The objective of this study is to introduce a type of concrete that is more cost effective when compared to other concretes, yet is still compliant with the BS8110 standards of mechanical characteristics for design and construction of structural concrete. This paper presents part of the test results of an ongoing research project to produce structural lightweight concrete, using the low cost solid waste material 'almond shell' (AS), as a coarse aggregate. Test results in this paper detail the compressive strength, bond strength, modulus of elasticity, and flexural behaviour of the AS concrete. Based on these experimental results, it was determined that although AS concrete has a lower modulus of elasticity than other test samples, middle-scale beam tests revealed that deflections under expected design service loads are within acceptable limits. The span-deflection ratios ranged between 252 and 263, which are within the allowable limits specified by the BS8110 standards. Laboratory investigations showed satisfactory performance of the AS concrete within the remit of the BS8110 code, and it can be concluded that almond shell has strong potential as a coarse aggregate in the production of structural lightweight concrete, especially for low-cost applications such as in housing construction and for use in earthquake prone areas.
Keywords: Structural Lightweight Concrete, Almond Shell, Bond Strength.
INTRODUCTION
The issue of resource reduction has become more prominent in recent years, and global pollution has resulted in a challenge for engineers to seek and develop new materials based on renewable resources. These include the use of waste materials, recycled items, and by-products from construction. Many of these by-products are used as aggregate materials in the production of lightweight concrete. There has been much research conducted on the structural performance of lightweight aggregate concrete, and this has been focused on naturally occurring aggregates, manufactured aggregates, and aggregates from industrial by-products.
Stone fruit, from the Rosaceae family, is closely related to the peach and originates from the Middle East where conditions are dry and hot. Almonds are the single seed from a type pf stone fruit, and they are grown throughout the entire Mediterranean Region, the USA (California), Northern Africa, Turkey, Iran, Australia and South Africa. This is because the almond is sensitive to wet conditions, and is therefore not grown in wet climates. Currently, research efforts have been directed towards the potential for using almond shell (AS) as a coarse aggregate in the production of structural lightweight concrete. In Iran, there is over 70 tonnes of Almond shell waste produced at each cropping, meaning it is a prime location for making use of this waste material.
Using this waste material leads to many benefits, such as maximisation of the use of almond shell, preservation of natural resources and maintenance of ecological balance. In addition, the current economic and social climate means there is an increasing demand for low-cost housing in many countries, and therefore AS can be used as a cheaper alternative to the conventional aggregates in fulfilling this demand.
Aggregates that have a dry unit weight of less than 1200 kg/m3 are commonly used, and are classified as lightweight aggregates (Owens, 1993). AS aggregate has a unit weight of 600-700 kg/m3, which is approximately 55% lighter when compared to the conventional crushed stone aggregates. Consequently, the resulting concrete will be even more lightweight. This lightweight concrete using AS coarse aggregate is still a relatively new construction material and its structural performance still requires investigation. This study attempts to determine some important characteristics of AS aggregate concrete, to result in wider acceptance of AS as a lightweight aggregate material alternative in producing concrete that can be used as a construction material for low-cost housing construction. The structural properties investigated in this paper are initial compressive strength, bond strength, modulus of elasticity, and flexural behaviour of reinforced AS concrete beams.
MIX DESIGN OF AS CONCRETE
The components of the AS concrete in this study included ordinary Portland cement (ASTM Type 1), with crushed sand as a fine aggregate, almond shell (AS) as a coarse aggregate, and a Type-F naphthalene sulphonate formaldehyde condensate based superplasticiser (Collepardi et al. 1993). The chemical structure of the SNF based superplasticiser is shown in Figure 1.
Figure 1. Chemical structure of naphthalene sulphonate formaldehyde based superplasticiser.
The AS aggregates for this study were acquired from local almond shell mills. AS is usually available in various shapes, which include roughly parabolic, and other irregular shapes as shown in Figure 2. Before the AS was used as aggregate, it was sieved and passed through the No.1/2 sieve, with the particles that were retained being discarded. The remaining AS was then passed through the No.4 sieve, with the particles retained from this second sieving being used for the experiment. The particle size distribution of the AS aggregate is shown in Figure 3, and the properties of the crushed sand and AS are shown in Table 1.
Figure 2. Various shapes of AS aggregate.
