CEMENT PHYSICAL TEST FOR CIVIL ENGINEERING PROJECT BASICS

(a) Soundness Test: It is conducted by sieve analysis. 100 gms of cement is taken and sieved through IS sieve No. 9 for fifteen minutes. Residue on the sieve is weighed. This should not exceed 10 per cent by weight of sample taken.

(b) Setting Time: Initial setting time and final setting time are the two important physical properties of cement. Initial setting time is the time taken by the cement from adding of water to the starting of losing its plasticity. Final setting time is the time lapsed from adding of the water to complete loss of plasticity. Vicat apparatus is used for finding the setting times [Ref. Fig. 1.5].

  FIG 1.5 – PARTS OF VICAT APPARATUS

Vicat apparatus consists of a movable rod to which any one of the three needles shown in figure can be attached. An indicator is attached to the movable rod. A vicat mould is associated with this apparatus which is in the form of split cylinder.

Before finding initial and final setting time it is necessary to determine water to be added to get standard consistency. For this 300 gms of cement is mixed with about 30% water and cement paste prepared is filled in the mould which rests on non porous plate. The plunger is attached to the movable rod of vicat apparatus and gently lowered to touch the paste in the mould. Then the plunger is allowed to move freely.

If the penetration is 5 mm to 7 mm from the bottom of the mould, then cement is having standard consistency. If not, experiment is repeated with different proportion of water fill water required for standard consistency is found. Then the tests for initial and final setting times can be carried out as explained below:

Initial Setting Time: 300 gms of cement is thoroughly mixed with 0.85 times the water for standard consistency and vicat mould is completely filled and top surface is levelled. 1 mm square needle is fixed to the rod and gently placed over the paste. Then it is freely allowed to penetrate.

In the beginning the needle penetrates the paste completely. As time lapses the paste start losing its plasticity and offers resistance to penetration. When needle can penetrate up to 5 to 7 mm above bottom of the paste experiment is stopped and time lapsed between the addition of water and end if the \ experiment is noted as initial setting time.

Final Setting Time. The square needle is replaced with annular collar. Experiment is continued by allowing this needle to freely move after gently touching the surface of the paste. Time lapsed between the addition of water and the mark of needle but not of annular ring is found on the paste. This time is noted as final setting time.

(c) Soundness Test: This test is conducted to find free lime in cement, which is not desirable. Le Chatelier apparatus is used for conducting this test. It consists of a split brass mould of diameter 30 mm and height 30 mm. On either side of the split, there are two indicators, with pointed ends.

The ends of indicators are 165 mm from the centre of the mould. another glass plate and a small weight is placed over it. Then the whole assembly is kept under water for 24 hours. The temperature of water should be between 24°C and 50°C.

Note the distance between the indicator. Then place the mould again in the water and heat the assembly such that water reaches the boiling point in 30 minutes. Boil the water for one hour. The mould is removed from water and allowed to cool.

The distance between the two pointers is measured. The difference between the two readings indicate the expansion of the cement due to the presence of unburnt lime. This value should not exceed 10 mm.

(d) Crushing Strength Test: For this 200 gm of cement is mixed with 600 gm of standard sand confirming to IS 650–1966. After mixing thoroughly in dry condition for a minute distilled potable water P/4 + 3 percentage is added where P is the water required for the standard consistency.

They are mixed with trowel for 3 to 4 minutes to get uniform mixture. The mix is placed in a cube mould of 70.6 mm size (Area 5000 mm2) kept on a steel plate and prodded with 25 mm standard steel rod 20 times within 8 seconds.

Then the mould is placed on a standard vibrating table that vibrates at a speed of 12000 ± 400 vibration per minute. A hopper is secured at the top and the remaining mortar is filled. The mould is vibrated for two minutes and hopper removed. The top is finished with a knife or with a trowel and levelled. After 24 ± 1 hour mould is removed and cube is placed under clean water for curing.

After specified period cubes are tested in compression testing machine, keeping the specimen on its level edges. Average of three cubes is reported as crushing strength. The compressive strength at the end of 3 days should not be less than 11.5 N/mm2 and that at the end of 7 days not less than 17.5 N/mm2.

