Option B is correct because, Calcium chloride is a rapid hardening agent. The acceleration of setting, hardening and evolution of this large quantity of heat in the early period of hydration makes the cement suitable for concreting in cold weather. The increased porosity of high alumina cement further develops more resistance to the frosting.
- 1 Which type of cement will be preferred during hot weather concreting?
- 2 Which of the following measures must be undertaken for cold weather concreting?
- 3 What is the MCQ answer?
- 4 Which admixture is associated with cold weather?
Which type of cement will be preferred during hot weather concreting?
Hot Weather Concreting Posted at 12:58h in by Durability of concrete structures are being widely discussed and deliberated across the globe amongst civil engineering fraternity in the last few decades. The costs towards repair and rehabilitation of concrete structures have been found to be in such high magnitude that the needs to construct durable structures have become imperative. Concrete/cementitious products have a similarity to Human beings in the sense that how a new born baby is being taken care so much by means of timely vaccinations and other protection measures during the first few years after birth, the same applies to taking care of the structures built of concrete / cement mortar during the initial hours/days after they are cast in the structure. In my experience in cement/ready mix concrete industry in South India last 19 years I have been seeing Plastic Shrinkage cracks in concrete being reported from lots of projects especially during the last decade or so. Hence the need to understand HOT WEATHER CONCRETING especially with respect to tropical and dry regions of our country. Hot weather may be defined as any period of high temperature in which special precautions need to be taken to ensure proper handling, placing, finishing and curing of concrete. Hot weather problems are mostly encountered in the summer but the associated climatic factors of high winds, relative humidity and solar radiation can occur at any time, especially in arid or tropical climates. Hot weather conditions can produce a rapid rate of evaporation of moisture from the surface of the concrete and accelerate setting time etc. Generally, high humidity tends to reduce the effects of hot weather in concreting. But then, what is hot weather concreting? The definition may vary from country to country. As per BUREAU OF INDIAN STANDARDS IS:7861(Part I), ” Any operation of concreting done at atmospheric temperatures above 40 degree Celsius or any operation of concreting ( other than steam curing) where the temperature of concrete at time of its placement is expected to be beyond 40 degree Celsius is termed as HOT WEATHER CONCRETING “. We need to understand, foremost, the effect of hot weather on concrete, in the absence of special precautions, than concreting done at normal temperatures. The principal general effects of hot weather concreting are as follows: Accelerated setting: Cement Hydration results in setting and hardening of concrete. High temperature increases the hydration of cement resulting in faster setting which means the time available for workability (concrete in plastic state) of concrete is reduced. Hydration of cement also releases heat. Hence, faster the cement hydration faster the heat generation and this increases concrete temperature, further speeding up the cement hydration or setting. While initial strength may not suffer, the long term strength (> 28 days strength) is affected. This is the reason why hot weather concreting needs special care especially on finishing and curing of concrete. Reduction in strength: High temperatures necessitates addition of water at project site to enhance the workability of concrete and thereby reduction in concrete strength. It may also results in formation of cold joints in the structure cast.
- Increased tendency to cracks: High temperatures, High wind velocity and low relative humidity can affect fresh concrete in two important ways: first, the high rate of evaporation may induce early plastic shrinkage cracks or cracks may be developed in the hardened concrete either by increased drying shrinkage resulting from greater mixing water used or by cooling of the concrete from its elevated initial temperature and secondly, the evaporation rate can remove surface water necessary for hydration and unless necessary curing practices done it may affect the cement hydration process in the initial stages resulting in lower concrete strengths.
- Thermal cracking also may result from rapid drops in the temperature of the concrete, from hot weather during concreting and the night cooler temperature.
- How to concrete in HOT WEATHER?
- The Key to successful hot weather concreting is:
- Recognition and understanding the factors that affect the concrete.
- Planning and necessary precautions to minimize their effects.
We will now see some useful measures that need to be taken during hot weather concreting without compromising important parameters such as workability, strength, crack-free surface etc for better performance in the long run i.e. high durable concrete structures.