Figure 3. Particle size distribution of AS aggregate.
Table 1. Properties of crushed river sand and AS
Properties
Crushed sand
Almond shell (AS)
Maximum grain size, mm
4.75
12.5
Shell thickness, mm
(Average shell thickness = 2.0 mm)
-
1.0- 3.0
Specific gravity
2.46
1.25
Bulk unit weight, kg/m3
1600-1650
600-700
Fineness modulus
1.6
5.7
Los Angeles abrasion value, %
-
4.2
Aggregate impact value, %
-
6.22
Aggregate crushing value, %
-
6.00
24-h water absorption, %
3.28
23
Based on the properties of the fine and coarse aggregate available for this study, the mix proportions were approximated, followed by the modification of trial mixes to achieve a practical and satisfactory result. The acceptable mix comprised 450 kg/m3 cement, 810 kg/m3 sand, and 440 kg/m3 AS, with a free water to cement ratio of 0.35. The cement content used in this research was within the acceptable range for lightweight concrete (Mindess et al., 2003). The quantity of superplasticiser used was 1.5% by cement weight (6.75 kg/m3), which was within the recommended range provided by the manufacturer. This mix proportion was used throughout the entire investigation. The AS aggregate used was mixed at the saturated surface dry (SSD) condition, based on 24 hours submersion in potable water.
TESTING METHODOLOGY
Several tests were conducted to determine the structural properties of AS concrete that were specified in the previous section. Compression strength tests on 100 mm cubes were performed according to the standard BS 1881: Part 116, and the initial modulus of elasticity of 150Ã-300 mm cylinders was determined as per ASTM C 469-87a. The bond strength of AS concrete was determined by carrying out a series of pullout tests on 100Ã-200 mm cylinders. Flexural tests on full-scale prototype beams were also conducted. Except for the prototype beam tests, which used beams of 3 different tension reinforcements, triplicate specimens were prepared for each test and the results were reported as an average.
Mixing of concrete was performed using a rotating drum mixer conforming to BS 1881: Part 125 (Section 6.3 for SSD aggregates). To prevent excessive evaporation from the fresh concrete, and subsequent shrinkage, a plastic sheet was placed on top of the moulds immediately after casting, and it was left for 24 ± 3 hours in the laboratory at ambient conditions of 24-28 °C and relative humidity of 85% - 95%. After this, the cube and cylindrical specimens were transferred into a 25-30 °C water tank until testing begun. For the prototype beams, the specimens were moist cured continuously for another 6 days after which they were left in the same ambient laboratory conditions until the test was to commence. The bond between the reinforcement and AS concrete was investigated by application of the bond-slip relationship. The pullout test was conducted using deformed bars of 10, 12, and 13 mm diameters and tested at an age of 3, 7, 28, 56, 90, and 180 days. The bond strength was computed by the following formula (1):
(1)
Ï„ = bond stress (MPa)
F = applied load (N)
d = nominal bar diameter
l = embedment length (mm)
Three separate reinforced sample beams were made up and tested. All test beams had rectangular cross-sections of 150Ã-230 mm, with a total length of 3200 mm and an effective span of 3000 mm. The test beam dimensions were chosen to be sufficiently large to simulate a real structural element. Tension reinforcements of 2Ø10, 2Ø12, and 2Ø13 were provided for beams S1, S2, and S3, respectively. Two 8 mm diameter mild steel hanger bars were provided for each beam. Sufficient shear links were provided to avoid failure in shear in order to only investigate the properties specified, and an all-round cover of 25 mm was maintained for each beam. The beams were tested under the loading profile known as 4-point bending. Three plunger travel LVDTs (linear voltage displacement transducers) capable of reading up to a maximum value of 100 mm were used to monitor the deflection of the beams in the pure bending region. The test set-up and beam details are illustrated in Figure 4.
2P
3200
A
A
3000
150
150
230
Ast=2Ø10
Beam 1
Ast=2Ø12
Beam 2
Section A-A
Ast=2Ø13
Beam 3
LVDT
600
Figure 4. Beam testing setup and details.
