Differences Between One-Way Slab and Two-Way Slab

Note: As per IS 456, when the ratio of long span to short span is 2 or more, the slab behaves as a one-way slab; when it is less than 2, it behaves as a two-way slab.

Active earth pressure ($P_a$). When a retaining wall yields or tilts away from the backfill, the soil expands sideways and its lateral pressure on the wall falls. The lowest value, reached when the soil is on the point of shear failure (Rankine's active state of plastic equilibrium), is the active earth pressure. The soil pushes the wall.

Passive earth pressure ($P_p$). When the wall is pushed towards the backfill, the soil is compressed and its lateral pressure rises. The highest value, reached when the soil is on the point of shear failure (Rankine's passive state), is the passive earth pressure. The soil resists the wall's push.

Figure (backfill on the left, wall on the right; the pressure always acts on the wall from the soil side)

   ACTIVE CASE                          PASSIVE CASE
   wall moves away from soil            wall moves towards soil
   wall  ------>                        wall  <------

   backfill |  wall                     backfill |  wall
    soil    |  |                         soil    |  |
    wedge   |  |                         wedge   |  |
    slides  |  |                         is      |  |
    down  ->|->| Pa (minimum)            pushed ->|<-| Pp (maximum)
            |  |   pushes wall out       up       |  |   resists wall
   ---------+--+---------                ---------+--+---------
   slip plane at 45 + phi/2 to           slip plane at 45 - phi/2 to
   the horizontal                        the horizontal

Since $K_p>K_a$ (indeed $K_aK_p=1$), $P_p>P_a$. Active pressure is the one used to design retaining walls, while passive pressure is relied on in front of the wall base (toe) for resistance against sliding.

Types of Soil Based on Particle Size

Soil is classified on the basis of the size of its individual mineral particles. The standard particle-size classification (IS / USDA system) divides soil particles into the following groups:

1. Gravel Particles larger than 4.75 mm in diameter. Gravel is further divided into coarse gravel (20–80 mm) and fine gravel (4.75–20 mm). It is a bulky, non-cohesive material with high permeability and no plasticity.

2. Sand Particles ranging from 0.075 mm to 4.75 mm. It is subdivided into:

• Coarse sand: 2.0–4.75 mm
• Medium sand: 0.425–2.0 mm
• Fine sand: 0.075–0.425 mm

Sand is a coarse-grained, cohesionless soil. It has high permeability, good drainage, and its strength depends mainly on internal friction.

3. Silt Particles ranging from 0.002 mm to 0.075 mm. Silt is a fine-grained soil, intermediate between sand and clay. It is non-plastic or slightly plastic, has low cohesion, and shows a tendency to become quick (lose strength) when saturated.

4. Clay Particles smaller than 0.002 mm. Clay is a fine-grained, cohesive soil with high plasticity. It has very low permeability, high water-holding capacity, and its strength depends on cohesion. Clay exhibits swelling and shrinkage with changes in moisture content.

Summary Table

In practice, natural soil is rarely a single type; it is usually a mixture of these fractions. Based on the relative proportions of sand, silt and clay, soils are also grouped as sandy soil, silty soil, clayey soil and loam (a balanced mixture). The particle size of a soil governs its permeability, compressibility, plasticity and bearing capacity, and hence its suitability for engineering works.

Internal effects developed in a structural member due to external loading

When external loads act on a structural member, internal forces (effects) are set up within the member to resist those loads and maintain equilibrium. These internal effects are:

1. Axial force (direct force) — It acts along the longitudinal axis of the member. It may be:

   • Tensile force — when the load tends to elongate the member (e.g., a tie rod).
   • Compressive force — when the load tends to shorten the member (e.g., a column).

2. Shear force (S.F.) — It is the internal force acting parallel to the cross-section of the member, tending to slide one part of the section over the adjacent part. It is measured in kN or N.

3. Bending moment (B.M.) — It is the internal moment developed at a section due to the external loads acting at a distance from that section. It tends to bend the member about an axis lying in the plane of the section. It is measured in kN·m or N·mm. A sagging moment causes tension at the bottom and compression at the top, while a hogging moment does the opposite.