The first immediate solution for concreting in arid and hot weather /low humid regions is to pour concrete during early morning or late evening. This may mitigate the bad effects to a large extent. In case, if the concreting pour has to be done during the day, other precautionary measures as below could be undertaken for better results.
The foremost amongst the possible measures are to lower the temperature of the concrete ingredients before they are used for concrete mixing. Primarily, in case of cement, High early strength cements to be avoided unless initial high strength of concrete is required at site.
In that case, it is advisable to use cementitious materials like flyash/slag to reduce the heat of hydration. But it is highly recommended to use BLENDED CEMENTS like Portland Pozzolana Cement (PPC) / Portland Slag Cement (PSC) in such cases not only from hot weather concrete point of view but also from overall DURABILITY OF CONCRETE STRUCTURES.
Aggregates may be protected from direct rays of the sun during concrete production by providing shades over the stockpiles. Sprinkling the stockpiles of coarse aggregates with water and keeping them moist is another option to be considered. Only precaution to be taken care is not to flood them so as to affect the moisture content considered during concrete mix design.
The mixing water has the greatest effect on temperature of concrete, since it has a specific heat of about 4 to 4.5 times that of cement or aggregate. Hence chilled water or ice is preferred during concreting at high temperatures. If at all ice is added to the mixing plant directly, it is added in the crushed state only and complete melting is to be ensured before mixing of concrete ingredients is complete.
In Hot weather condition the mix sets quicker and looses workability of slump faster due to rapid hydration and evaporation of water from the mix. This makes placing, compaction and finishing of concrete quite difficult leading to deterioration in the quality of concrete.
- Addition of water to maintain workability / slump leads to drop in concrete strength.
- Use of chemical admixtures like water reducers / retarders helps in delaying the setting of concrete.
- These are used in lots of projects these days in India and worldwide (Arid and tropical climates).
- However, use of these chemical admixtures need to be carefully done as per manufacturers specifications and expert engineers advice.
We need to plan adequate manpower and machineries to complete the concrete pour at the shortest time possible during hot weather. The effect of mixer machine surface as well as transporting trucks exposed to the hot sun should be minimized by painting white or lighter colour and covered with wet burlaps to minimize the radiation effects.
Also, it is advisable to minimize the time between water added during concrete mix and concrete pour to the minimum possible to negate the workability loss etc. The next important precautions to be taken are in PLACEMENT, FINISHING AND CURING of concrete during hot weather. Forms, Reinforcement and Sub-grade shall be sprinkled with cool water JUST prior to placement of concrete.
Speed of placement and finishing helps to minimize problems in hot water concreting. Since hot weather leads to rapid drying of concrete, protection and curing has to be started immediately after surface is hardened enough. This can be in the form of covering the concrete surface with wet burlap, curing compounds or sprinkling of water initially.
- RMC’s in South India have found brooming of concrete and sprinkling of water immediately to be suitable practical solution in case plastic shrinkage cracks have been observed within couple of hours after concrete pour.
- Finally, once concrete cubes are casted for testing of the concrete for compressive strengths, adequate protection to be made for cube specimens from being exposed to direct sunlight or else evaporation of water in the cube will lead to lower compressive strength reported when tested for 7 days and 28 days.
- To summarize, adequate measures at every step involved in concreting from material storage, concrete mixing, transportation, placing, finishing, curing and concrete cube casting as mentioned above to be undertaken if done during HOT WEATHER for better and durable concrete.
- Penna Cement is one of the ‘s practicing the concrete mixing and cement manufacturing processes to produce the best cement grades for construction distributing and providing across cement dealers in New Delhi, Pune, Mumbai, Hyderabad, Chennai, Bangalore, Maharastra, Andhra Pradesh, Kerala with a great cement manufacturing footprint in South, North, East and West regions of India.
: Hot Weather Concreting
Which of these the effect of cold weather on concrete?
Can You Pour Concrete in the Cold Winter? – You can pour concrete in the winter, but you should know a few things beforehand. Concrete forms from a chemical reaction between cement and water — cold weather can interfere with the reaction. Cold concrete takes more time to set and strengthen, and this larger time window brings other problems.
