Concrete specimen such as cubes and cylinders need to adjustment procedure in case of any irregularities found on the specimen surface. The concrete cubes and cylinders used for strength test in order to verify the quality of concrete, sometimes due to some factors the shape of the specimen does not conform with (EN 12390-Part 1); for this reason, the specimen need adjustment process. Adjustment is usually performing on concrete core samples when taken for testing purposes, and no further adjustment require for cubes and cylinders when casting in precise and firm molds.
When core samples were taken by core driller, usually many irregularities found at one end of the specimen as shown in (Fig-1).
Fig-1: Core specimen
Most of the core specimen used for compressive strength test, the load applied on both ends of specimen because of this reason both ends of specimen should be perfectly smooth, parallel to each other and perpendicular to loading axis, any irregularities can be eliminated by adjustment methods as described as follow:
1- Grinding: used for unlimited strength
2- Capping by using (Calcium aluminate cement mortar): used up to 50 Mpa.
3- Capping by using (Sulfur mixture): used up to 50 Mpa.
4- Sandbox: used for unlimited strength
Note: in case of dispute testing, grinding shall be a reference method.
1- Grinding Method:-
This method is a reference method in case of dispute testing, its much easier rather than other adjustment methods because of developing fully automated cutter machine , grinding is a process of cutting the irregular part of specimen, this method can be applied on all testing specimen such as (Cubes, Cast cylinder, Core cylinder), different kind of cutter can be used for grinding process as shown in (Fig-2).
Fig-2: Cutter Machine
2- CAPPING (USING Calcium aluminate cement mortar):-
"The capping material should consist of a mortar composed of three parts by mass of calcium aluminate cement to one part by mass of fine sand (most of which passes a 300 μm ISO 3310-1 woven wire sieve)"1.
Other cements conforming to EN 197-1 may be used provided that, at the time of test, the mortar has a strength at least equal to the strength of the concrete.
Place the specimen with one end on a horizontal metal plate. Rigidly clamp a steel collar of correct
dimensions and having a machined upper edge to the upper end of the specimen to be capped, in such a way that the upper edge is horizontal and just extends beyond the highest part of the concrete surface.
Fill the capping material into the collar until it is the form of a convex surface above the edge of the collar. Press a glass capping plate, coated with a thin film of mould oil down onto the capping material with a rotary motion until it makes complete contact with the edge of the collar.
3- CAPPING (USING SULFUR MIXTURE):-
"Proprietary sulfur capping mixtures are usually suitable. Alternatively, the capping material may consist of a mixture composed of equal parts by mass of sulfur and fine siliceous sand (most of which passes a 250 μm woven wire sieve and is retained on a 125 μm woven wire sieve conforming to ISO 3310-1). A small proportion, up to 2 %, of carbon black may be added."1
(Fig-3) shows the tools and equipment for sulfur mixture capping.
Fig-3: Required equipment for sulfur capping
Heat the mixture to the temperature recommended by the supplier or to the temperature when the require consistency reached, the mixture should stirring continuously to avoid sedimentation forming at the bottom of the melting pot, beer in mind the level of mixture shouldn't to low inside the melting pot to avoid of sulfur vapor because its risky for ignite or create some health problem.
once the mixture reached the required consistency, fill the capping pool\mold with sulfur mixture then lower one end of cylinder specimen into the capping mold, use capping frame as shown in (Fig-3) to ensure both capping surfaces are parallel to each other, allow the mixture harden enough before start capping other end of specimen.
if the capping sounds hollow, remove the capping material and repeat the procedure.
Note: Use exhausted fan during melting operation because sulfur vapor is danger for breathing system.
4- SAND BOX CAPPING:-
The sand used for capping shall be "fine siliceous sand, most of which passes a 250 μm woven wire sieve and is retained on a 125 μm woven wire sieve conforming to ISO 3310-1."1
Before capping, ensure that the surface of the specimen to be capped is clean and that all fine loose particles have been removed.
TOOLS AND EQUIPMENT:-
1- Steel box: steel box shall conforming to the shape and dimension as shown in (Fig-4), yield strength of steel should be reach to 900 Mpa to avoid shape changes under compression forces, maximum tolerance of the dimension should be less than 0.1-mm, the box shall have the opening at the side of the box to release the box after testing.
2- Positioning frame: used to keep the specimen perpendicular on the steel box, the frame should have two clamps to fix the specimen on its place during capping procedure, (Fig-5) show the positioning frame, a vibrator should be mounted under the frame to ensure the homogeneous distribution and compaction of the sand in the steel box, the frame assembly should isolated with vibrator to ensure that the vibrator doesn't transmitted to the entire frame.
3- Compressed air blower: used to release the steel box from the specimen after test completion.
4- Hotplate: used to melt the paraffin wax, should b thermostatically controlled to 110±10 °C.
5- Flask: used to hold the melting wax.
6-Calibration container: used to calibrate the required volume of sand corresponding to the height of 10±2 mm in the sand box.
1- Place the positioning frame on a horizontal working surface. Position one of the sand boxes on the frame and lock in position. Pour the required volume of sand, without spreading it, in the center of the box.
2- After wiping the bearing surfaces, place the specimen on the pile of sand and clamped in position.
3- Run the vibrator for (20 ± 5) s, making sure that the guide rollers bear correctly against the specimen.
4- Pour the paraffin wax up to the rim of the box and allow to set. Un-clamp the specimen and turn it over on the working surface. Repeat the operations for the second box.
Fig-4: Steel box dimensions and shape
Fig-5: Positioning Frame
Fig-6: Sand box capping
1- British Standard: BS EN 12390 - Part 3 \ Annex A
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BALEN ZRAR ABDULSAMAD BSc- Geotechnical Engineering\Koya University MSc- Infrastructure Engineering\ Szechenyi Istvan University
BALEN ZRAR ABDULSAMAD
BSc- Geotechnical Engineering\Koya University
MSc- Infrastructure Engineering\ Szechenyi Istvan University
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