Concrete Sweating: What is it and How to Prevent it

Concrete is used for all kinds of construction works these days. It is the first choice for building supports and beams because of its excellent load-bearing capacity, tensile strength, and longevity. However, nothing is a hundred percent impervious to weather effects, and neither is concrete.

Concrete Sweating, or more technically known as SSS (Sweating Slab Syndrome) occurs when water droplets are accumulated on the surface of a concrete structure. The phenomena occur generally overnight and may look like the concrete structure is sweating, hence the name. This can be a potentially dangerous issue in some cases.

Reasons for Concrete Sweating

There can be two main reasons why a structure is showing the SSS - dew point and subsurface moisture. Let us discuss them below.

Dew Point

When you take a soda out of the fridge and keep it out for a while, the soda bottle or can starts sweating. The same thing happens with concrete as well, especially overnight. A concrete surface is rather cooler than the air in contact with it. When the concrete is cooler than the dew point (the temperature at which the water vapor in the air starts to condense into liquid), the moisture in air sticks to the concrete. This, in turn, starts forming the water droplets. Which, to uninformed eyes, looks like as if the concrete has started sweating.

Subsurface Moisture

A much rarer phenomenon, this occurs when the concrete itself retains some moisture inside it. When the concrete is formed, much water is used to set the cement. All that water gets absorbed to form the crystals that harden the concrete. But the issue is, this process can go on for a long time. Initially, most types of cement can set in a day, but they harden over a long time - over weeks, even months. While the composite is hardening, the hydrostatic pressure inside them rises a lot. This, in turn, pushes the water out of the concrete slabs or structures. In this case, you can say that the concrete really does sweat, in view that the water comes from inside, not outside.

There can be some secondary reasons for SSS. For example, if the concrete slab is adjacent to wetness (wet soil or water body), the capillaries in a porous concrete can suck up moisture from the wet surface to the dry surface. Also, if the concrete mix has salts in it then due to their hygroscopic nature the salts will attract water into the concrete. The material on top of the concrete can also be the culprit in attracting water, like dust or rubber.

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Published By
Rajib Dey
www.constructioncost.co
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What is Portland Cement and How is it Used in Construction

What is Portland Cement

The Portland Cement is a particular type of cementing material used in building construction. It is essentially an amalgamation of clay and chalk. This blend, when subjected to water, hardens up and when it is hard, mimics the portland stone in color. The Portland Stone is found in quarries in Portland, Dorset in England initially. This type of hydraulic cement was patented in 1824.

The portland cement is exceptional in giving strength to structural properties. Most commonly, it is used in making concrete. However, the portland cement can also be directly used in creating stucco or be used as a mortar. Some non-specialty grout also uses this type of cement as one of the main ingredients.

ASTM 150 defines the Portland Cement as “hydraulic cement (cement that not only hardens by reacting with water but also forms a water-resistant product) produced by pulverizing clinkers which consist essentially of hydraulic calcium silicates, usually containing one or more of the forms of calcium sulfate as an inter ground addition.”

How is Portland Cement Made

Usually created from heating limestone and clay minerals in a kiln, portland cement needs a little bit of gypsum in it to give it the desired setting qualities and to prevent flash setting. The chemical constituents of portland cement and their ratio are as follows:

1. Calcium oxide (61-67%)
2. Silicon dioxide (19-23%)
3. Aluminum oxide (2.5–6%)
4. Ferric oxide (0–6%)
5. Sulfur oxide (1.5–4.5%)
6. Gypsum (2-10%)

The Mix is generally a thin, light powder. Depending upon the ratio of materials in the clinker and the cement, this powder can be gray, white or something in-between. The most common form, called the OPC, displays a soft gray color. After hardening, it resembles the portland stones found in Dorset.

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Published By

Rajib Dey

www.constructioncost.co

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How to analyze rate for brick masonry work

While analyzing the rate of brickwork, it is essential to ascertain the quantities of materials (bricks and mortar) and labors. There are different types of ratios for mortar which range from 1:2, 1:4, 1:6, 1:8 etc. Go through the following like to determine the quantity of mortar.

Estimating the quantity of bricks is necessary for the rate analysis.

Quantity Estimation for Brick Masonry - To analyze the rate of brick masonry, 1m3 of brick masonry is taken:

1. Number of bricks for 1 cubic meter of brick masonry:
For 1m3 of brick masonry, the number of typical size of bricks must be 494.

2. Quantity of mortar for 1m3 of brick masonry:

For 1m3 of brickwork, the quantity of the mortar should be 25 – 30%, i.e. 0.25m3 – 0.3m3 of cement masonry. For this purpose, cement mortar is selected as 0.3m3 .

Labor Estimation for Brick Masonry:
Labors which are essential for brick masonry belong to mason for brick work, labours for transmitting materials (sand, cement, bricks, and water), mixing and transporting mortar.

The quantity of labor is provided as requirement of labour in longer period for 1m3 of brick masonry.

