Laboratory Evaluation of Engineering and Strength Properties for Semi Rigid Mixtures

Semi rigid mixture is a unique type of mixture in recent years, which has two components:Open graded bituminous mix and cement mortar grout. In this study, engineering and strength properties of semi rigid mixtures with three selected gradations are investigated. The flowability, compressive strength and flexural strength are identified to measure the optimum cement mortar grout. Meanwhile for Open graded bituminous mix, volumetric analysis, drain down and cantabro abrasion loss tests are carried out for selected gradations by varying binder content to obtain optimum mixture. Further the compressive strength, flexural strength, stability, tensile strength and durability tests are evaluated for the selected gradations of optimised Semi Rigid mixtures. The results showed that cement mortar grout containing cement to M-Sand ratio of 1:1, water-cement ratio of 0.35 with super plasticizer (polycarboxylic ether polymer) dosage of 0.45% resulted in desired flow of 12 seconds, compressive strength and flexural strength of 27 MPa and 3.5 MPa respectively. The optimum binder contents of Open graded bituminous mix for the selected gradations are found to be in the range of 4.5-5.0%. Furthermore the obtained 4.5% binder content in Semi Rigid mixture have higher engineering and strength properties compared to 5.0% binder content .


Introduction
Over the past decades due to urbanization heavy trucks have increased enormously in the total traffic volume in developing countries like India where more than 90 percent of roads are bituminous pavement, demanding stronger and long lasting pavements. Bituminous pavements are considered for their satisfactory performance against traffic loads and also high resistance to skidding, good serviceability and riding quality, are used as highways [1]. On the other hand it suffered from major distresses like rutting, moisture damage and cracking due to high traffic volumes, overloading and variation in climatic condition. The disadvantages of bituminous pavement can be overcome by concrete pavements but initial cost is high, requires skills for construction and maintenance and also due to joints in concrete pavements riding quality is poor as well as the noise generated by the joints, the differential expansion/contraction needs to be accommodated [1]. Thus, a new technology of pavement need to be arise to overcome these distresses. One such type of pavement is Semi Rigid mixture, which is a reliable , joints free pavement, highly load carrying capacity, longer service life, easy to construct and repair and desirably having the nature of both rigidity and flexibility, moreover, offers riding quality and satisfactory serviceability. It has been adopted to furnish a durable and water proof surface for the

Bitumen
VG 30 grade bitumen used in this study and the basic tests were carried out according to IS 73 2013. Table 3.2 shows summary of VG-30 bitumen physical properties, the mixing and compaction temperature for this bitumen is 152°C-156°C and 138°C-142°C respectively. The temperatures corresponding to viscosity ranges of 0.17Pa-sec and 0.27Pa-sec were plotted in the graph and the resulting temperatures are chosen for compacting and mixing.

Cement
In this investigation Ultratech 53 grade cement were used. Table 3.3 shows summary of 53 grade cement properties.

M-Sand
The M-Sand was procured from a plant near Bidadi, Bengaluru. The sand should pass 0.6mm size and retained 0.075mm so that the grout will enter into the air voids easily. Table 3.4 shown the summary of M sand properties of as per IS 1542.

Super-Plasticizer
Super plasticizer helps in reducing the water requirement without reduction in strength of the grout, thus improving the flowablity and helps in early gain of grout strength. In this investigation we used Sulphonated Naphthalene Formaldehyde and Poly Carboxylic Ether Polymer for the cement grout.

Fibres
In this study, natural sugar cane fibre is used to enhance crack resisting property to the mixtures, 2.36mm passing size fibres were selected for preparing of samples.

Selection of aggregate gradations
The three aggregate gradations were identified for preparation of open graded bituminous mixes which allow cement mortar grout flows freely into the voids of mixture with selected quantity of bitumen and compaction. The gradation for the aggregate were adopted according to master gradation provided by the ASTM D7064 (OGFC), New Jersey MOGAC (12.5-mm NMAS) and Transit New Zealand (PA 14 HV) as shown in graph

Methodology
The first stage is to develop a cement mortar grout. The second stage is to open graded bituminous mix and finally the production of the Semi rigid mixtures. The following methods were carried on Semi rigid mixtures.
x Selection of Cement Mortar Grout x Preparation of Open Graded Bituminous Mix Specimens x Mix design for Semi rigid pavement x Strength properties x Durability test.