CEMENT PASTE CORROSION PREVENTION BASIC AND TUTORIALS

CEMENT PASTE CORROSION PREVENTION BASIC INFORMATION
How To Prevent Corrosion Of Cement Paste?


The corrosion of cement paste occurs because chemical reaction can happen between its external environment and internal environment. The corrosive compound must be the solution with a certain concentration, such as high temperature, proper humidity, fast flow, and the corrosion of steel bar.

Thus, the following measures can be adopted in the use of cement:

1) Select the cement varieties reasonably based on the features of the erosive environment. For example, the cement whose hydrates contains a little calcium hydroxide has high capacity to resist erosive effects of soft water and others; the anti-sulfate cement whose content of tri-calcium aluminate is less than 5% can resist the sulfate erosion.

2) Raise the Density of Cement Paste. The amount of mixing water for Portland cement should be strictly controlled in order to reduce the pore space.

The water theoretically needed in hydration of Portland cement is only 23% but much more mixing water (accounting for about 40%-70% of the cement mass) is needed in practical projects, and the pores connect to each other after the excessive water evaporates, so the erosive media go through the inner part of cement easily to accelerate the corrosion of cement.

The mix proportion should be designed reasonably in order to improve the compactness of cement concrete. Low water-cement ratio and the best construction method should be adopted as much as possible.

In addition, the insoluble calcium carbonate shell or calcium fluoride and thin silica gel film generated by conducting carbonization or fluosilicic acid treatment on the surface of concrete and mortar can increase the compactness of the surface and decrease the infiltration of erosive media.

3) Add a Protective Layer. The resistant stone, ceramic, plastic, and waterproof material are covered on the surface of cement paste, forming a impermeable layer for protection, to prevent the corrosion media contacting with cement paste directly.

CEMENT HYDRATION AND CONCRETE CURING BASICS AND TUTORIALS

CEMENT HYDRATION AND CONCRETE CURING BASIC INFORMATION
Cement Hydration And Concrete Curing Information

Concrete curing is not simply a matter of the concrete hardening as it dries out. In fact, it is just the opposite. Portland cement is a hydraulic material. That is, it requires water for curing and can, in fact, fully cure to a hardened state even if it is completely submerged in water.


Portland cement is anhydrous—it contains no water or moisture at all. The moment it comes in contact with water, a chemical reaction takes place in which new compounds are formed. This reaction is called cement hydration.

The rate of hydration varies with the composition of the cement, the fineness of the cement particles, the amount of water present, the air temperature, and the presence of admixtures. If the mixing water dries out too rapidly before the cement has fully hydrated, the curing process will stop and the concrete will not harden to its intended strength.

Curing will resume if more water is introduced, but at a slower rate. Hydration occurs more rapidly at higher air temperatures. Cement hydration itself generates heat, too. This heat of hydration can be helpful during cold-weather construction, and potentially harmful during hot-weather construction.

The chemical reaction between water and cement first forms a paste which must completely coat each aggregate particle during mixing. After a time, the paste begins to stiffen or set, and after a few hours has lost is plasticity entirely.

The rate of this setting, however, is not the same as the rate of hardening. A Type-III high-early-strength cement may set in about the same time as a Type-I general-purpose cement, but the Type III hardens and develops compressive strength more rapidly after it has set.

Concrete normally cures to its full design strength in 28 days. Curing is slower in cold weather, and at temperatures below 40°F, the concrete can be easily and permanently damaged if it is not properly protected.

Concrete must be kept moist for several days after it is placed to allow the portland cement in the mix to cure and harden properly. Concrete that is not kept moist reaches only about 50% of its design strength. Figure 2-19 shows the differences in concrete strength for various periods of moist curing.

If it is kept moist for at least three days, it will reach about 80% of its design strength, and for seven days, 100% of its design strength. If the concrete is kept moist for the full 28- day curing period, it will reach more than 125% of its design strength.

