How to assess acceptability of concrete structure based on cube strength

1) Clause 9.2.2 of IS 456 – 2000 :
a) “The mix shall be designed to produce the grade of concrete having the required workability and a characteristic compressive strength not less than appropriate values given in table 2. The target mean strength of concrete mix should be equal to the characteristic strength plus 1.65 times the standard deviation.”
b) Above clearly explains that the mix designed for use when made in lab condition should be able to give a strength of characteristic strength plus 1.65 times the standard deviation. Based on binomial theorem the meaning is that atleast 5% can still fall below the characteristic strength.

2) Clause 15.1.1, last line :
a) “In all cases, the 28 days compressive strength specified in table 2 shall alone be the criterion for acceptance or rejection of the concrete.”
b) Above clearly explains that characteristic strength is the limit for acceptance / rejection / review.

3) Different cases as guideline for interpretation :
a) Step 1 :
i) Check if the variance in cube results exceed 15% as given in clause 15.4. of IS 456 – 2000.
ii) If yes, remove that set of data from further statistical analysis and declare it as faulty sample preparation. Corrective action will be to tighten the sampling and moulding procedure as well as check the mould dimensions.
iii) If no, move to next step given below.
b) Step 2 :
i) If 28 days cube strength (average of 3 samples) is equal to or above characteristic strength
(1) Check if any of the individual value in the three results taken for averaging falls below Characteristic strength minus 4 Mpa.
(a) If yes, concrete is accepted. Corrective action will be to tighten the sampling and moulding procedure as well as check the mould dimensions.
(b) If no, concrete is accepted. It is wrong to say that they should have strength of Characteristic strength plus 4 Mpa.
c) Step 3 :
i) If 28 days cube strength (average of 3 samples) is below characteristic strength,
(1) Check if any of the individual value in the three results taken for averaging falls below Characteristic strength minus 4 Mpa.
(a) If yes, the structure has to be evaluated using NDT equipment.
(b) If no, move to the next step below,
(2) Check if for the same concrete proportion and similar material used earlier from the same plant, the mean of atleast four consecutive results at any time earlier from your plant cube test data (average of three cubes) is atleast Characteristic strength plus 4 Mpa.
(a) If yes, the mix has proved itself that it is capable of giving a higher strength under similar conditions and hence it is a problem with the material used or any batching error.
(b) If no, the concrete is bad and structure needs to be reviewed for the lower strength. The mix has not been designed for the bad degree of control in material quality and batching. The assumed standard deviation during initial design stage has to be increased so that the mix is capable of giving a higher strength with the existing material and plant

