete（back）

Fig. 6: Measuring points layout of strain gauge on concrete

During the test experiment, stick the 3mm × 5mm electric resistance strain gauge to the steel reinforcement and stick the 3mm × 100mm electric resistance strain gauge on the surface of the concrete. Adhesive is octyl 2-cyanoacrylate(502glue), at first, burnish patch parts with sandpaper, and clean it with absorbent cotton dips in aceton, then paste a thin AB glue homogenous, after 24 hours when the glue is dry, stroke gently into the direction of cascade 45° with needle point and clean it with absorbent cotton dips in acetone, dry this position with blower, when surface mounting, put a piece of polyethylene film on the strain gauge, roll it with finger, extrude redundant binder and bubbles. After completing the above steps, examine bond strength whether is enough, whether glue line has no spare bubbles, whether position is accurate, and whether resistance value is normal. If any of the above

701

abnormality is encountered, the should stick strain gauge again. Seal the glue after 15 min, before sealing the glue dry the surface of the strain gauge with blower. The strain gauge on the steel reinforcement should be wrapped tightly with cotton gauze before sealing glue. After sealing glue you need 4 ~ 6 hours to allow it to dry. The above process can ensure that 95% of the chip mounting reach the requirement in bonding strength and insulation resistance.

3.4 Crack observation Crack observation under each level of loading should follow the following methods: (1)

after loading stabilizes, clean the side of the test beam with acetone cotton, observe the appearance of crack and any situation which developed and read through microscope; (2) give numbers to new cracks, record all the developed situation near the cracks, and write down the load value and maximum crack width; (3) observe the maximum crack width and write down crack width under each level load for the new cracks which appear. Depict crack development trend and record the crack width with coordinate paper.

3.5 Test loading procedures The experiment was carried out in “Hunan City University Structure Laboratory”, from

early October 2008 to early February 2009, lasting 4 months. Before the experiment started, the beams were lifted to the corresponding location and

put on the support, the surface of beam was printed white, after it was fully dry, 50mm×50mm square grid were drawn on it, this is ensure position of every measuring point, and then the machinery dial indicator was installed, the strain gauges were tied to conducting line to BZ2208－Astatic resistance and examining the resistance whether to make sure it functions normal, using the method of member compensation for compensation of temperature. The mechanical jack with geometric method was installed, wire splice of 200 tons force sensor was tied to YJZ-16 intellectual digital static resistance and read the value of the strength. After all of the wire splice and equipment were found to be normal the test began.

When the test started, level 3 load was exerted, the value of each level is 2KN, a reading must be taken after each load-level is complete, the dial indicator is read, the reading of each strain gauge collected and recorded. Calculating and comparing the deflection of Number3 dial indicator under each load; Calculate and compare strain on both sides of symmetrical position. Then load shedding, after unloading according to the formal computed result regulate the position of mechanical jack in order to achieve the best physical alignment. In general, the error of deflection and strain should be controlled in 5%. Strict physical alignment is sometimes difficult, it is needed to shift again and again to meet the requirement.

After accomplishing physical alignment that can load with formal formulated system, and after completion of per level loading, the strain values are collected and recorded. The crack development situation and crack width are also read and recorded.

702

4. EXPERIMENT’S MAJOR RESULT AND ANALYSIS

4.1 Physical and mechanical capability of material Through the provision of 150mm×150mm×150mm cube test block and the

150mm×150mm×450mm prism blocks, respectively measure compressive strength, elastic modulus and tensile strength of recycled concrete. Tensile test reinforced specimens meet current national standards requirements. The results are shown in Table 2 and 3.