CEMENT MORTAR USED IN CIVIL AND STRUCTURAL CONSTRUCTION BASICS AND TUTORIALS

CEMENT MORTAR USED IN CIVIL AND STRUCTURAL CONSTRUCTION BASIC INFORMATION
What Are Cement Mortar? What Are The Applications Of Cement Mortar?


For preparing mortar, first a mixture of cement and sand is made thoroughly mixing them in dry condition. Water is gradually added and mixed with shovels. The cement to sand proportion recommended for various works is as shown is Table 2.1

Table 2.1 Cement To Sand Proportion 

Curing: Cement gains the strength gradually with hydration. Hence it is necessary to see that mortar is wet till hydration has taken place. The process to ensure sufficient moisture for hydration after laying mortar/concrete is called curing. 

Curing is ensured by spraying water. Curing normally starts 6–24 hours after mortar is used. It may be noted that in the initial period water requirement is more for hydration and gradually it reduces. Curing is recommended for 28 days.

Properties of Cement Mortar: The following are the important properties of cement mortar:

1. When water is added to the dry mixture of cement and sand, hydration of cement starts and it binds sand particles and also the surrounding surfaces of masonry and concrete.

2. A mix richer than 1:3 is prone to shrinkage.

3. Well proportioned mortar provides impervious surface.

4. Leaner mix is not capable of closing the voids in sand and hence the plastered surface is porous.

5. The strength of mortar depends upon the proportion of cement and sand. Strengths obtained with various proportion of cement and sand is shown in Table 2.2.

Uses of Cement Mortar
Mortar is used
1. to bind masonry units like stone, bricks, cement blocks.
2. to plaster slab and walls make them impervious.
3. to give neat finishing to walls and concrete works.
4. for pointing masonry joints.
5. for preparing building blocks.
6. as a filler material in ferro cement works.
7. to fill joints and cracks in walls.
8. as a filler material in stone masonry.

MODIFIED PORTLAND CEMENTS BASICS AND TUTORIALS

DIFFERENT TYPES OF MODIFIED PORTLAND CEMENTS BASIC
What Are The Different Types Of Modified Portland Cements?


Increasingly, modern concretes contain a blend of Portland cement and other cementitious materials. When other materials are added to Portland cement at the time at which the concrete is batched, they are referred to as mineral admixtures; however, there are also hydraulic cements, which are produced either by forming other compounds during the burning process or by adding other materials to the clinker and then intergrinding them.

The common types of such modified cements are described in the following sections.


Portland Pozzolan Cements
Portland pozzolan cements are blends of Portland cement and a pozzolanic material. The role of the pozzolan is to react slowly with the calcium hydroxide that is liberated during cement hydration.

This tends to reduce the heat of hydration and the early strength but can increase the ultimate strength of the material. These cements tend to be more resistant to sulfate attack and to the alkali–aggregate reaction.

Portland Blast-Furnace Slag Cements
Ground granulated blast-furnace slag (GGBFS), which is a byproduct of the iron and steel industry, is composed largely of lime, silica, and alumina and thus is a potentially cementitious material. To hydrate it, however, it must be activated by the addition of other compounds.

When the GGBFS is to be activated by lime, the lime is most easily supplied by the hydration of the Portland cement itself. Slags may be present in proportions ranging from 25 to 90%. They react slowly to form C–S–H, which is the same product that results from the hydration of the calcium silicates.

In general, because they react more slowly than Portland cement, slag cements have both lower heats of hydration and lower rates of strength gain.

On the other hand, they have an enhanced resistance to sulfate attack. When the GGBFS is to be activated with calcium sulfate (CaSO4), together with a small amount of lime or Portland cement, the material is known as supersulfated cement.

This cement is available mostly in Europe, where it is used for its lower heat of hydration and its resistance to sulfate attack.

Expansive Cements
Expansive cements were developed to try to offset the drying shrinkage that concrete undergoes. This is particularly important when the concrete is restrained against contraction or when it is to be cast against mature concrete in repair situations.

In both cases, severe cracking may occur as a result of the shrinkage. Expansive cements are based on the formation of large quantities of ettringite during the first few days of hydration; however, they are little used today, in large part because it is very difficult to control (or predict) the amount of expansion that will take place for a particular concrete formulation.

