Techshore Institute Blogs about Best Technical and Management Courses: concrete

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Sunday, February 5, 2017

Techshore Inspection Services : Half Cell Electrical Potential method

Principle

The half-cell electrical potential method is used to measure the potential of an embedded reinforcing bar relative to a reference half-cell placed on the concrete surface. The half-cell usually used is a copper/copper sulphate or silver/silver chloride cell but other combinations are also used. The concrete functions as an electrolyte and the risk of corrosion of the reinforcement near the test location may be related empirically to the measured potential difference. ASTM C876 - 91 gives a Standard method for testing Half-cell potential of uncoated reinforcing steel in concrete.

Equipment

The testing apparatus consists of the following components:

Copper-Copper Sulphate Half-Cell:

The cell consists of a rigid tube made of dielectric material which is non-reactive with copper or copper sulphate, a porous wooden or plastic plug which remains wet by capillary action, and a copper rod which is immersed within the tube in saturated copper sulphate solution. The solution is prepared by dissolving reagent grade copper sulphate crystals in distilled or de ionized water.

The inside diameter of rigid tube should not be less than 25 mm and that of porous tube should not be less than 13 mm. The immersed copper rod should have inner diameter of not less than 6 mm and length 50 mm.

Electrical junction device:

An electrical junction device is used to provide a low electrical resistance liquid path between the concrete surface and the half-cell. It consists of a sponge pre-wetted with a low electrical resistance contact solution. The sponge can be attached to the tip of the half-cell so that it provides electrical continuity between the porous tip and the concrete member.

Electrical contact solution:

Electrical contact solution is used to wet the electrical junction device so that the potential drop through the concrete portion of the circuit is standardized. It is a prepared by thoroughly mixing a 95 ml of wetting agent or a liquid household detergent with 19 L of potable water.

Voltmeter:

A battery operated voltmeter is used with ± 3% end of scale accuracy at the voltage ranges in use. The input impedance should be not less than 10 MW while operating at a full scale of 100 mV.

Electrical lead wires:

The electrical lead wire should be such that its electrical resistance for the required length does not disturb the electrical circuit by more than 0.0001 V.

Procedure

Measurements are made in a grid pattern. The spacing between the measurements is generally chosen such that adjacent readings are less than 150 mV with the minimum spacing so that there is at least 100 mV between readings. An area of high corrosion activity is indicated by the area with greater than150 mV. A direct electrical connection is made to the reinforcing steel using a compression clamp.

To get a low electrical resistance connection, the rod must be scraped before connecting it to the reinforcing bar. The reinforcement bar is connected to the positive terminal of the voltmeter. A lead wire is used to connect the half-cell to the negative terminal of the voltmeter. Before the test begins, the concrete surface has to be pre-wetted with a wetting agent. Areas in the member where corrosion activity may be occurring can be represented using an equipotential contour map.

The potential risks of corrosion based on potential difference readings are shown in Table 1 below:

Table 1. Risk of Corrosion against the potential difference readings

Potential difference levels (mv)	Chance of re-bar being corroded
Less than –500	Visible evidence of corrosion
-350 to -500	95%
-200 to -350	50%
More than -200	5%

Saturday, February 4, 2017

Techshore Inspection Services : SLUMP TEST

Slump test is a quality control test used to ensure the quality of concrete during manufacture. This test is conducted when the concrete is in the plastic state. It is mainly used to determine the consistency or work ability of concrete. The result of slump test is also taken into consideration while accepting a load of concrete. The first step in slump test is to take a test sample from the batch of concrete within 20 minutes of the arrival concrete in site. The sample is taken in one of the following two ways:

Sampling done after 0.2 m³ of the load has been poured (common method).

Sample taken from three places in the load, at equal intervals and equal portions, during the discharge.

Fig.1 Equipment

A Slump cone which is in the form of a frustum with height 30 cm, bottom diameter 20 cm and top diameter 10 cm.

A steel tamping rod with 16 mm diameter and 60cm length which is rounded at one end.

Slump Plate of size 50 cm x 50 cm.

A Ruler or scale

Float and trowel

Procedure

At first the slump cone is placed over the slump plate and both are cleaned and moistened. Then fill one-third of the cone with concrete and this layer is rodded by giving 25 strokes to it. While rodding make sure that the whole area is rodded uniformly. Rodding means pushing the tamping rod in and out of the concrete to compact it. Always rod in a definite pattern by which rodding is done from outside to inside. The slump cone must be held firmly by standing on the foot lugs while adding the concrete during rodding.

Fig. 2

After rodding the first layer, fill the slump cone with a second layer until two-third is full and rod this layer uniformly with 25 strokes just into the top of the first layer. Then fill the slump cone with next layer until it slightly overflows and rod this top layer uniformly with 25 strokes just into the top of the second layer. Then remove the excess concrete from the top using a straight edge so that the cone is exactly filled and the spilled concrete removed from around the bottom of the cone.

Fig. 3

Then lift the slump cone straight up very slowly. The cone is then turned upside down without further disturbing the concrete and the taming rod is placed across the up-turned cone. The distance from the rod is then measured to the top of the slumped concrete. If the top of the slump is irregular, average height is measured. If the slump is too high or too low compared to the specification, another sample must be taken. If this also fails then the remainder of the batch should be rejected.

Result

Slump for the given sample is measured in mm. While carrying out the slump test, following are the shape of the concrete slump that can be observed:

Fig. 4

True Slump: This is the only slump that can be measured in the test. The measurement is taken between the top of the cone and the top of the concrete after the removal of cone as shown in Fig.3.

Zero Slump: This slump is the indication of very low water-cement ratio, which results in dry mixes. Such type of concrete is generally used for road construction.

Collapsed Slump: This slump is an indication that the water-cement ratio is too high. This indicates that the concrete mix is too wet or it is a high workability mix, for which a slump test is not appropriate.

Shear Slump: This slump indicates that the result is incomplete, and concrete to be retested.

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Techshore Inspection Services : Bar bending schedule

Bar bending schedule

Bar bending schedule provides details of reinforcement and bending length as well as the total weight.
Bar bending schedule provides the steel quantity requirement much accurately and thus provides an option to optimize the design.

Standard weight coefficient for steel
Formula : D2 / 162
D : Diameter in mm
For Example:
If D=6mm

Weight for 1m

6X6/162 = 0.22Kg

Diameter	Weight for 1m
6mm	0.22
8mm	0.39
10mm	0.61
12mm	0.88
16mm	1.58
20mm	2.46
22mm	2.98

Development length

Development length is the amount of rebar length that is required to be embedded or to be projected into concrete to create desired bond strength between the two materials and to develop the required stress in steel at that particular section.

Code used for bending and fixing bars in concrete works is IS2502:1963

Techshore Institute Blogs about Best Technical and Management Courses

Sunday, February 5, 2017

Techshore Inspection Services : Half Cell Electrical Potential method

Saturday, February 4, 2017

Techshore Inspection Services : SLUMP TEST

Techshore Inspection Services : Bar bending schedule

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