CELE Construction Management & Methods — Construction Estimates and Quantity SurveyingSummary

The systematic measurement of materials, labor, and work items directly from construction drawings. For concrete work, volume is calculated as section area multiplied by length (m³). Reinforcing steel quantities are determined by counting bars, measuring lengths, and applying unit mass formulas. Formwork is measured as the contact area of concrete surface formed. Earthwork volumes are determined from cut and fill calculations. The take-off must be comprehensive, organized by work item, and cross-referenced to drawings to ensure nothing is omitted. Accuracy at this stage directly impacts all downstream cost calculations.

Concrete volume is calculated from geometric dimensions: V = length × width × height (m³). The cement requirement depends on the concrete class and mix proportion. Class A (1:2:4 proportion) requires approximately 9 bags of cement per m³; Class B (1:2.5:5) requires approximately 7.5 bags/m³. Other materials (sand, gravel) scale proportionally. The number of bags per m³ is derived from absolute volume calculations: one 40 kg bag of cement occupies about 0.0244 m³. For example, Class A 1:2:4 yields (1 + 2 + 4) = 7 parts total; 1 m³ ÷ 7 ≈ 0.143 m³ per part, requiring 0.143 ÷ 0.0244 ≈ 9 bags. ACI 318 and NSCP 2015 specify concrete strength classes and mix design procedures; estimates typically use standard class designations (Class A, B, C) per Philippine practice.

The mass of round reinforcing bars is calculated using the formula: unit mass (kg/m) = 0.006165 × d², where d is the bar diameter in millimeters. This formula derives from the density of steel (7850 kg/m³) and the cross-sectional area of a round bar. For example, a 25 mm diameter bar has unit mass = 0.006165 × 25² = 0.006165 × 625 = 3.85 kg/m. A 20 mm bar: 0.006165 × 400 = 2.47 kg/m. A 16 mm bar: 0.006165 × 256 = 1.58 kg/m. The total steel mass for a bar is calculated as: total mass (kg) = unit mass × length (m). Multiple bars are summed: total = Σ(unit mass × length) for all bars. AISC 360 and NSCP 2015 specify standard bar sizes and properties; the unit mass formula is universally applied in Philippine practice.

Formwork is measured as the contact area of concrete surface that requires forming in m². For a beam of rectangular section 0.3 m × 0.5 m and length L, the formwork area includes two sides (0.3 × L each) and one soffit (0.5 × L), totaling 1.1L m². For a slab of dimensions A × B × thickness, formwork is typically A × B m² (bottom surface) plus edge strips if required. Columns require formwork on all four sides; a square column 0.4 m × 0.4 m × height H requires 4 × 0.4 × H = 1.6H m². Formwork cost includes materials (plywood, lumber), labor for construction and stripping, and equipment (props, shores). The unit cost (₱/m² of formwork) varies by complexity, concrete face finish required, and reuse potential. Multiple uses reduce the effective unit cost per use.

Direct cost is the sum of all costs directly attributable to the work item: Direct Cost = Materials + Labor + Equipment. Materials include concrete, steel, formwork supplies, finishes, and any other consumables. Labor includes productive work hours (pouring, finishing, stripping formwork) at the applicable wage rate. Equipment includes small tools and major equipment (mixer, crane, vibrator) allocated to the work item. Labor productivity (hours per unit of work) depends on crew size, experience, working conditions, and complexity. A typical concrete finishing rate might be 0.8–1.2 m²/hour; a 100 m² slab requires 80–125 labor hours. Equipment costs may be allocated per unit (e.g., mixer cost per m³ concrete) or per time period (e.g., crane rental per day). All direct costs are expressed in the same unit (m³, m², m, kg, etc.) to calculate a unit rate.

The bid price is calculated as: Bid Price = Direct Cost × (1 + Markup Factor). The markup factor covers indirect costs (project overhead, site administration, supervision, safety, insurance, temporary facilities) and profit (the contractor's business margin). Contingency for unforeseen conditions is often included as a percentage of direct cost (typically 3–5% for routine work, higher for complex or uncertain projects). Overhead and contingency combined (OCM) typically range from 10–15% of direct cost; profit margins range from 5–15% depending on project risk and market conditions. Thus, markup = OCM + profit might be 15–25% total. For example, if direct cost is ₱500,000 and markup is 25%, the bid price = 500,000 × 1.25 = ₱625,000. In the Philippines, Value Added Tax (VAT) is typically added to the bid price per tax regulations; if VAT is 12%, the final price = 625,000 × 1.12 = ₱700,000. The markup factor should be clearly defined in each estimate to avoid ambiguity.

A complete estimate is organized in a structured format, typically following the CSI MasterFormat or local Philippine construction practice. Each work item (e.g., 'Concrete slab, 150 mm, Class A') lists quantity, unit of measure, unit rate (₱/unit), and extended cost (quantity × unit rate). Related items are grouped (concrete work, formwork, reinforcement) to facilitate review and cost control. Subtotals are taken for each major section. The estimate includes a summary sheet totaling all subtotals, identifying direct cost, markup, and bid price. Assumptions and exclusions are noted (e.g., 'excludes painting', 'includes all labor up to curing'). Supporting calculations and take-off sheets are attached. In large estimates, a contingency line is often included separately to reflect project risk. The format should be clear enough for a third party (estimator, cost engineer, project manager) to trace any line item back to the calculations and drawings.

Common errors include: (1) Incorrect unit mass formula for steel—using area instead of 0.006165d², resulting in errors of 20–50%. (2) Confusing concrete class and cement bags per m³—Class A vs. Class B difference of ~1.5 bags/m³ affects cost significantly. (3) Omitting or duplicating formwork, especially on complex sections or multi-level structures. (4) Failing to account for waste and breakage in materials (typically 5–10% allowance). (5) Inconsistent units (mixing m and cm, m³ and liters). (6) Forgetting labor for setup, cleanup, and ancillary tasks. (7) Underestimating equipment time and allocation. (8) Applying markup to components instead of total cost, resulting in double-counting. Verification includes: (a) Cross-checking quantities against multiple take-offs or scaled drawings. (b) Comparing unit rates against recent similar projects or supplier quotes. (c) Reviewing markup as a percentage of direct cost to ensure reasonableness (typically 15–25%). (d) Checking that all items on the drawings are captured in the estimate. (e) Ensuring units are consistent throughout (all m³, all m², etc.).