FIVE TYPES OF PORTLAND CEMENT BASIC AND TUTORIALS

PORTLAND CEMENT TYPES BASIC INFORMATION
What Are The Five Types of Portland Cement?


Portland cement has become the most widely used cement in the world. Portland cement got its name because the cured concrete it produced was the same color as a gray stone quarried in nearby Portland, England.

There are five types of portland cement, each with different characteristics.

■ Type I is a general-purpose cement and is by far the most commonly used, especially in residential work. Type I portland cement is suitable whenever the special characteristics of other types are not required.

■ Type II cement has moderate resistance to sulfates, which are found in some soil and groundwater, and generates less heat during hydration than Type I. This reduced curing temperature can be particularly helpful in large structures such as piers and heavy retaining walls, especially when the concrete is placed in warm weather.

■ Type III is a “high early strength” cement. High early strength does not mean higher strength—only that strength develops at a faster rate. This can be an advantage during winter construction because it reduces the time during which fresh concrete must be protected from the cold. Early strength gain can also permit removal of forms and shoring more quickly.

■ Type IV cement produces less heat during hydration than Type I or Type II and is used only in massive civil engineering structures such as dams, large highway pilings, or heavy bridge abutments. Its strength development and curing rates, though, are much slower than Type I.

■ Type V cement is used in concrete exposed to soil or groundwater that has high sulfate concentrations. This type of cement is usually available only in areas where it is likely to be needed. In the United States, Type V cement is common only in the southwestern states.

Types I, II, and III portland cement can also be made with a foaming agent that produces millions of evenly distributed microscopic air bubbles in the concrete mix. When manufactured in this way, the cements are said to be air entrained, and are designated as Types IA, IIA, and IIIA. Air-entrained cements require mechanical mixing.

Finely ground cement increases the workability of harsh mixes, making them more cohesive and reducing tendencies toward segregation. Coarsely ground cement reduces stickiness. Cement packages that are marked ASTM A150 meet industry standards for both physical and chemical requirements.

Portland cement comes in three colors—grey, white, and buff. The white and buff are more expensive and typically used in commercial rather than residential projects to achieve special color effects.

 Liquid or powder pigments can be added to a concrete mix, and liquid stains can be used to color the surface of cured concrete, but both will add to the cost. For most applications, ordinary gray concrete made with gray cement is suitable. Colored concrete should be reserved for special areas like a front entrance, a patio, or a pool deck

WATER TO CEMENT RATIO BASICS AND TUTORIALS

WATER TO CEMENT RATIO BASIC INFORMATION
What Is The Ideal Water To Cement Ratio?

For brittle ceramic materials, including cementitious systems, the strength has been found to be inversely proportional to the porosity. Often, an exponential equation is used to relate strength to porosity; for example,

fc = fcₒe⁻kt

where fc is the strength, fc0 is the intrinsic strength at zero porosity, p is the porosity, and k is a constant that depends on the particular system.

Equations such as this do not consider the pore-size distribution, the pore shape, and whether the pores are empty or filled with water; thus, they are a gross simplification of the true strength vs. porosity relationship.

Nonetheless, for ordinary concretes for the same degree of cement hydration, the strength does indeed depend primarily on the porosity. Because the porosity, in turn, depends mostly on the original w/c ratio, mix proportioning for normal-strength concretes is based, to a large extent, on the w/c ratio law articulated by D.A. Abrams in 1919: “For given materials, the strength depends only on one factor—the ratio of water to cement.” This can be expressed as: fc = K1/ [K2^(w/c)] where K1 and K2 are constants, and w/c is the water/cement ratio by weight.

In fact, of course, given the variability in raw materials from concrete to concrete, the w/c ratio law is really a family of relationships for different mixtures. As stated by Gilkey (1961a):

For a given cement and acceptable aggregates, the strength that may be developed by a workable, properly placed mixture of cement, aggregate, and water (under the same mixing, curing, and, testing conditions) is influenced by the: (a) ratio of cement to mixing water; (b) ratio of cement to aggregate; (c) grading, surface texture, shape, strength, and stiffness of aggregate particles; and (d) maximum size of aggregate.

