Specification for Mixing Concrete

 

Concrete shall be mixed at the construction site, at a central mixing plant, in a truck mixer, or by a combination of central plant and truck mixing. Hand-mixing may be used only when approved by the Engineer. No concrete shall be mixed, placed, or finished when the natural light is insufficient, unless an adequate and approved artificial lighting system is operated.

Mixing at site of concrete construction

Concrete shall be mixed in a batch mixer of the type and capacity approved by the Engineer. Mixing time shall be determined by the Engineer in accordance with Method of Test for Variation in Unit Weight of Air Free Mortar in Freshly Mixed Concrete. When results of the above tests are not available, the mixing time shall be longer than 1 1/2 minutes after all the materials have been introduced into the mixer, but in no case shall the mixing time exceed three times the mixing time prescribed above.

Charging of water into the mixer shall begin before the cement and aggregates enter the drum. During mixing, the drum shall be operated at speeds specified by manufacturers. Pick-up blades in the drum of the mixer which are worn down 20 mm or more at any part must be replaced.

The volume of a batch shall not exceed the manufacturer’s rated capacity of the mixer without written permission of the Engineer. No mixer whose rated capacity is less than a one-bag batch shall be used.

Concrete shall be mixed only in such quantities as are required for immediate use, and concrete which is not of the required consistency at the time of placement shall not be used. Re-tempering of concrete will not be permitted. Entire content of the mixer shall be removed from the drum before materials for the next batch are placed therein. Upon cessation of mixing for a considerable length of time, the mixer shall be cleaned thoroughly. Upon resumption of mixing, the first batch of concrete material placed in the mixer shall contain sufficient sand, cement, and water to coat the inside surface of the drum without diminishing the required mortar content of the mix.

Central plant mixing

Mixed concrete shall be transported from the central mixing plant to the site of work in agitator or non-agitator trucks approved by the Engineer.

Agitator trucks shall be equipped with a water-tight revolving drum, and shall be capable of transporting and discharging concrete without segregation. The agitation speed of the drum shall be between 2 and 6 revolutions per minute. The volume of mixed concrete permitted in the drum shall not exceed the manufacturer’s rating nor exceed 70% of the gross volume of the drum. Upon approval of the Engineer, truck mixers may be used in lieu of agitator trucks for transportation of central plant mixed concrete. Gross volume of agitator bodies, expressed in cubic metres, shall be as determined by the mixer manufacturer. The interval between introduction of water into mixer drum and final discharge time shall be a maximum of 45 minutes unless the use of additives has been approved. Depending on the type and usage of the approved additives this interval may be extended up to a maximum of 2 hours.

During this interval the mixture shall be agitated continuously. Bodies of non-agitator trucks shall be smooth and water-tight. Covers shall be provided when needed for protection against rainfall. The non-agitator trucks shall deliver concrete to the work site in a thoroughly mixed and uniform mass. Uniformity shall be deemed satisfactory if samples from the one-quarter and three-quarter points of the load do not differ more than 25 mm in slump. Placing of concrete shall be completed within 30 minutes after introduction of mixing water into the cement and aggregates or if admixture is used at a time to be determined by the Engineer.

Truck mixing

Concrete may be mixed in truck mixers of approved design. Truck mixing shall be in accordance with the following provisions. The truck mixer shall be either a closed, water-tight, revolving drum or an open-top revolving-blade or paddle type. It shall combine all ingredients into a thoroughly mixed and uniform mass, and shall discharge the concrete with satisfactory uniformity. A maximum difference of 25 mm between slumps of samples from the one-quarter and three-quarter points of the discharge load shall be deemed satisfactory.

Mixing speed for revolving drum type mixers shall not be less than 4 revolutions per minute of the drum nor greater than a speed resulting in a peripheral velocity of the drum of 1 metre per second. For the open top type mixer, mixing speed shall be between 4 and 16 revolutions per minute of the mixing blades or paddles. Agitation speed for both the revolving-drum and revolving blade type mixers shall be between 2 and 6 revolutions per minute of the drum or mixing blades or paddles. The capacities of truck mixer shall be in accordance with the manufacturer’s ratings except that they shall not exceed the limitation herein. Standard for normal rated capacity, expressed as percentage of the gross volume of the drum, shall not be more than 50% for truck mixing and 70% for agitating.