Which of the following measures must be undertaken for cold weather concreting?
Which of the following measures must be undertaken for cold Option 4 : Heating of water Free 20 Questions 20 Marks 20 Mins Explanation: Cold Weather Concreting:
Cold Weather Concreting is defined by ACI 306 as a period when for more than three successive days the average daily air temperature drops below 4°C. Concrete can freeze before it gains strength which breaks up the matrix Concrete sets more slowly when it is cold – very slow below 10°C; below 4°C the hydration reaction basically stops and the concrete doesn’t gain strength, Unless concrete gains sufficient strength, it cannot resist freezing and thawing action. Hence delay in setting time doesn’t make concrete more resistant to frost attack.
Suitable measures must be taken in cold weather concreting are as follows:
Heat the mixing water Avoid using frozen aggregates Heat the forms prior to and after casting If the concrete does not freeze and no heat is applied, do not strip the product until adequate strength is attained Monitor and record concrete temperatures during curing
Additional Information Hot Weather Concreting :Hot weather concreting is defined by ACI 305 as one or a combination of the following conditions that tend to impair the quality of freshly mixed or hardened concrete by accelerating the rate of moisture loss and rate of cement hydration.
High ambient temperature (more than 35° C) High concrete temperature Low relative humidity High wind speed
India’s #1 Learning Platform Start Complete Exam Preparation Daily Live MasterClasses Practice Question Bank Mock Tests & Quizzes Trusted by 3.4 Crore+ Students : Which of the following measures must be undertaken for cold
Is there cold weather cement?
Construction doesn’t stop–it’s a year-round industry. This means concrete mixes, pours and placements are needed all year and cannot come to a halt due to less-than-ideal weather conditions. While concrete can be mixed and poured during the cold weather, there are many things to keep in mind which we plan to detail throughout this blog.
First, we’ll explain what can go wrong while pouring concrete in cold weather conditions. Then, we’ll discuss how your ready mix provider can help you overcome the problems associated with cold weather concreting. Finally, we’ll warn you of the mistakes we frequently see when it comes to cold weather concrete pouring and tips to avoid making those mistakes.
Before we start, we feel it’s important to all get on the same page as to what exactly “cold weather” is considered. The ACI or American Concrete Institute says in their ACI 306R-10 “Guide to Cold Weather Concreting that “cold weather exists when the air temperature has fallen to, or is expected to fall below 40℉ during the protection period*.” So if you find yourself grabbing a jacket on your way out the door, consider the air temperature before mixing, pouring, or placing concrete.
What is meant by cold weather in concrete?
What is Cold Weather Concrete? The American Concrete Institute (ACI) defines cold weather concreting as, ‘ a period when the average daily ambient temperature is below (or expected to fall below) 40°F (5°C) for more than 3 successive days.
What chemical is added to concrete in cold weather?
Adding Calcium Chloride to Improve Cold Weather Concrete Pouring Calcium chloride is the most effective and least expensive cold weather accelerator for concrete. But its use may be limited by building codes or prohibited by some specifications. Contractors then have to ask one or two questions:
- Can we use calcium chloride admixtures?
- If so, what is an acceptable dosage?
Fortunately, the can be used to answer these questions.
How long does concrete set in cold weather?
Maintaining Ideal Temperature – To protect concrete in cold weather, the concrete should be kept warm during the curing process—over 5˚C for the first 48 hours. Concrete strength development is critical during the first 48 hours. But if it is below 5˚C, concrete will take longer to develop its required strength.
Ideally, you will maintain concrete temperatures above 10˚C (50˚F) for the first three to seven days. And for at least four more days after, maintain the concrete temperature above 4˚C (40˚F). Be careful not to let the concrete temperature drop more than 4˚C in 24 hours. Frost blankets and insulated formwork can help keep concrete warm enough and protect it from the cold.
So be sure to cover the concrete slab with a plastic sheet, and then cover the plastic sheet with insulating blankets. Concrete pouring in winter is possible. You just need to take the necessary precautions to ensure the concrete isn’t at risk of freezing, cracking, and not curing to its desired strength.