Table 2: Main typical character index experiment results of concrete

Specimen number

standard cube of concrete compressive

strength cuf (N/mm2)

Concrete compressive

strength ckf (N/mm2)

Axial tensile strength of concrete tkf (N/mm2)

Elastic modulus of concrete

Ec (×104 Mpa)

RCL1 29.1 19.0 2.46 3.78 RCL2 36.4 23.8 2.41 2.91 RCL3 35.8 23.4 2.38 2.83 RCL4 35.0 22.9 2.35 2.50

Table 3: Main typical character index experiment results of steel

Steel Type

yield strength ykf (N/mm2)

ultimate strength

limf (N/mm2)

elastic modulus Ec (×105 Mpa)

elongation (%)

φ 6 261.5 349.0 1.98 29.0 φ 12 339.0 592.4 1.99 32.1 φ 16 342.6 576.8 2.08 30.9

4.2 Crack development and failure modes

RCL1 (0%): When the load is small, the recycled concrete beam is basically in flexible working conditions, load and deflection of test beam is close to linear changes, concrete and steel strain is also small. As the load increased to 18KN, concrete of mid-span beam bottom edge fiber strain arrive tensile strain, concrete cracking (bottom longitudinal reinforcement contingency to a larger jump). When the load increased to 24KN, observed the first batch of vertical cracks. Cracks firstly appear near mid-span and concentrated load. Cracks first appeared, crack width is smaller, crack width is 0.04mm. As the load increased, cracks in the beams extended laterally, and continue to see more new small cracks, as the crack width increased, the number of cracks increased relatively fast. When the load reached about 74KN the crack is basically homogeneous. When the load reached about 84KN the longitudinal steel yielded. When the load increased to 88.1KN, the maximum width of vertical cracks is up to

703

1.46mm, cracks in the concrete mid-span compressive zone appeared, and the beam crushed. When the test beam reached bending resistance, deflection wass 8.292mm. Figure 7 shows the typical cracks.

a) Failure of RCL1

b) Crack development of RCL1

Fig. 7: Crack development and destruction of RCL1 RCL2 (50%): When starting to load, the load and the deflection was proportional, strain

of each measuring point at the beam section is small, beam work situation is similar with elastic beam. With the increase of load 18KN, concrete of mid-span beam bottom edge fiber strain arrive tensile strain, concrete cracking (bottom longitudinal reinforcement contingency to a larger jump). When the load increased to 28KN, observed the first batch of vertical cracks. Cracks firstly appear near mid-span and concentrated load. Cracks first appeared, crack width is smaller, crack width is 0.03mm, cracks in the beams extended laterally, and continue to see more new small cracks. Load continued to increase to 40KN, test beam emerge new cracks and development. When the load increased to 90kN, the crack is basically homogeneous, longitudinal reinforcement reached its yield. A moment later the test beam cracks and deflection increased dramatically, while the increased load at this time is not much. When the load increased to 94KN, the maximum width of vertical cracks is up to 1.50mm, test beam at the top of concrete is pressure crisp, the beam then crushed. When the test beam reached the bending resistance, deflection was 8.992mm. Figure 8 shows the typical cracks.

704

a) Failure of RCL2

b) Crack development of RCL2

Fig. 8: Crack development and destruction of RCL2 RCL3 (100%): When the load was initially small, the load and the deflection was

proportional, strain of each measuring point at the beam section is small, beam work situation is similar with elastic beam. As the load increased to 16KN, the bottom of the concrete beam edge fiber strain reached the tensile strain, concrete cracked(bottom longitudinal reinforcement contingency to a larger jump). When the load increased to 29KN, observed the first batch of vertical cracks. Cracks firstly appeared near mid-span and concentrated load. Cracks which first appeared had smaller crack width of only 0.06mm, and cracks in the beams extended laterally. With the increase of the load to 39KN, new cracks emerged on the test beam and development. When the load increased to 89kN, the cracks were basically homogeneous, longitudinal reinforcement reached its yield. A moment later the test beam cracks and deflection increased dramatically, while the increased load at this time was not much. When the load increased to 91KN, the maximum width of vertical cracks was up to 1.54mm. The test beam at the top of concrete was pressure crisp, then the beam collapsed. When the test beam reached the bending resistance, deflection was 9.246mm. Figure 9 shows the typical cracks.

