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Acceptable Rebar Rust | Rust on Reinforcing Steel

Q.: If reinforcing bars have been stored outside and have a coating of rust, how much rust is acceptable?A.: Section 12 of ASTM A 615-96a, "Standard Specification...

How to check suitability of water for concrete mixing ?

Suitability of water for concrete mixing objective.To assess suitability of water for use in concrete mix.Eqipment- Measuring cylinder- Cube moulds-...

Joints in concrete construction are construction, expansion, contraction and isolation joints. These joints are placed in concrete slabs and pavements at regular intervals to prevent development of cracks in concrete.

Types of joints in concrete constructions are:

1.Construction Joints

Construction joints are placed in a concrete slab to define the extent of the individual placements, generally in conformity with a predetermined joint layout.

Construction joints must be designed in order to allow displacements between both sides of the slab but, at the same time, they have to transfer flexural stresses produced in the slab by external loads.

Construction joints must allow horizontal displacement right-angled to the joint surface that is normally caused by thermal and shrinkage movement. At the same time they must not allow vertical or rotational displacements

2.Expansion Joints

The concrete is subjected to volume change due to many reasons. So we have to cater for this by way of joint to relieve the stress. Expansion is a function of length. The building longer than 45m are generally provided with one or more expansion joint. In india recommended c/c spacing is 30m. The joints are formed by providing a gap between the building parts.

3.Contraction Joints

A contraction joint is a sawed, formed, or tooled groove in a concrete slab that creates a weakened vertical plane. It regulates the location of the cracking caused by dimensional changes in the slab.

Unregulated cracks can grow and result in an unacceptably rough surface as well as water infiltration into the base, subbase and subgrade, which can enable other types of pavement distress.

Contraction joints are the most common type of joint in concrete pavements, thus the generic term “joint” generally refers to a contraction joint. Contraction joints are chiefly defined by their spacing and their method of load transfer. They are generally between 1/4 – 1/3 the depth of the slab and typically spaced every 3.1 – 15 m

4.Isolation Joints

Isolation joints have one very simple purpose—they completely isolate the slab from something else. That something else can be a wall or a column or a drain pipe.Walls and columns, which are on their own footings that are deeper than the slab subgrade, are not going to move the same way a slab does as it shrinks or expands from drying or temperature changes or as the subgrade compresses a little.

SNRN 7/15/2020 Admin Bandung Indonesia

Types of Joints in Concrete Constructions

Joints in concrete construction are construction, expansion, contraction and isolation joints. These joints are placed in concrete slabs and pavements...

Definition of cover block use in construction

Concrete cover, in reinforced concrete, is the least distance between the surface of embedded reinforcement and the outer surface of the concrete.Concrete...

When you see a crack in your concrete slab or wall, your first assumption is typically that something has been done wrong–but that’s not always the case. Actually, concrete cracks are very common, some are even inevitable.

American Concrete Institute touches on the issue of cracking concrete in their American Concrete Institute manual, ACI 302. 1-40:

“Even with the best floor designs and proper construction, it is unrealistic to expect crack-free and curl-free floors. Consequently, every owner should be advised by both the designer and contractor that it is normal to expect some amount of cracking and curling on every project, and that such occurrences do not necessarily reflect adversely on either the adequacy of the floor’s design or the quality of its construction”

We explain 6 of the most common types of concrete cracks below.

1. Plastic shrinkage concrete cracks

When concrete is still in its plastic state (before hardening), it is full of water. When that water eventually leaves the slab, it leaves behind large voids between the solid particles. These empty spaces make the concrete weaker and more prone to cracking. This type of cracking happens frequently and is referred to as “plastic shrinkage cracking”.

While plastic shrinkage cracks can happen anywhere in a slab or wall, they almost always happen at re entrant corners (corners that point into the slab) or with circular objects in the middle of a slab (pipes, plumbing fixtures, drains, and manholes). Since concrete cannot shrink around a corner, stress will cause the concrete to crack from the point of that corner.

Plastic shrinkage cracks are typically very narrow in width and barely visible. While nearly invisible, it is important to remember that plastic shrinkage cracks don’t just exist on the surface, they extend throughout the entire thickness of the slab.

An excessively wet mix is a contributing factor to shrinkage in concrete. While water is an essential ingredient in every concrete mix, there is such a thing as too much water. When the mix contains too much water, the slab will shrink more than if the correct amount of water was used. Hot weather is another big reason for plastic shrinkage cracks.

