Comparison of the load bearing capacity and behaviour of two earth rendered and one cement rendered straw bale wall

Building materials that are sourced from the wastes of an agricultural process show great merit in their applicability to smart and sustainable ways of construction. One such material is straw, the waste material of regularly grown and harvested cereal crops. Most straw bale walls are rendered with a cement/sand based mortar like mixture, sometimes with the addition of lime. This traditional form of render has been considered because of its known (and proven) strength and weathering characteristics due to the addition of the cement. However such finishing methods are not without their critics. Purists would prefer all straw bale constructions to be finished without a render containing any cement. Suitable earth based renders are a good proposition to finish these types of buildings. But there is a question about the strength and durability of such earth rendered straw bale walls. This paper examines the performance, behaviour and load-bearing capacity of three differently rendered straw bale walls tested at the University of Western Sydney. One wall was rendered using a traditional cement based render; the other two had an applied earthen render of soil mixed with wheat chaff and without the addition of any cement. One of the earth rendered walls had the render in direct compression due to the positioning of the top plate structure by design. The other earth rendered wall had the load to the top plate applied directly to the straw bales. The emphases were placed on the strength, compressibility, build ability and serviceability of the rendered straw bale walls. The full-scale wall tests were undertaken in laboratory conditions to failure. The test results were compared with recommendations provided by some of the codes of practice and other available test results. The tests show that the walls are well suited for "the normal" loads associated with house construction. Recommendations regarding issues of construct ability and serviceability were also made.