The concrete shall be delivered to the site of the work and discharge shall be completed within 45 minutes after the introduction of the mixing water into cement and aggregates unless the use of additives has been approved by the Engineer. Depending on the type and usage of the approved additives this interval may be extended up to a maximum of 2 hours During this interval the mixture shall be agitated continuously. When the concrete is mixed in a truck mixer, the mixing operation shall begin within 30 minutes after the cement has been mixed with the aggregates. Except that when intended for use exclusively as agitators, truck mixers shall be provided with a water measuring device which will measure accurately the quantity of water for each batch. The delivered amount of water shall be within plus or minus 1% of the indicated amount when the tank, if mounted on the truck mixer, is satisfactorily and practically level.

Hand mixing

Hand mixing will not be permitted, except in case of emergency, without written permission from the Engineer. When permitted, it shall be performed only on water-tight mixing platforms made of metal, etc. Concrete shall be turned and returned on the platform at least six times and until all particles of the coarse aggregate are covered thoroughly with mortar and the mixture is uniform.

Curing Concrete

Immediately after forms have been removed and finishing completed, all concrete shall be cured by one of the following methods. The Engineer will specify the concrete surface which may be cured by either method.

Curing concrete using Water method

The entire exposed surfaces other than slabs shall be protected from the sun and the whole structure shall be covered with wet burlap, cotton mats, or other suitable fabric for a period of at least seven days. These materials shall be kept thoroughly wet for the entire curing period. Curbs, walls, and other surfaces requiring a rubbed finish may have the covering temporarily removed for finishing, but the covering must be restored as soon as possible. All concrete slabs shall be covered as soon as possible with sand, earth or other suitable material and kept thoroughly wet for at least seven days. This covering material shall not be cleared from the surface of the concrete slabs for a period of twenty one days. If wood forms are allowed to remain in place during the curing period, they shall be kept moist at all times to prevent them from shrinking.

Membrane forming curing compound

All surfaces shall be given the required surface finish prior to application of the curing compound. During the finishing period, the concrete shall be protected by the water method of curing. Membrane curing compound shall be applied after the removal of forms, or after the disappearance of surface water. It can be sprayed or applied to the concrete surface by means of an applicator in one or more coats at the rate instructed by the manufacturer. Should the membrane seal be broken or damaged before the expiration of the curing period, the damaged area shall be immediately repaired by the application of additional membrane material. The Contractor’s proposals for the use of liquid membrane curing compound and the locations shall be subject to the approval of the Engineer.

Introduction to Sprayed Concrete

 

Sprayed concrete is an excellent tool for stabilization and support of structures in a very short time and for concrete application without using any molds. Sprayed concrete is also the interaction of man, machine and concrete. Sprayed concrete is a high-performance material which functions only as well as these “three components of success”. Man, personified in the work of the nozzle man, requires great technical skill and dedication to the job. The operator must be able to rely fully on the machine and the sprayed concrete material. It is the interaction and quality of these components that finally determines the success of the sprayed concrete application.

In times of rapidly increasing mobility and limited space, the need for underground infrastructure continues to grow. Sprayed concrete has an important role in this requirement. This method is economically outstanding and almost unlimited technically, making it obvious answer.

Sprayed concrete (or shotcrete)  is a single technical term that covers different components of a complete technology:

1. The sprayed concreting process
2. The material sprayed concrete
3. The sprayed concrete system

These three components define a complete technology which has a long tradition, huge potential for innovation and a great future. The material sprayed concrete is a concrete mix design that is determined by the the requirements of the application and the specified parameters. As a rule, this means a reduction in the maximum particle grading to 8mm or  maximum 16mm, an increase in the binder content and the use of special sprayed concrete admixtures to control the properties of the material. Sprayed concrete was used for the first time in 1914 and has been permanently developed and improved over recent decades.