What is the MCQ answer?
A multiple-choice question (MCQ) is composed of two parts: a stem that identifies the question or problem, and a set of alternatives or possible answers that contain a key that is the best answer to the question, and a number of distractors that are plausible but incorrect answers to the question.
- Students respond to MCQs by indicating the alternative that they believe best answers or completes the stem.
- There are many advantages to using MCQs for assessment.
- One key advantage is that the questions are easy to mark and can even be scored by a computer, which makes them an attractive assessment approach for large classes.
Well designed MCQs allow testing for a wide breadth of content and objectives and provide an objective measurement of student ability. The following suggestions for designing MCQs are organized into three sections: 1) general strategies, 2) designing stems, and 3) designing alternatives.
Which admixture is associated with cold weather?
HE200 Early Age Strength Admixture – Sikament HE200 is a new technology admixture that provides an effective superplasticising action on fresh concrete and rapidly accelerates its early age strength development without any negative effect on the final strength.
HE200 is ideal in colder temperatures where accelerated strength development is necessary. Allied Concrete staff are more than happy to help you with any problems or enquiries. For more information or assistance, please don’t hesitate to call. Your call will be automatically connected to our nearest plant.
(Calls from mobile phones will be directed for Auckland, Wellington or Christchurch.)
How do you protect concrete from cold weather?
MISTAKE #2: ALLOWING CONCRETE TO FREEZE – Plastic concrete freezes at about 25° F and doing so can reduce its final strength by more than 50%. Therefore, it is important to keep fresh concrete from freezing until it reaches a compressive strength of at least 500 PSI.
- It is very important to protect the concrete and keep it as warm as possible (at least 50° F).
- This can be accomplished with insulating blankets or heaters.
- A non-chloride or liquid chloride admixture can also help.
- Colder temps slow down concrete set times, but non-chloride admixtures can ensure your concrete sets at the normal rate.
Liquid chloride is also an excellent accelerator for concrete, helping your flatwork set faster while improving workability and finishability. Don’t use a liquid chloride accelerator if your flatwork will contain steel or another metal reinforcement, as liquid chloride can be corrosive to metals.
Can hydraulic cement be used in cold weather?
Hydraulic Cement Pros and Cons – Hydraulic cement will offer some advantages but it also has some downsides as well. Some of its advantages are:
- Provide durable repairs that will last for long periods of time.
- Sets and hardens fast, normally three minutes after being mixed with water.
- It is a cost-effective solution.
- Hydraulic cement is very easy to use.
- Hot water will accelerate the setting time and cold water will retard it.
- Can be used on vertical applications.
- It will maintain its strength even if it’s submerged in water.
- Will not corrode or become rusted.
- Hydraulic cement will not shrink.
- It can fix leaky pipes and basements without having to stop the leaking.
- It can be painted within one hour of it being applied.
But it also has some drawbacks:
- Once mixed, the hydraulic cement only remains workable for 10 to 15 minutes.
- Will not work on frozen surfaces or if the temperature will drop dramatically within 48 hours.
- Avoid using it when the temperature is below 40 degrees Fahrenheit.
What do you put in concrete in hot weather?
Water and Ice – To maintaining cool concrete in hot weather, the temperature within the concrete can be reduced by using cool water or ice as part of the mixing water. Additionally, spraying aggregates with water can help keep the concrete cool.
What is concrete in hot weather?
Hot Weather Concreting—Methods for Success – Injection of liquid nitrogen into concrete truck As a global leader in advancing concrete knowledge, ACI’s goal is to keep concrete industry professionals up to date with the latest information. For example, here’s a recent exchange from ACI’s Q&A section: Q.
- What are the ACI building code requirements for placing concrete in a tropical climate? Some people in our office say we should specify a maximum permissible concrete temperature of 90 ºF (32 ºC).
- Others say a temperature higher than 90 ºF (32 ºC) is allowable if the concrete contains a set-retarding admixture.” A.