Control joints can be incorporated into the slab to prevent shrinkage cracking. The joints will open up as the concrete slab gets smaller.

2. Expansion concrete cracks

Just like a balloon, heat causes concrete to expand. When concrete expands, it pushes against anything in its way (a brick wall or adjacent slab for example). When neither has the ability to flex, the expanding force can be enough to cause concrete to crack.

Expansion joints are used as a point of separation (or isolation), between other static surfaces. Typically made of a compressible material like asphalt, rubber, or lumber, expansion joints must act as shock absorbers to relieve the stress that expansion puts on concrete and prevent cracking.

3. Heaving concrete cracks

When the ground freezes, it can sometimes lift many inches before thawing and settling back down. This ground movement brought on by the freezing and thawing cycle is a huge factor contributing to concrete cracking. If the slab is not free to move with the ground, the slab will crack.

Large tree roots can have the same effect on a slab. If a tree is located too close to a slab, the growing roots can lift and crack the concrete surface. Always consider this when laying a slab.

4. Settling concrete cracks

On the other hand, ground settling below a concrete slab can also cause cracking.

Settling cracks typically occur in situations where a void is created in the ground below the concrete surface. Think about when a large tree is removed from nearby and the roots begin to decompose or when a utility company digs a trench for their lines, pipes, etc. and don’t compact the soil when they refill it–these are examples of instances where settling cracks are likely to happen.

5. Concrete cracks caused by overloading the slab

Although concrete is a very strong building material, it does have its limits. Placing excessive amounts of weight on top of a concrete slab can cause cracking. When you hear a concrete mix has a strength of 2000, 3000, 4000, or 5000+ PSI, it is referring to the pounds per square inch it would take to crush that concrete slab.

When it comes to residential concrete slabs, overload of the actual slab isn’t all that common. Instead, what is more likely to occur is excess overload on the ground below the slab.

After a heavy rain or snowmelt when the ground below is soft and wet, excessive weight on the slab can press the concrete down and result in cracks. Residential homeowners who place large recreational vehicles or dumpsters on their driveways are more likely to see this type of cracking.

6. Concrete cracks caused by premature drying

There are two common types of cracks brought on by premature drying.

Crazing cracks are very fine, surface cracks that resemble spider webs or shattered glass. When the top of a concrete slab loses moisture too quickly, crazing cracks will likely appear. While unsightly, crazing cracks are not a structural concern.

Crusting cracks typically happen during the concrete stamping process, which is a way of adding texture or pattern to concrete surfaces. On sunny or windy days where the top of the slab dries out quicker than the bottom, the top of the concrete surface can become crusty. When the stamp is embedded, it pulls the surface apart near the stamped joints and causes small cracks around the outside edges of the “stones”. Again, while they don’t look great, crusting cracks are not a structural issue to be considered about.

It’s often difficult to determine exactly what caused a particular crack. Proper site preparation, a quality mix, and good concrete finishing practices can go a long way towards minimizing the appearance of cracks and producing a more aesthetically pleasing concrete project.

We can’t stress the importance of a quality mix design in concrete crack controlling. Read our Concrete Checklist: Get The Best Mix For Your Project, which will guide you and your concrete supplier towards creating the best mix for your concreting project.

SNRN 7/11/2020 Admin Bandung Indonesia

Types of Cracks in Concrete

When you see a crack in your concrete slab or wall, your first assumption is typically that something has been done wrong–but that’s not always the case....

Which is the Better Building Material? Concrete or Steel

Concrete and steel are among the most common construction materials. Each material has advantages and limitations; this article provides a general comparison between steel and concrete. No material can be considered better than the other for all cases, and project conditions determine the best option. Both provide numerous benefits. As for whether or not one is better, Buildings lets you specify which side you’re on.

Steel Structure:

Steel is an alloy of iron and carbon and contains a small amount of silicon and phosphorous. Generally, for the steel structures, mild steel, medium carbon steel, and low alloy steel are used.

Advantages of steel structure:

1- Strength: Steel structures are stiffer, stronger, more durable, and more stable than concrete and timber structures. The strength to weight ratio of steel is also high. So no matter how large the overall structure is, the steel sections will be small and lightweight, unlike other building materials.

2- Durable: Steel is very durable. Structural steel structures can withstand external pressures such as earthquakes, thunderstorms, and cyclones. A well-built steel structure can last up to 30 years if maintained well.

3- Lightness: The steel structures are very light in comparison to other structures, such as concrete structures.

