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.

10 Things to Remember when doing Concrete Mix Design

Good quality concrete starts with the quality of materials, cost effective designs is actually a by-product of selecting the best quality material and good construction practices. Following are 10 Things to remember during Concrete Mix Design and Concrete Trials.

1. ACI and other standards only serves as a guide, initial designs must be confirmed by laboratory trial and plant trial, adjustments on the design shall be done during trial mixes. Initial design “on paper” is never the final design.

2. Always carry out trial mixes using the materials for actual use.

3. Carry out 2 or 3 design variations for every design target.

4. Consider always the factor of safety, (1.125, 1.2, 1.25, 1.3 X target strength)

5. Before proceeding to plant trials, always confirm the source of materials to be the same as the one used in the laboratory trials.

6. Check calibration of batching plant.

7. Carry out full tests of fresh concrete at the batching plant, specially the air content and yield which is very important in commercial batching plants.

8. Correct quality control procedures at the plant will prevent future concrete problems.

9. Follow admixture recommendations from your supplier

10. Check and verify strength development, most critical stage is the 3 and 7 days strength.

Important note:
Technical knowledge is an advantage for batching plant staff, even if you have good concrete design but uncommon or wrong procedures are practiced it will eventually result to failures.

Structural engineering

Structural engineering
Main article: Structural engineering


Burj Khalifa, the world's tallest building, in Dubai


Clifton Suspension Bridge, designed by Isambard Kingdom Brunel, in Bristol, UK

Structural engineering is concerned with the structural design and structural analysis of buildings, bridges, towers, flyovers, tunnels, off shore structures like oil and gas fields in the sea, and other structures. This involves identifying the loads which act upon a structure and the forces and stresses which arise within that structure due to those loads, and then designing the structure to successfully support and resist those loads. The loads can be self weight of the structures, other dead load, live loads, moving (wheel) load, wind load, earthquake load, load from temperature change etc. The structural engineer must design structures to be safe for their users and to successfully fulfill the function they are designed for (to be serviceable). Due to the nature of some loading conditions, sub-disciplines within structural engineering have emerged, including wind engineering and earthquake engineering.

Design considerations will include strength, stiffness, and stability of the structure when subjected to loads which may be static, such as furniture or self-weight, or dynamic, such as wind, seismic, crowd or vehicle loads, or transitory, such as temporary construction loads or impact. Other considerations include cost, constructability, safety, aesthetics and sustainability.