Selection of cement mortar grout
Around 22 cementious grouts were evaluated to identify the proportions of grouts for the suitability of semi rigid mixtures. Flowability and strength of the grouts were measured and proportions were changed progressively based on the obtained results. The ratio 1:1 proportion of cement and M sand was fixed and two types of super plasticizer with different dosage were used for water cement ratio of 0.30-0.40. Mixing of the grout was done in a mechanical mixer for three minutes. A summary of cement mortar grout mixtures are shown in Table 4.1.

Flowablity of cement mortar grout
The flowable properties of cement mortar grouts for two different types of superplasticizer were examined. The flow times of grout are analyzed using effect of superplasticizer dosage and superplasticizer type of flow characteristics are shown in Figure 4.1. From the obtained results, dosage of superplasticizer significantly influence the flowablity of cementious grout. According to the trails, the optimal flow time shall be 12 seconds.

Strength properties of cement mortar grout
The strength results for selected grout mixtures containing superplasticizer dosage with flowablity 12 seconds are shown in Table 4

Preparation of open graded bituminous mix specimens
Requisite binder content is determined by the method of Marshal Mix design according to codal provision conforming to ASTM-D6927-06. The mixtures were compacted by giving 50 no. of blows on only one face of the specimen using Marshall hammer, for 4 binder contents at an increment of 0.5%. Around 1000gm of aggregates is allowed to heat to a temperature of l60-l75°C.
Asphalt is melted to a temperature of 150-165°C with the initial experimental percentage of binder (say 4.0 to 5.5 percent by weight of the aggregates). The heated aggregates and binder is carefully batched at 160-165°C temperature. The mixture is then poured inside the heated mould & compacted using a rammer with the 50 number of blow at temperature 130°C to 140°C. The weight of mixed aggregates used for the casting of the sample can be suitably changed to get the desired compacted thickness of 63.5mm. Increase the bitumen content in next trial by +0.5% and repeat the above technique.

Determination of optimal bitumen content (OBC)
The optimal bitumen content was determined for all three gradations by ( )*100 (4.2) Where, Gmb= bulk specific gravity of the mix Gsb= bulk specific gravity of aggregates Gmm=theoretical specific gravity of the mix Ps= fraction of aggregates present, by total mass of mix

Cantabro Abrasion loss
Cantabro abrasion loss test was used to determine the abrasion loss value of unaged open graded bituminous mixes using the Los Angeles abrasion machine. The test specimen was placed in a Los Angeles abrasion drum without any abrasive charges, and the machine was operated at a speed of 30-33 revolutions per minute for 300 revolutions. The percentage of weight loss in the specimen when compared to its initial weight was expressed as the unaged abrasion loss (UAL). The temperatures recorded during the test procedure were within the range of 25 ± 5°C as specified in

Drain down test
The drain-down test is used to determine the correct amount of fiber needed for the mix. The test measures the potential for asphalt binder to drain from the coarse aggregate structure while the mix is held at an elevated temperature. The test is performed in accordance with ASTM D6390. The mass of the drained material is determined to calculate the amount of drain down as a percentage of the mass of the total asphalt mix sample are determined using

Permeability test
Falling head permeameter was used for conducting the permeability test as per IS Code 2720 part 17. The specimen for determining permeability is1prepared by Marshal Method and the specimen is1sealed around its periphery on both the1sides to prevent the loss of water from sides. Water is allowed1to flow through the specimen for1a certain head, the time1required for the water to pass through the1specimen is recorded using stop watch. The resulting permeability cm/s is converted to m/day is determined1using the Equation 4.5. Following the typical guidelines for OGFC's, a minimum value of 100 m/day for permeability should be ensured, so that the cement grout can penetrate the voids in the bitumen mix. Figure 4.6 shows the f permeability results for binder contents. Permeability=  x The open graded bituminous mix were prepared as discussed in the above section.
x The optimized cement mortar grout is identified as explained above section.
x The open graded bituminous mix specimens cooled down to ambient temperature, prepared cementious grout should be poured gradually on the surface of the specimen.
x The care should be taken to prevent spill of grout in mould while pouring.
x The electric vibrator is used for 30 seconds to provide mild vibration to ensure grout penetrated into the mix.

