Kuldeep Singh - Best Answers

Whenever testing for cube strength, it is always advised to make and test three specimens from the same mix. This is done to reduce or average out the error due to manually introduced differences in the three cubes.

Now, concrete and cement are always tested at both 7 days and 28 days for compressive strength. The compressive strength at seven days is known to be 65% of the final (28 days) strength. This double testing is done so that if the seven days strength is found inappropriate, the cubes may be straight away discarded to save 21 days’ time because the 28-day strength will also probably be below par. Now, a time even earlier than seven days is not chosen because, at that age, strength gain is very steep, and there’s a higher margin of error. Also, seven days is a round figure.

For both the tests, 3 cubes each are employed. Hence, a total of 6 cubes go through the compression test.

Bar bending schedule (BBS) is a comprehensive, detailed list of reinforcement bars to be used in an RCC construction along with their number, location, mark, type, size, lengths, details in correspondence with the drawings. It also includes the timing for the bending of each bar, and that is why it’s called a bending schedule.

Bars are needed to be bent in order to give them the desired reinforcement shape. Stirrups and anchorages are two places where bar bending is compulsory. In addition, when we need to join bars, we might want to bend them on the ends before welding or fastening.

BBS also gives a clear and logical understanding as to how much bars of what sizes will be needed to be procured and bent by what time so that no delay, all procurement is done with minimal transport expenses, and all bending is done with minimal machine and labor costs.

The electrical curing of concrete is done by employing the heat generated from an alternating current to raise the temperature of maturing concrete.

The concrete is heated up using an AC till up to about 90ºC because ten degrees more will cause the water to turn into steam, which can cause damage to concrete. With this heat, the hydration reactions of maturing concrete are thermally accelerated.

Now, the curing can be done in three ways. The first is the most popular one :

1. Electrode plates or a network of electrode plates are joined to the faces of the concrete member, and an AC of generally 30V or 60V is passed for a period of at most three days. Larger curing times have been seen to be uneconomical even in the most favorable conditions.
2. A larger magnitude and lower voltage AC as compared to method one is passed only through the steel reinforcement in the concrete. This provides for a slow but steady and uniform distribution of heat.
3. Large electric pads surround the concrete much like shuttering. Current is passed only through the pads, and they are heated up. The heat, in turn, heats up the concrete very uniformly.

Although it’s quite an impressive technique of concrete curing, it is rarely used because it’s generally uneconomical. It is only used in cold climates where concrete is very susceptible to freezing and thawing within the first week. Also, natural curing is not very easy in cold weather. The technique was even born in Sweden.

Another use is where cost is not, but time is the most important consideration. Maybe in large projects and in real estate.