Concrete and its grades

Concrete is a construction material that consists of cement (commonly Portland cement), aggregate (generally gravel and sand), water and admixtures.

Engineers usually specify the required compressive strength of concrete, which is normally given as the 28 day compressive strength in mega pascals (MPa) or pounds per square inch (psi). Twenty eight days is a long wait to determine if desired strengths are going to be obtained, so three-day and seven-day strengths can be useful to predict the ultimate 28-day compressive strength of the concrete.

Measures to avoid cracking in fresh concrete

Generally, the contractor shall allow for all necessary measures to monitor and avoid cracking in fresh hydrating concrete, regardless the size or volume of the pour. Such measures shall be to the satisfaction of the Engineer and shall be such that maximum surface crack width on hardened concrete measure immediately after the pour does not exceed 0.004 times the nominal cover of the main reinforcement.

The contractor shall allow for and provide approved instrumentation for the measurement of internal temperature changes in large pours. The maximum concrete temperature at the point of delivery shall not in general exceed the lower of either 37 degree C, or 6 degree C above the prevailing shade temperature in accordance with the recommendations of ACI. The limiting internal temperature differential measured across the extreme faces of concrete mass shall not exceed 25 degrees C at any time.

Curing of hardened concrete shall be executed in accordance with the curing specification. Generally, the element surface shall not be cooled to dissipate heat from the concrete. Curing methods, such as the wetting of heated concrete elements exposed to prolonged and direct radiation, which induce temperature gradients within the concrete mass are strictly prohibited.

For large pours, the contractor shall allow for and take extra precautions to reduce concrete temperature gradient and to prevent the loss of surface moisture. Such measures include but are not limited to:

• Keeping all mix constituents shaded where possible to reduce their temperatures in the stockpile

• Cooling of mixing water and/or replacing part or whole of the added water with ice.

• Reducing the cement content by the use of admixtures (but not below that required for the durability)

• Using a cement with a lower heat of hydration

• Injecting liquid nitrogen after mixing of concrete

• Restring the time between mixing and placing of the concrete to not more than 2 hours

• Providing approved surface insulation continuously over all exposed surfaces to prevent draughts and to maintain uniform temperature through the concrete mass

• Initiating curing immediately after final tamping and continue until the approved surface insulation system is fully in place

• Providing shade to the concrete surface to prevent heat gain from direct radiation.

If the surface exhibits crack after compaction, it shall be retamped to close the cracks while the concrete is still in plastic stage.

Requirements for transportation of Concrete

The contents of the mixer shall be discharged in one continuous operation and the concrete transported in such a manner that there shall be no segregation of its constituents. If, in the opinion of the Engineer, any segregation of the concrete materials has taken place during transport, the concrete shall be again turned over and mixed just before it is finally placed in position. No water shall be added to the concrete between the time of mixing and placing except on the written instructions of the Engineer.

Whilst being transported from the mixer to the site of placing, all concrete shall be properly protected from contamination by dust or sand and from excessive moisture gain or loss from rainfall or high temperature, and all equipment used shall be purpose-made for the correct transportation of concrete.

Each batch of concrete delivered to the Site shall be accompanied by a delivery ticket, a copy of which shall be handed to the Engineer’s representative at the time the delivery is made. The following minimum information shall be printed, stamped or written on the delivery ticket:

Name or number of ready-mixed concrete depot:

• Serial number of ticket
• Date
• Truck number
• Name of location of site
• Grade or mix description of concrete, including minimum cement content.
• Specified workability
• Type of cement
• Nominal maximum size of aggregate
• Type or name of admixture, if included, and the dosage quantity
• Quantity of concrete in cubic meters
• Time of loading, defined as the time of contract between cement and aggregates or when these are surface dry, between cement and added water.

The contractor shall complete the following items of information on the delivery ticket after the discharge of each batch of concrete:

• Arrival and departure times of the truck
• Time of completion of discharge
• Details of extra water or any other materials added on site
• Position in the Works where concrete is placed

In the case of truck mixed concrete, the water may be added either at the concrete supplier’s plant or under the Contractor’s supervision after arrival at Site, but not in transit. Transporting the concrete mixture manually is only applied for the distance not over 200m. If the concrete mixture is segregated, need to re-mix before filled into formworks. When using the hung containers to transport the concrete mixture, the concrete mixture poured into the hung containers is not exceed 90 – 96% of the container volume.

Transporting the concrete mixture by truck or specific vehicle, need to ensure for the following requirements:

• The thickness of the concrete layer in the truck body need to be greater than 40 cm if using self-dump truck;
• If using specialized vehicle, both in transportation and mix, the transportation technology is determined by technical parameters of using equipment.

When using concrete pump for transportation, need to ensure for the following requirements:

• The components and slump of concrete need to be tested and trial pumped in order to ensure for the concrete quality and constructing condition, concurrently, suitable for technical function of pump equipment.
• When constructing in the hot conditions, the exterior surface of pipe need to covered or white painted in order to limit the radiation from the sun to heat the concrete.