ACI 301-16 “Specifications for Structural Concrete ” and ACI 305.1-14 “Specification for Hot Weather Concreting ” limit the maximum concrete temperature to 95 °F (35 ºC) at the time of discharge. This limit is for general types of hot weather construction such as pavements, bridges, and buildings, not mass concrete.
Hot weather concreting is defined by ACI as ” one or a combination of the following conditions that tends to impair the quality of freshly mixed or hardened concrete by accelerating the rate of moisture loss and rate of cement hydration, or otherwise causing detrimental results: high ambient temperature; high concrete temperature; low relative humidity; and high wind speed.
” Various adverse effects on the properties and serviceability of concrete are brought on by hot weather. Just one of those is the effect on strength—concrete mixed, placed, and cured at elevated temperatures normally develops higher early strengths than concrete produced and cured at lower temperatures.
Hot Weather Concrete webinar scheduled for May 9 at 1 pm EST. Sign up now and reserve your spot! https://t.co/rNpfqyrjXx pic.twitter.com/g4mNZpngXy — AmericanConcreteInst (@ConcreteACI) April 20, 2017 Damage to concrete caused by hot weather can never be fully alleviated. Potential deficiencies to concrete in the hardened state can include decreased strengths resulting from higher water demand, and decreased durability resulting from cracking.
Typical hot weather concrete evaporation protection measures include fogging; evaporation retarders; wet burlap; wind breaks; and protection of quality control specimens. An upcoming May 9 webinar will cover best practices and examples from projects where pre-cooling measures, moisture controls, mixture adjustments, and admixture technologies have been implemented to reduce adverse effects to the concrete under hot, drying conditions.
- Add to your knowledge by signing up for: Hot Weather Concrete – Best Practices and Lessons Learned On-Demand Course,
- ACI 305R-10 Guide to Hot Weather Concreting defines hot weather, discusses potential problems, and presents practices intended to minimize them.
- These practices include selecting materials and proportions, precooling ingredients, and batching.
Other topics discussed include length of haul, consideration of concrete temperature as placed, facilities for handling concrete at the site, and, during the early curing period, placing and curing techniques, and appropriate testing and inspection procedures in hot weather conditions. Hot weather concreting can be a challenge – to minimize the disadvantages, and make your next hot weather concreting project a success, use ACI’s resources, including ACI University online courses, free online education presentations, publications, and journal articles.
Can cement be poured in hot weather?
High summer temperatures, low humidity, direct sunlight, high concrete temperatures and even modest wind velocity can cause concrete surface cracking (plastic shrinkage cracks). There are several easy steps that should be taken to reduce shrinkage cracking and to improve placement characteristics in hot, dry, or windy conditions.
- Plastic shrinking cracks occur when surface moisture evaporates too rapidly.
- In addition to the potential for shrinkage cracks, hot weather will tend to cause concrete to lose slump (workability) and set much more quickly.
- Concrete will typically set in about 4 hours in 80 o F temperatures, a 10 o F increase in ambient temperature will reduce the set time to about 2.5 hours.
An increase of 20 o will reduce the set time approximately 60% to 1.5 hours. Adding water to the mix to offset a loss in workability will have a negative affect on the ultimate strength of the concrete and can increase the rise of shrinkable cracking.
Dampen the substrate and form work with cool water prior to concrete placement. Use cool mixing water to reduce initial concrete temperatures. Store bags of QUIKRETE concrete mixes and aggregates in a shaded area. Construct temporary wind-breaks to reduce wind velocity and sun-shades to reduce concrete temperatures. Consider placing concrete early or later in the day when ambient temperatures are cooler. Apply QUIKRETE Acrylic Concrete Cure & Seal immediately after finishing procedures are completed to prevent surface moisture loss. Have plenty of help to allow for the reduced placement and finishing time of the concrete mix.
With proper attention to these steps, concrete can be successfully placed in hot, dry, windy and low humidity conditions.
How concreting is done during hot weather?
One of the most successful techniques in warm weather is to reduce the temperature of the concrete mix by cooling it down. This can be done in multiple ways, and ACI recommends the following techniques: Cooling the concrete with chilled mixing water. Cooling the concrete with ice.