Question

Explain why a high-carbon steel would not be suitable for reinforcement for concrete.

Suggest why lightweight premixed gypsum plasters have become very popular in recent years.

Why mortar used for clay bricks should not be stronger than necessary? Explain

List: Granite, Limestone, Sandstone, Travertine, and Marble according to their acid resistance (from best to worst).

.In order to design a timber sturcture, what kind of stress values do we need to know? Name 5 of them.

Is bleeding of fresh concrete a kind of segregation? Why?

What is the aim of aggregate sieve analysis test?

Explain underwater concreting from start to finish briefly.

Why alkali-silica reactions are harmful for concrete?

Suppose that you have “OPC, Rapid Hardening PC, High Alumina Cement, White PC, Portland Blast Furnace Slag Cement” at your construction site. Which type (only one) of these would you prefer to use for the conditions given below? (5%) a. Concreting very close to Golden Sand Beach, Karpaz Peninsula, Cyprus: b. Reinforced-concrete column: c. Concreting in Cyprus at an air temperature above 35 degrees Celcius: d. Reinforced concrete water tank construction: e. Reinforced-concrete slab at an air temperature of 2 degrees Celcius:

Put “T” for true and “F” for false statements below. (5%) a. Plaster of Paris is obtained by calcination of gypsum rock at 190 degrees Celsius. b. Slaking of lime is obtained to remove air and water completely from limestone. c. Mixture of uniform size aggregates causes friction at many points. d. Workability of fresh concrete can be measured indirectly by slump test. e. Sea water attack is an internal factor affecting durability of concrete

Answer 1

Explain why a high-carbon steel would not be suitable for reinforcement for concrete.

High carbon steel over traditional steel. Because it’s so strong, carbon steel is difficult to work with. It can’t be easily bent and molded into different shapes, thus limiting its utility in certain applications. Carbon steel is also more susceptible to rust and corrosion than other types to steel. So its not used in reinforcement concrete works.

Suggest why lightweight premixed gypsum plasters have become very popular in recent years.

Premixed lightweight plasters essentially consists of gypsum plaster and lightweight aggregate which are characterized by low density, high thermal insulation and sound absorption properties and can be readily used for building purposes.

Salient Features

• Premixed light weight plaster
• Easy workability : Easy to mix - Easy to apply
• Free from inherent shrinkage cracks
• Classified as non-combustible
• Low Thermal Conductivity
• Light Weight - Better strenght to weight ratio as compared to density
• Low density, Better sound absorption, Better paint coverage

Why mortar used for clay bricks should not be stronger than necessary?

The cost and quality of masonry work is significantly affected by the mortar used. Mortars may account for as little as 7% of the volume of the walls, but the role it plays and the influence it has on performance are far greater than the proportion indicates. Mortar provides a bed for laying; bond units together to give compressive and flexural strength and seals joints against rain penetration.

Explain Granite, Limestone, Sandstone, Travertine, and Marble according to their acid resistance (from best to worst).

GRANITE

Granite is the most durable of the stone surfaces, which makes it the best choice for a worry-free countertop. It is extremely durable, stain resistant and easy to care for. It is chip acid resistant and is unlikely to crack or scratch during normal use.Granite is not nearly as sensitive to the acid

MARBLE

Marble stone is relatively softer than other stones and offers less stain resistance. Thus it requires careful maintenance and should be treated as fine furniture. Marble is susceptible to damage from citric acids, alcohols and oils. Spills should be wiped up immediately. Treated properly, marble will remain elegant for decades.

SANDSTONE

Like slate, sandstone has very little sensitivity to acid in most cases. This earth-toned stone mimics the look of desert and beach sand with reds, golds, tans, and browns, making it a good choice for matching nearly any décor. The downside of sandstone, especially compared to slate and granite, is that it has a high absorption rate of 1-6 percent. This makes sandstone vulnerable to staining and bacteria growth. If you choose sandstone, staining might be an issue. Sealing sandstone is the best way to guard against pet stains.

TRAVERTINE

Travertine is considered a soft stone, which means it has a high level of calcium carbonate, a mineral that is very vulnerable to acids like citrus juice, coffee, pet urine etc...Sealing the travertine can reduce the risk of damage, but the most important thing to do is blot up accidents immediately.

LIMESTONE

About 10% of sedimentary rocks are limestones. The solubility of limestone in water and weak acid solutions leads to karst landscapes, in which water erodes the limestone over thousands to millions of years. Most cave systems are through limestone bedrock.

In order to design a timber sturcture, what kind of stress values do we need to know? Name 5 of them.

Design stress Properties

Extreme fiber stress in bending, Fb

Tension parallel to grain, Ft

Shear parallel to grain, Fv

Compression parallel to grain, Fc

Compression perpendicular to grain, FcL

Is bleeding of fresh concrete a kind of segregation? Why?

Bleeding and segregation

Bleeding is a form of segregation where some of the water in the concrete tends to rise to the surface of the freshly placed material. This arises due to the inability of the solid components of the concrete to hold all of the mixing water when they settle downwards (water being the lightest of all the mix constituents). Bleeding of the water continues until the cement paste has stiffened enough to end the sedimentation process.

Cement types can influence bleeding capacity, increased proportions of, for example, cement combinations containing ground granulated blastfurnace slag (ggbs), leading to an increase in the time for bleeding continue due to a longer setting time. The presence of an adequate proportion of very fine aggregate (smaller than 150µm) reduces bleeding. Similarly polypropylene micro-fibres are known to reduce bleeding.

What is the aim of aggregate sieve analysis test?

