Steel reinforced concrete pipe is a material unlike most other concrete products. It has a unique history, is made using specialised processes, and results in a material that provides strength, durability and sustainability. The topics below will give you plenty of detailed steel reinforced concrete pipe information.
History
The construction of pipes and underground conduits dates back thousands of years and is one of the earliest forms of civil engineering construction. The Romans developed cement and concrete similar to that used today. They mixed slaked lime with a pozzolanic volcanic ash from Mt. Vesuvius to produce hydraulic cement that hardened under water and would not deteriorate when exposed to moisture. Some pipelines and aqueducts constructed using this concrete are still in use today.
The oldest recorded modern-day concrete pipe installation is a sanitary sewer constructed in 1842 at Mohawk in New York State, USA. It remained in operation for over 100 years. The French were the first to incorporate steel reinforcement in concrete pipe in 1896 (known as the Monier patent). The concept was brought to America in 1905 and to Australia in 1910. Since then, over 300,000 kilometres of steel-reinforced concrete pipe have been laid in Australia and New Zealand in drainage, road culvert, sewer and pressure pipe applications. Many of these pipes are still in operation and attest to the long service life of spun steel-reinforced concrete pipe. Indeed asset owners can now confidently plan on a 100-year service life for steel-reinforced concrete pipe.
Australian and New Zealand Standards
Reinforced concrete pipe is recognised as the durable and economical solution for drainage pipelines. This has been further emphasised by the latest Australian and New Zealand Standard’s for concrete pipe which recognise a service life in excess of 100 years for this benchmark product.
Standards Australia and Standards New Zealand are each recognised as the peak standards body in their respective countries. These Standards are derived by specialist committees with knowledge and extensive experience in manufacturing, research, development and in service performance.
The concrete pipe Standards detail the specification of the product to ensure it will provide the intended service life in a range of conditions. The guidelines under which reinforced concrete pipe quality is achieved differ quite markedly from those set for other concrete products, as it is manufactured using unique production methods, and is placed in underground conditions not typical of the exposure conditions expected for above ground elements.
The reinforced concrete pipe industry is guided by two standards on this basis for manufacture, durability, design and installation:
AS/NZS 4058 “Precast concrete pipes – pressure and non-pressure” outlines the minimum requirements for materials and manufacture of precast reinforced concrete pipes. It classifies pipes on the basis of size, strength and application, and sets minimum requirements for sampling and testing. The Standard is the benchmark for concrete pipe manufacturers. This document is also essential for designers and specifiers to ensure correct specification for each application.
AS/NZS 3725 “Design for installation of buried concrete pipes” sets out the methods and data required for calculating working loads on buried concrete pipe, relating this to the correct selection of reinforced concrete pipe and specifying details of the installation. The Standard details design and installation criteria for a broad range of applications.
Uses
Concrete pipe has a long history of excellent performance as a durable product for stormwater drainage and sewer applications throughout the world. In Australia, concrete pipes have been manufactured for over 100 years, and there are pipes still in service that were made over 90 years ago.
Today, new technology is making concrete pipe more reliable than it has ever been before. Decades of research and development of many aspects of concrete pipe has enabled concrete pipe producers to implement concrete mixes and pipe design to provide products that can withstand a complete range of underground environments and effluent profiles.
This, combined with the application of a sound asset management approach to public infrastructure reinforces the choice of concrete pipe for sustainable stormwater drainage systems. A drainage pipeline built today with low-maintenance steel reinforced concrete pipe would last for over 100 years, if the system is planned and designed with full knowledge of existing and future effluent characteristics and loading. When projects are designed with life cycle costs in mind, concrete pipe is a product that easily falls within the accepted general notion of sustainability by meeting the needs of the present generation, without compromising the needs of future generations.
Installation practice, according to standards and accepted codes of practice, are proving to be able to reduce the installation costs of construction projects significantly. For pipeline systems that are expected to last 100 years or more, there is no doubt that a concrete pipe manufactured, designed, and installed in accordance with AS/NZS 4058 and AS/NZS 3725, will keep on functioning for many more years. Based on durability and performance, concrete pipe is the confident choice for stormwater drainage and sewer pressure systems.