There are now two different sprayed concrete processes:

• dry process sprayed concrete
• wet process sprayed concrete

The main mix requirements focus on the workability (pumping, spraying application) and durability; they are:

• high early strength
• the correct set concrete characteristics
• user-friendly workability (long open times)
• good pumpability (dense-flow delivery)
• good sprayability (pliability)
• minimum rebound

The sprayed concreting process designates its installation. After production, the concrete is transported by conventional means to the process equipment. Sprayed concrete or sprayed mortar is fed to the point of use via excess-pressure-resistant sealed tubes of hoses and is sprayed on and compacted. The following methods are available for this stage of the process:

• the dense-flow process for wet sprayed concrete
• the thin-flow process for dry sprayed concrete
• the thin-flow process for wet sprayed concrete

Before being sprayed, the concrete passes through the nozzle at high speed. The jet is formed and the other relevant constituents of the mix are added, such as water for dry sprayed concrete, compressed air for the dense-flow process and setting accelerators when required. The prepared sprayed concrete mix is the projected onto the substrate at high pressure which compacts so powerfully that a fully-compacted concrete structure is formed instantaneously.  Depend on the setting acceleration, it can be applied to any elevation, including  vertically overhead.

The sprayed concrete process can be used for many different applications. Sprayed concrete and mortar is used for concrete repairs, tunnelling and mining, slope stabilisation and even artistic design of buildings. Sprayed concrete construction has various advantages:

• application to any elevations because sprayed concrete adheres immediately and bears its own weight
• can be applied on uneven substrates
• good adhesion to the substrate
• totally flexible configuration of the layer thickness on site
• reinforced sprayed concrete is also possible (mesh/fibre reinforcement)
• rapid load-bearing skin can be achieved without forms (shuttering) or long waiting times

Sprayed concrete is a flexible, economic and rapid construction method, but it requires a high degree of mechanization and specialist workers are essential.

Total Quality Control in Construction

Quality control in construction typically involves insuring compliance with minimum standards of material and workmanship in order to insure the performance of the facility according to the design. These minimum standards are contained in the specifications described in the previous section. For the purpose of insuring compliance, random samples and statistical methods are commonly used as the basis for accepting or rejecting work completed and batches of materials. Rejection of a batch is based on non-conformance or violation of the relevant design specifications. Procedures for this quality control practice are described in the following sections.

An implicit assumption in these traditional quality control practices is the notion of an acceptable quality level which is a allowable fraction of defective items. Materials obtained from suppliers or work performed by an organization is inspected and passed as acceptable if the estimated defective percentage is within the acceptable quality level. Problems with materials or goods are corrected after delivery of the product.

In contrast to this traditional approach of quality control is the goal of total quality control. In this system, no defective items are allowed anywhere in the construction process. While the zero defects goal can never be permanently obtained, it provides a goal so that an organization is never satisfied with its quality control program even if defects are reduced by substantial amounts year after year. This concept and approach to quality control was first developed in manufacturing firms in Japan and Europe, but has since spread to many construction companies. The best known formal certification for quality improvement is the International Organization for Standardization’s ISO 9000 standard. ISO 9000 emphasizes good documentation, quality goals and a series of cycles of planning, implementation and review.

Total quality control is a commitment to quality expressed in all parts of an organization and typically involves many elements. Design reviews to insure safe and effective construction procedures are a major element. Other elements include extensive training for personnel, shifting the responsibility for detecting defects from quality control inspectors to workers, and continually maintaining equipment. Worker involvement in improved quality control is often formalized in quality circles in which groups of workers meet regularly to make suggestions for quality improvement. Material suppliers are also required to insure zero defects in delivered goods. Initally, all materials from a supplier are inspected and batches of goods with any defective items are returned. Suppliers with good records can be certified and not subject to complete inspection subsequently.

The traditional microeconomic view of quality control is that there is an “optimum” proportion of defective items. Trying to achieve greater quality than this optimum would substantially increase costs of inspection and reduce worker productivity. However, many companies have found that commitment to total quality control has substantial economic benefits that had been unappreciated in traditional approaches. Expenses associated with inventory, rework, scrap and warranties were reduced. Worker enthusiasm and commitment improved. Customers often appreciated higher quality work and would pay a premium for good quality. As a result, improved quality control became a competitive advantage.

Of course, total quality control is difficult to apply, particular in construction. The unique nature of each facility, the variability in the workforce, the multitude of subcontractors and the cost of making necessary investments in education and procedures make programs of total quality control in construction difficult. Nevertheless, a commitment to improved quality even without endorsing the goal of zero defects can pay real dividends to organizations.