4- Flexible: Massive production and fabrication of steel are smooth. Steel sections can be manufactured off-site at shop floors and then assembled onsite which, saves time and increases the efficiency of the overall construction process.

5- Cheaper: Structural steel is relatively inexpensive compared to other building materials like timber.

6- No sudden failure: Steel being a plastic material, does not have an unexpected failure. Instead, it gives a clear indication by deflection before the crash.

7- Recycle value: Recycle and re-use are more flexible for the components of steel structure than the concrete and timber structures.

8 Fire Proof: Fire hazards can be prevented through the use of steel structures instead of concrete and timber structures.

Disadvantages of steel structure:

1- Susceptible to corrosion: steel is vulnerable to corrosion. This problem can be solved using anti-corrosion applications, which again increases the cost.

2- High maintenance cost: Steel has high maintenance costs as it has to be painted to make it corrosion-resistant.

3- Skilled labour: The steel structure requires specialized work for their construction.

4- Buckling: As the length of the steel column increases, the chances of buckling also increases too.

5- Expansion: Steel has a high expansion rate with changing temperatures, which can be detrimental to the overall structure.

Concrete Structure:

Concrete is a composite material composed of aggregate bonded together with a fluid cement, which hardens over time. It is obtained by mixing cementing material, water, and aggregates and sometimes admixtures in required proportions.

Advantages of concrete structure:

1- Availability: Ingredients used in concrete, such as cement, aggregates, and water, are locally available and are cheap.

2- Easily moulded: Concrete can be mould, and it can be poured and cast into any shape.

3- Flexibility: Concrete, when used along with reinforcement, is capable of taking bending and tension forces.

4- High Compressive Strength: The compressive strength of concrete is very high, making it reliable for structures and components under compressive loads.

5- Rust, rot, or burn: Unlike steel, concrete is free from rusting.

Besides above, concrete is wind and water-resistant, non-combustible, requires low maintenance, and absorbs and retains heat.

Disadvantages of concrete structure:

1- Low tensile strength: Concrete is very weak in tension.

2- Creep: Continuous sustained loads develop creep in concrete structures.

3- Efflorescence: If salts are present in the concrete, then it will result in the efflorescence in a concrete structure.

4- Shrinkage: Shrinkage is contracting of a hardened concrete mixture due to the loss of capillary water. This shrinkage causes an increase in tensile stress, which may lead to cracking, internal wrapping, and external deflection before any loading ion the concrete.

5- High Self-weight: High Self weight of concrete is not always favorable for seismic prone structures.

SNRN 6/24/2020 Admin Bandung Indonesia

Which is the Better Building Material Concrete or Steel?

Which is the Better Building Material? Concrete or SteelConcrete and steel are among the most common construction materials. Each material has advantages...

There are three main types of cracks in concrete. Each has its own cause and strategies to prevent or minimize.

Plastic shrinkage cracks. These occur during the first few hours when the concrete is still in a “plastic” state. They are caused when the surface moisture evaporates too quickly, usually during hot or windy weather. Synthetic fiber additives can help reduce this type of cracking, but do little once the concrete has cured.

Drying shrinkage cracks. These occur as moisture leaves the concrete after the slab has hardened. The main cause is concrete that is too wet, referred to as a “high-slump” mix. The best solution is to use less water in the concrete mix. Concrete suppliers sometimes add water to make the concrete easier to work with, but this weakens the concrete.

Welded wire mesh can also help reduce shrinkage cracking, but only if it is placed in the middle or upper half of the slab, but at least 2 inches below the surface. Wire mesh also helps keep small cracks from growing. In too many cases, however, the wire mesh ends up on the bottom of the slab where it does nothing.

Shrinkage cracking can be managed by the use of control joints placed in the slab. Some contractors cut or form a grid of small grooves in the slab to keep the shrinkage cracks in an orderly grid, which looks better than random cracks, but functions the same way. If you are placing tile on the slab, it’s important the control joint line up with a control joint in the tile easier said than done. So random cracking might be a better approach for tile.

Structural cracks. Concrete can support a lot of weight in compression, but is weak in tension. For example, a concrete wall can support tons of weight from above, but will crack easily if pushed sideways forcing it to bend. Similarly, a slab will crack if too much weight is placed in one spot, or if the soil settles unevenly, bending the slab.

The best protection against structural cracking in residential structures is good compaction of the soil and gravel underneath the slab. In addition, rebar should be placed in the footings around the perimeter of the slab and at post bases within the slab.

