Production and transport of fresh concrete

As a reliable, universal, durable and versatile construction material whose production is globally estimated to amount of nearly 3 tons per person per year, double than all the other materials together, including wood, steel, plastic and aluminum, concrete became the second most consumed substance on Earth right after water.

Concrete constituents

Concrete is complex construction material which is made by mixing aggregate (sand and gravel), water, cement, and small amounts of admixtures for concrete. Fresh cement powder mixed with water produces cement paste. Right after the mixing starts chemically very complex process of hydration. Chemical reactions are followed by physical processes - hardening of the cement paste.

Aggregate accounts 70 – 80 % of concrete weight. The most commonly used aggregate is natural (river) gravel sand, which particles form has a beneficial effect on the workability and the processability, or crushed coarse and fine aggregate which is characterized by a better homogeneous composition, arris and better adhesion with hardened cement.

Concrete can be prepared using the other materials as well. For example, natural aggregates could be replaced by aggregates made of recycled concrete.

Types of natural stone graded aggregate that can be used for making concrete and which are subject of certification, the scope and testing procedures, the limit values of quality characteristics of aggregate, statistical analysis of test results, issuing the certificates and labeling of products placed on the market of the Republic of Serbia are determined by the Order of Mandatory Certification of natural stone graded aggregate for concrete and asphalt ("Official Gazette of SFRY", no. 41/87). (Download the Order)

Cement accounts in concrete with only 10 - 20 % by weight, but it has a very large influence on the concrete properties.

Production of Portland cement is a complex technological process that consists of two basic steps. In the first, through heat processing entering raw materials are fundamentally altered creating clinker. In the second, by grinding and milling clinker with gypsum the final product – cement – is produced.

The choice of cement should be made on the basis of its properties as follows:

• The strength and increment of strength (class of cement)
• The heat of hydration
• Chemical resistance.

The requirements in terms of quality of cement which is incorporate into the concrete structure, then the procedures for the conformity assessment of cement, conformity mark and labeling, as well as the requirements which conformity assessment body must met, are established by Rulebook On Cement Quality ("Official Gazette of the RS No. 34/2013 and 44/2014). (Download the Rulebook)

Admixtures for concrete are substances which by its physical, chemical or combined action affect the improvement of certain properties of fresh and/or hardened concrete. Dosage of the admixtures is usually about 5 % of cement weight, and they are added in the process of preparation of the concrete mixture.

Suitability of admixtures for use as addition for the preparation of concrete, confirms by checking the fulfillment of the requirements determined by Order of Mandatory Certification of admixtures for concrete ("Official Gazette of SFRY", no. 34/1985). (Download the Order)

The same regulation determines the types of admixtures which are subject of mandatory certification, quality characteristics of concrete admixtures which should be tested and test methods, then the process of certification and labeling of admixtures which are placed on the market of the Republic of Serbia.

Water is a necessary ingredient in the concrete mix, because only with its presence a process of cement hydration is possible. In addition, the water in the fresh concrete is also important as a component which provides required viscosity of concrete mixture, i.e. as a component which allows the placing and finishing of the concrete. Mixing water for concrete should not contain ingredients that could have negative affect to the process of cement hydration, as well as those ingredients that may cause corrosion of reinforcement (steel) in reinforced concrete structures.

Drinking water is practically always meets these conditions, so it can be used for concrete without special proof of suitability. However, in all other cases mixing water for concrete must meet the requirements determined by Serbian standard SRPS U.M1.058: 1985 - Concrete - Water for making concrete - Technical requirements and testing methods.

Mixing of concrete

In modern construction, preparation of the concrete mixture is carried out exclusively by machines, wherein this method is based on dosing and mixing of the concrete constituents, in order to obtain a homogeneous mixture.

Concrete could be made:

• on-site (B.I)
•  in concrete plants (> 15 m3 / h) or (> 10 m3 / h, SRPS U.M1.050) ( B.II and B.I).

The production process phases are:

• Delivering and storing of concrete constituents,
• Dosing and mixing,
• Transportation,
• Placing of concrete,
• Finishing of concrete's free surface,
• Curing concrete.

At all phases of the production process, the base tendency is to get a good mixed, homogeneous mixture of the planned composition and specified properties of hardened concrete.

