AAC-concrete light weight precast composite floor slab

Construction and Building Materials 40 (2013) 405–410

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AAC-concrete light weight precast composite floor slab Yavuz Yardim a,⇑, A.M.T. Waleed b, Mohd. Saleh Jaafar c, Saleh Laseima c a

Engineering Faculty, Epoka University, Albania Engineering Faculty, University of Nizwa, Oman c Engineering Faculty, UPM, Malaysia b

h i g h l i g h t s " We test nine different full scale slabs to find optimum solution for AAC precast slabs. " We examine changes in the layout of precast floor and amount of AAC. " The dead load of the slab can be reduced by using proposed composite slab 32-23 % compeered to solid RC. " Based on strain monitoring of the test specimens, structures perform in a fully composite manner until the ultimate load. " Ductility and maximum deflection of the all tested slabs are well enough to give warning before failure.

a r t i c l e

i n f o

Article history: Received 27 August 2012 Received in revised form 20 September 2012 Accepted 12 October 2012 Available online 12 December 2012 Keywords: Composite Light weight slab Aerated concrete Ferrocement

a b s t r a c t In this study, the use of Autoclaved Aerated Concrete (AAC) as an in fill material for semi precast panel is investigated experimentally. The effectiveness of proposed light weight slab is reached by comparing the behavior of specimens with that of conventional solid precast slab. The comparisons were based on structural performance and total weight reduction. The composite AAC slabs section chosen are one way slabs with a size of 1m 3m 0.130 m (Width Length Depth). The specimens vary in the AAC blocks layouts and total weight reduction ratio. The test results showed that the AAC composite precast panel provides reasonable weight reduction without sacrificing the structural capacity. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction A slab structure occupies the biggest percentage of total dead load and volume for an ordinary residential structure. A simple load calculation for a residential building shows that approximately 40–60% of dead load is self weight of slab structure [1]. Thus approximately 10% of self weight reduction from floor slab may lead to 5% of self weight reduction of entire building. Moreover, it directly faces the live load and transfers the load to beam and columns. Clearly, more mass means higher inertia force. Therefore, lighter buildings sustain the earthquake shaking better. Under horizontal shaking of the ground, horizontal inertia forces are generated at level of the mass of the structure, usually this situated at the floor levels [2]. These duties increase floor slab significance and complexity. The traditional solid precast slab is found to be challenging for large scale projects because of its heavy self weight which leads to dependency on heavier equipment, transportation ⇑ Corresponding author. Tel.: +355 672069 729; fax: +355 2222 117. E-mail address: [email protected] (Y. Yardim). 0950-0618/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.conbuildmat.2012.10.011

difficulties, expensive connections and joints solution. In addition, heavy precast slabs needs extra temporary supports during construction and larger beam and column size which result in the escalation of the overall cost [3,4]. In terms of better structural performance and lower cost, the development of varieties of light weight slab has become a crucial need. The use of semi precast panels is increasing rapidly due to it is versatile solution for transportation, handling and effective joint practice. In the recent past, a large number of semi precast panel have been developed using either ferrocement or composite cold steel deck with different type of toping concrete [5–9]. Insulating and light weight core panels were then developed which greatly increased the desirability of this type of construction. The panel consists of two thin skins high strength layers and elastic moduli separated by a core thick layer of normally much weaker and lower material density [10–13]. More than 15 different types of precast slab are being used successfully in construction market. Five general criteria has to be considered for the capacity of flooring units; bearing capacity, shear capacity, flexure; capacity, deflection limits, handling restriction [3]. There is no system fulfilling all of the

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Y. Yardim et al. / Construction and Building Materials 40 (2013) 405–410

above mentioned criteria. Nevertheless researches are going on to achieve the best fit slab system for different environments and projects. The composite slab systems were found structurally effective with thin layer of precast member taking into account of the benefits which include: shorter construction time, less dependent on heaver equipment on job site, less wastage of material, high quality smooth surface finish, in situ structural concrete topping and in-fill forming monolithic structures, eliminates or greatly reduces props, eliminate convention formworks [14–18]. Thinner precast structure of the composite slab could be achieved with ferrocement technology. Ferrocement provides considerable reduction in cracks number and their spacing (64–84%) was observed. Additionally, it enhances the ductility and energy absorption properties [1]. Ferrocement is not only an extension of reinforced concrete but also is now considered a member of the family of laminated composites, it can be reinforced with steel, or non-metallic meshes such as fiber reinforced polymeric (FRP) meshes [16]. The addition of fibers or micro-fibers as secondary reinforcement in the cement matrix, to improve performance, makes ferrocement a hybrid composite.

