High Performance Dry Premixed Concrete

Ferrocement - Innovatives Applications for Architecture

Rating and Rehabilitation of Steel Truss Bridges
 

Development of High Performance Dry Premixed Concrete

(M. Engg. Thesis)

Abstract
Dry premixed concrete is a relatively new product in the concrete industry. It contains a mixture of cement, aggregates and admixtures, all packed in a bag that can be mixed with water to produce concrete of a given strength and workability. The dry premixed concrete available in the market is of low strength (250 ksc), which limits its use in large construction projects where usually high strength and high workability is required. This experimental study was conducted to develop high performance dry premixed concrete, which could give both high strength and economy than other concrete production methods namely in-situ and    ready-mixed concrete production. The target compressive strength was 600 ksc at 28 days for normal workability. The parent materials were selected from Pakistan and Thailand based on the criterion of identical properties. Two types of coarse aggregate namely limestone and andesite were used in two maximum sizes of 10 mm and 20 mm. Fly ash was incorporated as cement replacement material in 20% and 30% amount by weight to reduce the cost. Silica fume was added as 10% by weight of cement to increase the early strength. All the mixtures were produced at three workability levels and tested for compressive strength at 3, 7, 28 and 56 days. Test results showed that dry concrete containing limestone aggregates of 10 mm maximum size and 20% fly ash replacement gave the desired performance for strength and workability. Cost analysis indicated that it was 18% cheaper than ready-mixed concrete but 10% more expensive than in-situ concrete. However, this high performance dry concrete takes only one-third of the time to produce the same quantity as compared to in-situ concrete. This is beneficial in improving the productivity at site and can reduce the overall cost of the project by cutting indirect costs. The selected mix was also tested for flexural strength and modulus of elasticity and found to be satisfactory.

Introduction
Concrete is the Planet’s most widely used man-made construction material. It has one uniqueness over the other materials that it is produced just about an hour before use in the structure in which it will serve for years. Its level of use is an important indicator of a country’s degree of industrial development. In South and Southeast Asia, the demand of good quality concrete is increasing day by day due to the rapid development rate of infrastructure. Concrete is either produced at the site by mixing various raw materials, called ‘in-situ concrete’, or supplied from ready mixed concrete plants through truck mixers, known as ‘ready-mixed concrete’. These methods have been used successfully over the past two decades but now several problems are arising which may offset their continuous use.

If in-situ concrete production is considered, the major associated problems are:

(i) Adequate time is needed before production to search and order suitable materials, which may require laboratory testing, too. Moreover, shortage of any material can cause serious delays in construction; (ii) A large area is required to store the materials. In big cities where construction sites are usually congested, this may become a vital problem; (iii) Quality control cannot be assured due to more human interaction; (iv) More labor is required for transportation and mixing processes as compared to ready mixed concrete; (v) Storage of raw materials increases dust pollution; (vi) More careful planning and scheduling is required to cope with any shortage of resources.

Fig. 1.1 shows a typical construction site in Pakistan where in-situ concrete production is in progress.
 
 

Fig.1.1: A typical construction site in Pakistan showing in-situ concrete production (ABN-AMRO Bank Building, Lahore)

On the other hand, if ready-mixed concrete is taken into account, some disadvantages are: (i) In big cities where traffic congestion is a major problem, any traffic jam can delay the delivery of concrete. This may cause a serious problem when concreting at site is in progress; (ii) Sometimes, the site constraints may prohibit the entry and parking of ready mixed concrete trucks; (iii) A truck mixer normally contains 5-7 m3 of concrete, so if less amount is needed, the rest will be wasted; (iv) Usually 2 days advance order is required, so in emergency, it cannot be used. Moreover, exact schedule has to follow at site; (v) Transportation time may affect the strength and durability of concrete; (vi) Tower crane or concrete pumps are needed to transport concrete into formwork.

To solve such problems, a new product called ‘Dry Premixed Concrete’ was introduced in North America and Canada about five years back. Dry concrete is a mixture of cement, fine and coarse aggregates, and chemical admixtures, all packed in bag that can be mixed with water to produce concrete of given strength and workability. In Thailand, dry concrete had been introduced by UBAU Co., Ltd., Bangkok in cooperation with Target Products, Canada since 1995 while in Pakistan, this product has not been introduced yet.