Hydraulics
The hydraulic capacity (the amount of water a pipe can convey) of all types of pipe depends on the smoothness of the interior pipe wall. The smoother the wall, the greater the hydraulic capacity of the pipe. Smoothness of pipe can be represented by any of the following:
- the Colebrook Roughness Coefficient “ks” mm
- the Hazen and Williams “c”
- Manning’s Roughness Coefficient “n”
In all, the lower value, the greater the volume of water that will flow through pipe.
Hydraulic analysis for drainage systems involves the estimation of the design flow rate based on climatological and watershed characteristics. The hydraulic design of a drainage system always includes an economic evaluation. A wide spectrum of flood flows with associated probabilities will occur at the site during its design life. The benefits of constructing a large capacity system to accommodate all of these storm events with no detrimental flooding effects are normally outweighed by the initial construction costs. An economic analysis of the trade-offs is performed with varying degrees of effort and thoroughness. Risk analysis balances the drainage system cost with the damages associated with inadequate performance. With concrete pipe, there is no risk. With its long service life and hydraulic efficiency, concrete pipe handles the requirements of a system’s hydraulic design.
The selection of appropriate roughness coefficients for stormwater drainage is not precise because of the necessity to assess the effects of any debris which is carried by the stormflows. Unfortunately, but understandably, there is a dearth of relevant test data for in-service stormwater drains. To design a stormwater drainage system without allowance for debris (that is, for clean water with “ks”= 0.06 mm for concrete pipe), represents an unlikely situation. Equally, the effect of debris on equivalent pipe roughness is unlikely to be as severe as the influence of biological slimes in a heavily slimed sewer. For these reasons the concrete pipe industry recommends the adoption of a “ks” value of 0.6 mm for most stormwater drain designs, but this value of “ks” should be modified through engineering judgment where additional data is available. A value of “ks” of 0.6 mm is conservative compared with the “ks” range (0.15 mm to 0.30 mm) recommended in Australian Rainfall and Runoff, but again it should be noted that generally the cost penalty for adopting “ks”= 0.6 mm compared with 0.06 mm is at most one step in pipe diameter.
Research has concluded that designs using concrete pipe can be downsized by at least one size in most cases when compared to steel, aluminium, and lined corrugated HDPE pipe. For design engineers and owners to select the proper drainage pipe for a specific culvert or sewer application, it is critically important that the applied roughness co-efficient values are design values rather than laboratory values
Manufacture
The machinery and equipment used for the manufacture of steel reinforced concrete pipe in Australia and New Zealand is characterised by an ability to handle and compact concrete that is low in water content but high in cement, and thus, has low workability. The methods used in the two countries include:
- centrifugal roller compaction and heavy vibratory methods
- centrifugal spinning in which the water/cement ratio of concrete is reduced by centrifugal action
- vertical dry-cast methods using bi-directional rollers to compact the concrete
The water/cement ratio of concrete in pipes made using these processes is always less than 0.4 and more commonly in the range 0.3 to 0.35. This combination of such low water/cement ratios and high levels of compaction commonly achieves concrete compressive strengths up to 60 MPa and above. The concrete thus produced is practically impermeable to water and has the highest level of durability which can be achieved by any commercial concrete casting process. High strength concrete, low in permeability is recognised by Standards Authorities as providing a durable material.
Testing
AS/NZS4058 outlines a number of performance tests that must be carried out by manufacturers to demonstrate compliance of finished concrete pipe.
Tests included in the Standard are:
- test proof load testing
- ultimate load testing
- water tightness (formerly know as hydrostatic testing)
- specified and ultimate pressure tests
- water absorption
- flexible joint assembly
- measurement of concrete cover to reinforcement
- measurement of dimensions other than concrete cover
Added to this, concrete pipe manufacturers have strict quality control procedures that are implemented to ensure the whole manufacturing process is accounted for. In particular this includes the mixing and batching of concrete, with appropriate aggregates, admixtures and binders, using:
- computer controlled weighing and proportioning systems
- computer controlled mixing systems
- automated recording systems
The quality systems are also required to ensure that the welding of steel reinforcement into cages is carefully monitored and tested to ensure compliance.
