Experimental Study on Hollow Blocks with Wastes ^†

Abstract

The article presents an experimental study on concrete blocks prepared by using waste types such as fly ash as a cement substitution, waste of plastic bottles and wood waste as replacements for sand and polyester fibers waste as a dispersed reinforcement. The mechanical characteristics of concrete with fly ash and polyester fibers were determined. The influence of the type and dosage of waste on the mechanical strength is discussed. The concretes with fly ash and different dosages of waste were used for manufacturing hollow blocks that were tested in compression, and the behavior under load was analyzed. Failure in compression of hollow blocks was gradual and ductile.

1. Introduction

Concrete is one of the most used materials in construction and engineers have worked to ensure concrete responds to new requirements related to environment protection [1,2]. Production of cement, an important component of concrete, is a cause of CO₂ emission (7%), and the huge quantities of natural aggregates used in the concrete composition result in important changes in the natural environment. Non-conventional concretes, with different types of materials in the mix, have emerged just for partially eliminating the above ecological problems. The cement is replaced partially or totally by different materials, such as fly ash, silica fume, slag, rice husk and banana leaves ash [3,4,5,6,7,8]. Aggregates have been replaced by steel slag, chopped plastic bottles, polystyrene granules, recycled aggregates, chopped sunflower, etc. [9,10,11,12,13]. Fibers of diverse types have also been added in the concrete mix: steel, polyester, hemp, etc. [14,15,16,17,18]. The main objective of the article was to analyze the behavior of hollow blocks manufactured with concrete prepared with a cement substitution with fly ash and waste types such as chopped plastic bottles and wood waste as replacements for sort 0–4 mm and polyester fibers as a dispersed reinforcement. The hollow blocks manufactured with non-conventional concretes will be used to make a non-load-bearing masonry wall. In the next stage, this masonry wall will be built and tested.

2. Experimental Program

2.1. Materials

In the research, a control mix of concrete (C₀) was used for preparing hollow blocks which had the following components: cement type CEM I 42.5 R [19] in a dosage of 360 kg/m³; and river aggregates in three sorts, namely 0–4 mm, 4–8 mm and 8–16 mm, which were in the following dosages: 803 kg/m³ of sand, 384 kg/m³ of sort 4–8 mm and 559 kg/m³ of sort 8–16 mm. We used water in a dosage of 172 L/m³, and 10% of the cement dosage was replaced with fly ash, from CET Holboca Iasi. Fly ash was used before in other experimental tests and presented by the authors in [15,20]. Waste types PET bottles and wood waste were chopped into sorts of 0–4 mm and used as replacements for 20% by volume of the dosage of aggregate sort 0–4 mm in the case of PET and 40% by volume of the dosage of the same aggregate sort in the case of wood. The chopped PET and wood waste had sizes between 0 and 4 mm. Waste from polyester fibers was used, which was cut into 30 mm-long filaments and dispersed as a replacement reinforcement in the concrete, in a dosage of 0.25% of the concrete weight. In the mixture, we used a superplasticizer (Master Glenium SKY 617 from BASF) in a dosage of 1% of the cement volume.

2.2. Samples

The control mix of concrete (noted C₀) and the mixes with fly ash and chopped PET (noted C₁), fly ash and wood waste (noted C₂) and fly ash and polyester fibers (noted C₃) were prepared by mixing all the components. The wood waste was moistened before being added to the mix. The samples were poured: cubes of 150 mm in size for determining the compressive strength fc, and prisms of 100 × 100 × 500 mm in dimension for determining the flexural strength f_ti and split tensile strength f_td [21,22,23]. The hollow blocks, one of which is shown in Figure 1, labeled HBF1–HBF3 were manufactured only for the concretes with waste (concretes C₁–C₃). After 24 h, the specimens were removed from the formwork and kept in the laboratory at a temperature of 20 °C until testing.

3. Testing Result and Discussion

3.1. Mechanical Strength of Concrete Mixes

The control mix and the concretes with waste were tested at 28 days for mechanical strength. The values are given in

Table 1. Mechanical characteristics of experimental concretes.

Concrete Sample	fc N/mm2	fti N/mm2	ftd N/mm2
Control C0	33.45	1.82	1.72
C1	25.27	1.74	1.82
C2	13.63	1.30	1.24
C3	29.80	2.01	1.94

.

