BS8007 Allowable Crack Width of 0.2mm for Severe Exposure in Water-Retaining Structures

Introduction to BS8007 and Crack Width Requirements

BS8007, the British Standard for the design of concrete structures for retaining aqueous liquids, establishes rigorous crack width limitations that are fundamental to ensuring the serviceability and durability of water-retaining structures such as reservoirs, water towers, swimming pools, sewage treatment tanks, and liquid containment basins. The standard specifies a maximum allowable crack width of 0.2 millimeters for structures subjected to severe or very severe exposure conditions, a requirement that has profound implications for reinforcement detailing, concrete mix design, construction practices, and long-term maintenance. Understanding the rationale behind this stringent crack width limit is essential for structural engineers designing water-retaining structures, as it directly affects the critical steel ratio calculations for thermal reinforcement and the overall structural configuration. This article examines the technical basis for the 0.2 mm crack width limit, the mechanisms of water leakage through cracks, the impact of exposure conditions on durability, and the design and construction measures required to achieve compliance with BS8007 requirements.

The Physical Basis for Crack Width Limitation

The fundamental reason for limiting crack width in water-retaining structures is the relationship between crack geometry and water leakage. Water leakage through a cracked concrete section depends on several factors including the crack width, the hydraulic pressure differential across the section, the crack path tortuosity, and the presence of any self-healing mechanisms. Research has demonstrated that cracks wider than approximately 0.2 millimeters create continuous pathways through the concrete section that allow water to flow under normal hydrostatic pressures encountered in water-retaining structures. Below this threshold, cracks tend to be discontinuous and self-healing mechanisms, including continued hydration of unreacted cement particles and precipitation of calcium carbonate, can effectively seal the crack over time. The 0.2 millimeter limit represents a balance between structural economy and serviceability, as tighter crack width limits would require significantly more reinforcement and stricter construction controls, while looser limits would compromise watertightness. The standard recognizes that some cracking is inevitable in reinforced concrete due to the low tensile strength of concrete, thermal and shrinkage strains, and applied loads. The objective is therefore not to eliminate cracks entirely but to control their width to acceptable levels. The purpose of indirect tensile strength requirements in water-retaining structures is closely related to crack control, as the tensile strength of concrete directly influences the cracking behavior and the amount of reinforcement needed for crack width control.

Table 1 shows the relationship between crack width and water leakage rate for typical concrete sections under various pressure heads.

Severe and Very Severe Exposure Conditions

The classification of exposure conditions in BS8007 is critical for determining the applicable crack width limit and the required concrete cover to reinforcement. Severe exposure conditions include environments where the structure is subjected to alternate wetting and drying, freeze-thaw cycles, or contact with aggressive chemicals, all of which are common in water-retaining structures. Very severe exposure conditions involve aggressive chemical environments such as sewage, industrial effluent, seawater, or deicing salts, where chemical attack can accelerate the deterioration of the concrete and reinforcement. The 0.2 mm crack width limit for these exposure classes is based on the understanding that crack widths exceeding this threshold allow aggressive agents to penetrate to the reinforcement more rapidly, accelerating corrosion and reducing the service life of the structure. The relationship between crack width and corrosion rate is particularly important for water-retaining structures because the constant presence of moisture and potentially aggressive chemicals creates an environment highly conducive to reinforcement corrosion. Chloride ions, which are present in many water environments and are the primary cause of reinforcement corrosion, can penetrate through cracks more readily when crack widths exceed 0.2 mm. Additionally, carbonation of the concrete, which reduces the alkalinity of the concrete and destroys the passive layer protecting the reinforcement, proceeds more rapidly at crack locations. The selection between mild steel versus high yield steel reinforcement significantly affects crack width control because the yield strength and bond characteristics of the reinforcement influence the crack spacing and width under service loads.

Design and Detailing Requirements for Crack Control

Achieving the 0.2 mm crack width limit under severe exposure conditions requires careful attention to reinforcement detailing, concrete mix design, and construction practices. The reinforcement design must ensure that the steel ratio is sufficient to control cracking, with the minimum reinforcement area calculated based on the section dimensions, concrete strength, and steel yield strength. BS8007 specifies a minimum steel ratio based on the concept of the critical steel ratio, which ensures that the reinforcement can resist the tensile forces released when concrete cracks without yielding, thereby controlling crack widths. The spacing of reinforcement is equally important, with closer spacing producing more numerous but narrower cracks, while wider spacing leads to fewer but wider cracks. The standard limits the maximum spacing of reinforcement to control crack widths, with typical maximum spacings of 150 to 200 millimeters for water-retaining structures. Crack control also depends on the type of movement joint provided, with proper joint spacing and detailing essential for accommodating thermal and shrinkage movements without causing uncontrolled cracking. The consideration of movement joints in water storage tank design is essential for accommodating the volumetric changes that occur due to temperature variations and concrete shrinkage without inducing excessive restraint stresses that could lead to cracking.

Table 2 summarizes the key design parameters for achieving 0.2 mm crack width control under severe exposure conditions.

Construction Practices and Quality Control

The successful achievement of the 0.2 mm crack width limit depends not only on correct design but also on meticulous construction practices that minimize the risk of uncontrolled cracking. Proper curing is perhaps the most critical construction activity for water-retaining structures, as inadequate curing leads to high early-age shrinkage, reduced tensile strength development, and increased cracking propensity. BS8007 requires that curing begin immediately after finishing and continue for a minimum of seven days, with wet curing or the application of curing compounds that prevent moisture loss. The sequence of concrete placement and the provision of construction joints must be carefully planned to avoid the development of high restraint stresses. Construction joints should be located at positions of minimum stress, typically at points of contraflexure in the structural elements, and should incorporate waterstops to maintain watertightness at the joint location. The quality of the waterstops and their proper installation are essential for preventing leakage at construction joints, which are often the most vulnerable locations in water-retaining structures. Temperature control during concrete placement is important for mass concrete elements such as reservoir walls and base slabs, where the heat of hydration can generate significant thermal gradients and tensile stresses. Techniques such as using low-heat cement, reducing cement content through the use of supplementary cementitious materials, pre-cooling the concrete mix, and providing cooling pipes in thick sections help control temperature rise and reduce the risk of thermal cracking. In conclusion, the 0.2 mm crack width limit in BS8007 for severe and very severe exposure conditions is a well-established requirement based on the relationship between crack width and water leakage, durability, and reinforcement protection. Compliance requires integrated design and construction practices that address reinforcement detailing, concrete mix design, joint provision, curing, and temperature control, all of which contribute to the serviceability and longevity of water-retaining structures.