The 1998 edition of IS 785 outlines the criteria for designing, selecting materials, manufacturing, testing, and labeling reinforced concrete poles intended for overhead electrical, traction, and telecommunication lines. It applies to poles fabricated through mechanical compaction and addresses factors such as transverse load resistance, embedding depth, reinforcement details, and curing processes. This standard is pivotal for producers, structural engineers, and utility operators ensuring the reliability and safety of concrete poles used in power and communication networks.
Overview
The 1998 edition of IS 785 outlines the criteria for designing, selecting materials, manufacturing, testing, and labeling reinforced concrete poles intended for overhead electrical, traction, and telecommunication lines. It applies to poles fabricated through mechanical compaction and addresses factors such as transverse load resistance, embedding depth, reinforcement details, and curing processes. This standard is pivotal for producers, structural engineers, and utility operators ensuring the reliability and safety of concrete poles used in power and communication networks.
Audience
Contents
Structure
IS 785 defines the technical and dimensional specifications for reinforced concrete poles used in overhead electrical and telecommunication lines. It mandates rounding off final test data following IS 2:1960, preserving the number of significant digits. Sampling and testing procedures specify sample sizes and permissible defective quantities based on lot size, with detailed tables guiding acceptance criteria. A failed transverse strength test requires doubling the sample size for retesting; absence of failure in this retest leads to lot acceptance. Notes clarify the flexibility in sampling numbers and reference relevant Indian Standards.
This section defines key terms employed throughout the standard to ensure consistency. It reiterates the requirement to apply IS 2:1960 for rounding off test outcomes. Formwork must sustain its shape during concrete placement and compaction, with tight joints to prevent leakage of cement slurry. Sampling and testing protocols reiterated here emphasize sample sizes and defect limits, along with procedures for retesting in case of failures.
IS 785 references several Indian Standards for materials utilized in pole manufacture, including Ordinary Portland Cement grades (IS 269, 8112, 12269), Portland Pozzolana Cement (IS 1489 Parts 1 & 2), Portland Slag Cement (IS 455), and Rapid Hardening Cement (IS 8041). Aggregates must comply with IS 383, and reinforcement steels with IS 432 and IS 1786. The section emphasizes adherence to material quality parameters, detailed mix design, and reinforcement detailing as per IS 456. It also notes the necessity of formwork integrity and appropriate rounding of test data.
This part prescribes minimum planting depths relative to pole lengths to ensure stability under transverse loads. For poles measuring 6.0 to 7.0 meters, a minimum embedment of 1.20 meters is required, while poles 7.5 to 9.0 meters long need at least 1.50 meters. The working transverse load is assumed to act 600 mm below the top of the pole. Sampling and testing include verification of overall length, cross-sectional dimensions, and vertical alignment. The overall pole length equals the sum of the above-ground length and the planting depth, guaranteeing structural stability.
Reinforcement for poles includes mild and medium tensile steel bars or wires complying with IS 432 (Parts 1 & 2) and high strength deformed bars/wires conforming to IS 1786. Placement must be precise with corrosion-resistant supports to maintain minimum concrete cover, which should be the greater of aggregate size plus 2 mm or 20 mm minimum. Reinforcement surfaces must be free from contaminants like rust, oil, or clay to ensure strong bonding with concrete. Proper fixation and cover are critical for durability and structural integrity.
The design section consolidates key formulas and acceptance tables for sampling and testing. It includes the load factor calculation as the ratio of ultimate transverse load capacity to the load at first crack, ensuring design loads do not exceed permissible limits. Sampling sizes and acceptance numbers are reiterated with procedures for retesting defective samples. Formwork requirements and rounding rules per IS 2:1960 are also addressed to maintain quality and consistency.
Concrete must be placed immediately after mixing, avoiding use once initial setting commences. After placement and mechanical compaction (via spinning, vibrating, or shocking), the concrete must remain undisturbed throughout setting. Hand compaction is strictly prohibited to prevent defects. Proper curing involves maintaining moisture for at least seven days (or 28 days for high-strength concrete) using water curing, wet coverings, or curing compounds. Formwork must retain shape and prevent leakage. These practices ensure the production of strong and durable poles.