Delivering and storing of concrete constituents

During delivering, concrete constituents should be protected from pollution. Special attention should be paid to the trucks or railway cars or tanks, in case they previously drove material, which in small quantities, may have harmful effect to the process of setting and hardening of the cement. All materials delivered to the concrete plant must have documentation which shows that they are suitable for the purpose, and that their supplier has attest / certificate on their conformity.

More about the delivery and storage of concrete constituents can be found here.

Dosing and mixing

In modern production, the constituents of concrete are dosed by weight, because the volumetric measuring of cement and aggregates results in significant deviations of the planned composition of the concrete. Volume of sand depends on the moisture content, so the mass of sand, in the unit of volume, may vary up to 30 %. In the calculation of a concrete mixture, aggregates are dosed on the basis of the density of saturated surface dry particles, so when it comes to the humidity measurement, that part should be removed because it does not participate in the free water for preparation of cement paste (water/cement ratio). Due to moisture, particle size of sand are stuck together, so usually it is difficult to dose sand from silos in measuring devices. Therefore, at the bottom of the silo are set devices for shaking during discharging. Opening and closing of the silo's bottom during the measuring should be relatively precise, in order of accuracy of measurements. Therefore closures are hydraulically triggered and managed electronically. It is particularly difficult to regulate it when it comes to largest fraction aggregate, where there is a risk that the some particle size gets stuck.

Measuring of cement and fractions of aggregates is usually cumulative, i.e. the batching dispenser fills respectively with individual constituents and measures with the required accuracy. The precision and accuracy of measuring must be checked daily, and calibration of scale carries out in accordance with national legislation which determines criteria for metrological requirements.

Besides the accurate measuring of concrete constituents, it is important that they are dosed in the correct order in the mixer. It is better mixing, with lower costs of energy, if aggregate and cement are first mixed in dry condition, and then with the addition of water. Batching, mixing and emptying the mixer should give a homogeneous mixture of uniform quality of concrete, in the shortest possible time. For some light concrete, in which density of aggregate or other fillings is highly variable, there volumetric batching is more precise.

In concrete plants dosing of liquid admixtures is much easier, than powdered ones. Admixtures are pre-diluted with water so they can be quickly and intensely homogenized in concrete.

More about types of mixers and mixing duration can be found here.

Transportation

The main motto of transportation is that concrete arrives to the place of use in a condition as it was when it left the mixer.

Transportation of fresh concrete is a very delicate operation, as part of the overall technology of concrete works, for the following reasons:

• Possibility of segregation due to:

  • inevitable shakes

  • inattention in filling and emptying the transport vehicles,

• Leakage of cement paste from the vehicle,
• Water evaporation during long transport (particularly in summer);
• Transportation period in function of the beginning of the setting time of cement in the concrete and maintaining the consistency (particularly in summer), - application of a retarders,
• Changes consistency over time while using plasticizers or superplasticizers.

Due to the risk of segregation, already in process of charging transport vehicles with concrete mixture, certain measures of protection against segregation should be taken.

Concrete is being transported from concrete mixer to the place of use in various ways, while there is difference between internal and external transportation concrete. An internal transport means transport of concrete within the site, from the place where concrete is produced to the place of its use, while the external transportation of concrete means the transportation of concrete from plant to the construction site.

Choice of transport vehicle at the construction site depends on the amount of concrete to be placed, the complexity of the concrete blocks, availability, concrete's workability and economy. Today, a common method of internal transport at the site is using a concrete pumps, alternatively, conveyor belts and carts. In special cases, for example in underground works, sprayed concrete and concrete cannons are used. During operation it is important to choose such means of transportation, with which the concrete with determined consistency remains homogeneous. Conveyors consist of appropriate dozers, conveyors cleaners and redirector of concrete, are suitable for the transport of very different consistency of the concrete and the largest maximum particle size aggregate.

Vehicles for external transportation are:

• Truck-mixers (car mixers)
• Silo-buses (transport vehicle with agitating concrete mixture),
• Dumpers,
• Dump trucks,
• Regular trucks with containers (baskets).

Car mixers and silo-buses are used for the transport of fresh concrete of all consistency, while the other transport vehicles are used for the transport of fresh concrete of rigid and low-plastic consistency (due to the possibility of segregation).