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How to solve no cell phone signal reception issues in your homeByJason Samuel

Bad cell phone signal reception is something everyone has to deal with, both home

users and business users. There is a lot of conflicting information out there and I often

have to explain how cell phone repeaters and other cell phone booster solutions work

and what doesn’t to my family, friends, and co-workers. I have very good first hand

experience with some solutions and since it is technology related, I have decided to

dedicate a post here on my blog about these cell phone reception solutions to point

everyone to. Saves me from having to say the same thing over and over when I could

just email them a link. 

First, for those people that are not regular readers of my blog, I have to let you know I

work in IT infrastructure so cell phone engineering is not my expertise. I do some

telecom and VoIP work but I am not an RF Engineer that works for a cell phone carrier

by any means so please don’t consider me an expert on this subject. I’m only going to

tell you about my own experience and research.

Second, I live in the Houston, Texas area. Cell phone coverage depends on your carrier

and how their cell tower coverage is in your city. Houston being the fourth largest city in

the United States has very good cell tower coverage from all the big carriers: AT&T, T-

mobile, Verizon, Sprint, & Nextel. So parts of this post may or may not apply to you

depending on where you live. Please do your research before spending your money

and don’t rely soley on my blog post.

The Cause of Bad Reception

The question I get asked a lot is how can I improve the cell phone signal in my house or

my office because I get 0 bars. In Houston, it is rarely a cell phone tower coverage

issue. Cell phone towers are everywhere in this city. People just usually don’t notice

them. But if you really start paying attention, you can’t go more than a few blocks before

spotting one. Most of the time I hear complaints from friends and family that have just

moved into a new house (newly constructed home) about cell reception being poor or 0

bars. They wonder what the cause could be in their brand new house because their old

house didn’t have signal issues. The reason for this is because new homes use better

radiant barriers in the attic than older homes. Especially in hot places like Texas. These

radiant barriers are the cause of the poor signal, not cell phone tower coverage. They

are blocking the cell phone signal from penetrating the house. Almost all home builders

in Houston use LP TechShield radiant barrier:

http://www.lpcorp.com/techshield/

David Weekly Homes, Newmark Homes, etc. to even smaller custom home builders all

use it here in Houston. TechShield can save you a TON of money per year. I went from

having a $900 per month electricity bill in the summer in a house with no TechShield to

less than $200 in a newly constructed home with TechShield installed. And the new

house has double the square footage! Though TechShield works great and will save

you a lot of money on your energy bill, the downside is it block RF signals (radio

frequency signals). RF signals is how your cell phone talks to the cell tower. Specifically

Ultra High Frequency or UHF. So what a lot of my friends, family, and co-workers find is

that when they go home, they go from having 5 bars outside to 0 bars inside their

homes. You can confirm TechShield is the cause by going into your attic and looking

up. You will see the silver radiant barrier and it will say TechShield on it like this:

With businesses, the taller your building is and the more steel, insulation, and other

dense material it is constructed with, the less signal you will get. In some cases like

hospitals and large medical centers there could be equipment causing poor signal too.

A lot of the big skyscrapers in Downtown Houston and the Medical Center rarely get

good a good cell phone signal without working with carriers to install commercial in-

building cell phone repeaters on every floor. I will go over what cell phone repeaters are

exactly in the next section.

The Wrong Solution

A quick Google search will tell you cell phone boosters or cell phone repeaters are the

way to go to solve bad cell phone signal issues. Those stickers and other solutions that

are “passive” are all garbage. You need something that is “active” and amplifies the

signal which is what a cell phone repeater is. Another term for a cell phone repeater is a

BDA or bi-directional amplifier. For home users, you can even buy cell phone signal

repeaters at electronic stores like Fry’s or Best Buy. They are cell phone amplifiers that

amplify the weak signal from cell phone towers. Usually you stick an antennae on your

roof that pulls the signal in, which is connected to the amplifier inside your home which

amplifies the signal, which is then connected to another antennae inside your home that

your cell phone connects to. Here is a diagram of how a simple cell phone repeater

system works:

One thing you will not be told about these is that though they are completely legal to

purchase, they are illegal to operate. The reason is cell phone carriers pay tons and

tons of money to the FCC to license radio frequency spectrums (cellular frequency

spectrums) for their use. You are not allowed to broadcast on these licensed

frequencies without permission from the carrier that has licensed the spectrum or you

will be fined by the FCC. The problem with a lot of cell phone repeaters is that they

amplify the signal, sometimes way too much. This can cause issues with a nearby cell

phone tower and causes problem for the carrier. For this reason, they do not want

consumers installing and operating in-building cell phone repeaters in their homes. I

speak from first hand experience so let me continue.

Amazon, Best Buy, Fry’s and a lot of other stores and websites sell dual band cell

phone repeaters and boosters. They are usually dual band meaning it will work with 800

Mhz and 1900 Mhz bands which is AT&T and T-mobile respectively in the US. My

friend’s 2 story 5,500 sq. ft house was currently getting 0 bars inside the house due to

TechShield radiant barrier. Up to the front door, we could get 5 bars on AT&T and T-

mobile phones. As soon as we stepped into the house, the signal drops. He specifically

wanted coverage in the kitchen and living rooms only because that is where most of the

family usually is. Having 0 bars in the house was unacceptable. If there was an

emergency at work or with family everyone has his cell phone number and will call it

first. Also he did not have a land line since his whole family had cell phones. So he

purchased the zBoost YX-545 unit for around $270 from Amazon.com.

http://www.amazon.com/Wireless-Extenders-YX545-Dual-Band-Booster/dp/B003VOW5WI/

ref=sr_1_1?s=wireless&ie=UTF8&qid=1301611766&sr=1-1

You will see a lot of reviews online ranging on this unit and it’s predecessor the YX-510

from horrible reviews to 5 stars. It all depends on your building, cell tower coverage, and

installation. I personally saw very poor results when my friend set it up. Within 5 ft. of

the unit, I went from 0 bars to 2 bars. As soon as I stepped out of this 5 ft radius, I got 0

bars again. He called their support line which was very helpful and they confirmed

everything about the install was correct. He even sent them pictures of how the

antennae was mounted outside on the roof and into the house. But, that’s the best he

could get in the house.

So both of us being geeks, I convinced him to try upgrading the unit. He bought the

premium kit with upgraded outdoor directional antennaes for $130. You can use this to

get a better signal from the cell tower because you can point it directly at it:

http://www.amazon.com/Wireless-Extenders-zBoost-Dual-Band-Directional/dp/

B003N3HDAQ/ref=sr_1_5?s=wireless&ie=UTF8&qid=1301611766&sr=1-5

We also upgraded the thin RG-59 coaxial cable the kit came with to some nice thick

RG-11 cable:

http://www.showmecables.com/viewItem.asp?idProduct=7326

The tradeoff is the RG-11 is less flexible so it is harder to run the cable through walls or

attic all the way up to the roof. Imagine RG-6 (the cable you use to connect your to your

TV or cable box for cable TV) but twice as thick.