a) Failure of RCL3

b) Crack development of RCL3

Fig. 9: Crack development and destruction of RCL3

705

RCL4 (100%, full regeneration): When the load was initially small, the recycled concrete beam is basically in flexible working conditions. As the load increased to 14KN, the bottom of the concrete beam edge fiber strain reached tensile strain. When the load increased to 24KN, observed the first batch of vertical cracks. Cracks first appeared near mid-span and concentrated load. Crack width was smaller, of 0.04mm. Cracks in the beams extended laterally, the new cracks emerged in the test beam. After concrete cracked, steel strain suddenly increased. Load continued to increase to 36KN, test beam emerge new cracks and constant continuation of cracking. When the load increased to 88kN, the cracks were basically homogeneous, longitudinal reinforcement reached its yield. A moment later the test beam cracks and deflection increased dramatically, while the increased load at this time was not much. When the load increased to 89KN, the maximum width of vertical cracks was up to 1.86mm, the top of the concrete test beam was pressured crisp. The beam then collapsed. When the test beam reached the bending resistance, deflection was 9.400mm. Figure 10 shows the typical cracks.

a) Failure of RCL4

b) Crack development of RCL3

Fig. 10: Crack development and destruction of RCL4 These 4 test beam crack development is basically similar. When the beam cracked, they

are accompanied by a slight sound of cracking, the crack width increases with increasing load, after longitudinal reinforcement yield, the crack width increases rapidly until the test beam collapses. The 4 beams’ damaged crack spacing are the same, about 100mm. However, the four test beams crack situation is different, recycled concrete beam cracking moment is slightly larger than the ordinary concrete beam, recycled concrete beam crack width is less than ordinary concrete beam at the same load; when the beam collapses, the crack width of the recycled coarse aggregates increased with the increase of recycled coarse aggregate replacement rate, in this test. 4.3 Steel strain

The 4 test beams’ load – mid-span steel strain curve is shown as Figure 11. It can be seen

706

from the graph, each curve roughly consists of 3 parts, before cracking is the line segment of flexible working stage, stage work with cracks and steel yield curve after the horizontal section. In comparing the eight curves it can be seen that before longitudinal steel yielded, the recycled concrete beams and concrete beam of reinforced strain have little differences. After longitudinal reinforcement yielded, the strain of the recycled concrete beam increase rapidly compared with ordinary concrete beam reinforced strain; then the replacement rate of regenerated coarse aggregate greater, reinforced strain increased greater, which also shows the ductility of recycled concrete beams are slightly higher than normal concrete beams.

a) NO. RCL1 b) NO. RCL2

c) NO. RCL3 d) NO. RCL4

Fig. 11: Load-strain of longitudinal steel in RCL1 、RCL2、 RCL3、RCL4 in bottom of recycled concrete beams

4.4 Strain of mid-span concrete

Figure 12 are load-strain curve of mid-span concrete which was measured by strain

707

gauge. We can see from the figure that from the beginning load to the vertical steel yield, at a particular load, concrete strain of the cross section point with the distance of the point to the neutral axis is approximate proportion. The resnlt showed that the plane section assumption is still right in bending process of recycled concrete beam.

a) No: RCL1 b) No: RCL2

c) No: RCL3 d) No: RCL4

Fig. 12: Load-strain of mid-span concrete in RCL1

4.5 Load–deflection curve RCL1 ~ RCL4 measured bending beam load-deflection curve is shown in Figure 13.

708

Fig. 13: Load-deflection diagram of flexural beam

It can be seen from Figure 13, that the same as with the ordinary concrete beam, during the loading process, recycled concrete beam also has significant flexibility, the work of crack stage and destruction stage.

In the elastic stage, the load deflection relationship is linear change; after cracking the longitudinal reinforcement yield and the load deflection relationship is non linear; after longitudinal reinforcement yield, the load deflection relationship was level. With comparison of the four beams of the load-deflection curves can be seen that before the test beam cracks, at the same loads, beam RCL4 (100%, full regeneration) deflection is larger, RCL3 (100%) and RCL2 (50% ) are second, while RCL1 (0%) is the least. This is due to the elastic modulus of recycled concrete is lower than ordinary concrete.

