Concrete Construction

Q: What are joints in concrete? Why are they necessary?
A: Joints are pre-planned cracks to accommodate the expansion and shrinkage of concrete, caused by changes in moisture and temperature. Although irregular cracks are unsightly and difficult to maintain, they generally do not affect the integrity of the concrete. Cracks in concrete can be controlled and minimised by properly designed joints. There are three types of common joints:

  • Contraction joints
  • Isolation or expansion joints
  • Construction joints


Q:
Why does concrete crack?
A: Concrete, like all other materials, will slightly change in volume when it dries out. In typical concrete, this change amounts to about 500 millionths. Translated into dimensions-this is about 1/16" for 10ft (0.4cm in 3m). The reason that contractors put joints in concrete pavements and floors is to allow the concrete to crack in a neat, straight line at the joint when its volume changes.

 

Q: Can it be too hot or too cold to place new concrete?
A: Temperature extremes make it difficult to properly cure concrete. On hot days, newly placed concrete losses too much water through evaporation. If the temperature drops too close to freezing, hydration slows to nearly a standstill. Under these conditions, concrete ceases to gain strength and other desirable properties. In general, the temperature of new concrete should not be allowed to fall below 50°F (10°C) during the curing period.

 

Q: What precautions must be taken during cold-weather concreting?
A: Cold weather concreting is a common and necessary practice; every cold weather application must be considered carefully to accommodate its unique requirements. The current American Concrete Institute definition of cold-weather concreting, as stated in ACI 306 is, “a period when for more than 3 successive days the average daily air temperature drops below 5°C (40°F) and stays below 10°C (50°F) for more than one-half of any 24 hour period.”
Rule number ONE is that ALL concrete must be protected from freezing until it has reached a minimum strength of 3.5MPa (500psi), which typically happens within the first 24 hours. In addition, whenever air temperature at the time of concrete placement is below 5°C (40°F) and freezing temperatures within the first 24 hours after placement are expected, the following general issues should be considered:
(1) Adjustment of construction schedule regarding loads imposed on the new concrete structure
(2) Placing and curing temperatures to produce quality concrete
The exposure of concrete to cold weather will extend the time required for it to gain strength. In structures that will carry large loads at an early age, concrete must be maintained at a minimum of 10°C (50°F) to accommodate stripping of forms and shoring and to permit loading of the structure. In many cases, achieving the required durability will require a protection period of more than 24 hours. This may not be an issue with residential applications where applied loads are typically small and may be applied in small increments over several days or weeks.
In no case should concrete be allowed to freeze during the first 24 hours after it has been placed. Since cement hydration is an exothermic reaction, the concrete mixture produces some heat on its own. Protecting that heat from escaping the system may be all that is required for good concrete quality, while more severe temperatures may require supplemental heat.

 

Q: What precautions do I have to take during hot-weather concreting?
A: It is true that hot weather conditions above approximately 25°C (77°F) can adversely impact the quality of concrete. The precautions that should be taken to assure a quality end product vary depending on the actual conditions during concrete placement and the specific application for which the concrete will be used. In general, if the temperature at the time of concrete placement will exceed 25°C (77°F), a plan should be developed to negate the effects of high temperatures. The precautions may include some or all of the following:
1) Moisten sub grade, steel reinforcement, and formwork prior to concrete placement
2) Erect temporary windbreaks to limit wind velocities and sunshades to reduce concrete surface temperatures
3) Cool aggregates and mixing water added to the concrete mixture to reduce its initial temperature
4) Use a concrete consistency that allows rapid placement and consolidation
5) Protect the concrete surface during placement with plastic sheeting or evaporation retarders to maintain the initial moisture in the concrete mixture
6) Provide sufficient labour to minimise the time required to place and finish the concrete, as hot weather conditions substantially shorten the time between initial and final set
7) Consider fogging the area above the concrete placement to raise the relative humidity and the satisfy moisture demand of the ambient air.
8) Provide appropriate curing methods as soon as possible after the concrete finishing processes have been completed.
9) In extreme conditions, adjusting the time of concrete placement to take advantage of cooler temperatures, such as early morning or night placement

With proper planning and execution concrete can be successfully placed and finished to produce high quality durable concrete at temperatures of 35°C (95°F) or more.