RESULT AND DISCUSSION
The fresh concrete density during this study ranged from 1863 to 1897 kg/m3, and the air content in the specimens was in the range of 4.2% to 4.9%, which is relatively high. This could be attributed to the highly irregular shapes of the individual AS particles, which prevented their full compaction with each other; however, this air content value for the AS concrete samples is still within the acceptable values of 4% to 8% specified in the ACI 213R-87 samples. The slump test results for the AS fresh concrete workability evaluation was performed, which determined that AS concrete was in the range of 6 to 8 cm. Analysis of these test results revealed that the AS concrete had a medium degree of workability, which was within the acceptable range of a workable concrete. The properties of the hardened AS concrete tested at an age of 28 days are presented in Table 2.
Table 2. Properties of AS concrete.
Air-dry density, kg/m3
1790
Compressive strength, MPa
32.5
Modulus of elasticity, GPa
6.73
Pullout bond strength, MPa
7.0-9.9
Concretes that have a density of less than 1900 kg/m3 are classified as lightweight concretes, and from Table 2 it can be seen the density of AS concrete is within this range, meaning it can be termed as a lightweight concrete. Compared to normal concretes, which have a density of around 2400 kg/m3, AS concrete is approximately 25.5% lighter. This shows that use of AS concrete would eliminate 25.5% of dead load when used in construction, whilst still maintaining satisfactory physical properties. Another benefit of this reduced weight is that the damaging effect on concrete structures exposed to disastrous earthquake forces and inertia forces can be ultimately reduced, as these forces are proportional to the weight of the structure.
In order to evaluate compressive bearing capacity of AS concrete, compressive strength tests were performed on 10 cm cubes at an age of 28 days. The results determine an average compressive strength of 32.5 MPa, which is approximately 90% higher than the minimum required compressive strength of 17 MPa for structural lightweight concrete provided by the ASTM C330 standard. Although AS is an organic material, tests revealed that biological decay did not have a negative impact on this result, as the cubes retained acceptable strengths even after 180 days. This has been showed in figure 5.
Figure 5. Compressive strength development of AS concrete.
Different damage and failure mechanisms were observed at different sample ages. At the earlier stages of testing, from 3 to 28 days, it was observed that the compression failure in the concrete was mainly due to failure in the interface between the cement paste and the AS aggregate, where the crack follows a path around the aggregate (Figure 6a). At later stages, from 42 to 180 days, this mortar-aggregate interface bond is stronger, and hence the crack grows through the aggregate as illustrated in Figure 6b.
Crack path around AS aggregates
AS aggregates
Crack path through
AS aggregates
(a)
(b)
Figure 6. Crack paths (a) at earlier ages (b) at later ages.
The bond strength development of AS concrete is illustrated in Figure 7. Based on the pullout test performed, the bond strength of AS concrete was found to be about 2.7 to 3.5 times higher than the design bond strength as recommended by BS 8110. All specimens failed by the same mechanism, which was splitting of the concrete cover. The failure was catastrophic and very sudden, and was accompanied by the formation of longitudinal cracks. It was observed that cracks progressed over the entire length of the sample before failure occurred. Splitting failure occurs when radial cracks form due to the bearing pressure developed by the projections of the steel bars on the surrounding concrete. When cracks start to form, the bond forces are directed outward from the bar surface and these forces cause anchorage failure by cracking of the confining concrete cover. The bond strength of the AS concrete was approximately 26% to 29% of the compressive strength, which is comparable to the bond strength of other lightweight concretes such as sintered pulverized fuel ash concrete (Orangun, 1967) and Aerocrete (Chitharanjan et al., 1988).
Figure 7. Bond strength development of AS concrete.
The modulus of elasticity is one of the most important parameters for structural concrete as it is required when evaluating deflections and cracking of a structure for design. Figure 8 shows a stress-strain curve for the AS concrete. The strain value corresponding to the maximum stress is approximately 0.005 micro-strains. One particular concern for AS concrete is the low value of elastic modulus and this was further investigated with the prototype beam testing.
Figure 8. Stress-strain curve for AS concrete.