Properties
Concrete pipe is known as a rigid pipe that provides both structure and conduit when it arrives on site. Flexible pipe systems such as high density polyethylene (HDPE) and polyvinyl chloride (PVC) drainage systems, provide conduit only. Backfill must be properly engineered and applied to provide structure. Imported fill is a must; required for flexible pipe systems.
Concrete pipe is recognised for quality of manufacturing, consistent strength, availability in designs and sizes to serve most installations, being easy to place, and providing a reliable and durable system, particularly under load.
Reinforced concrete pipe produced today is a result of:
- computer aided design and analysis based on Australian and New Zealand standards
- advanced concrete mix designs
- automated and computer controlled batching
- precision fabricated wire reinforcement
- quality driven manufacturing techniques
- improved water tight joints
- new installation standards
Strength
Concrete pipe is a rigid pipe system that relies mostly on the strength of the pipe and is only slightly dependent on the strength derived from the soil envelope. The inherent strength of concrete pipe can compensate for site problems not designed for such as construction shortcomings and higher fill heights and trench depths.
Concrete pipe is less susceptible to damage during construction, and maintains its shape, by not deflecting. Flexible pipe must deflect to reach its maximum installed performance. Flexible pipe is at least 95% dependent on soil support and the installation expertise of the contractor. This is the single most critical factor for using flexible pipe. Specifiers of flexible pipe products must consider design theory balanced against the practicality of installing the products in each application. Concrete pipe in comparison, has an unlimited range of pipe strengths from which to choose, and strength is demonstrated prior to installation.
Concrete pipe strength is standardised by AS/NZS 4058 “Precast Concrete Pipes”. Concrete pipe is expected to be strength-tested by the manufacturer to proof loads, or test loads, as nominated by the standard for particular diameter and class.
Steel reinforcement in concrete pipe adds significantly to its inherent strength. The steel reinforcement is shaped into cages, using precision measures to fabricate a steel mesh by automatic cage welding machines. The cage machines fabricate machine formed details, are dimensionally stable, and have close engineered tolerances.
Joints
Concrete pipes offer a variety of joints. They are not affected by the type of backfill used for the installation. Joint performance must be demonstrated in the plant prior to pipe installation, and joint integrity can be field tested in a variety of ways. With concrete pipe, deflection will not compromise field joint test capability. The cross sectional rigidity of concrete pipe makes joint assembly a simple operation. Rigid joint integrity will minimize the likelihood of embedment intrusion and subsidence of overfill, often referenced as infiltration. These joints include:
Rolling Rubber Rings start off round and are usually stretched over the spigot and positioned (untwisted) in the spigot groove. They then become flattened as they roll up the spigot of the pipe to seal the joint. They are assembled dry without the use of a jointing lubricant. The pipe surfaces must be dry to allow the ring to roll. If the surface is partly wet, it must be dried.
Skid Rings can be either round or v-shaped and are retained in a ring groove as the pipe slides into position. A lubricant is applied to the front face of the ring and the adjacent lead-in surface of the socket. The lubricant is supplied by the pipe manufacturer and is a special solution (often a soft soap mixture). Petroleum based products such as grease should never be used as a substitute as these may attack the rubber compound.
External Bands (“EB’s” or “sand bands”) are used on flush jointed pipe to stop soil entering the pipeline and eroding away the backfill. Such bands are therefore adopted in sandy backfill conditions extending over the joint of “flush jointed” pipe.
Installation
Sound installation practice is required to ensure that a concrete pipeline is provided every opportunity to perform to its ability. For analysis and design purposes the following construction cases are generally considered to provide support for the concrete pipe:
- trench Condition
- embankment Condition (positive projection)
- embankment Condition (negative projection)
Negative projection embankment conditions and induced trench conditions are often approximated by either a trench or positive embankment condition respectively.
Trench Condition
Trenches are defined as narrow excavations in earth or rock. When a pipe is installed in a trench and the trench backfilled, the backfill material will tend to settle over time. This settlement of backfill places a load on the pipe. This load is reduced by the upward acting frictional forces that develop between the fill and the trench sides. The fill (or dead) load acting on the top of the pipe is taken as the weight of the fill material in a rectangular prism over the total width of the trench, less the frictional forces developed at the trench walls. The adjacent natural material is considered to be self-supporting and therefore, does not transfer load to the pipe.