4. Torsional moment (Twisting moment) — It is the internal moment developed when the external loads act in a plane perpendicular to the longitudinal axis and tend to twist the member about its own axis (e.g., a shaft transmitting torque). It is measured in kN·m or N·mm.

5. Shear stress and bending stress — These are the internal stresses (force per unit area) produced by the shear force and bending moment respectively within the material of the member.

6. Deflection and deformation — Under the action of these internal effects, the member undergoes change in shape or length, i.e., axial deformation, bending deflection, or twisting (angle of twist).

Thus, the main internal effects are axial force, shear force, bending moment and torsional moment, which together keep the member in equilibrium and produce corresponding stresses and deformations.

Admixture is a material other than water, aggregate, or cement that is added to a concrete mix, either before or during mixing, in small quantities to modify the properties of fresh or hardened concrete as desired. Admixtures are used to improve workability, accelerate or retard setting, increase strength or durability, reduce water demand, and control shrinkage or cracking.

Four types of admixtures:

1. Accelerating Admixture — Speeds up the setting and hardening of concrete. Calcium chloride (CaCl₂) is the most common example. Used in cold weather concreting to prevent freezing of fresh concrete and to allow early removal of formwork. Excess use may cause corrosion of reinforcement, so its dose is limited (usually up to 2% by weight of cement).

2. Retarding Admixture — Delays the setting time of concrete. Examples include sugar, molasses, and commercial retarders like calcium lignosulphonate. Used in hot weather concreting, for long hauls, and where a long time is needed for placing and finishing before setting begins.

3. Water-Reducing Admixture (Plasticizer) — Reduces the quantity of mixing water needed for a given workability, or increases workability at the same water-cement ratio. Examples are lignosulphonates and sulphonated naphthalene formaldehyde. Superplasticizers are high-range water reducers used for flowing concrete and high-strength concrete.

4. Air-Entraining Admixture — Introduces millions of tiny air bubbles into the concrete. Examples include natural wood resins, animal fats, and synthetic detergents. These bubbles increase resistance to freezing and thawing and improve workability, but slightly reduce compressive strength. Used mainly in pavement and dam concrete in cold regions.

Other common types include air-detraining admixtures, grouting admixtures, colouring admixtures, and corrosion-inhibiting admixtures.

Characteristics of Contour

A contour line is an imaginary line on the ground joining points of equal elevation. Its main characteristics are:

1. Equal elevation: Every point on a contour line has the same reduced level (RL).
2. Closed curves: A contour is a closed curve, either within the map or beyond its limits.
3. No crossing: Two contour lines of different elevations can never cross or intersect each other (they may touch only in the case of an overhanging cliff or cave).
4. Close spacing = steep slope: Contours close together indicate a steep slope; widely spaced contours indicate a gentle slope.
5. Uniform spacing = uniform slope: Equally spaced contours represent a uniform (plane) slope.
6. Concentric closed contours: Successive closed contours one inside the other indicate a hill or a depression. A hill has higher values towards the centre; a depression has lower values towards the centre and is shown with hachures.
7. Perpendicular to flow lines: Contours cross the direction of flow of water (streams, valleys) at right angles.
8. Parallel contours: Contours running parallel to each other indicate a valley, ridge or a uniform slope.
9. Contour gradient: The slope between two contours is the vertical distance (contour interval) divided by the horizontal distance.
10. Contour interval is constant: On a given map, the vertical distance between successive contours (contour interval) is kept constant.

Helpfulness in the Engineering Field

1. Selection of site and alignment: Contours help select the most economical and suitable alignment for roads, railways, canals and transmission lines by following a gentle, balanced gradient.
2. Reservoir and dam site: They help locate a dam site and determine the reservoir capacity and the area submerged at different water levels.
3. Earthwork estimation: The volume of cutting and filling for roads, canals and levelling work is computed from contour maps.
4. Drainage and irrigation: Contours show the natural direction of water flow, which is essential for designing drainage systems, sewage lines and irrigation canals.
5. Site selection for structures: They help choose sites for buildings, airports, bridges and culverts where the ground slope is suitable.
6. Intervisibility of stations: Contours indicate whether one point is visible from another, useful in surveying and in fixing triangulation stations.
7. Military and town planning: They assist in defence works, and in planning roads, water supply and layout of towns.