4.6 Deformation and characteristics of Recycled Concrete Beams 4.6.1 Elastic stage

By observing the experimental phenomena of the four recycled concrete beams and the Load-deflection curve, it can be clearly seen that: at the beginning of loading, beam bending moment is very small, section of the strain is also small, the load-deflection curve into a linear increase, concrete is in the elastic stage, the stress and strain is proportional. With the further increase of load, stress and strain of concrete has also increased. When the load reached about 15% of ultimate load, the beam concrete strain of tensile zone reaches the ultimate tensile strain of concrete, small tensile cracks began to appear, showing a large plastic deformation. Then with the increased load, the tensile zone of concrete cracks further. At this time, the compressive stress of recycled concrete is far less than the compressive strength of recycled concrete, therefore, the concrete compression zone is still in the elastic stage. 4.6.2 Crack to the steel yielding stage

After bending moment arrive cracking moment, continue to increase the load, in pure bending, the tensile strength of concrete section of the weaker section of the first cracks appeared. As the load increased, the number of fractures also increased, fracture height

709

upward quickly, but little change in crack width. When RCL1 (0%) reached about 85% of ultimate load, the crack is basically homogeneous, the numbers of crackes is not increase. When the load is close to 90% of the ultimate load, the longitudinal steel reaches to the yield status, then the maximum crack width is about 1.4mm. When the RCL2 (50%), RCL3 (100%) and RCL4 (100%, full regeneration) reached about 95% of ultimate load, the crack is basically homogeneous, the numbers of crackes is not increase , longitudinal reinforcement will reach to the yield status, then the maximum width of crack is about 1.5mm. The stage of concrete strain increased, but under a larger gauge measuring the average strain of concrete means that section of the strain distribution is basically consistent with the plane section assumption (see Figure 1 and2). 4.6.3 Steel yield and recycled concrete beam failure

When the load was added up to about 95% of the ultimate load, the tensile steel yielded, crack is rapidly upward, crack width increased rapidly, the beam deflection also increased rapidly, with cracking sound, the pure bending section of the steel yield led to a crack width so much bigger that it rapidly developed to the top of the beam. At the same time, it still continued to bear the load beam. After recycled concrete beam is continuely loaded, in a certain area, the beam at the top of the main crack on both sides, produced a large concrete. compressed zones of congcrete produce a very big plastic deformation, forming a more concentrated area of plastic deformation, and there are levels of crack around the plastic zone, measured strain is also had a significant increase in the compression zone, then the beam deflection increased dramatically, neutral axis rapidly rise, a sharp increase in cross-section of the corner, concrete test beams declared destruction. 4.6.4 Status of recycled concrete flexural beam damage

Concrete beam flexural tensile reinforcement ratio is ρ = 0.976%, appropriate Beams, showing ductile failure. Tensile steel reinforcement reaches yield strength first, then steel strain rapidly increases, the tensile cracking further develops, compressed zone of concrete is further damage when achiev ultimate compressive stress cuε . Failure state results are as shown in Table 4 below. finally, compressed edge fibrous occurred press

Table 4: Destruction results of flexural beam

Beam number

Steel yield strain yε (×10－6)

Yield strain of concrete

cε (×10－6)

Ultimate load of beams

yP (KN)

beam ultimate moment

yM (KN·M)

RCL1 1518 -788 86 34.4

RCL2 1817 -1207 94 37.6

RCL3 1908 -1148 91 36.4

RCL4 2507 -1371 91 36.4

710

5. CACULATION METHOD 5.1 Affecting factors of normal section of flexural bearing load capacity

This test is replaced by renewable aggregate ratio variation parameter[12-15], the study of recycled concrete beams are performed. The results indicate that the parameters on the performance of the beam bending, regeneration aggregates shear-span ratio of recycled concrete influence the compressive strength and modulus of elasticity and the ultimate bearing capacity of beams. Recycled concrete compressive strength and regeneration of concrete beams' ultimate bearing capacity of common concrete was increased[17-20], including replacing rate for the aggregate 50% of recycled concrete to improve these two aspects most.

5.2 Calculation of the flexural capacity in the normal section In the same condition of the longitudinal reinforcement ratio, sectional dimensions and

concrete strength, for the convenience of structure design, this paper was combined with the criterion [65] of bending capacity calculation method. Based on the experiment data, this paper approaches calculation method of flexural capacity of recycled concrete normal section.

5.2.1 Basic assumptions According to the breaking characteristics of the test beams, considering the connecting of

the calculation methods which the flexural capacity of the sections in the reinforced concrete beams, we followed the assumptions that:

(1) Basically corresponds the assumptions with flat section. As illustrated in figure 12 and test data, from the beginning of loading to failure the entire section was flat, approximately remaining a flat section.