After getting all the new upgrades installed, we could get 3 bars within about 10 ft. As

soon as we leave that radius, the signal drops to 0. At this point zBoost support did not

have any other suggestions for him except to move the directional antennae on the roof

and point it directly at another cell tower. We had already mapped out where the AT&T

and T-mobile cell towers were by using Google Maps and a cell tower locator

application for our phones called “Cell Tower Locator”. We also drove by the cell tower

sites to confirm we were connected to it and measured the dbm as we approached and

left the tower. There are also websites online that document cell tower locations and we

confirmed the locations of the towers there as well, it was only 1.3 miles away. He even

rotated the antennae on a mast at 5 degrees each for the full 360 degrees testing and

that was the best he could do. So it wasn’t a problem with my friend’s antennae pointing

for sure.

So again, he opted to upgrade but this time to a business solution that is used for large

office buildings, warehouses, etc. One really popular cell phone repeater company that

caters to home users and business users is Wilson Electronics but he opted to go with

Cellphone-Mate that makes a comparable product with excellent reviews. There was a

company locally that installed both Wilson and Cellphone-Mate in office buildings and

they recommended going with Cellphone-Mate for his house. He decided to go with

their flagship product, the SureCall CM2020 68dB amplifier for $1200. They claim to

cover 60,000 sq ft on their website:

http://www.cellphone-mate.com/newp/68db.html

and that it is FCC approved to the max output of 3 watts. But keep in mind they are

talking about the output of the device being legal per FCC regulation. They do not say

operating the amplifier itself is FCC approved and as I mentioned above, it is illegal to

operate an unauthorized cell phone repeater/amplifier. He did not know this of course

when purchasing the unit for $1200. They told him on the phone as well as on their

website that their basic CM2020 kit can cover up to 10,000 square feet easily and they

have done many installs for businesses and hospitals. Up to 60,000 sq ft. that the

manufacturer’s website said was with multiple antennae upgrades and such that was

overkill for a home. He purchased a complete top of the line kit consisting of the

following:

-68 dB CM2020 amplifier

-Outdoor directional yagi antennae

-Indoor omnidirectional dome antennae

-LMR400 cables (really thick coaxial cable)

You can see it here:

http://www.wpsantennas.com/CM2020-Kit-Cellphone-Mate-65db-dual-band-system.aspx

My friend purchased it locally and got the company to come out to install it. Instantly his

5,500 sq ft. house was at 5 bars. Anywhere in the house was a solid 5 bars, no drops at

all. The company tuned down the 68 db gain using the dip switches on the front of the

amplifier to a level that was just enough to cover the inside of the house only.

Now the problem came a few days later when he received a certified letter from an RF

engineer from one of the major cell phone carriers. In the letter the RF engineer stated

that since the day the unit was installed, it has been causing interference with a nearby

cell tower. It was causing all sorts of problems for that carrier’s customers in the area.

So they had sent this RF engineer out to investigate. Using a directional antennae in his

van and some other hardware, he discovered the source of the signal was my friend’s

house which is why he sent the letter. He went on to explain cell phone repeaters (aka

BDAs) are not permitted to be installed for use on any cell phone spectrum in the US by

the FCC without consent from the carrier that licenses (T-mobile, AT&T, Verizon, etc.).

None of these carriers permit the use of a BDA by a home or business user. Only the

carrier themselves installs them and it’s a huge process and very expensive to have

them do it so only large corporations tend to do this. My friend called the RF engineer

and got more info on this. They typically install licensed BDAs themselves or

subcontract the work out to companies like the one that my friend had hired to install the

unit.

In fact a few months ago while working in one of the largest skyscrapers in Downtown

Houston, I ran into a T-mobile subcontractor that was going from floor to floor testing the

building’s repeater signal. They had several of them per floor that fed into an IDF closet

that ran throughout the building and into several amplifiers on one level. Something like

this:

Anyhow, operating a big system like above or a simple system like my friend started off

within his house on your own is illegal. Only the carrier which is the licensee of the

spectrum is allowed to do it. Quick research online shows several cases of FCC

inspectors imposing fines and seizing cell phone repeaters that were installed by home

and business users without authorization. They typically start of with cease and desist

letters like below:

http://www.fcc.gov/Daily_Releases/Daily_Business/2010/db0927/DOC-301700A1.pdf

http://www.fcc.gov/eb/FieldNotices/2003/DOC-300634A1.html

http://www.fcc.gov/eb/FieldNotices/2003/DOC-296857A1.html

http://www.fcc.gov/eb/FieldNotices/2003/DOC-296238A1.html

http://www.fcc.gov/eb/FieldNotices/2003/DOC-295061A1.html

http://www.fcc.gov/Daily_Releases/Daily_Business/2010/db1012/DOC-302031A1.txt

http://www.fcc.gov/eb/FieldNotices/2003/DOC-266448A1.html

http://www.fcc.gov/Daily_Releases/Daily_Business/2010/db1101/DOC-302541A1.pdf

Here is the important part in all these cease and desist notices:

“Licensees may install in-building radiation systems without applying for authorization or

notifying the FCC, provided that the locations of the in-building radiation systems are

within the protected service area of the licensee’s authorized transmitter(s) on the same

channel or channel block.”2 A licensee’s authority to install a BDA does not permit a

subscriber to install a BDA, unless that subscriber has received explicit authorization

from the licensee to do so. In response to an inquiry from an FCC agent, T-mobile

reported that it did not provide you authorization to install a BDA. Operation of radio

transmitting equipment without a valid FCC authorization or license is a violation of

Section 301 of the Communications Act of 1934, as amended,3 and may subject the

responsible parties to substantial monetary forfeitures, in rem arrest action against the

offending radio equipment, and criminal sanctions including imprisonment.”

These “notices” from the FCC go on and one if you do a Google search for “FCC BDA

notice”. I’ve found them from just a few weeks ago to even as far back as 10 years ago.

You will notice that most of the complaints above are to home or business users. Some

are even found on boats. I even found one to Sony Pictures Studios in Los Angeles so

even a huge company can land in hot water. One really interesting find, there are some

notices from the FCC stemming from carrier to carrier complaints! So even one carrier

installing a repeater that interferes with another carrier’s signal is a real issue and the

FCC has to step in to correct it.

Luckily, the RF engineer was extremely nice in informing my friend of the laws in his

letter and on the phone and the company that installed the unit had a 30 day return

policy and removed the unit within a day. Apparently, this happens a lot in the Houston

area. Consumers go and purchase cell repeaters (or BDA which is the “official” term)

from Amazon or Best Buy and install them. Then he has the headache of hunting them

down and requesting the business or home user to remove it. If they don’t, he then

requests help from the FCC and they said a notice to the user like above. Poor guy.

