Nine times out of ten the culprit which causes poor concrete durability is water. Water, whether it's in the plastic concrete or in the hardened concrete, is the basic fundamental, undeniable source of durability problems. Excess mixing water, commonly called water of convenience, results in higher-than-designed water-cement ratio. This in turn results in a lower strength, higher permeability, and reduced durability. If you need or want additional workability for your concrete, invest in a high-range water reducer, or as they are more commonly called--a superplasticizer. With plasticizers you can have your cake and eat it too. You can have additional workability (chemically induced rather than water induced) and still maintain a low water-cement ratio to ensure that the resultant concrete will have high strength, low permeability, and increased durability.