Thus, contours give a clear three-dimensional picture of the terrain on a two-dimensional map and are indispensable for economical planning and design in civil engineering works.

Plain Cement Concrete (PCC) vs Reinforced Cement Concrete (RCC)

Reasons for using steel as a reinforcing material

1. High tensile strength — steel has high tensile strength (about 1400–1600 kg/cm² in working stress), so it safely carries the tension which concrete cannot.
2. Similar thermal expansion — the coefficient of thermal expansion of steel (12 × 10⁻⁶ /°C) is nearly equal to that of concrete (10–12 × 10⁻⁶ /°C), so both expand and contract together without slipping or cracking.
3. Good bond with concrete — steel develops a strong bond (and in deformed bars, mechanical interlock) with concrete, so the two act as one composite material.
4. Durability and fire resistance — steel embedded in concrete is protected from rusting by the alkaline nature of concrete, and concrete also protects the steel from fire.
5. Easy to bend and join — steel can be easily bent, hooked and lapped into any required shape, and is available in many diameters.
6. Modulus of elasticity — steel is about 10–15 times stiffer than concrete, so it controls deflection and cracking of the member.

Definition of Optimum Moisture Content (OMC)

Optimum Moisture Content (OMC) is defined as the moisture content of a soil at which, for a given compactive effort, the soil attains its maximum dry density (MDD). At this moisture content, the soil particles are lubricated just enough to slide over one another and rearrange into the densest possible packing.

Compaction Curve and Relationship between Dry Density and Moisture Content

The relationship is obtained from the Standard Proctor Test (or Modified Proctor Test) and is plotted as a compaction curve with:

• X-axis: Moisture content, w (%)
• Y-axis: Dry density, γ_d (kN/m³ or g/cc)

The curve is obtained as:

$\gamma_d = \frac{\gamma}{1 + w}$

where γ = bulk (wet) density of soil.

Shape of the compaction curve:

1. Dry side of OMC (ascending limb): As moisture content increases from a low value, the water acts as a lubricant, reducing inter-particle friction. Soil particles pack more closely, and dry density increases. The air voids decrease.

2. At OMC (peak point): The dry density reaches its maximum value (MDD). At this point, the soil is at its densest state for the given compactive effort. The soil is nearly saturated but still contains some air voids.

3. Wet side of OMC (descending limb): Beyond OMC, further addition of water fills the voids but does not help in further packing. The excess water occupies space and pushes particles apart, so dry density decreases. The soil becomes more saturated and may become spongy.

Zero air voids (ZAV) line: A theoretical line representing 100% saturation (S = 100%) is drawn on the same plot:

$\gamma_{d,\text{ZAV}} = \frac{G_s \cdot \gamma_w}{1 + \frac{w \cdot G_s}{S}}$

The compaction curve always lies to the left (dry side) of the ZAV line, since it is impossible to remove all air by ordinary compaction.

Engineering Significance of OMC and MDD in Field Compaction

1. Control of field density: The MDD obtained from the laboratory Proctor test serves as the reference density for field compaction. Field density is specified as a percentage of MDD (usually 95–100% for embankments, subgrades, and fills).

2. Moisture control during construction: Soil must be compacted at or near OMC (usually within ±2% of OMC) to achieve the required dry density with minimum compactive effort. Compacting on the dry side requires more passes; on the wet side, the soil becomes unstable (pumping, rutting).

3. Selection of compaction equipment: The type and weight of rollers (sheepsfoot, smooth-wheel, vibratory, pneumatic) are chosen based on the soil type and the OMC–MDD relationship to achieve the target density economically.

4. Ensuring strength and stability: Higher dry density means lower void ratio, which increases:

   • Shear strength
   • Bearing capacity
   • Slope stability

5. Reducing settlement and permeability: Dense compaction reduces future settlement and permeability, which is critical for foundations, pavements, and earth dams.

6. Quality control in the field: The field dry density is checked by the Core Cutter method, Sand Replacement method, or Nuclear Gauge method, and compared with the laboratory MDD to ensure the specified degree of compaction is achieved.

7. Economy and durability: Proper moisture control at OMC reduces the number of roller passes, saving time and cost, and ensures long-term performance of the compacted structure.