Working in infrastructure myself, imagine end users running amok on your network

causing it to go down every day. This is what he had to deal with on a daily basis. I did

not envy him.

The Right Solution

So now my friend was stuck. After spending 2 months experimenting with cell phone

repeaters and finding out their operation was not allowed, he was back at square one.

So I ended up doing research for him and found another solution called a femtocell I

pointed him toward:

http://en.wikipedia.org/wiki/Femtocell

It is basically a small device about the size of your router that sits in your home. Your

cell phone will connect to the device instead of a cell tower. The device then connects to

your carrier’s network through the Internet using a secure tunnel. So you will be making

the phone call over the Internet and not rely on a cell tower anymore. Currently AT&T,

Sprint, and Verizon all use this technology. Since his family was on an AT&T family

plan, he purchased their solution called the AT&T MicroCell:

http://www.wireless.att.com/learn/why/3gmicrocell/

The device itself is manufactured by Cisco so you know it’s a solid piece of networking

equipment. It cost a one time fee of $150 and there were no charges after that. Plus

AT&T gave him a 30 day trial. I was there the day he got it in and after following the

simple setup procedures, we turned it on. It took a about 5 minutes for it to get a GPS

signal (for Enhanced 911 purposes) and to establish the VPN tunnel with AT&T’s

server. Within a few seconds of all lights going solid green, all the AT&T phones that

were registered to use the device had 5 bars inside the whole 5,500 sq ft. house! A

$150 femtocell unit was doing the same thing a $1200 cell phone repeater was doing!

Both of us were floored. All that time and money he spent on installing was a complete

waste. He’s been using the AT&T Microcell for about 8 months now and is extremely

happy with it.

One thing to note, I mentioned earlier in this post that dual band repeaters work on two

spectrums. But sometimes carriers that offer high speed data operate those services on

a different spectrum that is not amplified by the repeater so you will get little or no high

speed data when you are around a repeater. For example, T-mobile uses 1900mhz for

voice and slow EDGE data service in the US but 1700/2100 MHz for high speed 3G

data service. When you go with a femtocell from the carrier that doesn’t rely on cell

towers, you don’t have to worry about frequency bands and data, everything goes

through the Internet and you will get the high speed data service you pay for!

By the way I mentioned above every major carrier has femtocell except T-mobile. I read

an article that they were testing them though. For now, look into T-mobile’s feature

called “Wi-Fi calling” which is similar to femtocell but this requires you have a special

phone that can make calls over WiFi. Read about it below:

http://support.t-mobile.com/doc/tm24195.xml

Go to :

http://www.t-mobile.com/shop/phones/default.aspx

and click the “Wi-Fi and Mobile Calling” checkbox in the left hand bar to see all the

phone that have this feature.

Conclusion

Do your research before investing money. My friend and I are both geeks, and he

makes a lot more money than I do so he doesn’t care about throwing money away

experimenting. If you live in a rural area with no cell phone towers around, using an

unauthorized BDA or cell repeater will likely not mess with a carriers’ network and you

probably won’t get a letter from a carrier or the FCC. But if you live in a highly populated

city like Houston with cell towers everywhere, the chances of your repeater causing

issues is much higher. Cell phone repeater companies will say “FCC approved” on the

device but again as I mentioned earlier in the post, they are talking about the device

itself, not it’s operation by you. So it is risky to use one and I would personally never

attempt it.

Luckily for us, femtocell technology is available from most carriers and works just as

well as an enterprise level cell repeater and for a fraction of the cost. So now my

friends, family, co-works can read this article and I don’t have to keep repeating my

story over and over again.   And everyone else reading this, please do post if my

experience helps you any. I’d also really love to hear from any RF engineers that work

for any carriers about their own experiences and recommendations.

Q: What is cement?A: Cement is a fine, soft, powdery substance, made from a mixture of elements found in natural materials such as limestone, clay, sand and/or shale. When cement is mixed with water, it can bind sand and gravel into a hard, solid mass called concrete. Cement is usually grey. White cement is also available, but is usually more expensive. 1. Cement mixed with water, sand and gravel, forms concrete. 2. Cement mixed with water and sand, forms cement plaster. 3. Cement mixed with water, lime and sand, forms mortar. Cement powder is extremely fine; one kilo (2.2lbs) contains over 300 billion grains. The powder is so fine it will pass through a sieve capable of holding water.In India, Ordinary Portland Cement (OPC) is manufactured in three grades, viz. 33 grade, 43 grade and 53 grade. The numbers indicate the compressive strength obtained after 28 days, when tested as per the stipulated procedure.Apart from OPC, there are several other types of cement, mostly meant for special purposes, e.g. sulphate resistant cement, coloured cement, oil well cement etc. However, there are some general-purpose cements, the commonest one being Portland Pozzolana Cement (PPC). Q: What is natural cement?A: Natural cements are hydraulic cements, produced by mining natural deposits of limestone and clay with a specific chemical composition within a narrow range. When heated in a kiln and ground to a fine powder, a type of cement is produced, which through chemical reactions sets and hardens when mixed with water. The strength and uniformity of natural cements are lower than those of Portland cements; but these are more historically accurate materials for restoration projects, which is their primary application. Natural cements were extensively used in 19th and early 20th century construction in several historic structures. However, with improved technology for producing Portland cements, sales of natural cements began to decline in the late 1800s, stopping entirely by the mid 1970s.

