All Bridgeman concrete plants are externally audited and graded, subsequently all concrete produced complies with the NZS 3104 concrete standard. In addition Bridgeman concrete have a stringent internal Quality Assurance system coupled with qualified engineers and testing staff to ensure, product consistency, Quality and customer support where required.

 

For optimum results it is highly recommended that when commencing your project that an experienced concrete placer is engaged and that all aspects of the desired out-come(s) of your project are discussed with your concrete placer.

 

CRACKING

 

Prehardening Cracks

Plastic Shrinkage Cracks

Prevention of Plastic Shrinkage Cracks

 

Figure 1

Plastic Settlement Cracks

 

Most concrete after it is placed bleeds, i.e. water rises to the surface as the solid particles settle. The bleed water evaporates and there is a loss of total volume – the concrete has ‘settled’.

 

If there is no restraint, the net result is simply a very slight lowering of the surface level. However if there is something near the surface, such as a reinforcing bar which restrains part of the concrete from settling while the concrete on either side continues to drop, there is potential for a crack to form over the restraining element. See Figure 2

 

Differential amounts of settlement may also occur where there is a change in the depth of a section, such as at a beam/slab junction. See Figure 3

 

Settlement cracks tend to follow a regular pattern coinciding with a restraint, usually the reinforcement or a change in section. Generally the cracks are not deep, but because they tend to follow and penetrate down to the reinforcement, they may reduce the durability of a structure.

Factors which may contribute to plastic settlement include:

• rate of bleeding;

• the depth of reinforcement relative to total thickness;

• the total time of settlement;

• the depth of reinforcement/size of bar ratio;

 

 

Prevention of Plastic Settlement Cracking

Figure 3

Figure 2

Other procedures which may help reduce plastic settlement cracking include:

• using lower slump mixes;

• using more cohesive mixes;

• using an air entrainer to improve cohesive-ness and reduce bleeding; and

• increasing cover to top bars.

 

Where there is a significant change in section, the method of placing may be adjusted to compensate for the different amounts of settlement. If the deep section is poured first to the underside of the shallow section, this concrete can be allowed to settle before the rest of the concrete is placed. However the top layer must be well vibrated into the bottom layer.

 

Avoiding the use of retarders is sometimes suggested as a way of reducing plastic settlement cracking, but for hot-weather concreting, the advantages of retarders generally outweigh the disadvantages.

 

Cracks Caused By Formwork Movement

 

If there is deliberate or unintentional movement of the formwork after the concrete has started to stiffen but before it has gained enough strength to support its own weight, cracks may form. Such cracks have no set pattern.

 

To avoid cracking from this cause, formwork must be:

• suff iciently strong and rigid to support the weight of the concrete without excessive deflections; and

• lef t in place until the concrete has gained suf ficient strength to support itself.

 

Some guides for the stripping time of formwork assume that Type GP cement is being used. Concretes incorporating supplementary cementitious materials, such as fly ash, may take longer to gain strength and allowance should be made for this.

 

 

Cracks in Hardened Concrete

Prevention of Crazing

Drying Shrinkage Cracks

Preventing Cracking Due to Drying Shrinkage

 

Thermal Movement Cracks

 

Heat of Hydration

 

Heat of Hydration cracks

Internal Restraint

External Restraint

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• sufficiently strong and rigid to support the weight of the concrete without excessive deflections; and

• left in place until the concrete has gained sufficient strength to support itself.