1. Mason: The quantity of mason necessary for 1m3 of brickwork is provided as 0.94 days.
2. Labor: The quantity of labor for different types of works like carriage of materials, blending of mortar, carrying of mortar etc. are amassed. The labour necessary for 1m3 of brick masonry is for 1.57 days.

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Published By
Rajib Dey
www.constructioncost.co
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Some useful tips to work out the cutting Length of Bent Up Bar In Slab

For a site engineer, it is essential to work out the cutting length of bars based on the slab dimensions and provided instructions to the bar benders.

If the construction work is intended for the small area, the reinforcement detailing can be transferred to the bar benders. They will deal with the cutting length. But be careful that it may not be perfect as they do not consider the bends and cranks. They may provide some additional inches to the bars for the bends which are fully imperfect. Therefore, to get rid of this issue, a site engineer should try to compute calculate the cutting length independently.

In this article, detailed explanation is given for working out the length for reinforcement bars of slab.

The calculation is made on the following dimensions :-

Diameter of the bar = 12 mm
Clear Cover = 25 mm
Clear Span (L) = 8000
Slab Thickness = 200 mm
Development Length(Ld) = 40d

Process for computation

Cutting Length = Clear Span of Slab + (2 x Development Length) + (2 x inclined length) – (45° bend x 4) – (90° bend x 2)
Inclined length = D/(sin 45°) – dD/ (tan 45°) = (D/0.7071) – (D/1)= (1D – 0.7071D)/0.7071= 0.42 D
There exist four 45°bends at the inner side (1,2,3 & 4) and two 90° bends ( a,b ).
45 ° = 1d; 90 ° = 2d
Cutting Length = Clear Span of Slab + (2 X Ld) +(2 x 0.42D) – (1d x 4) – (2d x 2) [BBS Shape Codes]

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Published By
Rajib Dey
www.constructioncost.co
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Some useful methods for estimating building works

The following method are useful for working out various building quantities like earth work, foundation concrete, brickwork in plinth and super structure etc.

a) Long wall – short wall method
b) Centre line method.
c) Partly centre line and short wall method.

a) Long wall-short wall method: Under this method, the wall along the length of room is treated as long wall whereas the wall that is situated vertically to long wall is called short wall. To find out the length of long wall or short wall, initially compute the length of centre line for separate walls. Then compute the length of long wall, (out to out) once half breadth at each end is added to its centre line length. Therefore, the length of short wall is calculated into in and is built by subtracting half breadth from its centre line length at each end. The length of long wall normally declines from earth work to brick work in super structure whereas the short wall enlarges. In order to obtain quantities, multiply these lengths with breadth and depth.

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Published By
Rajib Dey
www.constructioncost.co
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Some useful tips to design a T-beam with the use of limit state method

This construction video briefly describes how to create the design of t-beam with the use of limit state method.

A T-beam (or tee beam) is applied in construction extensively. It is a structural element developed with reinforced concrete, wood or metal. It contains a t-shaped cross section that is formed by a stem and a flange of reinforced concrete or rolled metal.

The top of the t-shaped cross section acts as a flange or compression member in withstanding compressive stresses. The web (vertical section) of the beam underneath the compression flange contributes to withstand shear stress and offer better separation for the coupled forces of bending.

A T-beam has the capacity to tolerate large loads by providing resistance in the beam or by internal reinforcements. It operates similar to singly reinforced beam.

In order to enhance the structural strength of a T-beam, just utilize an inverted T-beam together with a floor slab or bridge deck linking the tops of the beams. If it is accomplished perfectly, the slab performs as the compression flange.

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Published By
Rajib Dey
www.constructioncost.co
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How to prevent segregation of concrete

Definition of Segregation of Concrete?
Major components of concrete are cement, sand, aggregate, and water. Normally, a coating is made to concrete aggregate with cement and sand for developing the consistent mass disjointing of these constituent materials of concrete from each other. It is known as segregation of concrete.

Type of segregation:

1. Partition of course aggregate from the paste.
2. Partition of course agreement paste from the concrete.
3. Partition of water from the cement mix which is also known as bleeding.

The segregation contains unwanted properties in the hardened concrete. It also leads to honeycombing in concrete. Segregation also adversely impacts on stability, compressive strength and all other characteristics of concrete. It also results in forming cavities in the concrete surface.

Reasons for segregation:

The segregation mainly occurs due to the variation in specific gravity and size of its constituent material. Specific gravity of water is 1. Specific gravity of cement is approx 3.15 whereas that of aggregate is 2.6 to 2.7. Because of the variation in specific gravity, lighter particle goes upwards and heavier particle goes downwards and consequently the segregation of concrete occurs.

Other reasons range from greater water cement ratio in concrete. Besides, when excessive water is added at the time of mixing concrete, it results in segregation. Improper vibration for a long period on wet mix of concrete also causes heavy material to setting down and lighter cement sand paste to move upwards. Imperfectly proportioned mix and inappropriate placing on site may also lead to segregation.

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Published By
Rajib Dey
www.constructioncost.co
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