(2) Do not consider the tensile strength of concrete. (3) For the rectangular cross-section, the compressive stress distribution of compressed

concrete adopt the reduced height, With high pressure zone measured according to article 0x .Based on the test case the compressive stress distribution of compressed concrete is 1 0xβ . 1β 's value for the concrete strength: when the grade of the concrete strength does not exceed

C50,it is 0.8.If it is not exceed C80, it's 0.74. During this period determined by linear interpolation.The ultimate compressive strain of concrete beams ε according to the following formula values:

uε ＝0.0033－（ Rcuf －50）×10－5 ≤ 0.0033 （1） where: Rcuf ——Measured compressive strength of recycled concrete cube（Mpa） （4）When the beam is damaged, the longitudinal steel has reached to yield strength of

tensile reinforcement.

5.3 Caculation and analysis of bearing capcity of flexural bending of the recycled concrete beam

In accordance with the norms of basic assumptions given the formula, and then compared with the experimental situation, and then make use of recycled concrete beam flexural capacity formula. Standard-method and testing of the actual results were shown in

711

Table 5

Table 5: Contradistinction between specification method and practical result of test

Component

Standard method

Test

compressive strength（Mpa）

Strength of the standard

value cf

1α Tensile strength

steel（Mpa）

As

(mm2)

x

(mm)

Bending design value

（KN·M）

ultimate load（KN）

ultimate bearing capacity（KN·M）

RCL1 29.1 19.0 1.0 342.6 402 48.6 34.5 86 34.4 RCL2 36.4 23.8 1.0 342.6 402 36.7 35.3 94 37.6 RCL3 35.8 23.4 1.0 342.6 402 36.2 34.5 91 36.4 RCL4 35.0 22.9 1.0 342.6 402 35.8 33.7 91 36.4

Above table available: standard [65] method of calculation and experimental results was

phase. Flexural capacity of the beam of recycled concrete calculated according to specifications given：

1

1 0 2

c y s

c

f bx f A

xM f bx h

α

α

= = −

（2）

6. The discussion on the adaptability for eurrent normal to recycled concrete

6.1 Degree of crack resistance Apply the current norms in concrete beams cracking the formula, that is,:

0cr tkM fγω= （3） h mγ β γ= （4）

In this formula: γ is the influence coefficient of concrete section resisting moment; mβ is the influence coefficient of the Section Height , hβ =0.7+120/h. Reference standard, taken

hβ =1.0; mγ is the influence coefficient of Plastic moment resistance of concrete structures. Reference standard, taken mγ =1.55。

According to the test beam dimensions, and recycled concrete material test data, the result of (3.4) shown in Table 6. As it can be seen from Table 3.6, specification, the cracking of concrete beam formula calculated on the ordinary concrete beam is in good agreement, it’s also safer; and calculated value of recycled concrete beams are relatively large.

712

Table 6 Contradistinction between calculated value and theoretical value of beam’s crack moment

Test beam number

Calculated cracking moment (kN.m)

Cracking moment measured (kN.m)

The ratio between calculated and measured values

error（%）

RCL1 5.4 7.2 1/1.33 33.3 RCL2 5.3 7.2 1/1.36 35.8 RCL3 5.2 6.4 1/1.23 23.1 RCL4 5.1 5.6 1/1.10 9.8

6.2 Ultimate moment Based on the test beam reinforcement material strength map, the measured data and

standard bending ultimate strength formula, we can get the ultimate flexural capacity of test beams standard value. The specific results in Table 7. Table 7: Contradistinction between calculated value and theoretical valueof beam’s ultimate moment

Test beam number

Calculated ultimate moment (kN.m)

Measured ultimate moment (kN.m)

The ratio between Calculated and measured alues

error（%）

RCL1 34.5 34.4 1 -0.3 RCL2 35.3 37.6 1/1.07 7.4 RCL3 34.5 36.4 1/1.06 6.0 RCL4 33.7 36.4 1/1.08 8.0

Seeing from Table 7, the same compressive strength of concrete and different replacement ratios of recycled coarse recycled aggregate concrete beams’ ultimate bearing capacity of small differences, and the calculated value is less than the measured values, so the ultimate bearing capacity of recycled concrete beams can be calculated according to the standard formula.