Thus, in some cases, simple reliance on the w/c ratio law may lead to serious errors. It should be noted that many modern concretes contain one or more mineral admixtures that are, in themselves, cementitious to a greater or lesser degree; therefore, it is becoming more common to use the term water/ cementitious material ratio to reflect this fact rather than the simpler water/cement ratio.

For ordinary concretes, the w/c ratio law works well for a given set of raw materials, because the aggregate strength is generally much greater than the paste strength; however, the w/c ratio law is more problematic for high-strength concretes, in which the strength-limiting factor may be the aggregate strength or the strength of the interfacial zone between the cement and the aggregate.

Although it is, of course, necessary to use very low w/c ratios to achieve very high strengths, the w/c ratio vs. strength relationship is not as straightforward as it is for normal concretes. Figure 1.8 shows a variety of water/ cementitious material vs. strength relationships obtained by a number of different investigators.

A great deal of scatter can be seen in the results. In addition, the range of strengths for a given w/c ratio increases as the w/c ratio decreases, leading to the conclusion that, for these concretes, the w/c ratio is not by itself a very good predictor of strength; a different w/c ratio “law” must be determined for each different set of materials.

REINFORCED CEMENT CONCRETE (RCC) BASICS AND TUTORIALS

REINFORCED CEMENT CONCRETE (RCC) BASIC INFORMATION
What Is Reinforced Cement Concrete (RCC)?

Concrete is good in resisting compression but is very weak in resisting tension. Hence reinforcement is provided in the concrete wherever tensile stress is expected. The best reinforcement is steel, since tensile strength of steel is quite high and the bond between steel and concrete is good.

As the elastic modulus of steel is high, for the same extension the force resisted by steel is high compared to concrete. However in tensile zone, hair cracks in concrete are unavoidable. Reinforcements are usually in the form of mild steel or ribbed steel bars of 6 mm to 32 mm diameter.

A cage of reinforcements is prepared as per the design requirements, kept in a form work and then green concrete is poured. After the concrete hardens, the form work is removed.

The composite material of steel and concrete now called R.C.C. acts as a structural member and can resist tensile as well as compressive stresses very well.

Properties of R.C.C./Requirement of Good R.C.C.

1. It should be capable of resisting expected tensile, compressive, bending and shear forces.

2. It should not show excessive deflection and spoil serviceability requirement.

3. There should be proper cover to the reinforcement, so that the corrossion is prevented.

4. The hair cracks developed should be within the permissible limit.

5. It is a good fire resistant material.

6. When it is fresh, it can be moulded to any desired shape and size.

7. Durability is very good.

8. R.C.C. structure can be designed to take any load.

Uses of R.C.C.

It is a widely used building material. Some of its important uses are listed below:

1. R.C.C. is used as a structural element, the common structural elements in a building where

R.C.C. is used are:

(a) Footings (b) Columns

(c) Beams and lintels (d) Chejjas, roofs and slabs.

(e) Stairs.

2. R.C.C. is used for the construction of storage structures like

(a) Water tanks (b) Dams

(c) Bins (d) Silos and bunkers.

3. It is used for the construction of big structures like

(a) Bridges (b) Retaining walls

(c) Docks and harbours (d) Under water structures.

4. It is used for pre-casting

(a) Railway sleepers (b) Electric poles

5. R.C.C. is used for constructing tall structures like

(a) Multistorey buildings (b) Chimneys

(c) Towers.

6. It is used for paving

(a) Roads (b) Airports.

7. R.C.C. is used in building atomic plants to prevent danger of radiation. For this purpose R.C.C. walls built are 1.5 m to 2.0 m thick.

POZZOLANS - POZZOLAN CEMENT USED IN CONCRETE CONSTRUCTIONS BASICS AND TUTORIALS

POZZOLAN CEMENTS BASIC INFORMATION AND TUTORIALS
What Are Pozzolans? What Is Pozzolan Cement?


