CELE Construction Management & Methods — Construction Estimates and Quantity SurveyingRevision Notes

Formulas

Exam Tips

• Board problems almost always give dimensions directly — apply V = A × L immediately without converting to cm or mm.
• Memorize the key unit masses: 12 mm → 0.888 kg/m, 16 mm → 1.578 kg/m, 20 mm → 2.466 kg/m, 25 mm → 3.853 kg/m, 28 mm → 4.834 kg/m, 32 mm → 6.313 kg/m.
• For slab formwork: area = plan area (soffit only, unless sides are also formed).
• Always re-read the problem to confirm whether waste is included or excluded from the answer.

Key Points

• Quantity take-off is the systematic measurement of all work items from contract drawings, specifications, and bills of quantities (BOQ).
• The QTO forms the foundation of any cost estimate; errors in quantities directly cause errors in the bid.
• Work items are measured in consistent SI units: volume (m³) for concrete and earthwork, mass (kg or tonnes) for steel, area (m²) for formwork and finishing, and length (m) for pipes and bars.
• Always apply a waste allowance: typically 5–10% for concrete, 3–5% for rebar, and 10–15% for timber formwork.
• Take-off sequence follows the CSI or DPWH trade format: earthwork → concrete → masonry → steel → finishes → MEP.
• In Philippine practice, DPWH Standard Specifications and the NSCP 2015 govern the minimum acceptable material mixes and bar sizes used in the take-off.

Definitions

Section Title

1. Quantity Take-Off (QTO) — Principles and Methods

Common Mistakes

• Computing formwork as a volume (m³) instead of contact area (m²).
• Using gross building dimensions instead of net member dimensions for concrete volumes.
• Forgetting to multiply unit mass by the number of bars AND bar length simultaneously.
• Neglecting lap splices and hooks when computing total rebar length.
• Applying the same cement bags/m³ factor regardless of mix class (e.g., using 9 bags/m³ for Class B instead of 7.5).

Formulas

Exam Tips

• Memorize: Class A = 9 bags/m³, Class B = 7.5 bags/m³, Class C = 6 bags/m³ (40 kg bags).
• When the problem says '1:2:4 concrete,' automatically recall 9 bags/m³.
• Always round cement bags UP to the nearest whole bag for procurement.
• If asked for cost of cement: Cost = N_bags × price per bag.

Key Points

• Concrete mix class is described by the cement : fine aggregate : coarse aggregate ratio by volume.
• Class A (1:2:4) is the most common class for structural elements in Philippine practice and yields approximately 9 bags of 40 kg cement per m³ of concrete placed.
• Class B (1:2.5:5) is used for lightly loaded members and yields approximately 7.5 bags/m³.
• Class C (1:3:6) is used for mass concrete and plain concrete fills; approximately 6 bags/m³.
• A standard bag of Portland cement in the Philippines weighs 40 kg (one bag = 40 kg).
• The DPWH Blue Book (Standard Specifications for Public Works and Highways) provides the official mix tables referenced in Philippine government projects.
• Water-cement ratio governs strength and durability per ACI 318-19 Table 19.3.2; lower w/c → higher strength.

Definitions

Section Title

2. Concrete Mix Proportions and Cement Content

Common Mistakes

• Using 50 kg per bag instead of the Philippine standard 40 kg per bag when computing cement cost.
• Applying Class A cement factor (9 bags/m³) to Class B or C mixes.
• Confusing volume-based mix ratios with weight-based ratios.
• Forgetting that the cement factor already accounts for the volume change from dry mix to placed concrete (yield factor).

Formulas

Exam Tips

• Tabulated unit masses for quick recall: d=10→0.617 kg/m, d=12→0.888, d=16→1.578, d=20→2.466, d=25→3.853, d=28→4.834, d=32→6.313 kg/m.
• Always confirm the bar diameter is in mm before substituting into the formula.
• For mixed diameters in one member, compute each diameter's contribution separately and sum.
• If the problem gives total bar length in metres, do not re-multiply by the number of bars.