To minimise loads on the pipe, trenches should be kept as narrow as possible. The trench width adopted and therefore the load will depend on:
- excavator bucket width
- trench depth
- pipe diameter
- access required at sides of pipe to install and compact pipe support materials
- need for trench support
Embankment Condition
Embankment condition for a pipe is created when the pipe is laid at or close to natural ground (or as an induced trench) and fill material is placed over the top in the form of an embankment. Irrespective of the nature of the fill material and its method of placement, some settlement of the fill can be expected. In positive projection situations, the pipe projects above the natural ground level and differential fill settlement over the top of the pipe will occur compared to that on each side. The fill load acting on the top of the pipe is taken as the mass of the fill material in a trapezoidal prism over the width (diameter) of the pipe, plus the frictional forces developed between the soil prism and the adjacent embankment fill (compare this with the trench condition). The load on a pipe under embankment conditions will therefore generally be dependent on:
- pipe diameter
- height of embankment fill
Support conditions
A stable and uniform foundation is necessary for satisfactory performance of any buried pipeline. The foundation must maintain the pipe in proper alignment and sustain the weight of earth, traffic and construction loading over the pipeline. The Bed Zone is the area between the pipe and the foundation. It is commonly 100 mm thick and assists in providing even support along the pipe. Its function is to provide support to the underside of the pipe and reduce the intensity of the reactive forces. For concrete pipes larger than 1500 mm in external diameter this thickness should be increased to 150 mm. The Haunch Zone is located directly above the Bed Zone and extends to a height of between 10% and 30% of the outside diameter of the pipe above the Bed Zone. It provides support to the underside of the pipe, thus reducing bending moment effects in the pipe wall by more effectively distributing the applied loads into the foundation. The Side Zone provides support to the pipe sides and extends from the top of the Haunch Zone to a level of at least 50% of the external pipe diameter above the top of the Bed Zone. An Overlay Zone extending to a level 150mm above the top of the pipe provides protection from physical damage by oversize material in the Backfill or Embankment Fill.
Durability
There are a number of concrete properties that influence the durability of the product. These properties include compressive strength, density, water absorption, water/cement ratio, alkalinity (the amount of cement in the concrete), cement type, and aggregates.
The compressive strength of concrete pipe made to Australian and New Zealand Standards is usually in the range of up to 60 MPa and above. The strength of the pipe is dependent on the materials used in the concrete mix, such as aggregates, cementitious material, and additives. It is also dependent on the mix design, manufacturing techniques and the curing process.
Water absorption is primarily used to check the density and impermeability of the concrete used in reinforced concrete pipe. Water absorption can be greatly influenced by both the aggregates and the manufacturing process used. AS/NZS4058-2007 specifies a maximum allowable absorption of 6% for all concrete pipes, and outlines the appropriate testing methods to be used by manufacturers.
A low water/cement (W/C) ratio is considered a trade mark for durable concrete pipe, particularly as high compressive strength is related to this criterion. Typical reinforced concrete pipe in Australia and New Zealand has a W/C ratio that will range between from 0.35 to 0.40. In some instances W/C rations can be achieved even lower than 0.35.
Alkalinity is influenced by the cementitious content in the mix, and includes both cement and fly ash. The key to high alkalinity and proper cementitious content is in the design of the mix, with consideration given to all the material properties used, as well as the manufacturing and curing processes. Alkaline concrete is usually indicated by pH values between 12 and 13.
Concrete pipe aggregates, both coarse and fine, meet the requirements of AS2758. Aggregates are a key element in producing quality concrete and in turn, quality pipe. With regards to strength, durability and performance, all aspects of the aggregates should be considered. These include gradation, absorption, specific gravity, hardness, and in some cases alkalinity.
AS/NZS4058 states that steel reinforced concrete pipe will last for 100 years when it is designed appropriately. This means specifying pipe for the right use, in a defined environment, produced with good quality control, placed with expertise, and cured properly and thoroughly.