Comparison of Trenching and Ducting Methods for Underground Cable Networks

Trenching means digging an open trench along the cable route, laying the cable directly on a prepared bedding (usually sand), and backfilling it. Ducting means laying pipes or ducts (concrete, PVC, HDPE, GI) in the ground first and then pulling the cable through these ducts. The two methods compare as follows:

Conclusion: Trenching (direct burial) is economical and gives better heat dissipation, so it suits rural and single-cable installations. Ducting has a higher initial cost but allows easy replacement, future expansion and better protection, so it is preferred in towns, road crossings and areas where excavation cannot be repeated. In practice, a combination is used — direct burial in open stretches and ducts at road crossings and entries.

In a construction organisation, the owner, the contractor and the engineer form a triangular contractual relationship in which each party has distinct rights, duties and responsibilities, and all three are bound together by the contract documents. The relationship should be professional, contractual, cooperative and based on mutual trust, with each party performing its own role without interfering in the others' domain.

1. Owner (Employer / Client)

• Provides the site, funds, drawings and decisions in time.
• Appoints the engineer to design, supervise and certify the work.
• Has a direct contractual link with the contractor (through the contract agreement) and with the engineer (through the consultancy agreement).
• Should not issue instructions to the contractor directly; instructions should normally pass through the engineer.

2. Engineer (Consultant/Engineer-in-Charge)

• Acts as the technical representative of the owner for design, supervision, quality control and certification of bills.
• Also acts as an independent and impartial adjudicator between the owner and the contractor while interpreting the contract, fixing rates for extra items and settling disputes.
• Must be fair, honest, competent and must not favour either party.

3. Contractor

• Executes the work as per drawings, specifications, schedule and contract conditions.
• Arranges labour, materials, plant and safety measures.
• Is entitled to timely payment, site possession, approved drawings and extension of time for delays not caused by him.

Nature of the relationship

• Contractual (legal): Owner–Contractor and Owner–Engineer are bound by separate contracts; the engineer has no direct contract with the contractor but derives authority from the owner's contract with the contractor.
• Professional and cooperative: All three should work as a team with a spirit of goodwill, prompt communication and joint problem-solving so that the work is completed within time, cost and quality.
• Impartial and trustworthy: The engineer must maintain a balance between the owner's interest (economy, quality, time) and the contractor's legitimate interest (fair payment, reasonable profit).
• Hierarchical but independent: The engineer is paid by the owner but must give decisions independently, as required by the contract.

Conclusion The relationship should be a balanced triangular relationship — the owner provides resources and decisions, the engineer provides design, supervision and impartial judgement, and the contractor provides execution. When each respects the other's contractual role and works with honesty, cooperation and mutual trust, disputes are minimised and the project is completed successfully in terms of time, cost and quality.

Significance of Specification of Construction Materials

A specification is a written description of the quality, grade, size, strength and method of use of materials and workmanship required for a construction item. For an Overseer, specification is as important as the drawing itself. Its significance is as follows:

1. Basis of contract and agreement. The specification, together with the drawings, forms the legal basis of the contract. It defines exactly what the contractor must supply and what the owner will pay for, so both parties know their obligations and disputes are minimised.

2. Basis of estimating cost. The estimator reads the specification to know the quality of materials (e.g. OPC 43 grade cement, Fe 500 steel) and thus prepares an accurate rate analysis and tender. Without specification, correct costing is impossible.

3. Guidance for procurement. It tells the storekeeper and purchasing officer the exact brand, grade, size and standard (NS, IS) of materials to be bought, preventing purchase of inferior or wrong materials.

4. Control of quality of work. During construction the Overseer compares the materials actually brought to site with the specified quality. Substandard materials can be rejected on this basis, so the specified strength and durability of the structure are ensured.

5. Uniformity and standardisation. Standard specifications bring uniformity in work throughout the organisation, so that the same quality is obtained in every project and comparison between projects becomes easy.

6. Basis of measurement and payment. The method of measurement and the unit of payment for each item are stated in the specification, which helps in preparing bills, checking contractor's claims and settling final accounts.

7. Basis for arbitration and settlement of disputes. If a dispute arises about quality or workmanship, the specification is the reference document on which the decision of the engineer or arbitrator is based.

8. Helps in supervision and inspection. It tells the supervisor what checks to apply — slump test, cube test, compaction test — and the permissible tolerances, making inspection systematic.

