- The tensile strength ratio of bituminous mixtures indicates their resistance to moisture susceptibility and a measure of water sensitivity.
- Moisture damage in bituminous mixes refers to the loss of serviceability due to the presence of moisture.
- A higher TSR value indicates good resistance to moisture.
- The higher the TSR value, the lesser will be the strength reduction by the water soaking condition, or the more water-resistant it will be.
SCOPE OF TEST:
- Hot Mix Aggregate becomes sensitive in the presence of water
- This method covers preparation of specimens and the measurement of the change of tensile strength resulting from the effects of water saturation
- Water will cause the binder to not adhere to the aggregate. rapid failure of the pavement can be expected if the binder cannot adhere to the aggregate. This is often referred to as stripping.
- The results may be used to predict long term stripping susceptibility of the asphalt mixture and evaluate liquid anti-stripping additives that are added to the asphalt binder or pulverulent solids, such as hydrated lime or Portland cement, which are added to the mineral aggregate.
- To help prevent stripping, additives such as hydrated lime or liquid anti-stripping chemicals may be required.
- AASHTO T 283 is a test method that can be used to determine if the materials may be subject to stripping and also to measure the effectiveness of additives.
- The test is performed by compacting specimens to an air void level of six to eight percent.
- The specimens are then tested for indirect tensile strength by loading the specimens at a constant rate and measuring the force required to break the specimen.
- The tensile strength of the conditioned specimens is compared to the control specimens to determine the tensile strength ratio (TSR).
This test may also be performed on cores taken from the finished pavement.
For laboratory-batched mixtures, 150 mm diameter and 63.5 mm thick specimens are normally used. Enough material is mixed to produce at least eight.
After mixing, the mixture is placed in the pans and spread to about 25 mm thick. The mix is then cooled to room temperature for 2 ± 0.5 hours. The mixture is placed in the oven for 2 hours at 135 ± 3°C, and stirred every 60 ± 5 minutes to maintain conditioning.
Some experimentation will be needed to find the correct compactive effort that will yield 7 ± 0.5 percent air voids. The specimens are required to be compacted
in accordance with ASTHO T312. After the specimens are removed from the molds, they are stored at room temperature for 24 ± 3 hours.
EVALUATING AND GROUPING OF SPECIMENS:
After curing, the following tests and measurements of each specimen are done:
1.The maximum specific gravity (Gmm).
2.The thickness (t) and diameter (D).
3.The bulk specific gravity (Gmb). The volume (E) of the specimens is determined by subtracting the specimen weight in water from the saturated, surface-dry weight.
- The percentage of air voids (Pa) is determined. Once determined, the specimens are separated into two subsets, of at least three specimens each, so that the average air voids of the two subsets are approximately equal.
- For those specimens to be subjected to vacuum saturation, a freeze cycle, and a warm-water
- soaking cycle, the volume of the air voids (Va) in cubic centimeters is calculated as follows:
Va = volume of air voids, cubic centimeters
Pa = air voids percent
E = volume of the specimen, cubic centimeters
RECONDITIONING OF SPECIMENS:
- At the end of the curing period, the dry sample is wrapped in a heavy duty plastic bag. The specimens are then placed in a 25 ± 0.5°C water bath for 2 hours ± 10 minutes with a minimum of 25 mm of water above their surface.
- The weight of the saturated, surface-dry specimen after partial vacuum saturation (B1) is determined.
- The volume of absorbed water (J1) in cubic centimeters is determined by the following equation
J1 = B1 – A
J1 = volume of absorbed water, cubic centimeters
B1 = weight of the saturated, surface-dry specimen after partial vacuum saturation, gm
A = weight of the dry specimen in air, gm
The degree of saturation (S1) is determined by comparing the volume of absorbed water (J1) with the volume of air voids (Va) using the following equation:
S1 = degree of saturation, percent
If S1 70 TO 80 percent conditioning by freezing
<70 percent the vacuum procedure using more vacuum and/or time is repeated
>80 percent specimen is considered damaged
For specimens with 70 to 80 percent saturation, the samples are each wrapped with a plastic film such as Saran Wrap and placed in a plastic bag containing 10 ± 0.5 ml of water and sealed. The plastic bags are placed in a freezer at a temperature of 0 ± 5°F (-18 ± 3°C) for 24 ± 1 hours. The specimens should have a minimum of 25 mm of water above their surface.
Once the specimens are placed in the water bath, the plastic bag and film is removed from each specimen.
After 24 ± 1 hours in the water bath, the specimens are removed and placed in a water bath at 25 ± 0.5°C for 2 hours ± 10 minutes. The specimens should have a minimum of 25 mm of water above their surface.
The specimen is removed from the bath, the thickness (t1) determined, and then placed on its side between the bearing plates of the testing machine .Steel loading strips are placed between the specimen and the bearing plates. A load is applied to the specimen by forcing the bearing plates together at a constant rate of 50 mm per minute.
The maximum load is recorded, and the load continued until the specimen cracks. The machine is stopped and the specimen broken apart at the crack for observation The approximate degree of moisture damage is estimated on a scale from 0 to 5, with 5 being the most stripped.
The tensile strength is calculated using the following equation:
St = tensile strength, kPa
P = maximum load, Newton
t = specimen thickness, mm
D = specimen diameter, mm
The tensile strength ratio is calculated as follows:
Tensile Strength Ratio (TSR) = S2/S1
S1 = average tensile strength of the dry subset, (kPa)
S2 = average tensile strength of the conditioned subset, (kPa)
The values of tensile strength(St) may be used to evaluate the relative quality of bituminous mixtures in conjunction with laboratory mix design, testing and for estimating the resistance to cracking.
FACTORS AFFECTING THE RESULT:
- Tensile strength can be improved by using additives because, the presence of additives in the bituminous mixture strengthens the bonding between the aggregate provided by the binder and as a result, the mixtures had the highest stiffness.
- The tensile strength increases as the additive content increases, reaches a maximum and then decreases.
- Water content, organic matter content are the main factors which affects the tensile strength of aggregates.
A minimum TSR value set forth by AASHTO T283 is 70%
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