Light weight semi precast composite slab systems have been practicing mostly for roof panel. Weight reduction is achieved by replacing the core of panel with low density concrete and some other type of light weight infill blocks [19,20]. Different types of composite roof panel with low density infill as core element have been practiced [21,22]. Composition of light weight aerated concrete and ferrocement in sandwich structure shows effective load carrying performance in some applications [23]. Compared to other conventional wall and roof systems, AAC composite panels reduce energy consumption of buildings significantly with its excellent insulation qualities. It is considered as environmental friendly, no pollutants or toxic by AAC products are released that could affect indoor air quality [24,25]. Moreover, AAC can be obtained in any dimension, it is easy to handle which increase the construction speed, and it is widely practicing and available in construction marked. However, there is no significant experimental works has been recorded for semi precast slab with ferrocement precast layer and AAC as in fill material where ferrocement work as precast layer and AAC as efficient thermal insulator and light weight core element. Therefore, this paper presents one of the attempts to develop a light weight composite floor system to address these

In situ Concrete AAC

Projecting steel from Beam Tie Steel

Precast Slab Precast Slab Precast Beam

Precast Slab Wire mesh Steel Bar for connection Fig. 1. Ferrocement–AAC composite Slab.

a b≥a a ≥ 6x or 6y L ≥ 3a or 150mm whichever is larger

b x

L

y

Additional 2 layer wire mesh for additional resistance during testing Fig. 2. Detail test sample for tension test of wire–mesh.

Steel Reinforcement

407


requirements. This study introduces a semi-precast floor slab system; ferrocement–AAC composite slab to address some of the above listed shortcomings in existing systems. The new system consists of a bottom ferrocement skin, AAC masonry and in situ mortar ribs (Fig. 1). The ferrocement layer is the precast part of the composite slab, which consists of a wire mesh and steel reinforcement, required to resist the tensile stresses. The thickness and reinforcement of this layer will depend mainly on the span of the slab. The AAC layer and the in situ ribs provide the necessary resistance to the compressive forces developed due to bending. The two layers are interconnected using interlocking and rough surface between precast and cast in situ layers. The advantages of this system, amongst others, are its relatively lighter weight compared to R.C which will reduce the load transferred to the beams/walls. The

masonry AAC act as light, effective insulation material and at the same time resisting partially the compression forces developed due to bending of the composite. On site, the construction of the composite slab does not require heavy equipments to handle the ferrocement layer. Furthermore, the construction does not need any formwork since the bottom layer of ferrocement is a precast unit that can be easily fixed in position, using simple crane, to provide a platform that acts as a formwork for the brick layer and the in situ concrete ribs. This experimental study is limited to investigate the structural performance of one way ferrocement–AAC composite slab subjected to two-lines loading. The study highlights the effects of AAC layout on its overall structural response in terms of load–deflection characteristic, ductility, strain distribution, composite action and failure load.

Table 1 Experimental test program for precast composite slab and weight reductions of the specimens. Group

a b

ID

Ferrocement layer (mm)

Self weight% Precast

AAC

In situ

Number of La. Rib

Number of Tb. Rib

Reduction in weight compared with solid R.C%

A

AS (21) AS (22) AS (23)

30 30 30

66.55 66.94 67.31

16.43 15.46 14.52

17.02 17.60 18.15

2 2 2

1 2 3

34.02 32.67 31.30

B

AS (31) AS (32) AS (33)

30 30 30

67.87 68.16 68.44

13.12 12.39 11.68

19.01 19.44 19.87

3 3 3

1 2 3

29.16 28.00 26.83

C

AS (41) AS (42) AS (43)

30 30 30

69.02 69.23 69.43

10.23 9.69 9.17

20.74 21.07 21.38

4 4 4

1 2 3

24.30 23.34 22.36

L: Longitudinal. T: Transverse.

Fig. 3. Stages of construction in cross-section view.

408


Fig. 4. Test set up for simply supported two line load test series.