9. Ensures safety and durability. By fixing minimum cement content, water-cement ratio, cover to reinforcement and grade of concrete, the specification ensures the required strength, safety and design life of the structure.

10. Guidance for maintenance. A proper record of specified materials helps later in repair and maintenance, as the same or compatible materials can be selected.

Conclusion. Specification thus serves as the written standard of quality, the legal and financial basis of the contract, and the daily working guide for the Overseer. A clear, complete and unambiguous specification is therefore essential for economy, quality and timely completion of any construction work.

Line diagram: Drinking water supply system from source to consumer household

        SOURCE (spring / river / lake / deep tubewell)
                          |
                          v
              +-----------------------+
              |   INTAKE STRUCTURE    |   (collects water from source)
              +-----------------------+
                          |
                          v
              +-----------------------+
              |   RAW WATER PUMPING   |   (if source is below treatment plant)
              |        STATION        |
              +-----------------------+
                          |
                          v
              +-----------------------+
              |  TREATMENT PLANT      |
              |  - Aeration           |
              |  - Sedimentation      |
              |  - Filtration         |
              |  - Disinfection       |
              +-----------------------+
                          |
                          v
              +-----------------------+
              |  CLEAR WATER RESERVOIR|   (service/storage reservoir)
              +-----------------------+
                          |
                          v
              +-----------------------+
              |  DISTRIBUTION MAIN    |   (gravity or pumping main)
              +-----------------------+
                          |
          +---------------+---------------+
          |               |               |
          v               v               v
   +-------------+ +-------------+ +-------------+
   |  Service    | |  Service    | |  Service    |
   |  Reservoir  | |  Reservoir  | |  Reservoir  |
   |  / OHT      | |  / OHT      | |  / OHT      |
   +-------------+ +-------------+ +-------------+
          |               |               |
          v               v               v
   +-------------+ +-------------+ +-------------+
   | Distribution| | Distribution| | Distribution|
   |    Main     | |    Main     | |    Main     |
   +-------------+ +-------------+ +-------------+
          |               |               |
          v               v               v
   +-------------+ +-------------+ +-------------+
   |   Street    | |   Street    | |   Street    |
   |    Main     | |    Main     | |    Main     |
   +-------------+ +-------------+ +-------------+
          |               |               |
          v               v               v
   +-------------+ +-------------+ +-------------+
   |   Consumer  | |   Consumer  | |   Consumer  |
   |  Household  | |  Household  | |  Household  |
   | (tap/ferrule)| | (tap/ferrule)| | (tap/ferrule)|
   +-------------+ +-------------+ +-------------+

Main structures shown

1. Source — spring, river, lake or deep tubewell.
2. Intake structure — collects and protects water at the source.
3. Raw water pumping station — lifts raw water to the treatment plant (needed when the plant is at a higher level).
4. Treatment plant — aeration, sedimentation, filtration and disinfection to make water potable.
5. Clear water reservoir — stores treated water and balances supply against demand.
6. Distribution main — carries water from the reservoir towards the service area by gravity or pumping.
7. Service reservoir / overhead tank — maintains pressure and provides emergency storage in the locality.
8. Street main (branch pipe) — distributes water along the street.
9. Consumer connection — ferrule, communication pipe, water meter and household tap.

For a short reinforced concrete column, if the minimum eccentricity does not exceed 5% of the lateral dimension, i.e.

$e_{\min} \le 0.05D$

the column may be designed as an axially loaded column.

The ultimate load carrying capacity is given by:

$P_u = 0.4 f_{ck} A_c + 0.67 f_y A_{sc}$

where:

• $P_u$ = ultimate load carrying capacity of the column
• $f_{ck}$ = characteristic compressive strength of concrete
• $f_y$ = yield strength of steel
• $A_{sc}$ = area of longitudinal reinforcement
• $A_c$ = net area of concrete = $A_g - A_{sc}$
• $A_g$ = gross area of the column section

The coefficient $0.4$ is used for concrete to account for the effect of minimum eccentricity and other uncertainties. The coefficient $0.67$ is used for steel because at the ultimate limit state the steel does not reach its full yield stress.

