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.

Pre-tensioning Systems and Devices

This section covers the following topics.
• Introduction
•  Stages of Pre-tensioning
•  Advantages of Pre-tensioning
•  Disadvantages of Pre-tensioning
• Devices

 Introduction

Prestressing systems have developed over  the years and various companies have
patented their products. Detailed information  of the systems is given in the product
catalogues and brochures published by companies.  There are general guidelines of
prestressing in Section 12 of IS:1343 - 1980.  The information given in this section is
introductory in nature, with emphasis on the basic concepts of the systems. 

The prestressing systems and devices are described for the two types of prestressing,
pre-tensioning and post-tensioning, separately.  This section covers pre-tensioning. 
Section 1.4, “Post-tensioning Systems and Devices”, covers post-tensioning.  In pre-tensioning, the tension is  applied to the tendons before casting of the concrete. The
stages of pre-tensioning are described next.

Stages of Pre-tensioning

In pre-tensioning system, the high-strength steel tendons are pulled between two end
abutments (also called bulkheads) prior to the casting of concrete. The abutments are
fixed at the ends of a prestressing bed.
 
Once the concrete attains the desired strength for prestressing,  the tendons are cut
loose from the abutments.

The prestress is transferred to the concrete from the tendons, due to the bond between
them. During the transfer of prestress, the member undergoes elastic shortening. If the
tendons are located eccentrically, the member is likely to bend and deflect (camber).
The various stages of the pre-tensioning operation are summarised as follows.
1)  Anchoring of tendons against the end abutments
2)  Placing of jacks
3)  Applying tension to the tendons
4)  Casting of concrete
5)  Cutting of the tendons.

During the cutting of the tendons, the prestress is transferred to the concrete with elastic
shortening and camber of the member.

The stages are shown schematically in the following figures.



 Advantages of Pre-tensioning

The relative advantages of pre-tensioning as compared to post-tensioning are as
follows. 
•  Pre-tensioning is suitable for precast members produced in bulk. 
•  In pre-tensioning large anchorage device is not present.

 Disadvantages of Pre-tensioning 

The relative disadvantages are as follows. 
•  A prestressing bed is required for the pre-tensioning operation.
•  There is a waiting period in the prestressing bed, before the concrete attains
sufficient strength. 
•  There should be good bond between concrete and steel over the transmission
length.

Devices

The essential devices for pre-tensioning are as follows. 
•  Prestressing bed 
•  End abutments 
•  Shuttering / mould 
•  Jack 
•  Anchoring device   



An extension of the previous system is the  Hoyer system. This system is generally
used for mass production. The end abutments are kept sufficient distance apart, and
several members are cast in  a single line. The shuttering is provided at the sides and
between the members. This system is also called the  Long Line Method.  The
following figure is a schematic representation of the Hoyer system  

 

The end abutments have to be sufficiently  stiff and have good foundations. This is
usually an expensive proposition, particularly when large prestressing forces are
required.  The necessity of stiff and strong foundation can be bypassed by a simpler
solution which can also be a cheaper option.   It is possible to avoid transmitting the
heavy loads to foundations, by adopting self-equilibrating  systems. This is a common
solution in load-testing. Typically, this is done by means of a ‘tension frame’.  The
following figure shows the basic components  of a tension frame.  The jack and the
specimen tend to push the end members.  But the end members are kept in place by
members under tension such as high strength steel rods.     

 
The frame that is generally adopted in a pre-tensioning system is called a stress bench.
The concrete mould is placed within the frame and the tendons are stretched and
anchored on the booms of the frame.  The following figures show the components of a
stress bench. 



The following figure shows the free body diagram by replacing the jacks with the applied
forces.


The following figure shows the stress bench after casting of the concrete.


Jacks 
The jacks are used to apply tension to the tendons. Hydraulic jacks are commonly used.
These jacks work on oil pressure generated by a pump. The principle behind the design
of jacks is Pascal’s law.  The load applied  by a jack is measured by the pressure
reading from a gauge attached to the oil inflow or by a separate load cell.  The following
figure shows a double acting hydraulic jack with a load cell. 



Anchoring Devices
Anchoring devices are often made on the wedge and friction principle. In pre-tensioned
members, the tendons are to be held in tension during the casting and hardening of
concrete. Here simple and cheap quick-release grips are generally adopted.

Harping Devices
The tendons are frequently bent, except in cases of slabs-on-grade, poles, piles etc.
The tendons are bent (harped)  in between the supports with a shallow sag as shown
below.