The sieve analysis, commonly known as the gradation test, is a basic essential test for all aggregate technicians. The sieve analysis determines the gradation (the distribution of aggregate particles, by size, within a given sample) in order to determine compliance with design, production control requirements, and verification specifications. The gradation data may be used to calculate relationships between various aggregate or aggregate blends, to check compliance with such blends, and to predict trends during production by plotting gradation curves graphically, to name just a few uses. Used in conjunction with other tests, the sieve analysis is a very good quality control and quality acceptance tool.

Explain underwater concreting from start to finish briefly.

There are several methods to carryout underwater concreting such as tremie method, Pump etc...

• Tremie

It consist of placing concrete through a vertical pipe, allowing the concrete to flow from the bottom. The bottom of the tremie is kept submerged in the concrete all the times, and the concrete flows into the mass of previously placed concrete.A tremie consist of a pipe usually 10"-12" inch diameter with a hopper or funnel at the upper end. The equipment is adequately supported and arranged so that it can be raised and lengths of pipe can be removed if necessary as the level of concrete rises in the form. The spacing of the tremie should not be more than 12 ft to 16 ft apart or 8' from the sides or ends of enclosure.

Concrete is delivered to the hopper by bucket, truck mixer, pump or conveyor. The delivery must be at a good rate and without interruption. Delay for any reason impairs the free flowing mobility of the mass into which concrete is being placed. Plugged pipe and cold joints can result from delays.There are two basic methods of starting a tremie, which may be called the " wet pipe" and "dry pipe" method.

• Pumps

Pumps can carry out under water concreting satisfactorily. Muck should be removed. The end of the concrete pump line should be plugged and lowered to the bottom. The line should be filled with concrete, forcing the plug out. Concrete pumping continues and pump line is raised periodically as resistance increases. Sections of pipe line are removed above water as work progresses.

Why alkali-silica reactions are harmful for concrete?

Alkali-silica reaction (ASR) is of more concern because aggregates containing reactive silica materials are more common. In ASR, aggregates containing certain forms of silica will react with alkali hydroxide in concrete to form a gel that swells as it adsorbs water from the surrounding cement paste or the environment. These gels can induce enough expansive pressure to damage concrete. Its known as concrete cancer.

Suppose that you have “OPC, Rapid Hardening PC, High Alumina Cement, White PC, Portland Blast Furnace Slag Cement” at your construction site.

Which type (only one) of these would you prefer to use for the conditions given below? (5%)

a. Concreting very close to Golden Sand Beach, Karpaz Peninsula, Cyprus:

b. Reinforced-concrete column:

c. Concreting in Cyprus at an air temperature above 35 degrees Celcius:

d. Reinforced concrete water tank construction:

e. Reinforced-concrete slab at an air temperature of 2 degrees Celcius:

a. Concreting very close to Golden Sand Beach, Karpaz Peninsula, Cyprus: High Alumina Cement

Uses of High Alumina Cement

• Due the property of rapid hardening and strength, it is widely used in marine construction and sewer infrastructure.
• High alumina cement is also used in refractory concretes where it requires more strength at very high temperature.

b Reinforced-concrete column: OPC

Its the most commonly used cement worldwide. The cost of production is not expensive making it the sought after cement in the building industry. They are widely used for the construction of high-rise buildings, roads, dams, bridges, flyovers. Also, Ordinary Portland Cement is used for making grouts and mortars.

c. Concreting in Cyprus at an air temperature above 35 degrees Celcius:

Rapid hardening cement

Shrinkage is reduced during curing and hardening of cement. Rapid hardening cement is used in areas like road pavements so that the traffic can be opened early. It is also used in manufacturing precast slabs, posts, electric poles, concreting in cold countries.

d Reinforced concrete water tank construction: Blast-Furnace Slag Cement

Used in ready mix concrete plants. Used for structures meant for water retaining such as retaining wall, rivers, ports, tunnels for improvement in impermeability. Used in mass concreting works such as dams, foundations which require low heat of hydration.

e. Reinforced-concrete slab at an air temperature of 2 degrees Celcius:

Uses. White Portland cement is used in combination with white aggregates to produce white concrete for prestige construction projects and decorative work. White concrete usually takes the form of pre-cast cladding panels, since it is not economical to use white cement for structural purposes.

TRUE OR  FALSE

a. Plaster of Paris is obtained by calcination of gypsum rock at 190 degrees Celsius. TRUE

2CaSO4 . 2H2 O → 2CaSO4 . ½H2 O +3H2 O Gypsum Plaster of Paris At still higher temperatures (About 200°C), gypsum loses all its water of crystallization and turns out into an hydrate gypsum.

b. Slaking of lime is obtained to remove air and water completely from limestone. FALSE

The formation of slaked lime (calcium hydroxide, Ca(OH)₂) when water is added to lime (CaO) is exothermic.CaO(s) + H2O (l) → Ca(OH)₂(s)This reaction occurs when water is added to dry portland cement to make concrete, and heat evolution of energy as heat is evident because the mixture becomes warm.

c. Mixture of uniform size aggregates causes friction at many points. FALSE

When the particles are of uniform size the spacing is the greatest,more the void spaces and the paste requirement is higher,not economical.As the workability is high due to the requriement of more quantity of cement.

d. Workability of fresh concrete can be measured indirectly by slump test. false

The slump test is the most well-known and widely used test for concrete workability since 1922. This is mostly due to its simplicity, low cost, and immediate results. Widely standardized on a global scale, the Slump Test is regulated by ASTM C143 in the United States and EN 12350-2 in Europe. It is a direct method to find the Workability of fresh concrete.

e. Sea water attack is an internal factor affecting durability of concrete TRUE

When concrete is exposed to alternate wetting and drying conditions such as tidal waves from sea etc. ... When chlorides or sulfates from sea water meets reinforcement corrosion occurs and durability of concrete is reduced. Use of low-permeable concrete, proper cover for reinforcement can prevent this type of problems.