For a circular column with helical reinforcement, the ultimate load capacity is taken as:

$P_u = 1.05 \left(0.4 f_{ck} A_c + 0.67 f_y A_{sc}\right)$

Thus, the required ultimate load capacity is obtained from the above expression, and the column is safe if the factored axial load does not exceed this value.

Things to be considered while designing a septic tank (5 marks)

1. Number of users and volume of sewage: The tank capacity is fixed from the number of persons using it and the daily flow of sewage. A minimum of 130–150 litres per person per day (or 0.13–0.15 m³) is generally taken, and the tank should hold at least 24 hours' flow plus the sludge storage volume.

2. Detention time: The sewage must remain in the tank long enough for proper sedimentation and digestion — usually 24 to 48 hours. Too short a time gives poor settling; too long a time makes the tank unnecessarily large and costly.

3. Sludge and scum storage (cleaning interval): Space must be kept for the accumulation of sludge at the bottom and scum at the top between two cleanings. The tank is normally designed for 1 to 3 years of sludge storage, taking the sludge accumulation rate as about 25–30 litres per person per year.

4. Depth and dimensions: A liquid depth of 1.0–1.5 m (minimum 1 m) is kept so that digestion proceeds well. The length-to-width ratio should be about 2:1 to 3:1 to avoid short-circuiting of flow. Freeboard of 0.3–0.5 m above the liquid level is provided.

5. Inlet and outlet arrangement: The inlet and outlet should be placed at opposite ends, with the outlet slightly lower than the inlet. A submerged inlet pipe (with a tee or baffle) and an outlet tee dipping about 0.3 m below the liquid surface prevent scum from escaping with the effluent.

6. Ventilation and gas escape: Gases such as methane and hydrogen sulphide are produced during digestion, so a vent pipe (about 50–75 mm diameter) must be provided to release them safely and to avoid odour and explosion risk.

7. Watertightness and material: The tank must be made watertight (RCC, brick masonry with cement plaster, or ferrocement) so that groundwater does not enter and sewage does not leak out and pollute the soil and groundwater.

8. Location and clearance: The tank should be located away from drinking water sources (generally at least 15 m from a well or tube well), away from buildings and boundaries as per the local byelaws, and at a place where the desludging tanker can easily reach.

9. Inlet/outlet levels and slope: Proper slope (about 1 in 40 to 1 in 60) in the incoming sewer and correct invert levels of inlet and outlet must be fixed so that flow enters and leaves by gravity without backing up.

10. Provision for desludging and inspection: An inspection cover/manhole of adequate size (about 500 × 500 mm) should be provided on top for inspection and removal of sludge, and the design should allow easy access for cleaning.

Note: In the exam, write any five of the above points clearly — number of users, detention time, sludge storage, depth and L:W ratio, inlet–outlet arrangement, ventilation, watertightness and location are the most expected ones.

Water-Cement Ratio

The water-cement ratio (w/c ratio) is the ratio of the mass (or volume) of water to the mass of cement used in a concrete mix. It is expressed as a decimal or as a ratio, e.g. 0.50 or 1:2.

$w/c = \frac{\text{Weight of water}}{\text{Weight of cement}}$

It is the single most important factor governing the strength of concrete. Abrams' law states that, for given materials and conditions of curing, the strength of concrete is inversely proportional to the water-cement ratio:

$f_c \propto \frac{1}{w/c}$

Only about 23–25% of water by weight of cement is needed for complete hydration. Any water beyond this remains as free water, which evaporates and leaves voids (capillary pores) that reduce strength. Hence, a lower w/c ratio gives higher strength and lower permeability, but too low a ratio makes the mix unworkable. Typical values are 0.40–0.60 for normal structural work.

Factors Affecting Strength Gain of Concrete

1. Water-cement ratio — The dominant factor. Lower w/c → less capillary porosity → higher strength. A 0.05 increase in w/c can reduce strength by about 10%.

2. Cement content and type — Higher cement content (at the same w/c) improves strength up to a limit. OPC gains strength faster than PPC; rapid-hardening cement gains early strength; sulphate-resisting cement gains strength slowly.

3. Aggregate characteristics — Grading, shape, surface texture, and maximum size affect strength. Well-graded, angular aggregates give better bond; flaky and elongated particles reduce strength. Larger aggregate size lowers water demand but reduces the surface area for bond.

