To construct a flexible and efficient panel as a first point of impact with a vehicle, several experiments have been performed to design the section type and its concrete technology. To get a clear view, the experiments performed related to the concrete technology and choosing between rectangular and semi-circular section is discussed here briefly.
To start, regular concrete with average strength of 30 MPA is used. Then samples with 0, 1, 1.5, 2, and 2.5 % of glass fibers are made and tested for their compressive strength and resulting compressive strength of specimens on 7, 28 42 days of age can be observed in Table 4-1.
Table 1 – Initial mixture ratio to choose the control sample
Considering the compressive strength results, it is observed that compressive strength is reduced with increasing glass fiber content and the reduced strength is almost stable from 1 to 2 % fiber used. The mixture with 1.5% glass fiber is selected as the optimal fibers used.
After choosing the control sample, 5 other mixture ratios are designed which are listed in table 1-2. The control sample is made using natural aggregates and rubber powder used is zero. In each of the mixture ratios designs, 5% of sand’s weight is replaced by rubber powder till 25% of natural aggregates are replaced by rubber powder.
Table 2 – The final mixture ratios containing 1.5% glass fibers and variable rubber powder percentage
The control sample is called OSR, in which SR stands for Sand replacement and the O is the sign of rubber powder. SR, SR20, SR10 and 25SR5 are also named in the same manner.
Concrete samples studied to devise the scheme: To design a proper cross-section two concrete samples with rectangular and semi-circular section are used. The rectangular sample is made in 15*30 centimeters and is shown in figure 1. The semi-circular sample is also schematically shown in figure 2.
Figure 1 – schematic view of rectangular sample
Figure 2 – schematic view of semi-circular sample
The results obtained from compressive strength test is listed in table 3 and charts 1 to 4 is observed based on different rubber powder content on 7, 28 and 42 days of age.
Table 3 – Compressive strength results
Chart 1- The compressive strength results on different ages based on various rubber powder content.
Chart 2- The compressive strength results on different ages based on various rubber powder content.
Chart 3- The compressive strength results on different ages based on various rubber powder content.
Chart 4- The compressive strength results on different ages based on various rubber powder content.
Projectile impact strength, Charpy impact test, Drop weight impact weight and Hopkinson’s bar impact test are few common tests suggested in ACI544-2R committee to analyze impact behavior of concrete. Drop weight with repetitive impacts is sued to measure the impact strength. Here, a dropping weight of 4.54 kg is used which is dropped from a 4.57 m height on the steel ball of 64mm diameter. The steel ball is placed on the concrete sample. The dropping weight is released on the steel bar several times and number of impacts that cause the first crack and final rupture of the sample is recorded. This experiment is common since it’s simple and cost-effective. To do this experiment, the following equipment are need according to ACI-544.2R recommendation:
- Standard compressor hammer (the dropping weight) weighing 4.54 kg
- A steel ball of 64 mm diameter
- A steel plate for placing the concrete disk on it
- Concrete disk of 150 mm diameter and 64 mm thickness
In this experiment, several disk-shaped concrete samples of 150 mm diameter and 64 mm thickness are constructed or cut from the standard 150*300 cylindrical samples, Although the latter is preferred. The concrete disks are placed on the lower surface, on the steel plate after preparation. The steel plate must be screwed on a rigid base or foundation. The weight is dropped from a 0.5 m height and the repetitive impacts are continued till a specific level of cracking (the first crack and final rupture.
The schematic view of dropping weight can be observed in figures 3 to 5.
Figure 3 – The steel ball along with the concrete disk to do the impact test.
Figure 4- The schematic configuration of impact test
Figure 5 – Schematic view of dropping weight system recommended by ACI 544 Code
Since the rubber powder’s elastic properties are higher than that of cement paste surrounding it, when the system is loaded, cracks appear around the rubber powder which in turn accelerates the cement-rubber powder matrix of rupture, so increasing the amount of rubber powder, compressive strength of samples is reduced. Since the strength of normal concrete depends on aggregate types and size, density, and stiffness of the aggregates, and the aggregates are replaced by rubber powder, reduction in the strength of the sample is normal. On the other hand, after the samples are broken under the compressive strength testing machine, it is observed that the failure happened around the zone containing rubber powder, so by increasing the amount of rubber powder, reduction in compressive strength is also increased.