6.3 Mid-span deflection During the test, recycled concrete beams showed large deflections, this is mainly due to

the low elastic modulus of recycled concrete. Put the elastic modulus of recycled aggregate concrete into the specification on behalf of the deflection formula, and the remaining parameters selected reference standard. Taking into account the standard formula in the practicality of the design of concrete beams, this article only approached 50% of the yield moment of stage to the test phase of the beam longitudinal reinforcement yield, and calculate the results shown in Table 8. As it can be seen from Table 8, normal formula is suitable for the ordinary concrete beam, but the calculting datas is smaller for the recycled concrete beams, so standard beam deflection formula does not apply to recycled concrete.

713

Table 8 Contradistinction betweem calculated value and theoretical value of beam’s mid-span deflection(mm)

Bending momentkN.m

RCL1 RCL2 RCL3 RCL4 Calcu-lated

Measu-red

C/M Calcu lated

Meas- ured

C/M Calcu-lated

Meas- ured

C/M Calculate

Meas-ured

C/M

15.6 2.25 2.40 0.94 2.85 3.39 0.84 2.88 3.70 0.78 3.44 4.23 0.81 17.6 2.75 2.97 0.93 3.30 4.02 0.82 3.25 4.28 0.76 3.86 4.88 0.79 19.6 3.25 3.45 0.94 3.75 4.61 0.81 3.62 4.90 0.74 4.28 5.54 0.77 21.6 3.75 4.03 0.93 4.20 5.19 0.81 3.99 5.52 0.72 4.7 6.12 0.77 23.6 4.25 4.60 0.92 4.65 5.90 0.79 4.36 6.11 0.71 5.12 6.77 0.76 25.6 4.75 5.37 0.88 5.10 6.46 0.79 4.73 6.70 0.71 5.54 7.40 0.75 27.6 5.25 6.01 0.87 5.55 7.13 0.78 5.10 7.31 0.70 5.96 8.03 0.74 29.6 5.75 6.60 0.87 6.00 7.76 0.77 5.47 7.90 0.69 6.38 8.67 0.74 31.6 6.52 7.59 0.86 6.45 8.38 0.77 5.84 8.58 0.68 6.8 9.30 0.72

6.4 Maximum crack width

According with standard formulas and reference [27], we can catch the maximum crack width:

max s l mw wτ τ= （5） Where: sτ — ratio of maximum crack width and average crack width in the load time, taken

sτ =1.66; lτ — Long-term load effect coefficient, taken lτ =1.0; mw — the Average crack width

In the calculation of mw , the tensile strength of concrete taking values in Table 3.2, 50% yield moment load to take to test beam longitudinal reinforcement yield moment of the situation, the results is shown in Table 9.

Table 9: Contradistinction between calculated value and theoretical value of beam’s maximum crack width (mm)

Mom-ent kN.m

RCL1 RCL2 RCL3 RCL4 Calcula

-ted Meas-ured

C/M Calcu-lated

Measu-red

C/M Calcul-ated

Meas-ured

C/M Calcu-lated

Meas-ured

C/M

15.6 0.09 0.04 2.25 0.08 0.03 2.67 0.08 0.06 1.33 0.08 0.04 2.00 17.6 0.15 0.13 1.15 0.15 0.14 1.07 0.15 0.15 1.00 0.15 0.16 0.94 19.6 0.22 0.19 1.16 0.21 0.22 0.95 0.22 0.22 1.00 0.22 0.23 0.96 21.6 0.29 0.26 1.12 0.28 0.29 0.97 0.28 0.28 1.00 0.28 0.29 0.97 23.6 0.58 0.54 1.07 0.56 0.57 0.98 0.56 0.56 1.00 0.56 0.57 0.98

714

25.6 0.97 0.95 1.02 0.95 0.96 0.99 0.95 0.94 1.01 0.95 0.97 0.98 27.6 1.31 1.19 1.02 1.30 1.32 0.98 1.30 1.28 1.02 1.30 1.30 1.00

29.6 1.48 1.32 1.02 1.48 1.49 0.99 1.48 1.46 1.01 1.48 1.50 0.97 31.6 1.92 1.46 1.02 1.90 1.50 1.27 1.90 1.54 1.23 1.90 1.86 1.02

As can be seen from Table 3.9, the value of normal formula for ordinary concrete beams

and recycled concrete beams are both larger to calculate, and taking into account the discrete nature of concrete beam crack, standard formula in the calculation of recycled concrete beam crack width, calculation of the width of cracks conservative should be appropriate.

