Concrete has been used for the construction of buildings and other structures for over 120 years and it is due to its ubiquitous use, that some regard concrete as passé. In the 1920’s concrete was regarded as a new and innovative material and some of the earliest structures have shown unsurpassed durability and abilities to be modified to meet today’s needs.
One of the early examples of concrete use was Ward’s Castle, on the border of New York and Connecticut, which was constructed in the 1870’s using reinforced concrete. The fire-proof building is made entirely of reinforced concrete, from the foundation to the mansard roof that caps the two-story main block.
In 1903, the first concrete high-rise was built in Cincinnati, Ohio. The 16-story Ingalls Building was considered a daring engineering feat at the time, but its success contributed to the acceptance of concrete construction in high-rise buildings in the United States. The builder chose concrete because it was fireproof and it would be less expensive to build a structure of this size with concrete than with steel. Still in use today, the building was designated a National Historic Civil Engineering Landmark. During its opening, reporters camped out near the building, expecting a spectacular collapse.
During the past 30 years, many changes have occurred to concrete and other materials used in concrete construction that have made this material more durable, economical and sustainable. The following material will focus on several important changes that demonstrate the versatility of this material.
During the 1970’s, any concrete mixtures which showed 40 MPa or more compressive strength at 28 days were designated as high strength concrete. As the time passed, more and more high strength concrete such as 60 – 100 MPa, were developed which were used for the construction of long-span bridges, skyscrapers etc. According to BS EN 1992-1-1, to be high-strength the concrete should have a compressive strength for design of greater than C50/60.
The term “high performance concrete” was developed by Lacroix and Malier in 1980. ACI defined high-performance concrete as a concrete meeting special combinations of performance and uniformity requirements that cannot always be achieved routinely using conventional constituents and normal mixing, placing, and curing practice.
Benefits of high strength concrete includes:
- Reduction in member size resulting in increased floor areas.
- Reduction in self-weight and super-imposed dead loads.
- Reduction in form-work area and cost with the accompanying reduction in shoring and stripping time due to high early-age gain in strength.
- Longer spans and fewer beams for the same magnitude of loading.
- Reduced axial shortening of compression supporting members.
- Reduction in the number of supports and the supporting foundations due to the increase in spans.
- Reduction in the thickness of floor slabs and supporting beam sections which are a major component of the weight and cost in the majority of structures.
- High-strength concrete can also be used to reduce slab depths and, therefore, a building’s overall height. This can result in significant cost savings whilst slimmer high-strength concrete columns can increase the overall net area; again, more for less.
Producing high-strength concrete is not so different from producing normal strength concrete. The target water/cement ratio should be in the range of 0.30 – 0.35 or even lower. High range water reducers are used to achieve water reduction. A wide range of aggregates can be used with crushed rock offering particular benefits for high-strength concrete. The addition and use of silica fume offers a dense structure with enhanced strength to levels of C90/105 with high levels of chemical and abrasion resistance.
In 1990, an annotated bibliography titled “High Performance Concretes” was published (SHRP-C/WP-90-001.) The definition used was as follows:
- “It should meet one of the following criteria
- A 3-hour strength not less than 3,000 psi
- A 24-hour strength not less than 5,000 psi
- A 28-day strength of not less than 10,000 psi
- A water-cement ratio (including pozzolans) less than 0.36
- It should also have a durability factor not less than 80 after 300 cycles of freezing and thawing”.
In 1993, the American Concrete Institute published the following definition:
“High-performance concrete (HPC) is defined as concrete which meets special performance and uniformity requirements that cannot always be achieved by using only the conventional materials and mixing, placing and curing practices. The performance requirements may involve enhancements of placement and compaction without segregation, long-term mechanical properties, early-age strength, toughness, volume stability, or service life in severe environments”.
The sustainability benefits of high-strength concrete of using less material are further underlined by the possibility of using by-products from other industries such as ground granulated blast furnace slag (GGBS), fly ash and silica fume.
New developments in Ultra High Performance Concrete (UHPC) offer significant potential for even thinner/larger floor slabs and small/longer columns. To be considered as UHPC, a concrete should have a compressive strength of over 150MPa and a flexural strength of over 20 MPa.
Given the recognized benefits of using high-strength concrete, it is surprising that its use is not more widespread. A number of factors may be contributing to this including the lack of specific design codes and unfamiliarity with high strength concretes particularly UHPC. This is especially apparent where current design codes are not readily available for concretes that have many times greater strength than conventional concrete. This will change as experience grows as engineers seek to exploit the optimal designs made possible by using high-strength concrete.
Increasingly, for many commercial and residential projects the only way is up. High-strength concretes will enable taller buildings that are built more efficiently and, therefore, more sustainable.
High-Strength Reinforcing Bars (Rebar)
Utilizing HSRB in concrete members may result in smaller bar sizes and/or a fewer number of bars compared to members reinforced with Grade 60 or lower bars. It may also permit smaller member sizes. By specifying HSRB, the following may be attained:
- Lower placement costs
- Less congestion, especially at joints
- Improved concrete placement and consolidation
- Smaller member sizes
- More useable space
Corrosion-Resistant Reinforcing Steel
Effective corrosion resistance is the product of appropriate design and material selection for conditions as well as responsible field handling.
Steel is manufactured by heating iron ores that are primarily iron oxides along with other ingredients. This heat transforms the iron oxides into the metallic iron. Corrosion of steel is a natural electrochemical process whereby the metal reverts back to its original oxide state.
