VIETNAMESE STANDARD TCVN 3118: 1993 HEAVY CONCRETE - METHOD FOR DETERMINING COMPRESSIVE STRENGTH Heavyweight concrete - Method for determination of compressive strength This standard specifies method for determination of compress strength for heavyweight concrete. 1. Test equipment Compressor; Metal measure tape; Transmission pad (using compressed air to compress half specimen after flexural test). 1.1 Compressor is installed and placed at a fixed position. After installation, periodically once every 1 year or after each repair, the machine must be checked and issued with a valid certificate by an authorized state measurement agency. 1.2 The force transmission pad (Figure 1) is made of steel of 20 ± 2mm thick with grooves of 30 ± 2mm from the sample. The size of the direct force transmission part corresponds to the specimen’s cross-section (100 x 100; 150 x 150; 200 x 200mm). 2. Preparation of test sample 2.1 Test samples are prepared in lot. Each lot consists of 3 specimens. For specimen which are drilled or cut from concrete structure, if it were incapable of taking 3 specimens, a lot of 2 specimens would be allowed. 2.2 Sampling, curing, cutting of concrete specimen and selection specimen sizes shall be conducted in accordance with TCVN 3105: 1993.
2.3 Standard sample for determining the compressive strength of concrete has cubic shape with size of 150 x 150 x 150mm. Cubic specimen whose sizes differ from the standard sample, and the cylinder specimen after compression test must be calculated to convert into the strength of the standard one. 2.4 Age and state of the specimen should represent the structure which requires test for the purpose of acceptance or operation start. 2.5 Two compression suffered sides of the specimen should be checked and selected so that: a. The maximum gap between sample’s surface and the straight ruler, which is closely placed on the surface in all directions, should not exceed 0.05mm per 100mm from the ruler’s placement point. h. The maximum gap between specimen’s bearing side and the right-angled ruler, in which the other bar of ruler is closely placed on the other side of ruler to the lateral side of the cubic specimen or the height of the cylinder specimen should not exceed 1mm per 100mm from the ruler’s placement point. c. For cubic specimens and cylinder specimen which had been through flexural test, the bottom side and the open face for sample casting will not be selected for compressive strength test. 2.6 In the event that the specimens do not meet the requirements specified in clause 2.3, the specimen shall be reworked by abrading or flattening with a hard layer of cement not exceeding 2mm thick. The strength of this cement layer when tested shall not be less than half the expected strength of the concrete specimen. 3. Test implementation 3.1 Determine the bearing area of the specimen 3.1.1. Accurately measure, up to 1mm, the pair of parallel lines on the bearing sides (for cubic specimen), and the pairs of diameters perpendicular to each other on the bearing sides (for cylinder specimen), identify the area of the two upper and lower bearing sides in accordance with the average values measured from the aforementioned pairs. The area under the compressive strength of the specimen is the arithmetic mean of the two bearing sides. 3.1.2. The bearing area of the half specimen under flexural test is calculated by the arithmetic mean of the area of the common sections between the upper and lower bearing sides and the respective transmission steel pads. 3.2 Determination of destructive load 3.2.1. Choose the appropriate compression scale of the machine so that the destructive load is in the range of 20-80% of the maximum load of the selected scale. Do not compress beyond the selected scale.
3.2.2. Place the specimen into the compressor so that the selected side of the specimen is located at the center of the lower bearing pad. Operate the machine so that the upper side of the specimen gently approaches the upper bearing pad of the machine. The compression force will be increased continuously at a constant speed and equal to 6 r 4 daN/cm2 per second until the occurrence of damage in specimen. Use small loading rates for low-intensity concrete specimens, and high loading rates for high-strength concrete specimens. 3.2.3. The maximum force achieved is the value of destructive load occurred in specimen.
4. Calculation of results 4.1 The compressive strength of each concrete specimen (R) is calculated in daN/cm 2 (KG/cm2) according to the formula:
In which: P - Damage load, in daN; F - Area of compression bearing of the specimen, in cm2; α - conversion coefficient to convert the results on the specimens which differ in sizes to the standard specimen with size 150 x 150 x 150mm. The value of α according to Table 1. Table 1 Shape and dimensions of the specimen (mm)
Cubic specimen 100 x 100 x 100
150 x 150 x 150
200 x 200 x 200
300 x 300 x 300
Cylinder specimen 71,4 x 143 and 100 x 200
150 x 300
200 x 400
Note: 1. Do not use D values which are lower than the values in Table 1. 2. It is allowed to use D values greater than the values in Table 1 when D is determined experimentally by the method recorded in the appendix of this standard. 3. For half specimen, the conversion coefficient is also applied as the cubic specimen of similar compressive bearing section. 4.2 For cylinder specimens drilled from structures or products whose height/diameter ratio is less than 2, the result is also calculated using the formula and D factor stated in the aforementioned Section 4. 1 but multiplied with the coefficient E taken from the table 2. Table 2 H/d
4.3 The compressive strength of concrete is determined from the compressive strength values of specimens of a Lot as follows: 4.3.1. Compare the maximum and minimum compressive strength values with the compressive strength of the average specimen. If both measurements do not deviate by more than 15% from the compressive strength of the average specimen, the compressive strength of the concrete will be calculated by the arithmetic mean of the three specimens. If either of these two values deviates more than 15% from the compressive strength of the average specimen, both maximum and minimum values will be omitted, then the compressive strength of the concrete is the compressive strength of the remaining specimen.
4.3.2. In case of a Lot with two specimens, the compressive strength of the concrete is calculated by the arithmetic mean of the two specimens. 5. Test records Test records should clearly specify: - Sign of specimen; - Place of sampling; - Concrete age, curing conditions, specimen’s state at test; - Designed grade of concrete; - Dimensions of each specimen; - Compression bearing area of each specimen; - Destructive load of each specimen; - Compressive strength of each specimen and the average compressive strength, - Signature of tester. Appendix Determination of conversion coefficient by experiment 1. Conversion coefficient is determined separately for each concrete grade, each type of concrete, each compressor and each lot whose mold dimensions different from the standard specimen. 2. To determine the conversion coefficient, it should test 8 lots (3 specimens each) of standard size and 8 lots corresponding to each type of molds different from the size of the standard specimen. 3. Standard specimens and non-standard specimens should be made from a same mixture, cured and compacted under same conditions and tested at same age. When tested, standard and non-standard specimens must have a mass difference of not more than 2%. 4. After compression test, perform following calculations: 4.1. Conversion coefficient applied for each pair of standard and non-standard specimens.
(1) In which: - The average concrete strength of the standard and non-standard lot numbered i. 4.2. Average conversion coefficient:
(2) 4.3. Variance Sα.
(3) 4.4. Value t
There cases with value t: a. If t ≥ 1,4, use coefficient D determined experimentally; b. If t < l, 4, use coefficient D in accordance with Table 1 of this standard; c. Regardless of t if < α, α will be identified according on Table 1 of this standard.
5. The value of the conversion coefficient determined at the preliminary laboratories must have the participation of the specialized staffs from laboratories under the Ministry. It is then approved by the competent organizations. 6. The inspection on the conversion coefficients by experiment should be conducted at least once in two years.