Also, since there’s no cohesion between rubber particles and the cement paste, we can consider soft rubber particles as holes in concrete mixtures that will decrease its strength.
Impact Strength of rectangular sample
Number of impacts needed for the first crack or final failure to appear, are shown alongside the energy absorbed by the hardened samples in tables 4 and 6.
Table 4- The first crack, final failure and the energy absorbed by the hardened samples of 7 days of age
Table 5- The first crack, final failure and the energy absorbed by the hardened samples of 28 days of age
Table 6- The first crack, final failure and the energy absorbed by the hardened samples of 42 days of age
Charts 5 to 7 shows information related to the first crack and final failure of samples of 7, 28, 42 days of age.
Chart 5- first crack, final failure of the hardened samples after 7 days
Chart 6- first crack, final failure of the hardened samples after 28 days
Chart 7- first crack, final failure of the hardened samples after 42 days
As it is shown in charts 8 to 10, impact numbers is observed to increase until the replacement ratio with rubber powder is reached to 10% and replacement ratio of higher amounts, the impact numbers needed for the first crack or the final failure to appear are reduced. This trend is observed in all ages.
Chart 8 – Increase/decrease percentage of first crack and final failure for hardened samples of 7 days of age
Chart 9 – Increase/decrease percentage of first crack and final failure for hardened samples of 28 days of age
Chart 10 – Increase/decrease percentage of first crack and final failure for hardened samples of 42 days of age
The highest increase/decrease in the number of impacts for the first crack or final failure is observed on the seventh day and the lowest is observed on the 42th day.
The maximum amount of reduction for the first crack is equal to 52% in 25SR sample on the 42th day and the highest increase on the 7th day is 241% which is observed for 10SR.
Chart 11 – number of impacts to reach the first crack and final failure for hardened samples of all ages
Chart 12 – number of impacts to reach the first crack based on the age of sample
Chart 13 – number of impacts to reach the final failure based on the age of samples
Chart 14 – number of impacts to reach the first crack based on the replacement ratio
Chart 15 – number of impacts to reach the final failure based on the replacement ratio
days is higher than that of 7 days and this trend is observed in all replacement ratios. Since the hydration process progressively evolves with increased age, higher strength for the sample is normal.
The maximum number of impacts for the first crack on 10SR on 42th day is 101 and the least number of impacts to induce the first crack is on 7th day for 25SR which is equal to 20 hits.
The maximum number of impacts for the final failure on 10SR on 42th day is 389 hits and the least number of impacts to induce the final failure is on 7th day for OSR which is equal to 42 hits.
The number of impacts to reach the first crack for 20SR is higher than that of control sample, while the number of impacts for 25SR is lower than that of control sample.
The number of impacts needed to see the first crack, final failure and the energy absorbed by the hardened samples is shown in tables 7 to 9 for different ages.
Table 7- The first crack, final failure and the energy absorbed by the hardened samples on the seventh day
Table 8- The first crack, final failure and the energy absorbed by the hardened samples on the 28th day
Table 9- The first crack, final failure and the energy absorbed by the hardened samples on the 42th day
Charts number 16 to 18 show the data for the first crack and final failure on 7th, 14th, and 28th day.
Chart 16- The first crack, final failure for the hardened samples on the 7th day
Chart 17- The first crack, final failure for the hardened samples on the 28th day
Chart 18- The first crack, final failure for the hardened samples on the 42th day
As it is observed on charts 16 to 18, similar to the rectangular sample, for replacement ratios of 10% and lower, the impact numbers are increased and when this ratio is reached above 10%, the number of impacts and final failure decreases. This trend can be observed in all ages.
On charts number 19 to 21, increase/decrease percentage of impact numbers to reach the first crack and final failure is shown.
Chart 19- Increase/decrease percentage of impacts for the first crack and final failure for hardened samples on 7th day
Chart 20- Increase/decrease percentage of impacts for the first crack and final failure for hardened samples on 28th day
Chart 21- Increase/decrease percentage of impacts for the first crack and final failure for hardened samples on 42th day.
The highest increase/decrease percentage for the first crack or final failure for hardened samples is observed on the seventh day and the lowest is observed on the 42th day.