Key Points

• The unit mass formula for round deformed bars (ASTM A615 / PNS 49) is derived from the density of steel (7850 kg/m³) and the circular cross-section.
• Unit mass = 0.006165 × d² kg/m, where d is the bar diameter in millimetres.
• This formula is valid for ALL standard deformed and plain round bars used in Philippine construction.
• Total steel mass = unit mass × total length of all bars of that diameter.
• The take-off must account for: main bars, stirrups/ties, splices, hooks, and development lengths per NSCP 2015 Section 425.
• Hooks and bends add to bar length: a standard 180° hook adds approximately 4d; a 90° hook adds approximately 12d (NSCP 2015 Section 425.3).
• Minimum development length ld for 20 mm Grade 60 bars in normal-weight concrete (f'c = 28 MPa) is approximately 300 mm per NSCP 2015 Table 425.5.2.

Definitions

Section Title

3. Reinforcing Steel — Unit Mass and Take-Off

Common Mistakes

• Squaring the radius instead of the diameter in the unit mass formula.
• Using 0.00617 (rounded) versus the exact 0.006165 — use 0.006165 for precision.
• Forgetting to multiply by the number of bars n.
• Ignoring stirrup and tie lengths in the total steel mass.
• Reporting the answer in m (length) instead of kg (mass).

Formulas

Exam Tips

• The formula Bid = Direct Cost × (1 + markup) must be memorized verbatim — it appears in almost every construction management board exam.
• If the problem says '18% OCM and 10% profit,' add them: markup = 28%, Bid = Direct × 1.28.
• Watch for problems that give the bid price and ask for direct cost: Direct = Bid / (1 + markup).
• VAT is always 12% in current Philippine tax law unless the problem specifies a different rate.
• Re-read whether the markup given is on direct cost or on total cost — different base = different answer.

Key Points

• The direct (field) cost includes: materials + labor + equipment (MLE). These are costs directly attributable to specific work items.
• Indirect costs (overhead) cover: project management, site office, utilities, permits, insurance, and head-office expenses — not tied to a single work item.
• OCM stands for Overhead, Contingency, and Miscellaneous — the combined indirect cost allowance expressed as a percentage of direct cost.
• Profit is the contractor's intended earnings, typically 10–15% of the direct cost in Philippine practice.
• The combined OCM + Profit markup in Philippine government bidding is commonly 15–25% of the direct cost.
• VAT (currently 12% in the Philippines) may be added to the total contract price depending on the contract type and the contractor's VAT registration.
• DPWH procurement follows RA 9184 (Government Procurement Reform Act) which requires ABC (Approved Budget for the Contract) computation using a Program of Work (POW) that includes all markups explicitly.
• RA 544 (Civil Engineering Act of the Philippines) governs the professional practice of civil engineers and implicitly underpins the legal basis for signing estimates and POWs.

Definitions

Section Title

4. Direct Cost, Markup, and Bid Price

Common Mistakes

• Applying markup to the bid price (circular) instead of to the direct cost.
• Forgetting to add VAT when the question asks for the final contract price inclusive of all taxes.
• Confusing markup (on cost) with margin (on selling price): margin = markup / (1 + markup).
• Using a markup decimal (e.g., 0.25) directly as the bid price instead of multiplying (1 + 0.25).
• Omitting equipment cost from the direct cost when it is explicitly given.

Formulas

Exam Tips

• Problem 1 (Slab): V = 5×4×0.12 = 2.4 m³; Cement = 9×2.4 = 21.6 → 22 bags (Class A).
• Problem 2 (Column rebar): w = 0.006165×(20)² = 2.466 kg/m; Mass = 2.466×8×3.5 = 69.05 kg.
• Problem 3 (Bid): Direct = ₱1.2M; Bid = 1,200,000×1.18 = ₱1,416,000.
• Problem 4 (Formwork): A = (2×0.5 + 0.3)×6 = 1.3×6 = 7.8 m² (sides + soffit).
• These four exercises correspond exactly to the reference document exercises — master them.

Key Points

• Board exam problems in this topic are typically 3–5 steps: identify work item → compute quantity → apply unit cost or mix factor → apply markup.
• Always write down the given data, formula, substitution, and answer with units before computing.
• Check units at every step: m³ for volume, kg for steel mass, m² for formwork, ₱ for cost.
• In multi-part problems, carry forward rounded intermediate answers carefully to avoid compounding errors.
• Time management: a typical construction management problem should be solved in 90 seconds on the board exam.

Definitions

Section Title

5. Worked Board-Style Problems — Full Solutions

Common Mistakes

• Not labeling intermediate answers with units, causing wrong formula selections in subsequent steps.
• Rounding too early in multi-step problems — keep at least 4 significant figures in intermediate steps.
• Mixing up number of bars with number of sets of bars when computing rebar mass.