7. CONCLUSIONS

By analyzing the experimental data, we can catch the following preliminary conclusions （1）The calculation of the amount of additional water for recycled concrete is very

important, its a key to workability of recycled concrete[16-21], establishing the use of a new calculation formulae, according to reference [22-25].

(2) The same as the ordinary concrete beam, recycled concrete beam still has significant flexibility, yield, ultimate force of four characteristics during the loading process cracking. Recycled concrete beam of ordinary concrete beams have the same mechanism. Recycled concrete beams used in engineering practice is feasible. （3）The change of strain normal in section of the recycled concrete in the loading

process meet the flat section assumption. （4）Recycled concrete beam cracking moment was a little less than normal concrete

beams. Recycled concrete beams and plain concrete beam have little differences in ultimate moment. In the same moment on, recycled concrete beam deflection is greater than normal concrete beams, the maximum crack width slightly was less than the ordinary concrete beam.

（5）The formula using the current specification of recycled concrete beam ultimate moment is feasible, but in the calculation of recycled concrete beam deflection and maximum crack width it should be amended. How to fix? It needs further study.

ACKNOWLEDGEMENTS This work is Project supported by Hunan Provincial Natural Science Foundation of China

(0 6 J J 4 0 5 8 ); Hunan province construction hall(No:[2007]425-30;[2008]459-8); Changsha sity science and technology office（NO: K042013-12） ; Zhuzhou sity construction office([2010]119-1); Central South University of Forestry and Technology; Huizhou Zhou Universiy nature science foundation(No:C2.10.0107), which are highly appreciated.

REFERENCES [1] Xiao Jianzhuan，Lan yang, Flexural performance of concrete beam for recycled coarse aggregate[J],

Table 9 continued

715

Special Structure ,2006.3(1):9-12 [2] Huang Qing, Test reseach and limit analysis for norm section perporty of cycled concrete[D],

haerbin：Haerbin Agricultural University,2005. [3] Ishill K.etal. Flexible characteric of RC beam with recycled coarse aggregate[R]. Proceeding of the

25# JSCE Annual Meeting. Kanto Branch 1998:886-887. [4] Andrzej B. Ajdukiewicz, Alina T.Kliszczewicz. Behavior of RC Beam from Recycled Aggregate

Concrete[R]. Proceedings of the American Concrete Instiute 2002.International Conference,Cancun,10-13 December 2002.

[5] Ippei Maruyama,Masaru Sogo,Takahisa Sogabe etal. Flexural Properties of Reinforced Recycled Concrete Beams[R]. Conference On the Use of Recycled Materials in Building and Structures,November 2004 Barcelona,Spain.

[6] Yagishita.M,Sano.M,Yamada.M. “Behaviour of Reinforced Concrete Beams Containing Recycled Aggregate”.1993:pp.331-343.

[7] Dolara E.Recycled aggregate concrete prestressed beams.Use of Recycled concrete Aggregate[J].Universtity of Dundee,Scotland,1998.

[8] Xue-bing Zhang, Shou-chang Deng, Yin-hui Qin. Experimental Reseach on Unit Water Use in Recycled Concrete [C]. Yingshe Luo, Qiuhua Rao & Yuanze Xu. Advances in Rheology and Its Applications. 4th Pacific Rim Conference on Rheology, Shanghai, 2005. New York: Science Press USA Inc, 2005: 939-943.

[9] Zhang Xue-bing, Deng Shou-chang, Qin Yin-hui. Additional adsorbed water in recycled concrete[J]. Journal of Central South University of Technology, Vol.14.1, Aug. 2007, 449-453.