 

Q: What are the different types of joints and where are they used?
A: Isolation/Expansion Joints: Isolation joints are used to relieve flexural stresses due to the vertical movement of slab-on-grade applications that adjoin fixed foundation elements, such as columns, building and machinery foundations etc. Expansion joints are used primarily to relieve stress due to the confinement of a slab. If a slab is placed adjacent to structures on more than one face, an expansion joint should be placed to relieve stress. For example, if a slab were placed between two buildings, an expansion joint should be placed adjacent to the face of at least one of the buildings. Confinement on three faces would normally be handled by placing expansion joints on all three faces, and confinement on four faces should be isolated on all faces. This allows for thermal expansion and contraction without inducing stress into the system.
Contraction (control) joints: Contraction (control) joints are placed to control random cracking and should be placed at 2 times the slab thickness in feet for a maximum aggregate size of less than ¾”.
For example for a 5” slab with a ¾” coarse aggregate the maximum joint spacing would be 10’. When the maximum coarse aggregate size is greater than ¾” the spacing could be increased to 2 ½” times the thickness. For the prior example this would increase to 13’.
Applications that require thick slabs of 8" or more and good load transfer across joints, due to heavy loading, should be limited to a 15' contraction joint spacing to ensure aggregate interlock.
Construction joints are stopping places in the process of construction. Construction-joint types (a) and (b) are also used as contraction joints.

 


Q: How are exposed-aggregate concrete finishes produced?

A: There are three ways of obtaining exposed-aggregate finishes on concrete slabs: seeding a select aggregate into the concrete surface; the monolithic technique where a select aggregate, usually gap-graded, is mixed throughout the batch of concrete and exposing gap-graded aggregates in a special topping course.
The process for producing monolithic exposed-aggregate finishes is as follows:
1) Place the concrete containing the chosen aggregate in a normal manner in which you fill the forms with the material and rod the surface with a straight edge (typically a straight 2" X 4" board) with a sawing motion, back and forth across the form from side to side. Then close the surface as usual.
2) Spray the surface with retarder. This can be obtained at any contractors supply house. Retarder does typically contain sugars but the formulations that are designed for use with concrete are strongly recommended for a more consistent performance. The retarder will slow the set characteristics of the discrete surface layer allowing the interior to harden while the exterior remains soft.
3) The third phase is the difficult part. When the concrete has become hard enough to carry your weight without displacing the aggregate the surface is washed with a hose and scrub brush to remove the top layer of cement paste. Care must be taken not to displace the aggregate and not to expose the aggregate too deeply (this can cause the aggregate to lose bond and/or be displaced). Caution: A too long delay in this part of the process can create extreme difficulty in the removal of the top paste layer. The retarder slows the set of the top surface but does not stop it completely. After the desired surface has been achieved the slab should be sealed with a clear sealer and curing compound (also found at your local contractor supply house).

 

Q: We are planning to build a concrete countertop. Can you tell me which type of cement to use for this application? 
A: The recommendations to use a Type II cement for countertop construction is usually based on the premise that concrete with a Type II cement will have lower shrinkage potential and, therefore, lower cracking potential. While there may be slight variations in shrinkage potential between cements from different manufacturers and between cement types, it is of little practical value to limit the cement type based on this fact.
As can be seen in the figure below, the shrinkage potential of a plain concrete mixture (no reinforcement) typically ranges between 600 millionths to 790 millionths. The spread of shrinkage data for concrete mixtures is similar across the cement types. For small dimension projects that have little restraint in service like countertops, cement type is probably not of critical importance. In general, if Type II is available locally in bags, use it; if not, a Type I or Type III cement should give close to the same results.

 

To reduce the potential of cracking, it is of greater importance that the concrete be properly cured. Concrete gains strength under adequate moisture, temperature and time conditions. Maintaining the moisture content of the concrete for curing has the added benefit of extending the time at which drying shrinkage takes place. If the concrete is allowed to begin to dry in the first few days after it has been cast, it shrinks, producing tensile stresses that can cause cracking (if the concrete still has low strength). However, if moisture is maintained, shrinkage takes place after the concrete has developed additional strength. This allows it to better resist the tensile stresses that cause cracking. With this in mind, it is important to maintain the moisture in the countertop for a period of 7 days to reduce cracking potential, than it is to be overly particular about cement type.

 

Q: We will be placing a 4-foot thick concrete floor in a cooling tower. Can you tell us how much additional curing time will be required due to the thickness?
A: Recommendations for actual curing time for mass concrete applications vary from 7 to 21 days (or more), depending on the reinforcement used, the cement and pozzolan or slag content in the concrete mixture and the physical dimensions of the structure (dams, locks etc.). The recommendation for heavily reinforced structures is typically 7 days; for concrete mass structures with Portland cement as the sole cementitious material, the curing period is typically 14 days; and for Portland-pozzolan mixtures, 21 days or more, depending upon the design strength development.