2. Material

Load-Deflection

Load (kN)

36 34 32 30 28 26 24 22 20 18 16 14 12 10 8 6 4 2 0

AS (21) AS (31) AS (41)

0

2

4

6

8 10 12 14 16 18 20 22 24 26 28 30 32 34

Load-Deflection

36 34 32 30 28 26 24 22 20 18 16 14 12 10 8 6 4 2 0

AS (22) AS (32) AS (42)

0

2

4

Deflection (mm)

Load (kN)

Load (kN)

For both topping and ferrocement layer, Ordinary Portland cement in accordance with Type I and natural sand (10 mm maximum size) were used for concrete in the ratio 1:3 with water/cement ratio of 0.5. The mortar mix was designed to give 28-day cube strength of 30 N/mm2. Welded steel wire mesh of opening size 12.7 mm 12.7 mm with an average wire diameter of 1.1 mm was used. Tension test on the specimens were carried out on the Universal Testing Machine Zwick/Roell Amsler HB1000. Load was ap-

plied in increments of 10 N. Tests were performed for direct tension on the wire mesh and embedding a rectangular coupon of mesh in mortar. Dimensions of the tensile test specimen of wire meshes were designed based on ACI 549 recommendation (Fig. 2.). The tensile strength of the mesh and steel bar were found 250 N/ mm2. Based on specifications in BS 8110 for quality control of AAC (BS8110-2 1998 clause 6.4.2), twelve 100 100 100 mm were tested to determine compressive strength of AAC. The density of aerated autoclaved concrete was found as 5.8 kN/ m3 and saturated compressive strength of from 12 specimens is 6 N/mm2.

6

8 10 12 14 16 18 20 22 24 26 28 30 32 34

Deflection (mm) Load-Deflection

36 34 32 30 28 26 24 22 20 18 16 14 12 10 8 6 4 2 0

AS (23) AS (33) AS (43)

0

2

4

6

8 10 12 14 16 18 20 22 24 26 28 30 32 34

Deflection (mm) Fig. 5. Load deflection curves for the tested specimens.

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Y. Yardim et al. / Construction and Building Materials 40 (2013) 405–410 Table 2 Ultimate moment, deflection and ductility of the specimens. Ultimate load (PULT) (kN)

AS21 AS22 AS23 AS31 AS32 AS33 AS41 AS42 AS43

Ultimate moment, MU (k Nm/m)

Theor.

Exper.

Theor.

Exper.

24.37 24.37 24.37 25.23 25.23 25.23 25.74 25.74 25.74

20.5 23.0 24.0 25.2 26.5 27.4 31.2 33.0 34.2

11.39 11.39 11.39 11.79 11.79 11.79 12.03 12.03 12.03

9.57 10.73 11.20 11.76 12.37 12.79 14.56 15.40 15.96

M U ðExptÞ M U ðTheorÞ

Yielding deflection oy (mm)

Ultimate load deflection ou (mm)

Ductility (ou/oy)

Mode of failure

0.84 0.94 0.98 1.0 1.05 1.08 1.21 1.28 1.33

9.6 7.6 6.5 9.1 7.9 7.1 8.1 7.3 14.8

23.74 20.63 18.55 27.68 26.68 24.53 33.51 31.7 29.53

2.48 2.71 2.85 3.04 3.38 3.45 4.14 4.34 4.61

Flexural Flexural Flexural Flexural Flexural Flexural Flexural Flexural Flexural

2.1. Specimen preparation and testing In order to determine the behavior of composite slab under flexural, one way slab specimens with size of 1m 3m 0.130 m (W L D) have been chosen for investigation. Precast layouts are designed based on, previous experimental and literature investigation. [26–28]. Table 1 contains dimension description and weight reductions percentages of flexural specimens. The constructions of the specimens can be summarized in three stages: preparation of precast layer, placing of AAC blocks and filling of cast-in situ topping (Fig. 3). The slab specimens have been cast on the level floor of the heavy testing structural laboratory. Flexural tests were carried out on simply supported 2800 mm clear span under two line loads (Fig. 4). During flexural test, strains on the specimen were carefully studied to observe the composite behavior of the slab panel. The electrical strain gauges and demec points were installed along the depth, at the bottom and top surface of the specimens on critical locations to monitor the strain throughout the experiment. The points on top of the specimens were placed in such way that relative strain or displacement could be monitored. Strain gage were placed on main steel bars at mid span to observe yielding stage.