The classes of pozzolans most likely to be available are classes F and C fly ash and silica fume. Class N may be considered at those sites where a source of natural pozzolan is available.

(a) Regulations governing use of fly ash. The Solid Waste Disposal Act, Section 6002, as amended by the
Resource Conservation and Recovery Act of 1976, requires all agencies using Federal funds in construction to allow the use of fly ash in the concrete unless such use can be shown to be technically improper.

The basis of this regulation is both energy savings and waste disposal, since most fly ash in use today is the result of the burning of coal for electrical power.

(b) General. The use of pozzolan should be considered coincident with the consideration of the types of
available cements. Portland cement to be used alone should always be considered in the specifications as well as blended hydraulic cements or the combination of portland cement with slag cement or pozzolan unless one or the latter is determined to be technically improper.

Classes F and C fly ash are generally accepted on all Corps of Engineers’ (CE) civil works projects, and their use should be allowed in all specifications unless there are technical reasons not to do so.

(c) Class F pozzolan. Class F pozzolan is a fly ash usually obtained from burning anthracite or bituminous coal
and is the class of fly ash that has been most commonly used to date. It must contain at least 70.0 percent of
Si02 + Al203 + Fe203 by chemical analysis.

(d) Class C pozzolan. Class C pozzolan is a fly ash that is usually obtained from the burning of lignite or
subbituminous coal. It must contain at least 50.0 percent of Si02 + Al203 + Fe203 .

(e) Other considerations. Class C fly ashes often contain considerably more alkalies than do Class F fly
ashes. However, when use of either class in applications where alkali-aggregate reaction is likely, the optional
available alkali requirement of ASTM C 618 (CRD-C 255) should be specified. Use of Class F fly ash in replacement of portland cement results in reduction of heat of hydration of the cementitious materials at early ages.

Use of Class C fly ash in the same proportions usually results in substantially less reduction in heat of hydration. An analysis of the importance of this effect should be made if Class C fly ash is being considered for use in a mass concrete application.

Class F fly ash generally increases resistance to sulfate attack. However, if the portland cement is of high
C3A content, the amount of improvement may not be sufficient so that the combined cementitious materials are equivalent to a Type II or a Type V portland cement. This can be determined by testing according to ASTM C 1012 (CRD-C 211).

Class C fly ashes are quite variable in their performance in sulfate environments, and their performance
should always be verified by testing with the portland cement intended for use. Both Class F and Class C fly ashes have been found to delay for initial and final set. This retarding action should be taken into consideration if important to the structure.

Most Class C and Class F fly ashes are capable of reducing the expansion from the alkalisilica reaction. Use of an effective fly ash may eliminate the need to specify low-alkali cement when a reactive aggregate is used.

The effectiveness of the fly ash must be verified by ASTM C 441 (CRD-C 257). For additional information, see Appendixes D and E.

(f) Class N pozzolan. Class N is raw or calcined natural pozzolans such as some diatomaceous earths, opaline cherts, tuffs, and volcanic ashes such as pumicite.

(g) Silica fume. Silica fume is a pozzolan. It is a byproduct of the manufacture of silicon or silicon alloys.
The material is considerably more expensive than other pozzolans. Properties of silica fume vary with the type of silicon or silicon alloy produced, but in general, a silica fume is a very finely divided product and consequently is used in concrete in different proportions and for different applications than are the more conventional pozzolans discussed in the previous paragraphs.

Applications for which silica fume is used are in the production of concrete having very high strengths, high abrasion resistance, very low permeability, and increased aggregate bond strength.

However, certain precautions should be taken when specifying silica-fume concretes. Use of silica fume
produces a sticky paste and an increased water demand for equal slump. These characteristics are normally counteracted by using high-range water-reducing admixtures (HRWRA) to achieve the required slump. This
combination, together with an air-entraining admixture, may cause a coarse air-void system.

The higher water demand for silica-fume concrete greatly reduces or eliminates bleeding, which in turn tends to increase the likelihood of plastic shrinkage cracking. Therefore, steps should be taken as early as possible to minimize moisture loss, and the curing period should be increased over that required for conventional concrete.