Basic Guides for Hot Weather Concreting

 Dear Readers,

This article is in continuation with my previous posts of Hot Weather Concreting and Concrete Precautions. Following points are the simple basic guides for Hot Weather Concreting.

While hot weather conditions are commonly encountered in summer, combinations of high temperatures, winds and low humidity could result in conditions leading to problems with concrete placement and finishing at any time. Hot weather is any combination of:

High ambient temperature;

Low relative humidity;

High wind

AS 13791 places a 35°C limit on the maximum concrete temperature at the time of delivery. However, when the air temperature rises above 30°C, it is usually recommended that precautions be taken, particularly if there is also hot dry wind.

1.     Plan in advance. Have equipment and materials ready before the hot weather arrives during the pouring day.

2.       Keep the subgrade and forms moist so they do not absorb water from the mix.

3.       Keep sunshades and windbreaks available and use them whenever possible.

4.       Have everything prepared before the ready-mix truck arrives. Don’t make the truck wait for you.

5.     Keep in constant communication with the ready-mix concrete provider. Coordination between the contractor and the producer is key.

6.       Concrete should be placed, struck off and Darby immediately.

7.     Use evaporation retardants, fogging or misting with water, or cover with a vapor-proof sheet after screeding (Finishing works). This will help prevent rapid drying, crusting, plastic shrinkage and rubber sets.

8.     Temporary covers, like continuously moistened burlap, may be placed over the fresh concrete and removed in small sections immediately ahead of the finishers.

9.      Substituting a burlap drag or broom finish will eliminate other high-risk finishing practices, such as smooth trowel finish.

10.   Curing should take place when the surfaces are hard enough to resist marring.

11.   Seal with a good, quality sealer for a minimum of 30 days after placing and curing.

12.   Remember to protect the crew in high temperatures. Drink plenty of fluids and be careful with long exposure to the sun.

13.   Use PPE’s for workmen and rest shed (minimum 15% additional workforce)

14.   Ensure the machinery technicians at work location to attend minor repairs immediately.

15.       Have standby equipment and manpower for all stages

16.       Programme concreting for the cooler parts of the day, or even schedule night-time placement if possible.

17.       Plan the locations of construction joints ahead of time with hot weather contingencies in mind.

18.       Keep all equipment that touches the concrete cool (chutes, conveyors, pump lines, tremies, reinforcement and buggies). Protecting equipment from the direct sun will If it can’t be kept continuously cool, spray-cool it as necessary with water.

19.       Use a thermometer to monitor the temperature at which concrete is being delivered, and call for adjustments at the plant if necessary.

20.       Avoid delays at all stages. 

Wish You all the Best for Excellent Concreting during Hot Weather. Have a Nice Day!!! 

Purpose of Admixtures

Dear Readers,

In continuation with Admixtures article, let us understand the functions

Functions of Admixtures

Concrete admixtures have various functions and they are as follows:

(a) To  increase  the rate  of strength development at early ages – calcium chloride is the most widely  used accelerator,                              

(b) To retard the initial setting time while pumping concrete over a long distance,

(c) To  increase the workability without changing the water content – pozzolana such as fly ash is used,

(d) To increase the strength,

(e) To  increase  the  resistance  to  freezing  and  thawing  – vinyl  resin  is  an  air entrainment admixture which is used for this purpose,

(f) To decrease heat evolution,

(g) To increase water tightness,

(h) To decrease capillary flow of water, and

(i) To decrease rate of bleeding and segregation. 

Chemical admixtures are various types and they are:

(a) Accelerating admixture – A substance that increases the rate of hydration of hydraulic cement, reduces the setting time, or in other words, increases the rate of strength development.

(b) Retarding admixture – A substance which delays the setting time of cement paste.

(c)   Water – reducing admixture – A substance which either increases the workability of freshly mixed mortar or concrete without increasing water-cement ratio or maintains workability with reduced water-cement ratio.

(d) Air – entraining admixture – A substance that causes air to entrap/Cd in the form of tiny bubbles in mortar or concrete during mixing to increase its workability and resistance to freezing and thawing.

(e)  Super plasticizing admixture   – A   substance that has very high workability with a large decrease in water content (at least) for a given workability. High range water reducing admixture (HRWRA) is also referred to as a superplasticizer.

To Know more about concrete admixtures click Here

Types of Cement

Dear Readers,

As you are well aware that without Cement modern infrastructure is not possible or even unimaginable now. Cement is a critical component in the construction of buildings, roads and other brick-and-motor creations and it plays vital role in construction Industry. Cement is a grey colored powder that resembles ash in texture. It is a binding substance that is used to harden and set two materials, typical bricks or rocks together. Before using it, cement is mixed with water and other materials in varying ratios and placed between two substances as “glue”. 

It’s time to learn about the many types of cement available in the market to cater to different construction purposes. Cements are broadly classified into two categories – Hydraulic and Non-Hydraulic, depending on how they react to the presence of water.

Hydraulic Cement: This cement gets hardened in a very short span and are widely used in construction industry for saving time and effort.