The benefits achieved from a such a product can be summarized as: (i) It can be used at any time, any place and for any type of construction; (ii) Better quality control can be assured due to less human interaction; (iii) It can be used in small quantities and according to demand, so no chances of wastage; (iv) It reduces dust pollution specially as compared to in-situ concrete; (v) No trials mixes are required before use, as the strength is already specified; (vi) No skilled labor is required.

Dry Premixed Concrete Bag                   Mixing Operation

The dry premixed concrete available in the market is of ordinary strength (250 * 20 ksc), therefore it cannot be utilized in large construction projects where usually high strength and high workability is required. To increase its use, development and production of high performance dry premixed concrete is essential.

High performance concrete may be defined as one with properties superior than conventional concrete. These properties can be early or long-term compressive strength, durability, workability, low heat of hydration or any other property, which is significant for a particular project. In this research, high performance is defined in terms of high compressive strength. There is no universally accepted definition of high strength concrete. It was few years ago when 400 ksc concrete was considered high strength concrete. In the early 1980’s concrete of 500 ksc was utilized but by the end of the decade 600 and 700 ksc concretes became available. Currently, design strengths of 700 to 1000 ksc are being used for commercial applications in buildings. Strengths as high as 1400 ksc have been used, although their commercial applications are limited to a few locations.

In addition to strength and durability, high performance concrete also gives major cost savings by reducing the size of structural members thereby reducing the dead load. It has been estimated that core (e.g. lift shafts) construction costs can be reduced by 12.5% by using 600 ksc rather than 400 ksc concrete. The additional capitalized value from added floor space gives an even more dramatic return. The building owner can expect a return of about one third of the core construction cost. On a 50 storey building, the core construction cost would be reduced from US$ 10 million to US$ 8.7 million and 1350 m² of extra floor space would be provided.

Thus the development and large-scale production of ‘High Performance Dry Premixed Concrete’ seems to be an economical and structurally beneficial solution for today’s construction industry.

[Top]
 

Ferrocement

Ferrocement is a highly versatile form of reinforced concrete, constructed
of hydraulic cement mortar reinforced with closely spaced layers of
continuous and relatively small diameter wire mesh. The mesh may be
made of a metallic or other suitable material. Ferrocement primarily
differs from conventional reinforced or prestressed concrete by the
manner in which the reinforcing elements are dispersed and arranged.

Advantages of Ferrocement

Ferrocement is a suitable technology for developing countries for the
following reasons:

(a) Its basic raw materials are readily available in most countries.
(b) It can be fabricated into any desired shape.
(c) The skills for ferrocement construction can be acquired easily.
(d) Heavy plants and machinery are not involved in ferrocement construction.
(e) In case of damage, it can be repaired easily.
(f) Being labor intensive, it is relatively inexpensive in developing countries.
 
 

                                        Some Ferrocement Projects

For more details, visit International Ferrocement Information Center, Bangkok, Thailand.

[Top]

Rating and Rehablitation of Steel Truss Bridges

(B.Sc. Engg. Project)

Rating is performed to determine the existing life of a bridge or any other structure. Rating usually involves the (i) Physical inspection of the bridge; (ii) Testing of specimens taken from the critical sections; (iii) Analysis of bridge sections under current loads and stress conditions; and (iv) Comparison of stress resultants with available capacity.

After rating, various rehablitation techniques can be suggested such as (i) Strengthening existing sections by using additional steel plates; (ii) External prestressing; (iii) Addition of secondary members etc.

For this project, Nari Bridge, 30 km on Sibi-Quetta Section, Pakistan was selected which is almost 90 years old consisting of 7 spans of 110 m each. Rating was performed and suitable rehablitation techniques were suggested. A computer program was also developed to facilate rating analysis.

As an example, a highway girder bridge was also analysed and its existing life was determined.

All results were varified by suitable laboratory tests of specimens.

[Top]