Properties of grouted mix
The saturation degree and remaining air void (after filling the air void of open graded bituminous mix with grout) in Semi Rigid mixtures are critical parameters concerning the performance and durability of Semi Rigid mixtures under traffic loading and climate conditions. Therefore remaining air void of 3 to 6% for Semi Rigid mixtures should be maintain to avoid bleeding of bituminous mixture. In order to calculate the parameters of grouting saturation degree and remaining air void are calculated using Equation 4.6. The density of grout 1.963 gm/cc is calculated as per BS EN 1015-6 (1999). The Table 4.3 shows summary of volumetric analysis of mixes before and after grouting. Saturation degree = ௐଶିௐଵ ఘ * * (4.6) Where, W1 and W2 are specimen mass before and after grouting in gram, respectively, ߩis the density of grout in gr/cm 3 , V is the specimen volume in cm 3 , Va is the air void of mixtures

Compressive strength
Compressive strength of compacted asphalt mixtures is measured as per ASTM D1074 by Compressive Testing Machine (CTM). The same method was adopted to test Semi Rigid mixture samples, the sample preparation is explained in above section and dimensions of the cylindrical samples are 101.6mm diameter and 63.5mm thickness.

Flexural Strength
ASTM C 78 test method provides a method for measuring flexural strength of the concrete. The same method was adopted to test Semi Rigid mixture samples, the dimensions of mould size are 500mm*100mm*100mm.The open graded bituminous mix for the optimum binder contents (4.5-5.5%) is poured on the beam mould, CTM is used to compact the mix to obtain target air voids 20-30% and optimized cement mortar grout (1:1:0.35:0.45) is poured on the mixture when it cools down to ambient temperature. After 24 hours the beam sample were demould and kept for 28 days water curing. Two point loading test carried on the sample, point of failure at maximum load recorded for calculating flexural strength test.

Marshall Stability
Marshall Method is used almost everywhere in the world for its easy procedure and economy compared to other methods. Strength behavior of samples at optimum binder contents were studied using Marshall Stability. ASTM D6927 test method used for testing procedure of the samples. The cylindrical samples of 101.6mm diameter and 63.5mm thickness were cast to test the stability with 50blows of compaction on single side to obtain target air voids 20-30%. The point of failure is defined when the maximum load reading obtained and maximum load value is expressed in kN is recorded as the Marshall Stability value of the samples. Figure 5.3 shows the stability values of binder contents.

Indirect Tensile Strength
Indirect tensile strength test performed to study the tensile behavior of Semi rigid mixtures. The Indirect Tensile Strength (ITS) was determined as per ASTM D6931-17 on cylindrical samples by loading them along a diametrical axis at a deformation rate of 51 mm per minute. The same method was adopted to test Semi Rigid mixture samples. The maximum load P, taken by the specimen was used to calculate the indirect tensile strength. The ITS value can be determined by Equation 5.3 and Figure 5.4 shows the ITS value of conditioned and unconditioned for binder contents.

Durability
Cantabro abrasion test conducted at temperature of 25ºC in Las Angeles drum, rotated for 300 revolutions. This test is generally conducted on both dry and wet conditioned specimen as specified in the ASTM D 7064 -08. To study worst condition, aged samples were used in the present study. The samples were conditioned for 168 hours at 40°C and samples are then cooled to 25°C and stored for 4 hours prior to abrasion. The result of the test is determined using the Equation 5.4.

Conclusions
This current study investigated the engineering and strength properties of Semi Rigid mixtures. In the first stage of this study, the effect of flowability and strength properties of cement mortar grout was evaluated. In the next stage, the open graded bituminous mix, which is the skeleton of the Semi Rigid mixtures, was designed to find optimum binder content by air voids drain down and abrasion loss was evaluated. Finally, Semi Rigid mixtures specimens were prepared by grouting cement mortar into an open graded bituminous mix. Mechanical properties including compressive strength, flexural strength, Marshall Stability, tensile strength and durability were evaluated for optimum binder content. From the test results it was clearly recommended that the air voids 20-30% and filling rate of Semi Rigid mixtures specimens were greatly influenced by the flowability of cement grout within 12 seconds. Through evaluating the strength characteristics compressive strength, flexural strength, Marshall Stability, tensile strength and durability it was found that the 4.5% binder content have higher values compare to 5.0% binder content due to increase in void content and cement grout. 16