 

Common recommendations for mass concrete applications also include limiting the maximum internal temperature to 70°C (160°F) and the thermal gradient from the interior to the exterior of the concrete section to 20°C (36°F) (higher limits are possible with aggregates that have a low coefficient of thermal expansion).

 

Q: I have heard that concrete can be made to a wide variety of strengths. What are typical strength ranges of concrete and where is it appropriate to use them?
A: Concrete can be proportioned to meet a wide variety of strength requirements. It is important to note that there is more than one type of strength property used to design concrete projects. The most commonly used design properties are:
Flexural strength: used for design of pavements (slab-on-grade)
Compressive strength: used for design of foundations, building elements (walls, columns, slabs), bridges (abutments, columns, decks) etc.

Flexural Strength
Flexural strength increases in direct proportion with compressive strength. This property is used specifically for pavement design. The flexural strengths of interest fall in a range of 3.9MPa (570psi) to 5.1MPa (750psi). These flexural strengths correspond approximately to compressive strengths of 28MPa (4000psi) to 48MPa (7000psi). While concrete can attain much higher flexural strengths, it is not required for pavements and use of higher strengths would have an adverse effect on the economics of the project with little benefit in performance.

Compressive Strength
The compressive strength of structural concrete begins at 17MPa (2500psi) and can be produced commercially at 138MPa (20,000psi) or more. Residential and light commercial building projects typically use concrete strengths ranging from 17MPa (2500psi) to 34MPa (5000psi). It is important to bear in mind that the lower strength concrete is only appropriate for mild environmental exposures and interiors protected from the elements. Severe environmental exposures (freezing and thawing cycles and deicer chemical exposure) require a minimum strength of 4000psi to assure durability. Local codes commonly provide guidance for the minimum requirements, but in many cases do not address long-term durability issues.

Heavy commercial and special structures (high rise buildings, long span bridges, slabs exposed to heavy abrasion etc.) typically require concrete strengths of 28MPa (4000psi) or more. The structural loading, durability requirements, special property requirements (low permeability, high abrasion resistance, etc.) or a combination of these factors control the actual required strength. Concrete design professionals should always be consulted for guidance regarding these structures.

 

Q: What is the best way to store bags of cement on site?
A: The primary concern when storing Portland cement is to prevent exposure to moisture. Cement stored in damp air or moisture sets slower and has less strength than cement that is kept dry.
Bagged cement stored on a job site should be stacked closely together on pallets to reduce air circulation and to raise the cement above any moisture sources; it should also be covered with tarpaulins or other waterproof covers for further protection from moisture.
If possible, cement should be stored inside a structure to further reduce the potential of exposure to moisture, but should never be placed directly next to an exterior wall. The relative humidity within the structure should be reduced as much as possible.

Q: What construction practices can affect the air content of concrete?
A: Construction practices have a significant effect on the air content of concrete. These include load volume versus drum capacity, haul time, mixing or agitating speed, total revolutions of the drum before discharge, retempering, pumping and conveyors (any high energy process during placement including large drops), internal vibration, temperature and finishing practices. The materials used in the concrete mixture and the batching process also affects the air content of concrete.

 

Q: What is the effect of high concrete temperature (above 77°F) on transporting, placing and finishing concrete, and on hardened concrete properties?
A: When the temperature of freshly mixed concrete exceeds 77°F (25°C) there may be a number of effects on the fresh and hardened properties of concrete.
One of the effects on fresh concrete properties is accelerated setting, which leads to a shorter time period for transporting the concrete to the job site, and a shorter window for placement, consolidation and finishing of the material.
The higher temperature also leads to a higher water demand to maintain the concrete at the specified slump, which can tempt the contractor to add water to retemper the mixture leading to lower than expected compressive strength of the hardened material. In addition mixtures at high temperatures may require higher dosages of air-entraining admixtures to produce the required air content for durability in severe climates.

The effect on hardened concrete properties are a high early strength but low ultimate strength compared to mixtures placed with a lower temperature. In addition, if no precautions are taken, there is an increased potential for plastic shrinkage cracking during the finishing operations, and increased potential for cracking due to volume changes caused by drying shrinkage and thermal effects.