[10] Deng Shouchang，Zhang Xuebing, Addition water use influencing strength and fluidity of the recycled concret[C], The first national academic exchange meeting of research and application of recycled concrete technology，China ,shanghai, Tonji University 2008.7：203-207

[11] Xue-bing Zhang, Shou-chang Deng, Yin-hui Qin., Calculation of Water Usage Per Unit Volume of Recycled Concrete[J], 2005, (10).102-104

[12] Xue-bing Zhang, Shou-chang Deng, Xu-hua Deng, etc. Approaching Regression Coefficients in Recycled Concrete Bolomey Formula [C]. Yingshe Luo, Qiuhua Rao & Yuanze Xu. Advances in Rheology and Its Applications. 4th Pacific Rim Conference on Rheology, Shanghai, 2005. New York: Science Press USA Inc, 2005: 730-733.

[13] Xue-bing Zhang, Shou-chang Deng, et al. Experimental research on a few main factors influencing strength of the recycled concrete [J]. Nature Science Journal of Xiangtan University, 2005, 27(1): 129-133.

[14] Deng Shouchang,Zhang Xuebing, Experiment Research on Strength Calculation Formula of Recycled Concrete[C], The first national academic exchange meeting of research and application of recycled concrete technology，China ,shanghai, Tonji University 2008.7：195-202

[15] Yinhui Qin, Shouchang Deng, Xue-bing Zhang, etc. The Frost Resistance of Recycled Concrete [C]. Yingshe Luo, Qiuhua Rao & Yuanze Xu. Advances in Rheology and Its Applications. 4th Pacific Rim Conference on Rheology, Shanghai, 2005. New York: Science Press USA Inc, 2005: 884-887.

[16] Shouchang Deng, Keying Chen, Xue-bing Zhang, etc. Analysis of Rheological Characteristic of Fresh Mixing Concrete for Pumping in Pipeline [C]. Yingshe Luo, Qiuhua Rao & Yuanze Xu. Advances in Rheology and Its Applications. 4th Pacific Rim Conference on Rheology, Shanghai, 2005. New York: Science Press USA Inc, 2005: 959-962.

[17] Qin Yin-hui, Shou-chang Deng, Xue-bing Zhang . Frost Resistance of Recycled Roncrete [J]. Nature Science Journal of Xiangtan University, 2006.9(3): 117-121.

716

[18] Zhang Xue-bing, Deng Shou-chang, Luo Yingshe. Abandon the concrete present condition of the reborn exploitation analysis and the research outlook [J]. Concrete，2006.11(11): 20－24.

[19] Qin Yin-hui, Shou-chang Deng, Xue-bing Zhang . Current Situation Utilization of Recycled Roncrete[J]. Coal Ash China, 2006，(6): 41- 43

[20] Zhang Xue-bing, Deng Shou-chang, Deng Xu-hua, Qin Yin-hui. Experimental research on regression coefficients in recycled concrete Bolomey formula[J]. Journal of Central South University of Technology, Vol.14.1, Aug. 2007, p 314-317.

[21] Deng Shou-chang, Zhang Xue-bing, Qin Yin-hui, Luo Guan-xiang. Rheological characteristic of cement clean paste and flowing behavior of fresh mixing concrete with pumping in pipeline[J]. Journal of Central South University of Technology, Vol.14.1, Aug. 2007, 462-465.

[22] Qin Yin-hui , Deng Shou-chang , Zhang Xue-bing . Frost resistance of recycled concrete[J]. Journal of Central South University of Technology, Vol.14.1, Aug. 2007, p 409-413.

[23] Deng Shouchang，Wang Zhengping , Zhang Xuebing ,Zhang xinsheng , Yu Fang , Circulation Utilization of Building Solid waste and Custainable Development of Recycled Resources[C], The first national academic exchange meeting of research and application of recycled concrete technology，China ,shanghai, Tonji University 2008.7：41-52 e

[24] Zhang Xuebing ,Fang Zhi,Deng Shouchang, Cheng Ke, Qin Yinhui, Addition water use influencing strength and fluidity of recycled concrete[J], Journal of Central South University of Technology, Vol.15 suppl.1, Sept. 2008, 221-224

[25] Deng Shouchang，Li Wei, Wang Zhengping,Zhang Xuebing, Recycling of Solid waste and the Building of Circular Economical System [J]. Journal of Central South University of Forestry & Technology(Social Sciences).2009.12（6）：58-61；65