All of the tested beam samples exhibited typical failure in flexure. Failure occurred gradually, and since all the beams were under-reinforced, it resulted in yielding of the tensile reinforcement occurring before crushing of the concrete cover in the pure bending zone. The ultimate moments of the beams were predicted using a rectangular stress block analysis as recommended by the BS 8110 standard. It was observed that the experimental ultimate moments for the test beams were about 22% to 31% greater when compared to the predicted moments for normal weight concrete was applied. This shows that the BS 8110 standard can be used to give a conservative estimate of the ultimate moment capacity for singly reinforced AS concrete beams. The amount of deflection under loading is one of the main criteria for the serviceability requirements of a structural member. Under the design service load, which includes the dead load and live load, the mid span deflection obtained was 11.43mm, 11.62mm, and 11.87mm for beams S1, S2, and S3, respectively. Although AS concrete has a low modulus of elasticity, the deflection under the design service load is acceptable as the span-deflection ratios ranged between 252 and 263, which are within the allowable limits provided by BS 8110. Figure 9 illustrates the typical moment-deflection curve for beams S1, S2, and S3.
Figure 9. Moment-deflection curves for the test beams.
CONCLUSIONS
From the results of this study, it has been determined that AS has a good potential as a coarse aggregate in structural concrete production, and can be used for low to moderate strength applications such as structural members for low-cost houses. By integrating this waste material into concrete mixtures, not only can a reduction in concrete weight and production costs be achieved, but there is the added benefit that the ecological cyclic system can be preserved. Based on this research study, the following conclusions can be drawn:
The evaluation of the workability of fresh AS concrete by the slump test was 6 to 8 cm, which showed that the AS concrete had a medium degree of workability. It could therefore be defined as workable concrete.
The air content percentage in the concrete, which was 4.2 - 4.9%, could be reduced by optimising the gradation curve. However, the air content in the AS concrete is within the allowable range of 4-8% recommended by ACI 213R-87.
The compressive strength of the AS concrete was 32.5 MPa at an age of 28 days, which satisfies the requirement for structural lightweight concrete.
Despite the varying shapes of AS aggregate particles, the bonding properties of AS concrete are comparable to other types of lightweight concretes. It should be noted that this bond and its mechanical properties were obtained by using a cement content of 450 kg/m3.
Although AS concrete has a low modulus of elasticity of 6.73 GPa, full scale beam tests showed that the deflection under the expected design service loads was acceptable. The ratios of the effective span length to maximum mid span deflection ranged from 252 to 263, which are within the allowable limits provided by BS 8110.
Based on the beam test results, it was observed that the experimental ultimate moments for the singly reinforced beams were about 22% to 31% higher compared to the predicted moments from BS 8110.
REFRENCES
ACI 213R-87, Guide for Structural Lightweight Aggregate Concrete, American Concrete Institute.
ASTM C330, Standard Specification for Lightweight Aggregates for Structural Concrete, Annual Book of ASTM Standards ASTM C 469-87a, Standard Test Method for Static Modulus of Elasticity and Poisson's Ratio of Concrete in Compression, Annual Book of ASTM Standards.
BS 1881, Part 116, Method for Determination of Compressive Strength of Concrete Cubes, British Standards Institution, London.
BS 1881, Part 125, Methods for Mixing and Sampling Fresh Concrete Samples in the Laboratory, British Standards Institution, London.
BS 8110, Structural use of Concrete Part 1, Code of Practice for Design and Construction, British Standards Institution, London, 1985.
Chitharanjan N., Sundararajan R. and Manoharan P.D., \Development of Aerocrete: A New Lightweight High Strength Material", The International Journal of Cement Composites and Lightweight Concrete, 10, 27-38, 1988.
Collepardi M., Coppola L., Cerulli T., Ferrari G., Pistolesi C., Zaffaroni P., and Quek F., «Zero Slump Loss Superplasticized Concrete», Proceedings of the Congress "Our World in Concrete and Structures", Singapore, pp 73-80, 1993.
Mannan M.A. and Ganapathy C., \Behavior of Lightweight Concrete in Marine Environments", Proceedings of the International Conference on Ocean Engineering, Chennai, India, 409-413, 2001.
Mindess S., Young J.F. and Darwin D., Concrete, 2nd Edition, Prentice Hall, USA, 2003. Orangun C.O., \The Bond Resistance between Steel and Lightweight-Aggregate (Lytag) Concrete", Building Science, 2, 21-28, 1967.
Owens P.L., Lightweight Aggregates for Structural Concrete, Structural Lightweight Aggregate Concrete, edited by J.L. Clarke, Blackie Academic & Professional, London, 1993.