Transverse strength testing is conducted following IS 2905 standards. Poles must withstand ultimate transverse loads equal to or exceeding design ultimate loads, factoring in wind and snow loads where applicable. Sampling for this test adheres to specified lot sizes. The load factor ensures the design accounts for safety margins, and concrete strength verification follows IS 456 and IS 516. Testing confirms the pole’s ability to resist transverse forces encountered during service.
Sampling sizes and acceptance criteria for dimensional and transverse strength tests are detailed based on lot size. If initial tests reveal failures in transverse strength, the sample size is doubled for retesting; absence of failure in this retest results in lot acceptance, otherwise rejection. Test results must be rounded consistent with IS 2:1960, preserving specified decimal places. This systematic approach guarantees quality control through statistically valid sampling.
Poles must bear clear, permanent marks applied either during or immediately after production, prior to testing. Markings should remain visible after installation and include the manufacturer’s identity, month and year of fabrication, serial number, and the center of gravity indicated as 'C.G'. Optionally, poles may carry the BIS Standard Mark subject to licensing conditions. This facilitates traceability, inventory management, and quality assurance.
This annex lists essential Indian Standards referenced by IS 785 related to materials, testing, and design, such as IS 269, IS 383, IS 1786, IS 432, IS 455, IS 456, IS 516, IS 875, IS 8112, and IS 12269. These standards collectively ensure compliance with quality and performance requirements. Users should verify the latest editions for current validity. The annex also highlights the legal framework governing the use of the BIS Certification Mark.
The Cement Matrix Products Sectional Committee CED 53, responsible for IS 785, comprises representatives from industry, municipal bodies, rural electrification agencies, research institutions, and government departments. The chairman and members hail from diverse organizations ensuring comprehensive expertise. Alternates are assigned to members. This multidisciplinary composition ensures thorough technical review and consensus during standard formulation.
Frequently Asked
IS 785 permits various cement types including 33, 43, and 53 grade Ordinary Portland Cement (referenced in IS 269, IS 8112, IS 12269), Portland Slag Cement (IS 455), Portland Pozzolana Cement based on fly ash and calcined clay (IS 1489 Parts 1 & 2), and Rapid Hardening Portland Cement (IS 8041). Aggregates must comply with IS 383 specifications, with manufacturers required to submit samples for purchaser approval when requested. These materials ensure the concrete poles achieve the necessary durability and strength for overhead power and communication applications.
Transverse strength testing for reinforced concrete poles under IS 785 is performed according to IS 2905. The poles must demonstrate ultimate transverse load capacity equal to or greater than the design ultimate load, which includes wind and snow loads multiplied by a load factor. Sampling sizes for testing are defined based on lot size. The concrete’s compressive strength is verified per IS 456 and IS 516 to ensure material adequacy. This testing guarantees the poles can reliably resist transverse forces encountered in service.
Reinforcement in concrete poles must use mild or medium tensile steel conforming to IS 432 (Parts 1 & 2) and high strength deformed bars per IS 1786. The concrete grade should be at least M25. Though IS 785 does not explicitly state cover thickness, common practice is to maintain a minimum concrete cover between 25 mm and 40 mm depending on exposure conditions to protect reinforcement from corrosion. Reinforcement must be precisely positioned and secured with corrosion-resistant supports, ensuring holes for cross arms and attachments do not compromise structural integrity.
IS 785 allows mechanical compaction methods including spinning, vibrating, and shocking to ensure thorough consolidation of concrete. Hand compaction is prohibited because it cannot achieve uniform density or adequately eliminate voids and honeycombing, especially in densely reinforced or precast elements. Mechanical methods provide consistent energy application leading to higher concrete strength and durability essential for pole performance.
Poles must be indelibly marked either during curing or immediately after production but before testing. Markings, which must remain visible after installation, should include the manufacturer's identity, manufacturing month and year, a unique serial number, and indication of the pole’s center of gravity denoted as 'C.G'. This practice aids traceability, quality assurance, and inventory management throughout the pole’s service life.
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