 Q: How is cement made?A: 1) Limestone, the major ingredient needed for making cement is quarried. Small quantities of sand and clay are required as well. Limestone, sand and clay contain the four essential elements required to make cement: calcium, silicon, aluminium and iron. 2) Boulder-size limestone rocks are transported from the quarry to the cement plant and fed into a crusher, which crushes the boulders into marble-size pieces. 3) The limestone pieces then go through a blender where they are mixed with the other raw materials in the right proportion.  4) Raw materials are then ground to a powder. This is sometimes done with rollers that crush the materials against a rotating platform. 5) This mixture then goes into a huge, extremely hot, rotating furnace to undergo a process called ‘sintering’. Sintering means: to cause to become a coherent mass by heating without melting. In other words, the raw materials become partially molten. The raw materials reach about 2700° F (1480°C) inside the furnace. This causes chemical and physical changes to the raw materials and they come out of the furnace as large, glassy, red-hot cinders called ‘clinker’. 6) This clinker is cooled and ground into a fine grey powder. A small amount of gypsum is added during the final grinding. The finished product is Portland cement. The cement is then stored in silos (large holding tanks) where it awaits distribution. The cement is usually shipped in bulk in purpose-made trucks, by rail or even by barges and ships. Some is bagged for those who want small quantities. Q: What are the different types of Cements?A: Portland cement: Portland cement is made by heating limestone with small quantities of other materials (such as clay) to 1450°C in a kiln, in a process known as calcination. The resulting hard substance, called ‘clinker,’ which is then ground with a small amount of gypsum into a powder to make ‘Ordinary Portland Cement,’ the most commonly used type of cement (often referred to as OPC).Portland cement is the basic ingredient of concrete, mortar and most non-speciality grout. Its most common use is in the production of concrete. Concrete is a composite material consisting of aggregate (gravel and sand), cement, and water. As a construction material, concrete can be cast in almost any shape desired, and once hardened can become a structural (load bearing) element. Portland cement may be grey or white.Portland cement blends: These are often available as inter-ground mixtures from cement manufacturers, but similar formulations are often also mixed from ground components at the concrete mixing plant.Portland Blastfurnace Cement contains up to 70% ground granulated blast furnace slag, Portland clinker and a little gypsum. All compositions produce high ultimate strength, but as the slag content is increased, the early strength is reduced, while the sulphate resistance increases and heat evolution diminishes. Portland Blastfurnace Cement is used as an economic alternative to Portland sulphate-resisting and low-heat cements.Portland Flyash Cement contains up to 30% fly ash. The fly ash is pozzolanic, so that ultimate strength is maintained. Because fly ash addition allows for lower concrete water content, early strength can be maintained. This can be an economic alternative to ordinary Portland cement where good quality, cheap fly ash is available.Portland Pozzolan Cement includes fly ash cement, since fly ash is a pozzolan, in addition to cements made from other natural or artificial pozzolans. In countries where volcanic ashes are available (e.g. Italy, Chile, Mexico, the Philippines) these cements are often the most common form in use.Portland Silica Fume Cement is produced by the addition of silica fume to cement, and exceptionally high strength substance. Cements containing 5–20% silica fume are occasionally produced. However, silica fume is more usually added to Portland cement at the concrete mixer.Masonry Cements are used for preparing bricklaying mortars and stuccos, and must not be used in concrete. They are usually complex proprietary formulations containing Portland clinker and a number of other ingredients that may include limestone, hydrated lime, air-entrainers, retarders, waterproofers and colouring agents. They are formulated to yield workable mortars that allow rapid and consistent masonry work. Subtle variations of Masonry cement in the US are Plastic Cements and Stucco Cements. These are designed to produce controlled bonds with masonry blocks.Expansive Cements contain, in addition to Portland clinker, expansive clinkers (usually sulfoaluminate clinkers) and are designed to offset the effects of drying shrinkage that is normally encountered with hydraulic cements. This allows large floor slabs (up to 60m2) to be prepared without contraction joints.

White blended cements may be made using white clinker and white supplementary materials such as high-purity metakaolin.Coloured cements are used for decorative purposes. Some standards allow the addition of pigments to produce ‘coloured Portland cement’. In other standards (e.g. ASTM), pigments are not allowed constituents of Portland cement, and coloured cements are sold as ‘blended hydraulic cements’.Very finely ground cements are made from mixtures of cement with sand or slag or other pozzolan type minerals, which are finely ground together. Such cements can have the same physical characteristics as normal cement but with 50% less cement, particularly due to their increased surface area for the chemical reaction. Even with intensive grinding they can use up to 50% less energy for fabrication than ordinary Portland cements.Non-Portland hydraulic cementsPozzolan-lime cements: Mixtures of ground pozzolan and lime were the cements used by the Romans, and are found in Roman structures still standing (e.g. the Pantheon in Rome). They develop strength slowly, but their ultimate strength can be very high. The hydration products that produce strength are essentially the same as those of Portland cement.Slag-lime cements: Ground granulated blast furnace slag is not hydraulic on its own, but is ‘activated’ by the addition of alkalis, most economically using lime. They are similar to pozzolan lime cements in their properties. Only granulated slag (i.e. water-quenched, glassy slag) is effective as a cement component.Supersulphated cements: These contain about 80% ground granulated blast furnace slag, 15% gypsum or anhydrite and small quantities of Portland clinker or lime as an activator. They produce strength by formation of ettringite, with strength growth similar to a slow Portland cement. They exhibit good resistance to aggressive agents, including sulphates.Calcium aluminate cements are hydraulic cements made primarily from limestone and bauxite. The active ingredients are monocalcium aluminate CaAl2O4 (CA in Cement chemist notation) and Mayenite Ca12Al14O33 (C12A7 in CCN). Strength forms by hydrating calcium aluminate hydrates. They are well adapted for use in refractory (high-temperature resistant) concretes, e.g. furnace linings.Calcium sulfoaluminate cements are made from clinkers that include ye'elimite (Ca4(AlO2)6SO4 or C4A3 in CCN) as a primary phase. They are used in expansive cements, in ultra-high early strength cements, and in ‘low-energy’ cements. Hydration produces ettringite, and specialised physical properties (such as expansion or rapid reaction) are obtained by adjustment of the availability of calcium and sulphate ions. Their use as a low-energy alternative to Portland cement has been pioneered in China, where several million tonnes per year are produced. Energy requirements are lower because of the lower kiln temperatures required for reaction and the lower amount of limestone (that has to be endothermically decarbonised) in the mix. In addition, the lower limestone content and lower fuel consumption leads to a CO2 emission around half that associated with Portland clinker. However, SO2 emissions are significantly higher.‘Natural’ Cements correspond to certain cements of the pre-Portland era, produced by burning argillaceous limestone at moderate temperatures. The level of clay components in the limestone (around 30–35%) is so that large amounts of belite (the low-early strength, high-late strength mineral in Portland cement) are formed without the formation of excessive amounts of free lime. As with any natural material, such cements have highly variable properties.Geopolymer cements are made from mixtures of water-soluble alkali metal silicates and aluminosilicate mineral powders such as fly ash and metakaolin. Q: How is Portland cement made?A: Materials that contain appropriate amounts of calcium compounds like silica, alumina and iron oxide are crushed, screened and placed in a rotating cement kiln. Ingredients used in this process are typically materials such as limestone, marl, shale, iron ore, clay and fly ash.The kiln resembles a large horizontal pipe with a diameter of 10–15ft (3–4.1m) and a length of 300ft (90m) or more. One end is raised slightly and the raw mix is placed in the high end; as the kiln rotates, the materials move slowly toward the lower end. Flame jets are at the lower end and all the materials in the kiln are heated to high temperatures that range between 2700 and 3000°F (1480 and 1650°C). This high heat drives off, or calcines, the chemically combined water and carbon dioxide from the raw materials and forms new compounds (tricalcium silicate, dicalcium silicate, tricalcium aluminate and tetracalcium aluminoferrite). For each ton of material that goes into the feed end of the kiln, two thirds of a ton of clinker comes out the discharge end. This clinker is in the form of marble sized pellets. The clinker is very finely ground to produce Portland cement. A small amount of gypsum is added during the grinding process to control the cement’s set or rate of hardening. 