Non-Hydraulic Cement: They have a very long setting period and can only work in dry conditions. Non-hydraulic is not practical for commercial constructions, as they cannot be used in sites that are open to weather challenges.

Selection of cement based on suitability for different concrete construction is important for durability considerations of structures. Thus, making wise choice of cement type for specific construction site that prevents structure from deteriorating and saves much repair and rehabilitation cost later. 

A wide variety of cement types are available in market, which are suitable for use under certain conditions due to its unique properties. They are as follows.

    1. Ordinary Portland cement (OPC)

    2. Portland pozzolana cement (PPC)

    3. Rapid hardening Portland cement (RHPC)

    4. Low Heat Portland Cement (LHPC)

    5. Portland Slag Cement (PSC)

    6. Sulphate-resisting Portland Cement (SRPC)

    7. Hydrophobic Portland Cement (HPC)

Other Types of cements are based on its either unique property or its application.

    1. High Alumina Cement

    2. Quick-setting Cement

    3. White Cement

    4. Coloured Cement

    5. Air Entraining Cement

    6. Expansive Cement

    7. Extra rapid hardening cement

    8. Super sulphated cement

    9. Oil-well Cement

    10. Ultra-high strength concrete (UHSC)

Ultra-high strength concrete (UHSC) is the latest spectacular development in the domain of Portland cement-based materials.

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Concrete Admixtures

 Dear Readers,

Concrete is widely used in civil engineering and construction. In the concrete mixing process or before the concrete mixing can make the concrete hardening, mixing better materials, called concrete admixtures.

Various types of admixtures are used in concrete to enhance the performance of concrete. Concrete admixture is defined as the material other than the aggregate, water and cement added to the concrete. Admixtures are artificial or natural, and it may be added to the concrete mix, just before or during the mixing, to change one or more properties of the concrete in the plastic or hardened state as required in our structure. 

The successful use of admixtures depends on the proper combination of ingredients and concrete. Most admixtures are supplied in ready-to-use liquid form and are added to the concrete at the factory or construction site. Some additives such as pigments, extenders, and pumping agent, etc., used only in extremely small amounts, generally from the vessel manually pre-measured ingredients.

Types and Functions of admixtures are as Follows.

There are two types of admixtures:

Mineral admixtures 

Chemical admixtures

(1)    Mineral admixtures 

Followings are the types of Mineral admixture:

Fly ash

Silica fume

Ground granulated blast furnace slag

Rice husk ash

(2)    Chemical admixtures 

Chemical admixtures are various types, and they are:

Accelerating admixture

Retarding admixture

Water – reducing admixture

Air – entraining admixture

Super plasticizing admixture

Special Admixtures based on the application

Pozzolanic Admixtures

Damp-proofing Admixtures

Gas forming Admixtures

Air detraining Admixtures

Alkali Aggregate Expansion Inhibiting Admixtures

Anti-washout Admixtures

Grouting Admixtures

Corrosion Inhibiting Admixtures

Fungicidal, Germicidal, Insecticidal Admixtures

Coloring Admixtures Bonding Admixtures

Hot Weather Concreting

The intention of this article is to provide knowledge to general users with simple actionable works. To understand requirements, without going in to details of theoretical concepts, scientific and engineering analysis.

Concreting continuous throughout the year in construction industry but doing concreting in any extreme weather is a biggest challenge. Doing concreting in cold weather we don’t have much problem since concrete hardening being an exothermic reaction during cold weather.

To protect concrete from heat, common understanding of using chilled water is there or alternate is to do concreting in night shift. But with higher temperature in most part of the day, such strategy does not work for a durable concrete. General understanding in which concrete evaporates more than 1Kg/Sqm/Hr water.  It is perfect time to have a deep look in understanding hot weather concreting during summer.

Care during Designing

Care during Concrete Production

Care during Transportation

Care during Pouring

Care during Finishing surface and

Care during Curing

Care During Designing

Summer Concrete Shall be deigned with low heat of hydration materials.

PPC or Slag Cement, Low heat cement will help in many ways to have a good concrete with lower rate of reaction

Good quality Admixtures especially Polycarboxylic ether (PC) based can help to maintain good workability for relatively longer time and can also slowdown evaporation of water from concrete

Concrete shall be designed for the atmosphere in which the concreting is going to happen. (Maximum permissible fresh Pour Concrete is 35 Degree)

Care during Concrete Production

Care shall be taken to keep the fresh concrete temperature as lower as feasible. It is easier to care for fresh concrete rather than placed concrete

Mixing system shall not induce more heat and preferably insulation of mixing system shall be done

Cement (Bags) shall be stored in shaded area for sufficient time

Coarse aggregates shall be sprinkled with cold water

For Fine aggregates sprinkling of water if done shall be stopped couple of hour early to reduce moisture variation

For Higher temperature reduction, ice flakes shall be used

If ice flakes used shall be mixed with weighment to maintain the water cement ratio in line with design mix

Use of Liquid nitrogen is an option (Not Cost Effective and not feasible due to practical reasons)

Care during Transportation

Transportation time shall be kept minimum as much as possible (Batching location should be very nearer)

Transit Mixer drum shall be insulated to reduce the heat gain during transportation

Hessian cloth shall be used as an insulation material

Care during Pouring

During Pouring of concrete, pump pipe lines should be insulated to reduce temperature gain.