When steel is placed into concrete, it develops a surface passive oxide film, due to the high pH of the concrete. This passive film prevents further corrosion of the encapsulated metal. The film may be disrupted by carbonation of the cement paste, which reduces the pH, or through the ingress of chloride ions into the concrete, from either deicing salts or sea water.
As the steel corrodes, the released iron ions react with oxygen and water to form expansive iron oxides that may occupy up to seven times the volume of the initial metal. This expansion may cause the surrounding concrete to crack, leading to delamination and spalling.
There are many ways to reduce the risk of corrosion-related distress in concrete. One way is to use reinforcing bars with improved corrosion resistance over traditional unprotected carbon steel reinforcing bars. When selecting a particular type of corrosion resistant bar, issues such as level of corrosion resistance, cost, and availability should be considered.
Lap joints are commonly used to ensure that reinforcing bars are fully developed. As bar sizes increase, so does the length of the required lap, and this may become unwieldy or unfeasible. In these cases mechanical couplers may be provided.
According to CRSI, there are two types of Mechanical Splice;
- Type 1: “tension-compression” mechanical splice
- Type 2: “compression-only” mechanical splice
A Type 2 mechanical splice is required to develop, in tension or compression, at least 125 percent of the specified yield strength of the reinforcing bar.
A Type 2 mechanical splice is required to meet the requirements of a Type 1 mechanical splice and also develop the specified tensile strength of the reinforcing bar.
Rebar Coupler system performs like continuous reinforcement. Independent of concrete, Splicing develops strength mechanically. Therefore, provides ductility in RCC structures independent of condition of concrete. The continuity of spliced rebar offers excellent provision for grounding electrical current.
In dense reinforcement members, congestion of rebars is largely reduced by using coupler system. This helps improves concrete flow & consolidation. Rebar Coupler system provides greater flexibility in design options. The simplicity in detailing of reinforcement, particularly in reinforcement congestion zones minimizes the reinforcement fixing errors, detailing and fixing of seismic reinforcement becomes effortless.
Quality, Cost and Time Advantages
Rebar Coupler system offers quality, cost and time saving. No special skills or equipment are required for fixing couplers. Simple mechanical ways in adopting mechanical splicing compared to lapping, accelerates construction schedules for optimum cost and efficiency. Handling the rebar in convenient sizes saves on valuable crane time. In case of high diameter rebars, considerable length of rebars saves making the Rebar Coupler system economical.
Reinforcing steel needs to be fully developed within the concrete in order for it to be effective. This is either done by extending the bar, termed development length, or by using hooked or bent bars.
However, use of hooked or bent bars is not practical due congestion. In the mid-1980s research was conducted in Norway by Metalock Industries on headed reinforcing steel. In the early 1990s research was undertaken in the USA by Berner, Gerwick and Hoff.
Headed bars are covered by ASTM A970, originally developed in 1998. This requires bar heads to have a diameter 10 times that of the bar and that the concrete used has a strength of 4300 psi or greater.
Pre-fabricated rebar cages can reduce construction times and the tight quality controls of prefabrication can deliver a more reliable product. Further, reduced congestion of the reinforcing steel can allow for better field inspection and easier concrete placement. Combined with high-strength rebar and high-performance concrete, these innovations have the potential to dramatically accelerate construction schedules and reduce fabrication costs while also facilitating more reliable quality control.
Formwork is one of the most critical, but expensive parts of building construction. This cost may be in the range of 40 to 60 percent of the structure cost. Historically, formwork was constructed out of timber, and plywood or moisture-resistant particleboard.
More and more structures are now utilizing engineered formwork systems. In these systems, formwork is built using prefabricated modules with metal frames (usually steel or aluminum) and covered on the application (concrete) side with material having the wanted surface structure (steel, aluminum, timber, etc.). Formwork systems increase the speed of construction and provide for lower life-cycle costs
Re-usable plastic formwork is also available, which consists of interlocking and modular systems. They are lightweight and are suited for low-cost, mass housing schemes.
Insulated concrete forms have also found a marketplace in residential and smaller commercial buildings. This formwork generally is constructed from closed cell expanded polystyrene and is left in place after the concrete has cured. This provides improved thermal and acoustic insulation and voids in the forms provided space for utilities.
Prior to the development of concrete pumping, concrete was delivered in buckets lifted by cranes. The concept of pumping concrete was developed in 1927 by Giese and Hull. They were able to pump concrete to a height of 38 meters (125 ft) and a distance of 120 meters (130 yd). Shortly after, a concrete pump was patented in Holland in 1932 by Kweimn.
In 2008, concrete was pumped 91 stories (1125 ft) at the Trump Building in Chicago. Later that year, a new world record height of 606 m (1984 ft) for pumping concrete was set on the Burj Dubai Tower in Dubai.
One of the major advantages of concrete pumping compared to bucket lifting is the speed at which concrete can be moved. Due to the portability of most concrete pumping machines, concrete can be moved around the construction site with minimal delays. Concrete pumps can extend up and over, and even through other buildings. They can even deliver concrete below ground. Concrete pumps can be placed in areas that do not disrupt other construction operations or adjoining traffic.
Through the use of concrete pumps, less labor is required on the jobsite.
Voided Flat Plate Slabs
The concept of reducing the amount of concrete in a slab has been utilized for many years. One of the more recent innovations is voided slabs, where plastic voids are cast within concrete after being placed in a precise grid. This reduces the dead-load of the slab, which may be as much as 35 percent, enabling longer spans to be covered. In general voided slab systems are economical for spans between 35 and 50 feet.