Q: What is Fibre Reinforced Concrete? A: Low Fibre volume composite concrete contains less than 1% fibre. It is used for field applications involving large volumes of concrete. The fibres do not significantly increase the strength of the concrete. Low fibre volume concrete is used for paving roads. High Fibre Volume Concrete: Typically used for thin sheets with cement mortar mix. The fibre volume in this mix ranges from 5% to 15%. High Fibre Volume Composite: The fibre volume in this mix can be as high as 40%. This significantly increases the strength and toughness of the mix. The reinforcement in High Fibre Volume Composite concrete is usually in sheet form. This reinforced concrete type is used in roof and wall panels. Q: What is the difference between cement and concrete?A: Concrete should not be confused with cement because the term cement refers only to the dry powder substance used to bind the aggregate materials of concrete. Upon the addition of water and/or additives the cement mixture is referred to as concrete, especially if aggregates have been added. Q: What is concrete? A: Concrete is a mixture of cement, water, sand and gravel (stones, crushed rock). The mixture eventually hardens into a stone-like material. Cement and water are the two ingredients that chemically react; the gravel and sand give strength.  Q. How was concrete made in the earlier times?A: During the Roman Empire, Roman concrete (or Opus caementicium) was made from quicklime, pozzolanic ash/pozzolana and an aggregate of pumice. Its widespread use in many Roman structures, a key event in the history of architecture termed the Concrete Revolution, freed Roman construction from the restrictions of stone and brick material and allowed for revolutionary new designs both, in terms of structural complexity and dimension. Concrete, as the Romans knew it, was in effect a new and revolutionary material. Laid in the shape of arches, vaults and domes, it quickly hardened into a rigid mass, free from many of the internal thrusts and strains, which troubled the builders of similar structures in stone or brick. Q: How is modern structural concrete different from the earlier form of concrete?A: Modern structural concrete differs from Roman concrete in two important details. First, its mix consistency is fluid and homogeneous, allowing it to be poured into forms rather than requiring hand layering together with the placement of aggregate, which in Roman practice often consisted of rubble. Second, integral reinforcing steel gives modern concrete assemblies great tensile strength, whereas Roman concrete could depend only upon the strength of the concrete bonding to resist tension.

Q: What does ‘curing’ concrete mean? A: Curing is one of the most important steps in concrete construction, because proper curing greatly increases concrete strength and durability. Concrete hardens as a result of hydration: the chemical reaction between cement and water. However, hydration occurs only in the presence of water and if the concrete’s temperature stays within a suitable range. During the curing period-from, five to seven days after placement for conventional concrete, the concrete surface needs to be kept moist to permit the hydration process. New concrete can be wet with soaking hoses, sprinklers or covered with wet burlap, or can be coated with commercially available curing compounds, which seal in moisture.

Q: What is Reinforced concrete? A: Reinforced concrete contains steel reinforcing that is designed and placed in structural members at specific positions to cater for the stress conditions that the member is required to accommodate.

Q. What is Prestressed concrete? A: The principle behind Prestressed concrete is that compressive stresses induced by high-strength steel tendons in a concrete member before loads are applied will balance the tensile stresses imposed in the member during service. For example a horizontal beam will tend to sag down. However, if the reinforcement along the bottom of the beam is prestressed, it can counteract this.In pre-tensioned concrete, prestressing is achieved by using steel or polymer tendons or bars that are subjected to a tensile force prior to casting; and for post-tensioned concrete, after casting. Q. What are the sought after properties of concrete?A. 1. The concrete mix is extremely workable. It can be placed and consolidated properly. 

2. Desired qualities of the hardened concrete are met. For example, resistance to freezing and thawing and deicing chemicals, watertightness (low permeability), wear resistance and strength. 3. Economy. Since the quality depends mainly on the water to cement ratio, the water requirement should be minimised to reduce the cement requirement (and thus reduce the cost).The following steps reduce water and cement requirements:Use the stiffest mix possible Use the largest size aggregate practical for the jobUse the optimum ratio of fine to coarse aggregate Q: What is the composition of Concrete A: 11% Cement (usually Portland) 16% Water 6% Air 26% Sand 41% Gravel or crushed stoneQ: Descriptive composition of Concrete.A: There are many types of concrete available, created by varying the proportions of its main ingredients.The mix design depends on the type of structure being built, how the concrete will be mixed, delivered and how it will be placed to form the structure.

CementPortland cement is the most widely used cement. It is the basic ingredient in concrete, mortar and plaster. English engineer, Joseph Aspdin patented Portland cement in 1824; it was named because of its similar colour to Portland limestone, quarried from the Isle of Portland and used extensively in London architecture. It consists of a mixture of oxides of calcium, silicon and aluminium and is manufactured by heating limestone (source of calcium) and clay, then grinding this product (clinker) with a source of sulphate (most commonly gypsum). The manufacturing of Portland cement creates about 5% of human CO2 emissions.

WaterCombining water with a cementitious material forms a cement paste by the process of hydration. The cement paste glues the aggregate together, fills voids within it and allows it to flow more easily.Lower amounts of water in the cement paste will yield a stronger, more durable concrete; more water will give an easier-flowing concrete with a higher slump.Impure water used to make concrete can cause problems when setting or premature failure of the structure.Hydration involves many different reactions, often occurring at the same time. As the reactions proceed, the products of the cement hydration process gradually bind the individual sand and gravel particles with other components of the concrete to form a solid mass.

ReactionCement chemist notation: C3S + H2O → CSH(gel) + CaOH Standard notation: Ca3SiO5 + H2O → (CaO)•(SiO2)•(H2O)(gel) + Ca(OH)2 Balanced: 2Ca3SiO5 + 7H2O → 3(CaO)•2(SiO2)•4(H2O)(gel) + 3Ca(OH)2

AggregatesFine and coarse aggregates make up the bulk of a concrete mixture. Sand, natural gravel and crushed stone are mainly used for this purpose. Recycled aggregates (from construction, demolition and excavation waste) are increasingly used as partial replacements of natural aggregates, while a number of manufactured aggregates, including air-cooled blast furnace slag and bottom ash are also permitted.Decorative stones such as quartzite, small river stones or crushed glass are sometimes added to the surface of concrete for a decorative ‘exposed aggregate’ finish, popular among landscape designers.

ReinforcementConcrete is strong in compression, as the aggregate efficiently carries the compression load. However, it is weak in tension as the cement holding the aggregate in place can crack, allowing the structure to fail. Reinforced concrete solves these problems by adding either metal reinforcing bars, glass fibre or plastic fibre to carry tensile loads.

Chemical admixturesChemical admixtures are materials in the form of powder or fluids that are added to the concrete to give it certain characteristics not obtainable with plain concrete mixes. In normal use, admixture dosages are less than 5% by mass of cement, added to the concrete at the time of batching/mixing.