The maximum amount of reduction for the first crack is equal to 76% in 25SR sample on the 42th day and the highest increase on the 7th day is 160% which is observed for 10SR. This is while the maximum amount of impacts to reach the final failure is 374% for 10SR on the 7th day and the highest decrease in final failure is observed for 25SR as 35% on the 42th day. Also for the rectangular samples the highest decrease for the first crack and final failure is observed in 25SR.
Chart 22- number of impacts for the first crack and the final failure for hardened samples of all ages
Chart 23- number of impacts for the first crack based on the age of samples
Chart 24 number of impacts for final failure for hardened samples of all ages
Chart 25- number of impacts for the first crack based on the replacement ratio
Chart 26- number of impacts for final failure based on the replacement ratio
As it is observed in charts 28 to 32, the number of impacts to induce the first crack on the 42th day is higher than that of 28 days, and these number for 28 days is higher than that of 7 days and this trend is observed in all replacement ratios. Since the hydration process progressively evolves with increased age, higher strength for the sample is normal.
The maximum number of impacts for the first crack on 10SR on 42th day is 161 and the least number of impacts to induce the first crack is on 7th day for 25SR which is equal to 23 hits.
The maximum number of impacts for the final failure on 10SR on 42th day is 583 hits and the least number of impacts to induce the final failure is on 7th day for OSR which is equal to 86 hits.
The number of impacts to reach the first crack for 15SR is higher than that of control sample, while the number of impacts for 20SR and 25SR are lower than that of control sample.
Charts 27 to 29 show the impact strength of first crack and final failure for the rectangular and semi-circular samples.
Chart 27- comparing the 7th days old sample; impact strength of rectangular and semi-circular sections based on replacement ratios.
Chart 28- comparing the 28th days old sample; impact strength of rectangular and semi- circular sections based on replacement ratios
Chart 29- comparing the 42th days old sample; impact strength of rectangular and semi- circular sections based on replacement ratios
As it is shown, with all replacement ratios, semi-circular sections show higher impact strength than rectangular ones while the first crack appears or the final failure is reached. Also the highest amount of impact strength on 10SR is shown for both samples, which is increased 216% for the rectangular sample and 161% for semi-circular sample on 42th day. The increase related to the first crack for the rectangular and semi-circular sample is 110 and 32% respectively. In general, increasing influence the amount of rubber ball in all samples, on all ages and for both types of samples when in complete failure, is higher than that of the first crack appearance.
The relationship between compressive strength and the energy obtained from impacts to reach the first crack and final failure is shown in charts 30 to 35.
Chart 30- The relationship between compressive strength and impact strength while in final failure mode after 7 days
Chart 31- The relationship between compressive strength and impact strength while in final failure mode after 28 days
Chart 32- The relationship between compressive strength and impact strength while in final failure mode after 42 days
Chart 33- The relationship between compressive strength and impact strength for the first crack after 7 days
Chart 34- The relationship between compressive strength and impact strength for the first crack after 28 days
Chart 35- The relationship between compressive strength and impact strength for the first crack after 42 days
As it is shown in charts 30 to 35, the relationship between Compressive strength and the energy absorbed by the sample is a cubic equation and the correlation factor for all charts is bigger than 0.9. The peak amount of the first crack and final failure charts move towards higher compressive strength with increased age of samples. For example, for the final failure between 12-14, the chart for 28th day and 42th day show 19-21 and 22-24 respectively and this increase is also observed in charts related to the first crack.
The relationship between compressive strength and the energy obtained from weight impacts to reach the first crack and final failure is shown in charts 36 to 37.
Chart 36- The relationship between compressive strength and impact strength in final failure mode after 7 days
Chart 37- The relationship between compressive strength and impact strength in final failure mode after 28 days
Chart 38- The relationship between compressive strength and impact strength in final failure mode after 42 days
Chart 39- The relationship between compressive strength and impact strength for the first crack after 7 days
Chart 40- The relationship between compressive strength and impact strength for the first crack after 28 days
Chart 41- The relationship between compressive strength and impact strength for the first
The optimal sample for constructing the final concrete panel is of semi-circular section, with 1.5 % glass fiber and 10% rubber powder.
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