Effects of Alloying Elements in Steel

Effects of Alloying Elements in Steel
Steel is basically iron alloyed to carbon with certain additional elements to give the required properties to the finished melt. Listed below is a summary of the effects various alloying elements in steel.
• Carbon
• Manganese
• Chromium
• Nickel
• Molybdenum
• Titanium
• Phosphorus
• Sulphur
• Selenium
• Niobium
• Nitrogen
• Silicon
• Cobalt
• Tantalum
• Copper
Carbon
The basic metal, iron, is alloyed with carbon to make steel and has the effect of increasing the hardness and strength by heat treatment but the addition of carbon enables a wide range of hardness and strength.
Manganese
Manganese is added to steel to improve hot working properties and increase strength, toughness and hardenability. Manganese, like nickel, is an austenite forming element and has been used as a substitute for nickel in the A.I.S.I 200 Series of Austenitic stainless steels (e.g. A.I.S.I 202 as a substitute for A.I.S.I 304)
Chromium
Chromium is added to the steel to increase resistance to oxidation. This resistance increases as more chromium is added. 'Stainless Steel' has approximately 11% chromium and a very marked degree of general corrosion resistance when compared with steels with a lower percentage of chromium. When added to low alloy steels, chromium can increase the response to heat treatment, thus improving hardenability and strength.
Nickel
Nickel is added in large amounts, over about 8%, to high chromium stainless steel to form the most important class of corrosion and heat resistant steels. These are the austenitic stainless steels, typified by 18-8, where the tendency of nickel to form austenite is responsible for a great toughness and high strength at both high and low temperatures. Nickel also improves resistance to oxidation and corrosion. It increases toughness at low temperatures when added in smaller amounts to alloy steels.
Molybdenum
Molybdenum, when added to chromium-nickel austenitic steels, improves resistance to pitting corrosion especially by chlorides and sulphur chemicals. When added to low alloy steels, molybdenum improves high temperature strengths and hardness. When added to chromium steels it greatly diminishes the tendency of steels to decay in service or in heat treatment.
Titanium
The main use of titanium as an alloying element in steel is for carbide stabilisation. It combines with carbon to for titanium carbides, which are quite stable and hard to dissolve in steel, this tends to minimise the occurrence of inter-granular corrosion, as with A.I.S.I 321, when adding approximately 0.25%/0.60% titanium, the carbon combines with the titanium in preference to chromium, preventing a tie-up of corrosion resisting chromium as inter-granular carbides and the accompanying loss of corrosion resistance at the grain boundaries.
Phosphorus
Phosphorus is usually added with sulphur to improve machinability in low alloy steels, phosphorus, in small amounts, aids strength and corrosion resistance. Experimental work shows that phosphorus present in austenitic stainless steels increases strength. Phosphorus additions are known to increase the tendency to cracking during welding.
Sulphur
When added in small amounts sulphur improves machinability but does not cause hot shortness. Hot shortness is reduced by the addition of manganese, which combines with the sulphur to form manganese sulphide. As manganese sulphide has a higher melting point than iron sulphide, which would form if manganese were not present, the weak spots at the grain boundaries are greatly reduced during hot working.
Selenium
Selenium is added to improve machinability.
Niobium (Columbium)
Niobium is added to steel in order to stabilise carbon, and as such performs in the same way as described for titanium. Niobium also has the effect of strengthening steels and alloys for high temperature service.
Nitrogen
Nitrogen has the effect of increasing the austenitic stability of stainless steels and is, as in the case of nickel, an austenite forming element. Yield strength is greatly improved when nitrogen is added to austenitic stainless steels.
Silicon
Silicon is used as a deoxidising (killing) agent in the melting of steel, as a result, most steels contain a small percentage of silicon. Silicon contributes to hardening of the ferritic phase in steels and for this reason silicon killed steels are somewhat harder and stiffer than aluminium killed steels.
Cobalt
Cobalt becomes highly radioactive when exposed to the intense radiation of nuclear reactors, and as a result, any stainless steel that is in nuclear service will have a cobalt restriction, usually aproximately 0.2% maximum. This problem is emphasised because there is residual cobalt content in the nickel used in producing these steels.
Tantalum
Chemically similar to niobium and has similar effects.
Copper
Copper is normally present in stainless steels as a residual element. However it is added to a few alloys to produce precipitation hardening properties.

Effect of Phosphorus in carbon steel

Effect of Phosphorus on the Properties of Carbon Steels

Phosphorus in steel can have beneficial as well as harmful effects. Phosphorus is one of the most potent solid-solution strengtheners of ferrite. The addition of only 0.17% phosphorus increases both the yield and tensile strength of low-carbon sheet steel by about 62 MPa (9 ksi) while also improving the bake hardening response and deep drawability. Because of these properties, rephosphorized high-strength steels are widely used for cold-forming applications. Phosphorus is also used as an additive in steels to improve machining characteristics and atmospheric corrosion resistance

Surface Dressing failures on roads - Fatting up - Causes

SURFACE DRESSING  FAILURE - "FATTING UP" - CAUSES

PURPOSE

The object of surface dressing is to create a stable mosaic of chippings securely attached to the road surface, this provides a comprehensive seal to prevent the ingress of water in to the road pavement, and a fresh hard wearing, well textured, skid resistant surface,This is achieved by spraying the correct amount of bitumen onto the road surface followed by the appropriate amount of the correct size of chippings according to the softness of the road surface.