Mineral admixtures and blended cementsThere are inorganic materials that also have pozzolanic or latent hydraulic properties. These very fine-grained materials are added to the concrete mix to improve the properties of concrete (mineral admixtures) or as a replacement to Portland cement (blended cements).A by-product of coal fired electric generating plants, Fly ash is used to partially replace Portland cement (up to 60% by mass). The properties of fly ash depend on the type of coal burnt. In general, silicious fly ash is pozzolanic, while calcareous fly ash has latent hydraulic properties.

Ground granulated blast furnace slag (GGBFS or GGBS), a by-product of steel production, is used to partially replace Portland cement (up to 80% by mass). It has latent hydraulic properties.

Silica fume is one of the by-products of the production of silicon and ferrosilicon alloys. Silica fume is similar to fly ash, but has a particle size 100 times smaller. This results in a higher surface to volume ratio and a much faster pozzolanic reaction. Silica fume is used to increase strength and durability of concrete, but generally requires the use of superplasticisers for workability.

High Reactivity Metakaolin (HRM): Metakaolin produces concrete with strength and durability similar to concrete made with silica fume. While silica fume is usually dark grey or black in colour, high reactivity metakaolin is usually bright white, making it the preferred choice for architectural concrete where appearance is important.

Q: What is the moisture content of concrete? A: The moisture content of concrete is viewed from the context of total water content of the fresh concrete mixture and the available moisture content of the hardened concrete. The total water content of a fresh concrete mixture is a function of the total cementitious materials and water cement ratio (w/cm). Typical fresh concrete mixtures vary in cementitious material content in a range of 279 kg/m3 to 415 kg/m3 (470 lb/yd3 to 700 lb/yd3). Water cement ratios typically vary from 0.4 to 0.55. To estimate the available moisture content of hardened concrete one must begin with the total water content of the fresh mixture and define the service condition of the hardened concrete with regard to relative humidity (%). In addition, the water that is chemically bound with the cement in the hydration process must be accounted for. The water bound with the cement is in the range of 0.22 to 0.24 of the cement content. 

As an example, the moisture content of a concrete mixture with 334 kg/m3 (564 lb/yd3) of cement and a w/c of 0.45 and in a service environment with a 50% relative humidity could be estimated as follows:

Total water content: 334kg cement/m3 times 0.45 w/c ~ 150kg water/m3 (564lb cement/yd3 times 0.45 w/c ~ 254lb water/yd3)Chemically bound water at 0.24 w/c:334kg cement/m3 times 0.24 w/c ~ 80kg water/m3(564lb cement/yd3 times 0.24 ~ 135lb water/yd3)Moisture content: 150kg water/m3 – 80kg water/m3times .50 relative humidity ~ 35kg water/m3 (254lb water/yd3 – 135lb water/yd3 times .50 relative humidity ~ 60lb water/yd3)In reality, the relative humidity of the concrete will only reach 50% at the near surface of the concrete and the moisture gradient with depth will increase toward 100% relative humidity; hence, this method of estimation would typically overstate the quantity of moisture available to leave the concrete due to the initial mixing of water. This is only an estimate of the moisture available to leave the concrete, but it may help in gaining a perspective to the limited amount of water that the concrete can contribute when considering the drying time of hardened concrete.

Relative Humidity Profile 

Q: When was concrete first made?A: 500BC

Q: What is the purpose of cement in concrete?A: It acts as a primary binder that joins the aggregate into a solid mass.

Q: Why does concrete harden?A: The chemical process called cement hydration produces crystals that interlock and bind together.

Q: How strong can concrete or cement be (in pounds per square inch (psi))?A: 50,000 Q: How long can concrete last (in years)?A: 50,000 Q: What are Type I/II or Type II/V cements?A: Type I/II and Type II/V cements simply means that the cement complies with the requirements of ASTM C 150, Standard Specification for Portland Cement. It is quite common to find cements that comply with multiple cement designations such as Type I/II and Type II/V. Q: How is white cement different and why is it used in decorative concrete? A: There are only slight chemical and physical differences between grey Portland cement and white Portland cement. This is due to raw material differences and sometimes, though not always, slight differences in manufacturing. White cement has small amounts of the oxides (particularly iron and manganese) that impart the greyish colour normally associated with Portland cement. Q. What are the decorative finishes that can be applied to concrete surfaces? A: Adding pigment before or after the concrete is placed and using white cement rather than conventional grey cement, using chemical stains or exposing colourful aggregates at the surface may add colour to concrete. Textured finishes can vary from a smooth polish to the roughness of gravel.Geometric patterns can be scored, stamped, rolled, or inlaid into the concrete to resemble stone, brick or tile paving. Other interesting patterns are obtained by using divider strips (commonly redwood) to form panels of various sizes and shapes rectangular, square, circular or diamond.Special techniques are available to make concrete slip-resistant and sparkling.

Q: What are the different forms of sulphate in Portland cement and how can we analyse cement for SO3?A: Sulphates in Portland cement can be broadly categorised as:1.  Added sulphates – gypsum, hemihydrates, anhydrite, several synthetic forms of sulphates (typically by-products like flue gas desulphurisation materials). Clinker sulphates include arcanite, aphthitalite, calcium langbeinite and thenardite. Although normally reported as SO3 (% by mass) for consistency, sulphur can be found in any combination of forms. Elemental sulphur is almost never found in Portland cement, except in trace amounts.Added sulphates are blended with clinker during the final grinding of the cement, in amounts needed to control early setting properties as well as shrinkage and strength development. The amount needed varies depending on the chemistry and fineness of the cement, but is typically on the order of 5% by mass. The most common form of sulphate added to Portland cement is gypsum, some of which is intentionally dehydrated by the heat of grinding to form hemihydrates, which are more soluble and therefore available to control early hydration reactions.Clinker sulphates form naturally during clinker production. These sulphates tend to volatilise at the temperatures of cement kilns (up to about 1450ºC) and condense on the outer surface of clinker nodules as alkali sulphates, during the last stage of clinker production (rapid cooling). Again, the amount depends on the chemistry of the raw materials and kiln operating conditions, making the cement somewhat unique. These alkali sulphates also are soluble enough to help control early hydration reactions. Some clinker sulphate is also incorporated into other cement phases.

Since cement is unique, chemical analyses are the best method of determining the SO3 content of cements. Typically the total SO3 content is measured (or elemental S measured and converted to SO3) through methods in ASTM C 114 (or AASHTO T 105). XRF analysis is probably the most common technique. 