Q: In the design stage for a new high rise condominium project the architect has suggested the use of aluminium railings for the balconies. I had heard that this material may have some risks for this type of application. Can you provide some guidance on this issue?
A: Aluminium is an amphoteric material, meaning it will chemically react with either an acid or an alkali. While Portland cement hydrates, it releases free calcium hydroxide, which is a very strong alkali. The pH of a fresh concrete mix can be more than 13. Consequently, any material that reacts chemically with an alkali should be carefully considered if it is to be embedded in or placed adjacent to fresh concrete.
Aluminium posts and flashings are commonly used adjacent to or embedded in concrete that will be dry in service. While there is a reaction between the aluminium and the fresh concrete, the reaction slows or stops as the concrete hardens and dries. In addition, it is common for aluminium to be coated with a protective layer to prevent the alkalinity of the concrete from reaching its surface.
When this material is to be used in an exterior application like a balcony, the risks of problems considerably increase for two reasons. First, the concrete is exposed to multiple wet cycles, which transport alkalinity in the concrete to the surface of the aluminium and continually corrodes it. Second, steel reinforced concrete if used as the structural component of the balcony, would produce a galvanic cell (a galvanic cell is created when dissimilar metals, each having a different thermodynamic instability are placed in contact with each other) leading to an accelerated rate of corrosion.
In general, placing aluminium in or next to concrete for exterior applications should be avoided due to the increased risk of corrosion and accompanying concrete cracking issues caused by the increased volume of the metal as it corrodes.
Note that chloride based accelerators should never be used in concrete that contains any steel reinforcement or metal embeds.

Q: What is the method and means for concreting in water logged area?
A: Trimmi Pipe Method.

 

Q: What method needs to be adopted for designing Water Retaining structures?
A: The reinforced water retaining structures should be based on working stress method of design. Secondly the crack thickness and the width of the crack is also to be watched for.
Use of water bars during construction of the same also makes the water retaining structures economical.
If the water retaining structures are to be made of steel then the material of the structure is the thing to watched for as the same should be corrosion resistant.

 

Q: What can be the slump value in the case of following members as per IS recommendations
1. Slab
2. Beam
3. Column
4. Dam 
A: Slab: 90mm
Beam: 80mm
Column: 95 mm
Dam: 110 – 130mm
Today zero slump concrete (slumpless concrete) is being used in dams

 

Q: Why we need to do PCC?
A: PCC is done so as to provide a base for the foundation so as to avoid direct contact with soil.
As the grade of concrete of PCC and the foundation are different, it provides material difference for ground bacterial effect on the foundation.

 

Q: Why is the grade of PPC not mentioned or defined as per Indian Standards IS 1489–1991?
A: PPC is manufactured by blending a mixture of ordinary Portland cement and pozzolan material such as fly ash, not less than 15 by weight of cement. The fineness of PPC will be greater than OPC.

 

Q: What causes cracks in a building? What is the lasting solution to stop the cracks?
A: In walls
1. Shear cracks
2. Settlement cracks
These created cracks are overcome by:
1. Filling adhesive materials.
2. Using chicken net or polymer strips if the crack exceeds 2mm
In beams
1. Design calculation failure
2. Sagging failure
To overcome this cracks, consider the cover space (at bottom level) not exceeding 25mm.

 

Q: When we go for inverted T- beam for slab?
A: We provide inverted T-beam in places like portico. the condition is when there is compression in upper zone & tension in bottom zone then we provide inverted T-beam.

Q: What is Early-Age Cracking and how can it be checked?

A:
Early-age cracking can be a significant problem with concrete. Early age, for concrete, is the first seven days starting with final set, which is when the concrete has obtained a benchmark level of stiffness. During this time, concrete undergoes a significant amount of volume change caused by many variables, such as the hydration reaction (chemical shrinkage), water content (drying shrinkage and swelling) and temperature changes (thermal dilation).

Volume changes in concrete will drive tensile stress development when they are restrained, which is the case with most concrete. Tensile stresses are forces trying to pull the concrete apart and are opposite from compressive stresses. Cracks develop when the tensile stress exceeds the tensile strength. While concrete is strong in compression, the tensile strength is generally only 10% of the compressive strength. At early ages, this strength is still developing while stresses are generated by volume changes. Controlling the variables that affect volume change can minimise cracking and create a higher quality concrete placement.

Q: Are Concrete Floors Loud and Hard on the Feet?
A: Yes, concrete is a hard material and won’t cushion or ‘give’ under bare feet. Concrete floors can be loud and produce an echo effect, but not more than ceramic tile, natural stone flooring and some hardwood and bamboo floors.

 

Q: Are Concrete Floors Cold and Damp?
A: Yes, concrete can be cold, but no more so than ceramic tile or natural stone flooring. And yes, concrete floors can transmit moisture vapour if they aren’t insulated properly or if the slab is built on a poorly drained sub-base.
Concrete doesn’t have to be cold. Its thermal properties give it the ability to store and radiate heat. By embedding radiant heating cables in concrete floors, for example, you can keep floors toasty warm in the winter and control the temperature level. If the home is built to take advantage of solar radiation entering through windows, concrete floors will absorb the heat from the sun to keep rooms warmer in the winter. In summer and in hot climates, a cooler floor can be an advantage and can actually help lower air-conditioning costs.

