反撥硬度法에 의한 旣存 철근콘크리트 構造物의 壓縮强度 推定에 관한 硏究 : 再建築 대상 共同住宅을 中心으로
(A) Study on Compressive Strength Estimation of Existing Reinforced Concrete Structures using Rebound Hammer Test Method
반발경도법 철근콘크리트 압축강도 콘크리트강도;
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Among nondestructive evaluation techniques, a hammer method (Rebound Test Hammer Method)is to calculate an estimated compression by using an estimation equation of concrete compression which was studied with values arisen by the nondestructive test of concrete mainly for test pieces of laboratory. However, most of estimation equations proposed as the nondestructive evaluation technique have been mainly studied with indoor test up to now. Consequently, because its volatility is very big owing to many factors of existing concrete buildings, it is difficult to estimate the actual intensity. Accordingly, this study was intended to verify accuracy and reliance of Rebound Test Hammer Methodin estimating the compression of the superannuated concrete buildings which its age was 20 years (7,300 days). For the study, the method to compare the estimated compression using three compression estimation formulas using the rebound hardness by indoor test proposed in Japan with compression through destruction evaluation of Core extracted from the existing concrete buildings, was selected. Also, this study verified a validity and reliance of the age coefficient which was applied to three estimation formulas of rebound hardness methods by measuring the intensity through gathering Core sampling for materials of old age which have the same intensity, which can be hardly tested in normaltest. Also, this study investigated whether it wasproper to apply the age coefficient considering the increase of surface hardness based on the age for cutting plan of Core sample tested by interrupting contact with air by finished materials or by cutting parts of structure. Also, it investigated that the removal of finished materials affected the rebound value. To achieve the above objective, 65 compression destruction tests were conducted with 1300 measurement values of rebound hardness for 65 Core samples of 5 places in 2 superannuated public apartments(20 years have passed up to now) located in Masan and Changwon, as a reconstruction target, because there was the actual difficulty in gathering large samples for numbers of places. The following summarizes results on the compression of steel concrete building evaluated based on the construction test and estimated compression by nondestructive test. 1. When measurement values which exceeded ±20% of average rebound values were removed and rebound measures were calculated on the basis of 20 times among rebound hardness values, there was approx. 1.6% difference of the average rebound value, compared with the case conducted for all rebound values. This study indicated that it didn't affect the intensity estimation. 2. Because the estimated compression using the rebound hardness had different values by applied formula, the compression based on each estimation formula was calculated to calculate reliable compression in the practical field and then the average of the sum total was applied. In this study, all three estimation equations of compression using the rebound hardness value were summed up and then the calculated average value had the very close value to the average compression of the target buildings. Among the estimation formulas, the estimation value of Fc = 13 Ro 184 which was the formula of the Materials Research Society of Japan, was very close to the average compression. The estimation value of Fc = 10 Ro 110 which was the formula of Japanese Construction Materials Inspection Center, showed a low value which was included in the confidence interval of the average compression. The estimation of Fc = 7.3 Ro + 110 which was the formula calculated by the joint testing result of AIJ (Architectural Institute of Japan) showed the high value which was included in the confidence interval of the average compression. 3. In this study, the rebound value by vertical downward hammer conducted in the site after removing finished materials by general method was lower by approx. 3~10%to the rebound value by the horizontal hammer conducted for a section of Core. Accordingly, when the rebound hardness test is conducted in the site, if finished materials of the target building are completely removed and the section is not exposed, the rebound value may be low because an unevenness of remaining finished goods has a shock-absorbing effect. Consequently, the removal of finished materials and unevenness handling must be completely conducted to estimate the reliable compression. If not, a proper correction of the measured rebound value needs to be required. 4. Generally, it is considered that there is no increase of Surface Hardness even though several years pass for a cutting plan of testing core sample by interrupting contact with air by finished materials or by cutting parts of structure. Accordingly, it is predicted that applicable age factors are not considered on the basis of 3,000 days. However, as the result of this study, in spite of the above estimation, there could a little bit difference in the value of age factors like the below 5. However, it is concluded that the age factor to convert with intensity of 28 days must be applied to superannuated building. 5. The different age coefficient must be applied for superannuated buildings (20 years)according to the estimation equation for the study as follow; 0.65 in the formula of the Materials Research Society of Japan, 0.76 in the formula of Japanese Construction Materials Inspection Center, and 0.54 in the formula of AIJ (Architectural Institute of Japan). Accordingly, it can show the closest value to the average compression of the structure. Because this result comes from only the restricted test body used in this study, it is hard to generalize it. However, the age coefficient needs to be applied with different values for estimation expressions of compression in estimating the compression by nondestructive test of the building that its age is more than 3,000 days especially the building with the age of 20 (7,300 days) years. The age coefficient based on the estimation expression of compression will have to be applied after other age coefficient is determined additionally by the estimation expression through further research and test.