Dense Hessian cloth with sponge layer wrapping followed with wetting chilled water shall be done

Temporary shed shall be used for direct exposure to sun and wind. If shed is not feasible at least covering with thick plastic sheet shall be covered for shuttering and reinforcement before and during the concreting

Concrete pour area shall be water sprinkled (If Feasible) to cool shuttering and reinforcement. Care shall be taken that there is no standing water anywhere.

Care during Finishing surface

Late finishing shall be done before concrete starts being in semi plastic state

Evaporation reducing admixtures shall be used

Fogging of Concrete surface can be done during concrete green stage

Care during Curing. (Post Initial Set, Hardening of Concrete)

Once concrete can take a person’s weight, it is understood that concrete has initial set

Mist sprinkling of water can be done with a cover of hessian cloth

Water curing can continue either by ponding or intermittent watering surface

Time of form removal and start of curing is an important factor. Curing shall start with in 45 minutes of form removal

In short, Mass concreting during summer requires a detailed analysis, review and action plans for concrete care.

Concreting Precautions

Dear Readers,

No construction activities will be taken without concreting, Let us see the precaution works before concreting. Proper concreting of RCC Columns, pedestals and footings is needed to ensure desired strength and durability of these structural members. Improper method of concreting may lead to corrosion of steel reinforcement due to pores, cracks in concrete and concrete may not achieve its target strength on setting. This may also lead to failure of columns, pedestals or footings on during occupancy of the structure.

Things to keep in mind before starting concrete works at site are:

1. Formwork inspection for strength, leakage and surface finish.

2. Reinforcement inspection as per drawing, lap length, correct lapping guidelines as per code,

3. Inspection for concrete cover to reinforcement

4. Inspection for alignment of structural member as per drawing

5. Availability of sufficient construction material at site

6. Availability of concrete vibrators,

7. Proper safety PPEs and safety measures,

8. Proper ratio of concrete mix should be confirmed,

9. Availability of slump testing equipment,

10. Availability of cube casting equipment.

After the above inspection is done and found satisfactory, concreting work should start. 

Concreting of RCC columns, pedestals and footings should start from the center of vertical bars and go towards the ends. After placement of sufficient quantity, concrete should be vibrated at regular intervals so that the concrete spreads evenly on all sides. Over vibration of concrete should be avoided. Over vibration of concrete leads to segregation of coarse aggregates which settles at the bottom making the mix weak.

Proper keys should be provided in the center of column reinforcement and walls. if the concreting has to be stopped for the day, all upper surfaces of column and walls should be made rough by wire brush after initial setting of concrete for joints with later pour of concrete.

 

This is required to provide a proper grip between concrete of stem part with footing and walls and foundations. Cement and sand slurry of 1:1 mix should be applied on the footing and foundations before column stem and wall concreting is started, to avoid separation of stem/ wall where a cold joint may be formed.

After the concreting of footing is complete, immediately in a day or two starters for column and wall should be casted with proper alignment and again the upper surface of starter should be made rough. This can be easily achieved simply by spreading and light pressing coarse aggregates particles when concrete is still green in such a way that part of coarse aggregate is outside and part goes inside.

This method of making top surface rough is to be adopted along with providing key at all places in columns, pedestals, vertical walls which are always concreted after concreting of footing and rafts concreting is over (approximately two or three days letter). These measures provide a proper grip between the surfaces. In addition to above dowels in between outer main bars of wall and column are inserted when concrete is wet to provide further grip between old and new concrete surfaces.

 These dowels are 600 to 800 mm cut pieces of reinforcement bars, which are inserted in green/wet concrete such a way that half length is projecting and half length is inside concrete.

 

Wish you all the best for excellent concreting. Have a Nice Day.

Unit Weight of Rebar

Construction of Civil works with out Cement & Reinforcement steel ia an unimaginable issue. Let us see about the rebar.

Rebar (short for reinforcing bar) is mild steel rod that comes in various thickness. Rebar is usually manufactured with deformations and the rebar is not perfectly round. The rebar is typically "textured" i.e. ribbed. That means that it is not smooth and therefore will grip better.

Rebar is also known as reinforcing steel, reinforcement rod, rerod, or a deformed bar, is a common steel bar,In Australia, it is colloquially known as reo.These bars are produced in sizes from 4 mm to 50 mm in diameter.



Above weights are as per clause 5.2 of IS 1786 - 1985.

For any steel reinforcement bar, weight per running meter is equal to d^2/162 Kg [(d X d )/162], where d is diameter of the bar in mm.

For example, 10 mm diameter bar will weigh 10x10/162 = 0.617 Kg/m.

For more details of Building materials unit weight Click BUILD_MAT'L

Best Quote

"A successful man is one who can lay a firm foundation with the bricks that others throw at him". Have a Nice Day