3.1.1. Compressive Strength

The value of f_c for concretes with waste was influenced by the type of waste. All values of f_c were lower than that of the control mix. The replacement of sort 0–4 mm with chopped plastic in a dosage of 20% had reduced f_c, with 24.5%, in comparison with the control mix. In the case of the replacement of sand with 40% sawdust, f_c was reduced, with 52%, and mix C₃ (only with fly ash and polyester fibers) presented a decrease in f_c of only 11% in comparison with the control mix. For f_c, the highest value was obtained for concrete C₃.

3.1.2. Flexural Strength

The value of flexural strength was influenced by the type and dosage of waste. When the aggregates of sort 0–4 mm were replaced, a decrease in flexural strength was obtained. The addition of polyester fibers increased the flexural strength by 10% in comparison with the control mix. For f_ti, the highest value was obtained for concrete C₃.

3.1.3. Split Tensile Strength

The value of split tensile strength was influenced by the type and dosage of waste. The waste type chopped PET as a replacement for sort 0–4 mm in a dosage of 20% resulted in an increase in the strength. The waste type wood waste as a replacement for sort 0–4 mm in a dosage of 40% resulted in a decrease in the strength. The dispersed polyester fibers increased the split tensile strength in comparison with the control mix by 12.8%.

The mechanical strengths of concretes with different waste types as replacements for aggregates were lower than those of the control mix. In the case of concrete with fly ash and polyester fibers, the compressive strength was lower than that of the control mix, but the flexural strength and split tensile strength were highest.

3.2. Hollow Blocks Experimental Test

The blocks of concrete were subjected to axial compression. The compression force was applied along with the height of the block. The maximum value of the compression load was divided by the gross contact area of the block, including holes, noted f_cb1, and by the net area, noted f_cb2.

The indirect tension stress, according to [24], was computed with the following relation (1):

f_tb = 2P/πLh,

(1)

where P is the value of the maximum compression load, h is the height of the block (140 mm) and L is the split length (82 mm) if the holes are neglected, or 240 mm if the total length is considered.

The results of the experimental tests are given in

Table 2. Experimental results of the compression test on the hollow blocks.

No.	Block	Sizes of Blocks mm	Maximum Compression Force kN	fcb1 N/mm2	fcb2 N/mm2	ftb1 N/mm2	ftb2 N/mm2
C1	HBF1	240 × 290 × 140	815.00	11.71	14.29	4.53	1.54
C2	HBF2	240 × 290 × 140	390.42	5.71	6.85	2.17	0.74
C3	HBF3	240 × 290 × 140	925.21	13.3	16.21	5.13	1.75

.

The compressive strengths f_cb of the blocks had different values, depending on the type of concrete. According to [24], the minimum compressive strength must be 7 N/mm² and the blocks with fly ash and PET waste (HBF1) and those with fly ash and polyester fibers (HBF3) satisfy this condition for their use in masonry also in seismic areas, as a self-weight masonry for realizing partitioning walls [24]. The block HBF2 can be used for self-weight masonry.

The split tensile strength f_tb of blocks also had good values which are in concordance with values given by other authors [11].

The mechanical characteristics of hollow blocks recommend them to be used in construction for realizing masonry walls.

3.3. Failure Mode

During the tests in compression, the blocks failed gradually, and vertical cracks developed throughout the entire depth, especially near holes. The blocks had a ductile failure until the complete damage, as shown in Figure 2.

4. Patents

For manufacturing the blocks, the following types of waste were used for preparing concrete: fly ash that replaced 10% of cement in all mixes with waste, chopped plastic bottles (PET) that replaced aggregate sort 0–4 mm in a dosage of 20% by weight, waste of wood that replaced aggregate sort 0–4 mm in a dosage of 40% by weight and waste of polyester fibers that was added in the mix with fly ash.

The compressive strength, tensile strength and split tensile of the concretes with waste were determined. The type and dosage of waste influenced the mechanical properties. For all types of concrete, the value of compressive strength was lower than that of the control mix without waste. In the case of concrete with polyester fibers, the flexural strength and split tensile strength were higher than those of all others mixes. For concretes with saw dust, the lowest values of all mechanical strengths were obtained.

When tested in compression, the hollow blocks presented values of compressive strength and tensile strength comparable with other types of blocks, which means we can recommend their use for realizing walls.