3. Result and discussions In these experiments, ultimate flexural capacity of the proposed system with different AAC layout and amount, therefore different weight reduction, was investigated. Moreover, composite action between concrete and AAC, and ductility of the specimens were studied. The crack pattern shows that the system gives enough warning before fail. Full composite behavior was observed for the proposed system until ultimate load. The specimens show classical reinforced concrete slab flexural failure characteristics and cracks were observed between two line rods. Load deflection curves of specimens which are having the same number of transfer ribs but different numbers of longitudinal ribs,

are shown in Fig. 5. The effect of longitudinal ribs could be seen clearly in the figure. Reductions of weight as compared to solid RC slab for AS (22) and AS (42) are 32% and 23% respectively, however ultimate load capacities were 23 kN and 33 kN respectively. Therefore, 30% of the ultimate load capacity of the proposed slab could be upgraded with considerably small increase of weight by adding more longitudinal ribs. Table 2 summarizes the deflections measured at the yielding and ultimate load and the ductility of each specimen. The ductility (defined as the ratio of deflection at ultimate load to the deflection at yielding load) of each precast specimen is calculated and presented in the same table. All the specimens show satisfactory ductility above 2.4. The number of the longitudinal ribs has a significant effect on the ductility as the specimens with four longitudinal ribs show 61% and 29.5% increase in ductility compared to the slab specimens with two and three longitudinal ribs respectively. The proposed structure contains three layers: ferrocement, masonry and concrete. Therefore, the strain depth relationship is one of the important indicators of the structure behavior. The compression zone of the composite slab is combination of AAC and toping concrete. Careful strain inspection proves that the strain at the top surface of the composite slab is equal for both the toping concrete and masonry unit. Strain measurements of all the slabs were carried out. Similar results were obtained for the all specimens; only one specimen’s (AS32) strain results were presented to illustrate the behavior. Due to the load caring capacity a stronger structure bears more load with same strain. The top strains of the specimens were found to be same (Fig. 6). AAC composite structures’ strain along the depth relation is recorded as in Fig. 7. The result were found similar

1000

130

1.5 KN

Insitu layer with AAC

900 800

14 KN 19.5 KN

700

500 400

Width (mm)

600

26.5 KN

120 110 100 90 80 70

Precast Rip with AAC

60 50

300

40

200 100 0 -0.0045

-0.0035

-0.0025

-0.0015

-0.0005

0.0005

Strain Fig. 6. Load-top fiber strain diagram for AS32.

30

Ferrocement layer -0.005 -0.004 -0.003 -0.002 -0.001

20 10 0

0.001

0.002

0.003

Strain Fig. 7. Strain along the depth of slab AS32.

0 0.004

Depth (mm)

ID

410


with assumptions are drawn from early experimental results and literature for different light weight composite slab [21]. Based on careful inspection of the intersection joints between composite layers; there was no evidence of a horizontal defect such as cracks, spoiling or chipping. The proposed systems behave as a full composite slab. The strain of the compression zone was carefully observed by demec point placed top fibers of the composite. Fig. 6 shows that the structures’ strain responses at the compression zone are perfectly equal until possible service load. Strain on the concrete and AAC were found to be the same at the same amount of loading. Slight changes were observed while getting closer to the ultimate load. After yielding of steel, the final failures took place by gradually crushing of both concrete and AAC at the extreme top fiber. 4. Conclusions The proposed composite slab can be used as a structural floor for residential buildings. The dead load of the slab can be reduced by using proposed composite slab 32–23% compared to solid RC. The number of the longitudinal ribs has a significant effect on the ductility as the specimens with four longitudinal ribs show 60% and 28.4% increase in ductility compared to the slab specimens with two and three longitudinal ribs respectively. Based on strain monitoring of the test specimens, structures perform in a fully composite manner until the ultimate load. There were no horizontal cracks observed between the two layers of the composite slabs at any stage of loading. All the slab specimens show ductile behavior. Ductility and maximum deflection of slabs are well enough to give warning before failure. References [1] Yavuz Yardim. PhD thesis, development of light weight composite slab system for residential building, Malaysia, UPM; 2008. [2] Chopra AK. Dynamics of structures – a primer, EERI monograph. USA: Earthquake Engineering Research Institute; 1980. [3] Kim S. Elliott. Precast concrete structure, Jordan Hill, Oxford OX2 8DP 225 Wilwood Avenue, Woburn, Butterworth Heinemann, MA 01801-2041. [4] Alfred A, Yee PE, Hon D. Eng Structural and economic benefits of precast/ prestressed concrete construction. PCI J 2001;46(4):34–42. [5] Tas omorodion-Ikhimwin. Analysis and design of ferrocement ribbed slabs, Doctor of philosophy thesis, University of New York; 1983.

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