Q: What is air-entrained concrete? A: Air-entrained concrete contains billions of microscopic air cells per cubic foot. These air pockets relieve the internal pressure on the concrete by providing tiny chambers for water to expand into when it freezes. Air-entrained concrete is produced through the use of air-entraining Portland cement, or by the introduction of air-entraining agents, under careful engineering supervision. The amount of entrained air is usually between 4% and 7% of the volume of the concrete, but may be varied as required by special conditions. Q: What are recommended mix proportions for good concrete? A: Good concrete can be obtained by using a wide variety of mix proportions if proper mix design procedures are used. The general custom is the rule of 6’s: A minimum cement content of 6 bags per cubic yard of concrete A maximum water content of 6 gallons per bag of cement A curing period (keeping concrete moist) a minimum of 6 days An air content of 6% (if concrete will be subject to freezing and thawing) Q: Will concrete harden under water? A: Portland cement is a hydraulic cement, which means that it sets and hardens due to a chemical reaction with water. Consequently, it will harden under water. Q: What does 28 -day strength mean? A: Concrete hardens and gains strength as it hydrates. The hydration process continues over a long period of time; beginning rapidly and progressively slowing down. To measure the ultimate strength of concrete would require a wait of several years. This would be impractical, so a time period of 28 days was selected, by specification writing authorities, as the age that all concrete should be tested. At this age, a substantial percentage of the hydration has taken place. Q: What is 3,000 pound concrete? A: Concrete that is strong enough to carry a compressive stress of 3,000psi (20.7MPa) at 28 days is 3,000 pound concrete. Concrete may be specified at other strengths as well. Conventional concrete has strengths of 7,000psi or less; concrete with strengths between 7,000 and 14,500psi is considered high-strength concrete. Q: How do you control the strength of concrete? A: The easiest way to add strength is to add cement. The factor that most predominantly influences concrete strength is the water to cement ratio in the cement paste that binds the aggregates together. The higher this ratio is, the weaker the concrete will be and vice versa. Every desirable physical property will be adversely affected by adding more water. Q: What is alkali-silica reactivity (ASR)? A: Alkali-silica reactivity is an expansive reaction between reactive forms of silica in aggregates and potassium and sodium alkalis, mostly from cement, but also from aggregates, pozzolans, admixtures and mixing water. External sources of alkali from soil, deicers and industrial processes can also contribute to ASR. The reaction forms an alkali-silica gel that swells as it draws water from the surrounding cement paste, thereby inducing pressure, expansion and cracking of the aggregate and surrounding paste. This often results in map-pattern cracks, sometimes referred to as alligator pattern cracking. ASR can be avoided through

Proper aggregate selection Use of blended cements Use of proper pozzolanic materials Contaminant-free mixing water

 Q. What are Supplementary Cementations Materials (SCM)? A: Supplementary Cementations Materials (SCM) like silica fumes, meta-kaolin, fly ash, slag are the substances which improve the properties of concrete and enhance its durability, by reducing pore size in concrete through better particle distribution and through increased packing density of the concrete. Q: Are there different types of Portland cement? A: Though all Portland cement is basically the same, eight types of cement are manufactured to meet

different physical and chemical requirements for specific applications: Type I is a general purpose Portland cement suitable for most uses. Type II is used for structures in water or soil containing moderate amounts of sulphate, or when heat build-up is a concern. Type III cement provides high strength at an early state, usually in a week or less. Type IV moderates heat generated by hydration that is used for massive concrete structures such as dams. Type V cement resists chemical attacks by soil and water high in sulphates. Types IA, IIA and IIIA are cements used to make air-entrained concrete. They have the same properties as types I, II and III, except that they have small quantities of air-entrained materials combined with them. White Portland cement is made from raw materials containing little or no iron or manganese. Q. Is there any shelf life of cement? A: Cement is a hygroscopic material, meaning that in presence of moisture it undergoes chemical reaction termed as hydration. Therefore cement remains in good condition as long as it does not come in contact with moisture. If cement is more than three months old then it should be tested for its strength before being employed. Q. How fineness of cement affects strength gain? A: Finer cement particles imply more particles in unit weight. This enhances the reaction rate, which in turn will result in faster gain of strength at earlier stages. Q: Why do concrete surfaces flake and spall? A: Concrete surfaces can flake or spall for one or more of the following reasons:In areas subjected to freezing and thawing, the concrete should be air-entrained to resist flaking and scaling of the surface. If air-entrained concrete is not used, there will be subsequent damage to the surface.The water/cement ratio should be as low as possible to improve durability of the surface. Too much water in the mix will produce a weaker, less durable concrete, in turn leading to early flaking and spalling of the surface.The finishing operations should not begin until the water sheen on the surface is gone and excess bleed water on the surface has had a chance to evaporate. If this excess water is worked into the concrete because the finishing operations are begun too soon, the concrete on the surface will have too high a water content and will be weaker and less durable  Q: How do you remove stains from concrete? A: Stains can be removed from concrete with dry or mechanical methods, or by wet methods using chemicals or water.Common dry methods include sandblasting, flame cleaning, shotblasting, grinding, scabbing, planning and scouring. Steel-wire brushes should be used with care because they can leave metal particles on the surface that later rust and stain the concrete.Wet methods involve the application of water or specific chemicals according to the nature of the stain. The chemical treatment either dissolves the staining substance so it can be blotted up from the surface of the concrete or bleaches the staining substance so it will not show.To remove bloodstains, for example, wet the stains with water and cover them with a layer of sodium peroxide powder. Let stand for a few minutes, rinse with water and scrub vigorously. Follow with an application of a 5% solution of vinegar to neutralise any remaining sodium peroxide. Q: What is Self-Consolidating concrete (SCC)? A: SCC is a high-performance concrete that can flow easily into tight and constricted spaces without segregating and without requiring vibration. The key to creating SCC, also referred to as self-compacting, self-levelling, or self-placing concrete, is a mixture that is fluid, but also stable to prevent segregation. To achieve the desired flowability a new generation of superplasticisers based on polycarboxylate ethers works best. Developed in the 1990s, they produce better water reduction and slower slump loss than traditional superplasticisers. The required level of fluidity is greatly influenced by the particular application under consideration. Obviously the most congested structural members demand the highest fluidity. However, element shape, desired surface finish, and travel distance can also determine the required fluidity. Generally, the higher the required flowability of the SCC mix, the higher the amount of fine material