POSSIBLE CAUSES OF "FATTING UP" Wrong assessment of road hardness Too high a rate of spread of bitumen, and there can be a number of reason for this other than poor design Incorrectly calibrated "fifth wheel" or speedometer of spraying vehicle Incorrect speed chart for binder sprayer, Incorrect tank pressure Change in the base viscosity of the binder, Change in the spraying viscosity of the binder Using a smaller chipping than the road hardness testing indicated Roads where surface dressing as been the only maintenance treatment for many years. Roads where the underlying bituminous material has a low viscosity (i.e. high penetration grade or cut-back binder, this can often show itself in patches where a bituminous mixture with a cutback binder has been used for ease of laying by hand.

Tacoma Narrows Bridge

Tacoma Narrows Bridge

The Tacoma Narrows Bridge was the first suspension bridge across the Narrows of Puget Sound, connecting the Olympic Peninsula with the mainland of Washington, and a landmark failure in engineering history. Four months after its opening, on the morning of Nov. 7, 1940, in a wind of about 42 miles (68 km) per hour, the 2,800-foot (853-meter) main span went into a series of torsional oscillations the amplitude of which steadily increased until the convolutions tore several suspenders loose, and the span broke up. The bridge was designed to have acceptable horizontal displacement under the static pressure of a much larger wind, but was not designed to handle the dynamic instability caused by an interaction of the winds and the high degree of flexibility of the light, narrow, two-lane bridge. Modeling this type of fluid/structure interaction, a particularly simple type of flutter, was within the technical capability of engineers at the time, but was evidently not considered. A modern analysis would likely view the fluid/structure flutter as a bifurcation problem, and analyze the nature of the bifurcation as the wind speed increased. Immediately after the accident, numerous investigators were able to create both simple mathematical and scale physical models that exhibited the same failure as the actual bridge, and very simple models were able to predict the wind speed that would cause the collapse.

HVFAC advantages

Corrosion of reinforcements has been one of the major challenges that the civil engineers have been facing. Corrosion leads to the formation of rust which results in the spalling of concrete which in turn leads to the exposure of rebars to the aggressive environment. This will accelerate the ill effects and ultimately leads to the break down of the structure. Corrosion mainly occurs in areas of aggressive environment such as coastal regions. It is very important that corrosion of reinforcement must be prevented in order to have a durable structure. Even though there are many methods to prevent corrosion, most of them are uneconomical and requires great skill. Some of the recent studies in various parts of the world have revealed that High Volume Fly Ash (HVFA) concrete can protect the steel reinforcement more efficiently, so that it can resist corrosion, and thus the structure as a whole. HVFA concrete is a type of concrete in which a part of the cement is replaced by fly ash, which is an industrial waste. Thus the implementation of H V F A concrete can minimize corrosion in an effective way. Moreover it can lead to much durable structure without considerable increase in cost.

Third generation admixtures / poly carboxylic ether

Chemistry and mechanism of Polycarboxylic ether admixture action

It is a new, unique mechanism of action that greatly improves the effectiveness of cement dispersion. Traditional superplasticisers based on melamine and naphthalene sulphonates are polymers which are absorbed by the cement granules.They wrap around the granules' surface areas at the very early stage of the concrete mixing process. The sulphonic groups of the polymer chains increase the negative charge of the cement particle surface and disperse these particles by electrical repulsion. This electrostatic mechanism causes the cement paste to disperse and has the positive consequence of requiring less mixing water to obtain a given concrete workability.

It consists of a carboxylic ether polymer with long side chains. At the beginning of the mixing process it initiates the same electrostatic dispersion mechanism as the traditional superplasticisers, but the side chains linked to the polymer backbone generates a steric hindrance which greatly stabilize the cement particles' ability to separate and disperse. Steric hindrance provides a physical barrier (alongside the electrostatic barrier) between the cement grains. With this process, flowable concrete with greatly reduced water content

is obtained.