4. Degree of compaction — Proper vibration removes entrapped air and increases density. Poor compaction leaves voids, reducing strength by as much as 30%.

5. Curing — Adequate moisture and temperature must be maintained for hydration to continue. Inadequate curing (especially in the first 7 days) severely limits strength gain. Ideal curing temperature is about 20–25°C.

6. Age of concrete — Strength increases with age. Concrete gains about 70% of its 28-day strength in 7 days and nearly full strength in 28 days; slow gain continues for years.

7. Temperature — High temperature accelerates early strength but lowers ultimate strength; low temperature retards hydration and strength gain.

8. Admixtures — Accelerators (e.g., CaCl₂) speed up early strength; retarders slow it; water-reducing and air-entraining admixtures affect strength indirectly through water demand.

9. Air content — Entrapped or entrained air reduces strength, roughly 5% loss for every 1% of air.

10. Handling and placing — Segregation, excessive water addition on site, and delayed placing all reduce the final strength.

Definition of Contract in Construction Works

A contract in construction works is a legally binding agreement between two or more parties (usually the employer/owner and the contractor) by which the contractor agrees to execute a defined work — construction, repair, or supply — within a specified time, in accordance with agreed drawings, specifications and conditions, in return for a consideration (payment) made by the employer. It creates legal rights and obligations enforceable by law (Contract Act, 2000 in Nepal).

Types of Contracts

1. Item Rate Contract (Unit Price Contract)

• The contractor quotes a rate per unit for each item of work listed in the bill of quantities (e.g., Rs per m³ of concrete, Rs per m² of plaster).
• Payment is made on the basis of actual quantity of work measured at the quoted rate.
• Suitable for: works where quantities cannot be accurately estimated in advance, repair/maintenance works, road and building works.
• Advantage: variation in quantities is easily adjusted; Disadvantage: total cost is not known beforehand, risk of unbalanced bidding.

2. Lump Sum Contract

• The contractor quotes a single fixed sum for completing the entire work as per drawings and specifications.
• Payment is made in instalments/stages (or at completion), not by measurement of individual items.
• Suitable for: works of well-defined scope, small buildings, where detailed drawings are available and changes are unlikely.
• Advantage: total cost known in advance, simple administration; Disadvantage: risky for contractor if quantities or scope change, unsuitable where design is incomplete.

3. Cost Plus Percentage Contract

• The contractor is reimbursed the actual cost of labour, materials, plant and overheads, plus an agreed percentage as profit/commission.
• Suitable: emergency works (flood damage repair, earthquake reconstruction), works of uncertain nature where design is not ready.
• Advantage: work can start immediately, contractor is protected against loss; Disadvantage: no incentive to reduce cost — the contractor earns more as cost rises, so it is the costliest type.

4. Percentage Rate Contract

• The contractor quotes a percentage above or below the estimated cost (PWD/DoR schedule of rates) prepared by the department.
• Payment = estimated cost of work done ± the quoted percentage.
• Suitable: works where departmental schedule of rates exists and quantities are fairly known; commonly used in government works.
• Advantage: tendering is quick and comparison easy; Disadvantage: depends on accuracy of the departmental estimate.

Essential Conditions of Contract

1. Offer and acceptance — a valid proposal and its unconditional acceptance.
2. Lawful consideration — payment or price agreed between the parties.
3. Capacity of parties — parties must be competent (major, of sound mind, not disqualified by law).
4. Free consent — no coercion, undue influence, fraud or misrepresentation.
5. Lawful object — the work must be legal and not against public policy.
6. Certainty of terms — scope, drawings, specifications and quantities must be clear.
7. Time of completion — start date, completion date and extension clause.
8. Contract sum and mode of payment — rates, running bills, retention money, mobilisation advance.
9. Rights and obligations of parties — duties of employer and contractor clearly stated.
10. Quality and workmanship — materials, tests and workmanship standards.
11. Liquidated damages / penalty for delay and defect liability period.
12. Settlement of disputes — arbitration, engineer's decision, jurisdiction.
13. Signatures of competent authority and witnesses, with date.
14. Compliance with prevailing laws — Public Procurement Act/Rules, Labour Act, tax laws.