needed to produce a stable mixture. However, in some cases, a viscosity-modifying admixture (VMA) can be used instead of, or in combination with, an increased fine content to stabilize the concrete mixture. Q: The size of concrete cube is 150mm x 150mm x 150mm as per Indian Standards. Why? A: Because the shape effect is the least for the 15cm cube and we get a fairly accurate idea of the strength of the concrete as such. Q: How do you protect a concrete surface from aggressive materials like acids? A: Many materials have no effect on concrete. However, there are some aggressive materials, such as most acids, that can have a deteriorating effect on concrete. The first line of defence against chemical attack is to use quality concrete with maximum chemical resistance, followed by the application of protective treatments to keep corrosive substances from contacting the concrete. Principles and practices that improve the chemical resistance of concrete include using a low water-cement ratio, selecting a suitable cement type (such as sulphate-resistant cement to prevent sulphate attacks), using suitable aggregates, water- and air-entrainment. A large number of chemical formulations are available as sealers and coatings to protect concrete from a variety of environments; detailed recommendations should be requested from manufacturers, formulators or material suppliers.  Q: Why does concrete crack?A;Concrete, by nature, shrinks as it hardens. When concrete is placed on supporting soil or around steel reinforcement, the concrete mass is prevented from shrinking. This restraint creates internal forces exceeding the strength of concrete; cracks form to relieve these forces. Q: Does the presence of cracks indicate a structural problem?A: In most instances, the answer is no. Very narrow ‘hairline’ cracks are aesthetic in nature and do not indicate any structural problem. Cracks that have movement, i.e. where one side of the crack moves relative to the opposite side, should be investigated by a professional engineer. Q: Why does concrete harden?A: Concrete solidifies and hardens after mixing with water and placement due to a chemical process known as hydration. The water reacts with the cement, which bonds the other components together, eventually creating a stone-like material. Q: What is concrete used for?A: Concrete is used to make pavements, pipe, architectural structures, foundations, motorways/roads, bridges/overpasses, parking structures, brick/block walls and footings for gates, fences and poles.Concrete is used more than any other man-made material in the world. As of 2006, about 7.5km3 of concrete is made each year—more than 1m3 for every person on earth. Q: What are the more popular types of concrete in use?A: Reinforced concrete and prestressed concrete are the most widely used modern kinds of functional concrete extensions. Q: What evidence is there for the long life of concrete? A: The widespread use of concrete in many Roman structures has ensured that many of them have survived. The Baths of Caracalla is just one example of the longevity of concrete, which allowed the Romans to build this and similar structures across their Empire. Many Roman aqueducts and Roman bridges have masonry cladding to a concrete core, a technique they used in structures such as the Pantheon, the dome of which is concrete. Q: Who discovered concrete?A: The Romans used concrete in their structures but the secret had been lost for 13 centuries until 1756, when the British engineer John Smeaton pioneered the use of hydraulic lime in concrete, using pebbles and powdered brick as aggregate. Portland cement was first used in concrete in the early 1840s. This version of history has been challenged however, as the Canal du Midi was constructed using concrete in 1670.  Q: What is the role of water in concrete mix? A: Combining water with cementitious material forms a cement paste by the process of hydration. The

cement paste glues the aggregate together, fills voids within it and allows it to flow more easily.Less water in the cement paste will yield a stronger, more durable concrete; more water will give an easier-flowing concrete with a higher slump. Impure water used to make concrete can cause problems when setting or in causing premature failure of the structure.Hydration involves many different reactions, often occurring at the same time. As the reactions proceed, the products of the cement hydration process gradually bind the individual sand and gravel particles with other components of the concrete, to form a solid mass. Q: How do aggregates affect the strength of concrete? A: Concrete has a high compressive strength, as the aggregate efficiently carries the compression load. However, it is weak in tension as the cement holding the aggregate in place can crack, allowing the structure to fail. Reinforced concrete solves these problems by adding either metal reinforcing bars, steel fibres, glass fibre or plastic fibre to carry tensile loads. Q. What are the reasons for slow or fast setting of concrete or mortar?A: the rate of setting normally depends on the nature of the cement. It could also be due to extraneous factors not related to the cement. Ambient conditions also play an important role. In hot weather, concrete sets faster, whereas in cold weather, setting is delayed. Some salts, chemicals, clay etc., if inadvertently mixed with the sand, aggregate and water could accelerate or delay the setting of concrete. Q: What do grade numbers indicate? A: The grade number indicates the minimum compressive strength of cement sand mortar in N/mm2 at 28 days. Q. What is slag?A: Slag is a non-metallic product, essentially consisting glass containing silicates, alumino-silicates of lime and other bases, and is obtained as a by-product in the manufacture of pig iron in blast or electric furnaces. Granulated slag is used in the manufacture of Portland Slag Cement (PSC). Q. How is PSC made?A: PSC is made by intergrading clinker, granulated blast furnace slag and gypsum or by blending ground slag with Portland cement.

Q. Where can PSC be used?A: Slag cement can be used for all plain and reinforced concrete constructions and mass concreting structures such as dams, reservoirs, swimming pools, river embankments, bridge piers etc. It is used with advantage where low heat of hydration and resistance to alkali-silica reactions are desired; for structures in aggressive environments where chemical and mildly acidic waters are encountered (where the use of OPC is not recommended) and for marine constructions, dykes, wharves, etc where sulphuric water is encountered. In short, PSC can be used wherever OPC is used.http://www.foundationsakc.com/process/basics

The Importance of Sand in ConcreteBy Tim Plaehn, eHow Contributor

Sand is a major component of concrete and without the sand, concrete will not function as intended. The properties of a specific concrete mix will be determined by the proportion and type of sand used to formulate the concrete. Sand is usually a larger component of the mix than cement.

1. Components of Concrete

o The major components of concrete are cement (typically Portland cement);

sand; gravel or stones; and water. The larger stones and gravel are called

coarse aggregate and the sand is referred to as fine aggregate. Air may also

be considered a component of concrete. Air bubbles are formed in finished

concrete through the addition of special additives to the mixture. Sand

typically makes up about 25 percent of a wet concrete mixture.

Function of Aggregate

o The total aggregate in a concrete mixture comprises up to three-quarters of

the mixture; the Portland cement is approximately 10 to 15 percent. The

coarse aggregate is the volume of finished concrete, and the sand fills in the

spaces between the larger stones of the coarse aggregate. The cement in its

wet form should coat the individual pieces of aggregate and as the cement

dries and hardens, it locks the gravel and sand into a matrix of small-to-

larger aggregate pieces, giving strength to the concrete.

o

Properties of Sand

o Sand for concrete can be classified as soft or sharp sand. Soft sand has a

smooth surface on the individual granules. It is natural sand formed by

erosion factors such as water movement on a beach. Sharp sand granules

have a rough surface. This sand is the result of manufacturing by crushing

larger forms of aggregate. The most important factor concerning sand used

in concrete is that it must be clean sand. Impurities in the sand such as silt

or organic matter will weaken the final hardened concrete.

Technical

o The aggregate--including the sand--used in large commercial or government

projects such as road building must meet rigid standards for size and

cleanliness. Government inspectors will take core samples of the concrete to

determine if the sand and other aggregate is of the proper size and

composition. The strongest concrete comes from aggregate labeled as well-

graded. This means the sand and coarse aggregate mixture is composed of

granules and gravel of different sizes so the concrete mixture has uniform

voids between the aggregate particles. The voids fill with cement to give the

concrete a uniform structure.