SNRN 4/30/2020 Admin Bandung Indonesia

Type of Cracks in Concrete Slab

There are three main types of cracks in concrete. Each has its own cause and strategies to prevent or minimize. Plastic shrinkage cracks. These occur...

Advantages of Post Tension Slab

What are the advantages of post-tensioned slabs? 1. Architectural Benefits Post-Tensioned Slab has an advantage over others as it makes a very efficient...

Curing plays an important role on strength development and durability of concrete. Curing takes place immediately after concrete placing and finishing, and involves maintenance of desired moisture and temperature conditions, both at depth and near the surface, for extended periods of time. Properly cured concrete has an adequate amount of moisture for continued hydration and development of strength, volume stability, resistance to freezing and thawing, and abrasion and scaling resistance.

The length of adequate curing time is dependent on the following factors:

- Mixture proportions

- Specified strength

- Size and shape of concrete member

- Ambient weather conditions

- Future exposure conditions

Slabs on ground (e.g. pavements, sidewalks, parking lots, driveways, floors, canal linings) and structural concrete (e.g. bridge decks, piers, columns, beams, slabs, small footings, cast-in-place walls, retaining walls) require a minimum curing period of seven days for ambient temperatures above 40 degrees Fahrenheit.

American Concrete Institute (ACI) Committee 301 recommends a minimum curing period corresponding to concrete attaining 70 percent of the specified compressive strength. The often specified seven-day curing commonly corresponds to approximately 70 percent of the specified compressive strengths. The 70 percent strength level can be reached sooner when concrete cures at higher temperatures or when certain cement/admixture combinations are used. Similarly, longer time may be needed for different material combinations and/or lower curing temperatures. For this reason, ACI Committee 308 recommends the following minimum curing periods:

ASTM C 150 Type I cement seven days

ASTM C 150 Type II cement ten days

ASTM C 150 Type III cement three days

ASTM C 150 Type IV or V cement 14 days

ASTM C 595, C 845, C 1157 cements variable

There are three main functions of curing:

1) Maintaining mixing water in concrete during the early hardening process

Ponding and immersion

Ponding is typically used to cure flat surfaces on smaller jobs. Care should be taken to maintain curing water temperature at not more than 20 degrees Fahrenheit cooler than the concrete to prevent cracking due to thermal stresses. Immersion is mainly used in the laboratory for curing concrete test specimens.

Spraying and fogging

Spraying and fogging are used when the ambient temperatures are well above freezing and the humidity is low. Fogging can minimize plastic shrinkage cracking until the concrete attains final set.

Saturated wet coverings

Wet coverings saturated with water should be used after concrete has hardened enough to prevent surface damage. They should be kept constantly wet.

Left in Place Forms

Left in place forms usually provide satisfactory protection against moisture loss for formed concrete surfaces. The forms are usually left in place as long as the construction schedule allows. If the forms are made of wood, they should be kept moist, especially during hot, dry weather.

2) Reducing the loss of mixing water from the surface of the concrete

Covering concrete with impervious paper or plastic sheets

Impervious paper and plastic sheets can be applied on thoroughly wetted concrete. The concrete surface should be hard enough to prevent surface damage from placement activities.

Applying membrane-forming curing compounds

Membrane-forming curing compounds are used to retard or reduce evaporation of moisture from concrete. They can be clear or translucent and white pigmented. White-pigmented compounds are recommended for hot and sunny weather conditions to reflect solar radiation. Curing compounds should be applied immediately after final finishing. Curing compound shall comply with ASTM C3094 or ASTM C13155.

3) Accelerating strength gain using heat and additional moisture

Live steam

Live steam at atmospheric pressure and high-pressure steam in autoclaves are the two methods of steam curing. Steam temperature for live steam at atmospheric pressure should be kept at about 140 degrees Fahrenheit or less until the desired concrete strength is achieved.

Heating coils

Heating coils are usually used as embedded elements near the surface of concrete elements. Their purpose is to protect concrete from freezing during cold weather concreting.

Electrical heated forms or pads

Electrical heated forms or pads are primarily used by precast concrete producers.

Concrete blankets

Concrete insulation blankets are used to cover and insulate concrete surfaces subjected to freezing temperatures during the curing period. The concrete should be hard enough to prevent surface damage when covering with concrete blankets.

SNRN 4/12/2020 Admin Bandung Indonesia

Concrete Curing Role

Curing plays an important role on strength development and durability of concrete. Curing takes place immediately after concrete placing and finishing,...