 

Q: Can the wire mesh be replaced with fibres?
A: The answer depends on the intended purpose of installing wire mesh. If control joint spacing exceeds 30 times the concrete thickness, then wire mesh should be used to hold random cracks together. If not, wire mesh can be omitted and fibre can be used to reduce surface cracking resulting from rapid evaporation (plastic shrinkage cracking) and improve fatigue strength.  

 

Q: When does shrinkage cracking happen in concrete?
A: Shrinkage cracks occur when concrete members undergo restrained volumetric changes (shrinkage) as a result of either drying, autogenous shrinkage or thermal effects. Restraint is provided either externally (i.e. supports, walls and other boundary conditions) or internally (differential drying shrinkage and reinforcement). Once the tensile strength of the concrete is exceeded, a crack will develop. The number and width of shrinkage cracks are influenced by the amount of shrinkage that occurs, the amount of restraint present and the amount and spacing of reinforcement provided.
Plastic-shrinkage cracks are immediately apparent, visible within 0 to 2 days of placement, while drying-shrinkage cracks develop over time. Autogenous shrinkage also occurs when the concrete is quite young and results from the volume reduction resulting from the chemical reactions.

 

Q: When does tension cracking happen in concrete?
A: Concrete members may be put into tension by applied loads. This is most common in concrete beams where a transversely applied load will put one surface into compression and the opposite surface into tension due to induced bending. The portion of the beam that is in tension may crack. The size and length of cracks depends on the magnitude of the bending moment and the design of the reinforcing in the beam at the point under consideration. Reinforced concrete beams are designed to crack in tension rather than in compression. This is achieved by providing reinforcing steel which yields before failure of the concrete when compression occurs, allowing remediation, repair or if necessary, evacuation.

 

Q: What is Creep? How does it affect the structure?
A: Creep is the term used to describe the permanent movement or deformation of a material in order to relieve stresses within the material. Concrete, which is subjected to long-duration forces, is prone to creep. Short-duration forces (such as wind or earthquakes) do not cause creep. Creep is capable of reducing the amount of cracking in a concrete structure or element, but it also must be controlled. The amount of primary and secondary reinforcing in concrete structures contribute to a reduction in the amount of shrinkage, creep and cracking.

 

Q: How does temperature affect concrete?
A: Three factors control the set time, strength gain and overall durability of concrete: time, temperature and moisture. These factors are interdependent on each other. The ideal temperature for concrete is 50oF/10oC. If the temperature is too high, the concrete can dry out prematurely and not reach its desired strength. It can also crack from rapid shrinkage.
If the temperature is to low, the concrete set time and strength gain can be reduced and the concrete can be subjected to freezing while water is present and have severe freeze/thaw damage.

 

Q: Why is curing necessary for concrete?
A: Concrete develops strength ONLY in the presence of moisture, therefore it is absolutely important that it is kept moist after casting. If concrete is allowed to dry out, it will not develop to the strength it was designed for.

 

Q: What are the causes of Honeycombing?
A: Honeycombing is caused by a number of different reasons:
Insufficient compacting or vibrating
Segregation of the concrete when it is allowed to free-fall from heights over three feet
When the steel reinforcement is too close to the formwork (boxing)
When the formwork is not nailed tight and the grout from the concrete leaks out
When too stiff a concrete is used without the proper vibrating equipment

 

Q: What will happen if shape of aggregates used in concrete is not rounded/cubical?
A: If the shape of aggregates used in concrete is not rounded or cubical, proper interlocking of the aggregate will not occur and the strength of concrete will decrease. If the aggregates are elongated or flaky the strength of concrete will seriously be affected, resulting in porous concrete.

 

Q: What is the importance of water cement ratio
A: W/c ratio is very important in making concrete to satisfy the strength and durability criteria. If w/c ratio is more, concrete strength will be reduced due to void formations. The effect of w/c ratio on compressive strength of concrete is as shown in the table below:


Water Cement (w/c)

0.40

0.50

0.60

0.70

0.80

Probable Compressive Strength (%)

100

87

70

55

44

 

 

 

Importance of Water-Cement Ratio:

Factors

Low w/c ratio

High w/c ratio

Compressive strength

High

Low

Water permeability

Low

High

Shrinkage

Low

High

Water Bleeding

Low

High

 

Q: What is segregation?

A: Segregation is the separation of coarse aggregate from cement paste. It mainly occurs due to excess water, resulting in the non-cohesiveness of concrete.